homeostasis review Flashcards
Intro to Homeostasis
Regulation of body functions like blood sugar, body temp, blood pressure and pH
Changes in the environment can cause bodily functions to fluctuate. They must stay within acceptable ranges. Our bodies must be able to adapt accordingly.
Homeostasis: The process by which a constant internal environment is maintained despite changes in the external environment.
Sensor or monitor: detects a change in a variable
Control Centre or coordinating centre: receives a message from the monitor; directs a response via a regulator
Regulator or effector: carries out response initiated by the coordinating centre
Feedback systems in the body maintain homeostasis. Two types:
- Negative feedback: reverses the change – >maintenance. E.g: Body temperature increases 🡪 Skin blood vessels dilate 🡪 Body temperature decreases
- Positive feedback: increases change –> moves the variable even further away from its steady state. E.g., uterine contractions are stimulated by oxytocin 🡪 baby moves towards cervix 🡪 more oxytocin is released
Body Systems
The trillions of cells that make up your body can be organized into about 100 different types. Similarly specialized types of cells that perform a common function make up a tissue. Tissues of different types are organized as organs, which themselves are organized structurally and functionally as systems, as shown in Figure 8.1. Organ systems work together to perform functions necessary to sustain and maintain the human organism. The systems can be organized into groups based on functions with a common purpose.
Human Body Systems:
The circulatory system and lymphatic system both transport materials throughout the body. The heart and blood vessels pump and carry blood through the body. Blood transports nutrients and oxygen to cells and removes waste molecules excreted by cells. In the lymphatic system, vessels absorb fat from the digestive system and collect excess tissue fluid, which is returned to the blood and, thus, the circulatory system. The circulatory system and the lymphatic and immune systems are also involved in protecting the body against disease and substances that are foreign to the body.
Three systems—the digestive, respiratory, and excretory systems—add and/or remove substances from the blood. The digestive system processes food into nutrient molecules that are absorbed by the small intestine and enter the blood. The respiratory system brings oxygen into the body and removes carbon dioxide from the body. It also exchanges gases with the blood. The excretory (urinary) system rids the body of wastes and helps regulate the fluid level and chemical content of the blood.
The sensory receptors in the integumentary system, which consists of the skin, nails, hair, and glands, communicate with the brain and spinal cord via nerve fibres. The muscular and the skeletal systems enable the body and its parts to move. These two systems, along with the integumentary system, also protect and support the internal environment of the body.
The nervous system allows the body to respond to both external and internal stimuli. The endocrine system consists of the hormonal glands that secrete chemicals that serve as messengers between body cells. Both the nervous and the endocrine systems coordinate and regulate the functions of the body’s other systems.
The reproductive system involves different organs in the male and the female body. However, in both genders, the reproductive system produces and transports gametes and produces sex hormones.
Homeostasis
Whether you are sleeping, studying, enjoying a nice meal, exercising, working outside on a hot day, or hiking in below freezing temperatures, your body is working to maintain your internal temperature near a set point of 37°C. During these activities, your body works to maintain your blood glucose level around 100 mg/mL. Several processes in your body will help keep the pH of your blood near 7.4. Regardless of external conditions, the internal environment of your body remains stable or relatively constant.
The tendency of the body to maintain a relatively constant internal environment is known as homeostasis. Homeostasis is critical for survival because, like other vertebrates, the human body can survive only within a narrow range of conditions.
Homeostasis is a dynamic process. What this means is that any given variable, such as body temperature, blood glucose levels, or blood oxygen levels, may rise and fall around an average value throughout the course of a day, but still be considered to be in balance.
For example, blood glucose levels change in response to consuming food or going long periods without eating. Figure 8.2 shows how blood glucose levels may change throughout the day in a healthy individual. After a meal, blood glucose levels can rise quickly, especially if you’ve eaten something with lots of carbohydrates, such as pasta or potatoes. The endocrine system then reacts to bring glucose levels back to a normal value. If you were to go a prolonged period of time without eating, such as skipping a meal, blood glucose levels would start to fall. When that happens, the endocrine and nervous systems work to keep glucose levels within a normal range.
Whether it is from changes in the external environment or changes within the body, homeostasis is disturbed continually. Body systems respond by constantly monitoring any internal changes and maintaining homeostasis through feedback systems. A feedback system is a cycle of events in which a variable, such as body temperature, blood glucose level, or blood pH, is continually monitored, assessed, and adjusted. A feedback system consists of three components:
* a sensor, which detects a change in the internal environment and sends a signal to a control centre
* a control centre, which sets the range of values within which a variable should be maintained, receives information from the sensor, and sends signals to effectors when needed
* an effector, which receives signals from a control centre and responds, resulting in a change to an internal variable
The body uses two types of feedback systems to regulate its internal environment: negative feedback systems and positive feedback systems.
In a negative feedback system, the body works to reverse a change detected in a variable so that the variable is brought back to within a normal range.
Figure 8.3A compares negative feedback to the way a seesaw moves. A seesaw is level when the forces acting on it are balanced. If a change occurs to disrupt this balance, the seesaw can be made level again by applying a force to reverse the change.
In terms of negative feedback, a sensor detects a change that disrupts a balanced state and signals a control centre. The control centre then activates an effector, which reverses the change and restores the balanced state.
You know that when you engage in moderate to vigorous exercise for more than a few minutes, you start to sweat. But do you know why? As shown in Figure 8.3B, sweating is part of a negative feedback system your body uses to keep your internal temperature as close to 37°C as possible. As you exercise, your muscles produce heat, which raises the temperature of the blood. As a result, signals are sent to the control centre. The control centre directs a response to several effectors, including blood vessels and sweat glands. The blood vessels dilate, resulting in heat loss through radiation and conduction. The sweat glands release sweat. As sweat evaporates from the skin, heat is released from the body. These responses continue until body temperature returns to normal.
When you are outside on a cold day, such as when you are waiting for the bus to pick you up, you may start to shiver. Like sweating, shivering occurs in response to a change in body temperature; in this case, body temperature has begun to fall below normal. As shown in Figure 8.3B, sensors in the skin and brain send messages to the control centre. The control centre sends messages to several effectors. Blood vessels in the skin constrict, decreasing heat loss through the skin. Hormones are released that lead to an increase in body metabolism, which generates heat. Muscles begin to contract repeatedly, which results in shivering, which increases heat production. When body temperature is restored to normal, the feedback cycle and these responses stop.
Unlike a negative feedback system, a positive feedback system tends to strengthen or increase a change in a variable.
One example of a body process that is controlled by a positive feedback system is blood clotting. After an injury occurs, the affected tissues release chemicals that activate platelets. The platelets begin the clotting process. As well, they release chemicals that stimulate further clotting until the bleeding stops.
As shown in Figure 8.4, a positive feedback system also regulates contractions during childbirth. When a woman is giving birth, the uterus contracts, forcing the baby’s head or body into the cervix. The head of the baby presses against the cervix. This stimulates sensors in the cervix. Impulses are sent to the brain (control centre), which causes the pituitary gland (a gland in the endocrine system) to release oxytocin. Oxytocin is a hormone that causes muscles in the wall of the uterus (effectors) to contract. As labour continues, sensors in the cervix continue to send impulses to the brain, which leads to the release of more oxytocin. The release of more oxytocin leads to ever-stronger contractions until birth occurs.
Positive feedback systems are far less common than negative feedback systems. As exemplified above, positive feedback systems tend to be involved in processes that have a definitive cut-off point. In the case of clotting, the feedback cycle stops when bleeding stops. In the case of childbirth, the feedback cycle stops when the birth of the baby occurs. As you read this chapter and the two that follow, you will learn about more examples of negative and positive feedback systems in the human body.
Nervous System part and functions
Two Major Divisions
- Central Nervous System (CNS): Brain and spinal cord. Controls all of the body’s activities
- Peripheral Nervous System (PNS): All Nerves and sense organs. Messenger between CNS and rest of body. Somatic system = voluntary control (i.e. skeletal muscle(you decide to move)). Autonomic system = involuntary control (i.e. glands, smooth muscle, cardiac muscle(responses that are involuntary like releasing hormones or breathing))
Neurons:
- nerve cells that respond to stimuli, conduct electrochemical signals(messages), and regulate body
Consist of:
- Dendrites that branch out to receive nerve impulses from other neurons and relay them
- Cell body (nucleus) the site of metabolic reactions and dendrite input relay to axon
- Axon conducts impulses away from cell body to communicate with other tissues
- Myelin sheath fatty insulator protects axon
Glial Cells:
- Support cells that surround neurons
- Provide framework to sustain all nervous system tissue
- Nourish neurons, remove wastes, and defend against infections
- Schwann cells: specialized glial cells that wrap myelin sheaths around axons
Neuron Types:
- Bipolar (Inter): Two processes extend from cell body: one main dendrite before branching, one axon (tells motor what to do)
- Unipolar (Sensory): One axon branches from cell body (sensning and deteting stimuli)
- Multipolar (Motor): Many dendrites and one axon branch from cell body (wokrs to respond to stimuli)
Sensory Neurons: Receive stimuli and form nerve impulses to transmit them to CNS
Interneurons: Part of CNS that processes incoming sensory information and relays it to motor neurons
Motor Neurons (in peripheral nervous system): Transmit information from CNS to effectors (i.e. muscles, glands, etc.)
Reflex arc:
A simple connection of as few as three neurons that produces a reflex action in response to a stimulus
Moves directly to and from brain and spinal cord before voluntary control can respond
Enables body to react rapidly and involuntarily to potentially dangerous stimuli, even before you are conscious of the threat
E.g. jerking hand away from hot/sharp objects, blinking
Spinal Cord:
Anatomy: column of nerve tissue extending out of skull from the brain, downward through the backbone
Role: communication link between brain and PNS
Sensory nerves carry messages from body to the brain
Interpretation of message and preparation of response
Motor nerves relay response message to effectors
Grey matter: butterfly-shaped core of unmyelinated spinal neurons, and their cell bodies and dendrites
White matter: outer myelinated nerve fibres that surround grey matter
Brain:
Hindbrain:
- Cerebellum, Behind cerebrum, posture, reflexes, fine motor
- Medulla oblongata, Base of brain stem, connects brain & SC
Reflex coordination, bodily functions
- Pons Above M. oblongata, Relay centre between L&R halves of cerebrum
Midbrain:
- Midbrain, Above pons, Process/control information to/from eyes, ears, nose
Forbrain:
- Thalamus, Base of forebrain
Relay forebrain and hindbrain info
- Hypothalamus, Below thalamus, Internal environment & behaviour
- Cerebrum, Largest portion
Intellect, learning
Meninges: three tough, elastic tissue layers in skull and spinal column that enclose brain and SC for protection
Blood- Brain Barrier (BBB): glial cells (astrocytes) and blood vessels separate blood from CNS to allow selective control of substances that enter brain from blood
Cerebrospinal Fluid (CSF): transports hormones, white blood cells, nutrients across BBB for brain and SC; also acts as protection for brain and SC, shock absorbent
Cerebral cortex:
Folded layer of grey matter covering on each hemisphere
Each hemisphere is further divided into four lobes:
Occipital: receive/analyze visual information
Temporal: auditory reception, speech recognition
Parietal: receive/process information from skin
Frontal: critical thinking, memory, personality, speech
Corpus Callosum:
White matter linking two hemispheres, sends messages
Peripheral NS:
- Somatic: voluntary control of skelatl mucles
- Autonmic system - involuntary control of smooth, cardian muscle, glands
- sympatjetic - stress
- parasympathetic - rest
Structures and Processes of Nervous System
The human nervous system is equipped to sense and respond to continuous change within both the body and the external environment. The nervous system performs the vital function of regulating body structures and processes to maintain homeostasis despite fluctuations in both the internal and the external environment.
To maintain homeostasis, the human body must react to differences in temperature as well as respond to various internal and external stimuli, and it must regulate these responses. The human nervous system can regulate tens of thousands of activities simultaneously.
The nervous system monitors and controls body processes, from automatic functions (such as breathing) to activities that involve fine motor coordination, learning, and thought (such as playing a musical instrument). The brain and spinal cord, and the nerves that emerge from them and connect them to the rest of the body, make up the nervous system.
Together, the central nervous system and the peripheral nervous system control sensory input, integration, and motor output.
The central nervous system, which consists of the brain and spinal cord, integrates and processes information sent by nerves.
The peripheral nervous system includes nerves that carry sensory messages to the central nervous system and nerves that send information from the CNS to the muscles and glands.
The peripheral nervous system is further divided into the somatic system and the autonomic system, also shown in Figure 8.6. The somatic system consists of sensory receptors in the head and extremities, nerves that carry sensory information to the central nervous system, and nerves that carry instructions from the central nervous system to the skeletal muscles. The somatic system is under voluntary control. The autonomic system controls glandular secretions and the functioning of the smooth and cardiac muscles. These processes are involuntary. Involuntary processes, such as heartbeat and peristalsis, are those that do not require or involve conscious control.
The sympathetic and parasympathetic divisions of the autonomic system often work in opposition to each other to regulate the involuntary processes of the body.
The nervous system is composed of only two main types of cells: neurons and cells that support the neurons, which are called glial cells.
Neurons are the basic structural and functional units of the nervous system. They are specialized to respond to physical and chemical stimuli, to conduct electrochemical signals, and to release chemicals that regulate various body processes. Individual neurons are organized into tissues called nerves.
The activity of neurons is supported by another type of cells called glial cells. The word glial comes from a Greek word that means “glue.” Collectively, glial cells nourish the neurons, remove their wastes, and defend against infection. Glial cells also provide a supporting framework for all the nervous-system tissue.
Neurons have many of the same features as other body cells, such as a cell membrane, cytoplasm, mitochondria, and a nucleus.
In addition, neurons have specialized cell structures that enable them to transmit nerve impulses. Different types of neurons are different shapes and sizes.
In general, however, they share four common features: dendrites, a cell body, an axon, and branching ends, all shown in Figure 8.8.
Dendrites are short, branching terminals that receive nerve impulses from other neurons or sensory receptors, and relay the impulse to the cell body. The dendrites are numerous and highly branched, which increases the surface area available to receive information.
The cell body contains the nucleus and is the site of the cell’s metabolic reactions. The cell body also processes input from the dendrites. If the input received is large enough, the cell body relays it to the axon, where an impulse is initiated.
The axon conducts impulses away from the cell body. Axons range in length from 1 mm to 1 m, depending on the neuron’s location in the body. For example, the sciatic nerve in the leg contains neuronal axons that extend from the spinal cord all the way to the muscles in the foot, a distance of over 1 m.
The terminal end of an axon branches into many fibres, as shown in Figure 8.8. To communicate with adjacent neurons, glands, or muscles, the axon terminal releases chemical signals into the space between it and the receptors or dendrites of neighbouring cells.
The axons of some neurons are enclosed in a fatty, insulating layer called the myelin sheath, which gives the axons a glistening white appearance. The myelin sheath protects neurons and speeds the rate of nerve impulse transmission.
Schwann cells, a type of glial cell, form myelin by wrapping themselves around the axon.
Neurons can be classified based on their structure as well as their function.
Structurally, neurons are classified based on the number of processes that extend from the cell body.
Table 8.1 describes three types of neurons based on structure: multipolar, bipolar, and unipolar neurons.
Multipolar Neuorn: * Has several dendrites * Has a single axon * Found in the brain and spinal cord
Bipolar Nueron: * Has a single main dendrite * Has a single axon * Found in the inner ear, the retina of the eye, and the olfactory area of the brain
Unipolar Neuoron: * Has a single process that extends from the cell body * Dendrite and axon are fused * Found in the peripheral nervous system
Functionally, neurons are classified as one of three main types: sensory neurons, interneurons, or motor neurons. These three main types of neurons form the basic impulse- transmission pathway of the entire nervous system. This pathway, shown in Figure 8.9, depends on three overlapping functions: sensory input, integration, and motor output.
1. Sensory input: Sensory receptors, such as those in the skin, receive stimuli and form a nerve impulse. Sensory neurons transmit impulses from the sensory receptors to the central nervous system (brain and spinal cord).
2. Integration: Interneurons are found entirely within the central nervous system. They act as a link between the sensory and motor neurons. They process and integrate incoming sensory information, and relay outgoing motor information.
3. Motor output: Motor neurons transmit information from the central nervous system to effectors. Effectors include muscles, glands, and other organs that respond to impulses from motor neurons.
Some neurons are organized to enable your body to react rapidly in times of danger, even before you are consciously aware of the threat.
These sudden, involuntary responses to certain stimuli are called reflexes.
Reflex arcs are simple connections of neurons that explain reflexive behaviours. They can be used to model the basic organization of the nervous system.
Reflex arcs usually involve only three neurons to transmit messages. As a result, reflexes can be very rapid, occurring in about 50 ms (milliseconds).
Withdrawal reflexes, for example, depend on only three neurons. Figure 8.11 illustrates a typical neural circuit, as well as a withdrawal reflex from a potentially painful situation. Receptors in the skin sense the pressure of the cactus needle and initiate an impulse in a sensory neuron. The impulse carried by the sensory neuron then activates the interneuron in the spinal cord. The interneuron signals the motor neuron to instruct the muscle to contract and withdraw the hand.
A reflex arc moves directly to and from the brain or spinal cord, before the brain centres involved with voluntary control have time to process the sensory information.
we know that neurons use electrical signals to communicate with other neurons, muscles, and glands. These signals, called nerve impulses, involve changes in the amount of electric charge across a cell’s plasma membrane.
Resting Membrane Potential:
When microelectrodes are inserted in an inactive, or resting, neuron, measurements from a voltmeter indicate an electrical potential difference (voltage) across the neural membrane.
In a resting neuron, the cytoplasmic side of the membrane (inside of the cell) is negative, relative to the extracellular side (outside of the cell).
The charge separation across the membrane is a form of potential energy, or membrane potential. The resting membrane potential is the potential difference across the membrane in a resting neuron. The resting membrane potential of most unstimulated neurons is about −70 mV (millivolts).
The electrical charge is negative on the inside of the cell, relative to the outside of the cell, as shown in Figure 8.12. The resting membrane potential provides energy for the generation of a nerve impulse in response to an appropriate stimulus.
Three factors contribute to maintaining resting membrane potential.
First, large protein molecules that are negatively charged are present in the intracellular fluid but not outside of the cell. These proteins are so large that they cannot pass through the cell membrane, contributing to the negative charge in the interior of the cell.
Second, the plasma membrane contains ion-specific channels that allow for the passive movement of ions, such as sodium (Na+) and potassium (K+), across the membrane. In particular, K+ channels tend to be open more often at resting potential. This means that potassium can move out of the cell more readily, whereas sodium cannot move into the cell as easily, making the interior of the cell more negative relative to the exterior.
Third, and most important, is the sodium-potassium pump, which actively transports Na+ and K+ in ratios that leave the inside of the cell negatively charged compared to the outside of the cell. The process of generating a resting membrane potential of −70 mV is called polarization.
Sodium Potassium Pump:
The most important contributor to the separation of charge and the resulting electrical potential difference across the membrane is the sodium-potassium pump.
This system uses the energy of ATP to transport sodium ions out of the cells and potassium ions into the cells. The process is shown in Figure 8.13.
Notice that for every three sodium ions transported out of the cell, two potassium ions are transported into the cell. As a result, an excess of positive charge accumulates outside of the cell.
Recall that the cell membrane is not totally impermeable to sodium and potassium ions, so they also leak slowly by diffusion across the membrane in the direction of their concentration gradient. However, potassium ions are able to diffuse out of the cell more easily than sodium ions can diffuse into the cell.
The overall result of the active transport of sodium and potassium ions across the membrane, and their subsequent diffusion back across the membrane, is a constant membrane potential of −70 mV.
You might wonder why the −70 mV potential difference across the neuronal membrane is called the resting membrane potential when the sodium-potassium pump is constantly using energy to transport these ions. The term “resting” means that no nerve impulses are being transmitted along the axon.
The resting potential maintains the axon membrane in a condition of readiness for an impulse to occur. The energy for any eventual impulses is stored in the electrochemical gradient across the membrane.
Action Potential:
A nerve cell is polarized because of the difference in charge across the membrane—specifically that the inside of the cell is more negative than the outside of the cell.
Changes in membrane potential are changes in the degree of polarization. Depolarization occurs when the cell becomes less polarized, meaning that the membrane potential is reduced to less than the resting potential of −70 mV. During depolarization, the inside of the cell becomes less negative relative to the outside of the cell.
What causes a nerve cell to become depolarized, and how does the change in charge occur? An action potential causes depolarization to occur.
An action potential is the movement of an electrical impulse along the plasma membrane of an axon. It results in a rapid change in polarity across the axon membrane as the nerve impulse occurs. An action potential is an all-or-none phenomenon. If a stimulus causes the axon membrane to depolarize to a certain level, referred to as the threshold potential, an action potential occurs. Threshold potentials can vary slightly, depending on the type of neuron, but they are usually close to −50 mV. In an all-or-none response, the strength of an action potential does not change based on the strength of the stimulus. However, a strong stimulus can cause an axon to start an action potential more often in a given time period than a weak stimulus.
Figure 8.14 illustrates the changes in the membrane potential that occur during an action potential.
The events are also sequenced below. Notice that all of these events occur within a period of a few milliseconds. As well, they occur in one small region of the axon membrane.
* An action potential is triggered when the threshold potential is reached.
* When the membrane potential reaches threshold, special structures in the membrane called voltage-gated sodium channels open and make the membrane very permeable to sodium ions. The sodium ions on the outside of the axon suddenly move down their concentration gradient and rush into the axon. Within a millisecond or less, enough positively charged sodium ions have crossed the membrane to make the potential difference across the membrane in that tiny region of the axon +40 mV.
* As a result of the change in membrane potential, the sodium channels close and voltage- gated potassium channels open. The potassium ions now move down their concentration gradient toward the outside of the axon, carrying positive charge out of the neuron. As a result, the membrane potential becomes more negative again. In fact, the membrane potential becomes slightly more negative than its original resting potential, becoming hyperpolarized to about −90 mV. At this point, the potassium channels close.
* The sodium-potassium pump and the small amount of naturally occurring diffusion quickly bring the membrane back to its normal resting potential of −70 mV. The membrane is now repolarized—that is, returned to its previous polarization.
* For the next few milliseconds after an action potential, the membrane cannot be stimulated to undergo another action potential. This brief period of time is called the refractory period of the membrane.
The entire process described above continues down the length of an axon until it reaches the end, where it initiates a response at the junction with the next cell.
Myelinated Nerve Impulses:
A nerve impulse consists of a series of action potentials.
How does one action potential stimulate another?
Recall from earlier in this section that the axons of some neurons are enclosed in a fatty, insulating layer called a myelin sheath. At regular intervals, the axons of myelinated neurons have exposed areas known as nodes of Ranvier, shown in Figure 8.15.
Nodes of Ranvier contain many voltage-gated sodium channels. The nodes of Ranvier are the only areas of myelinated axons that have enough sodium channels to depolarize the membrane and elicit an action potential.
When the sodium ions move into the cell, the charge moves quickly through the cytoplasm to the next node. When the sodium ions reach the neighbouring node of Ranvier, the positive charges reduce the net negative charge inside the axonal membrane. The presence of the positively charged sodium ions causes the membrane at the node to become depolarized to threshold. Since an action potential just occurred at the node to the left, that membrane is refractory, which means that it cannot yet be stimulated to undergo another action potential.
This mechanism prevents impulses from going backward. The membrane of the node of Ranvier to the right is not refractory, so the depolarization initiates an action potential at this node.
The same process occurs at each node until it reaches the end of the neuron. This process of one action potential stimulating the production of another one at the next node constitutes the nerve impulse.
Because action potentials are forced to “jump” from one node of Ranvier to the next due to the myelin sheath, the conduction of an impulse along a myelinated neuron is called saltatory conduction. The word saltatory comes from a Latin word that means to jump or leap.
In unmyelinated neurons, conduction of a nerve impulse is continuous. Rather than jumping from one section of an axon to another, action potentials in unmyelinated neurons cause the release of sodium along each adjacent portion of a membrane.
As a result of this step-by-step conduction along the axon, the transmission of an impulse along an unmyelinated axon is much slower than the saltatory conduction along a myelinated axon—about 0.5 m/s, compared with as much as 120 m/s in a myelinated axon.
Signal Trasnmission across a Synapse:
The simplest neural pathways have at least two neurons and one connection between the neurons. Other neural pathways can involve thousands of neurons and their connections as an impulse travels from the origin of the stimulus, through the sensory neurons to the brain, and back through motor neurons to the muscles or glands.
The connection between two neurons, or a neuron and an effector, is called a synapse. A neuromuscular junction is a synapse between a motor neuron and a muscle cell.
An impulse travels the length of the axon until it reaches the far end, called the synaptic terminal. Neurons are not directly connected, but have a small gap between them called the synaptic cleft. Although the synaptic cleft is only about 0.02 μm wide, neurons are not close enough for the impulse to jump from one to the other.
How, then, does the impulse proceed from the presynaptic neuron, which sends out information, to the postsynaptic neuron, which receives the information?
Chemical messengers called neurotransmitters carry the neural signal from one neuron to another. Neurotransmitters can also carry the neural signal from a neuron to an effector, such as a gland or muscle fibre. Figure 8.16 shows the sequence of events in the movement of an impulse across a synapse.
When an action potential arrives at the end of a presynaptic neuron, the impulse causes intracellular sacs that contain neurotransmitters to fuse with the membrane of the axon. These sacs, called synaptic vesicles, release their contents into the synaptic cleft by exocytosis.
The neurotransmitters then diffuse across the synapse, taking about 0.5 to 1 ms to reach the dendrites of the postsynaptic neuron, or cell membrane of the effector.
Upon reaching the postsynaptic membrane, the neurotransmitters bind to specific receptor proteins in this membrane. As Figure 8.16 illustrates, the receptor proteins trigger ion-specific channels to open. This depolarizes the postsynaptic membrane and, if the threshold potential is reached, initiates an action potential. The impulse will travel along the postsynaptic neuron to its terminal and to the next neuron or an effector.
Neurotransmitters have either excitatory or inhibitory effects on the postsynaptic membrane.
If the effect is excitatory, the receptor proteins will trigger ion channels that open to allow positive ions, such as sodium, to flow into the postsynaptic neuron. As a result, the membrane becomes slightly depolarized.
If the neurotransmitter is inhibitory, the receptor will trigger potassium channels to open, allowing potassium ions to flow out. This results in a more negative membrane potential, resulting in hyperpolarization.
Nervous system communication
Parts of the Neuron:
- Cell body
- Dendrites (start)
- Axon (end)
Motor neurons stimulate muscle tissue
Dendrites receive impulses and conduct them toward the cell body
Myelinated axon:
-A single long, thin extension of varying lengths that sends impulses between neurons
- Surrounded by a layered lipid-protein myelin sheath, made of Schwann cells
(transmits nerve impusles away from cell body to dendirte of neighboring cells dendrite . schwann cells provide quicker transmission of impulses)
Resting membrane potential:
- A resting neuron is one that is not conducting an impulse
- Resting membrane potential: the difference in electrical charges (RECALL: voltage drop) between the outside and inside of the neuron membrane
- Typically, resting membrane potential = -70mV
- The outside has a positive charge and the inside has a negative charge
Na+and K+ pump:
- Different numbers of potassium ions (K+) and sodium ions (Na+) on either side of membrane
- When a nerve cell is at rest, each ATP hydrolysis makes the pump actively transport 3 Na+ out of the cell and 2 K+ into the cell through gates
- These gates are found between Schwann cells in the Nodes of Ranvier
(atp hydrolysis is inpuy of energy —> at rest you maintain the rmp of more neg on inside and pos outside → for 3 pos going out you have 2 pos coming in making inside more neg → voltage gated active pumps → find them in between schwann cells in node of ranvier )
Na+ and K+ gradient:
- Pump creates electrochemical gradient for Na+/K+
- So small amounts of K+ and Na+ also tend to diffuse back out of and into cell across the membrane
- Membrane is more permeable to K+ than Na+
- Overall result is a polarized membrane with a constant resting potential of -70mV
(3 out and 2 in produces an electrochemical gradient → some leakage of ions back into or out of cell → more permeable for to k so neg inside and na stays out)
Action potential:
- Rapidly depolarizes the membrane by sending an electrical impulse along the axon membrane when triggered, making membrane potential less negative
- If a stimulus causes depolarization to the threshold potential of -50mV first, an action potential occurs
- an action potential is triggered when the threshold potential is reached
- na+ channels open at -50mV and na+ rushes into the cell and depolarizes the membrane even more
- na+ channels are inactivated at +40mV and K+ channels open at +40mV so potassium ions exit the cell and hyperpolarizes the membrane slightly below the resting potential
- k+ channels close and na+ channels go from inactivated to closed. the resting membrane potential is restored and normal pumping occurs
All or none principle:
- During the depolarization, Na+ rushes into the cell until the action potential reaches its peak and the sodium gates close
- If depolarization is not enough to reach threshold, then an action potential and impulse do not occur
- This is called the All-or-None Principle
- you either detect stimulus and respond or you don’t
Speed of nerve impulses:
- The presence of a myelin sheath greatly increases the velocity at which impulses are conducted along the axon of a neuron
- In unmyelinated fibres, the entire axon membrane is exposed and impulse conduction is slower.
Synapses: transmitting impulses
- Synapses are the connecting junctions between two neurons, which is how impulses travel: Stimulus → sensory neurons →brain →motor neurons →effectors (muscles and glands)
- Synaptic cleft: 0.2μm gap between pre- and post-synaptic neurons that interrupts signal transmission
So how does the impulse travel between the neurons?
- Neurotransmitters (NTs): inhibitory or excitatory chemical messengers secreted by neurons to carry signals between neurons, or from neuron to effector
- Synaptic vesicles: sacs that carry NTs and release them via exocytosis in the synaptic cleft
Excretory System
Four Major Functions
Metabolic Waste Excretion: mostly nitrogenous wastes (i.e. ammonia is converted to urea in the liver, uric acid)
Salt-Water Balance: concentration of ion salts (Na+, K+, HCO3-, Ca2+) determines blood volume, pressure
Acid-Base Balance: maintain blood pH ~7.4 by excreting H+ ions and/or reabsorbing HCO3- ions as needed; for this reason urine pH ~6
Hormone Secretion: two hormones. Calcitrol: active form of vitamin D for Ca+ absorption. Erythropoietin (EPO): stimulates red blood cell production when O2 demands increase or O2-carrying capacity is low
Kidneys (x2): produce urine, surrounded by protective fat cushion, in the lower back region just above hips
Urinary bladder: temporarily stores urine until its release through two sphincters 🡪 inner and outer
Ureters (x2): muscular tubes that carry urine from the kidneys to the bladder; each is approx. ~28cm long
Urethra: tube through which urine is passed outside; in males it merges with ductus deferens and is ~20cm long; in females it has its own separate opening, is ~4cm long
Kidneys:
Have a renal artery 🡪 delivers O2- blood from aorta, and a renal vein 🡪 drains dO2-blood, returns solutes & H2O back to the body; both have several branches in kidneys
Three Regions:
Renal Cortex: outer layer, contains top part of nephrons (outer)
Renal Medulla: inner layer, contains bottom of nephrons (inner)
Renal Pelvis: central space that leads to ureters (central)
Nephrons:
The functional units of kidneys
Microscopic, tube-like structures that filter & reabsorb substances in blood and produce urine; 1 million/kidney
Three Main Regions
Filter: top of nephron, Bowman’s Capsule, has network of branched capillaries from r. artery, a glomerulus 🡪 impermeable to large molecules, produces filtrate
Tubule: Bowman’s Capsule connected to long, narrow, looped tubule that reabsorbs useful substances; three sections 🡪 proximal tubule, loop of Henle (close to nephron), distal tubule (further)
Duct: tubule empties into collecting duct, to reabsorb water from filtrate to minimize water loss from body; filtrate that remains becomes urine
Urine Production:
Glomerular Filtration: water and solutes become filtrate. Increased blood pressure forces water and solutes in blood plasma to move from the glomerulus to the Bowman’s capsule via the porous, semi-permeable capillary walls
Tubular Reabsorption: water, good solutes return to body. 65% of filtrate reabsorbed via active/passive transport as it goes through proximal tubule, which contain mitochondria (recall: ATP source) to provide energy to move ions
Tubular Secretion: more waste collected from blood. Distal tubule removes K+ and H+ ions via active transport. Foreign bodies (e.g. drugs, antibiotics) also removed
Water Reabsorption: minimizes water loss/dehydration. Reabsorbed ions increases blood plasma concentration, which drives water reabsorption via osmosis in the Loop of Henle and collecting duct
Water reabsorption is driven by demands and conditions of the body 🡪 i.e. if you are dehydrated, [blood plasma] increases, driving increased water reabsorption
Osmoreceptors in hypothalamus signal pituitary to release ADH in response to increased [blood plasma], causing increased permeability of distal tubule and collecting duct (therefore retains more water)
Osmoreceptors also signal pituitary to stop ADH release in response to decreased [blood plasma] (i.e. too dilute)
Diabetes insipidus and caffeine and alcohol consumption inhibit ADH activity
Urinary Tract Infections (UTIs) (bladder infection)
More common in females than males due to anatomy (since shorter in females, it’s easier for it to get dirty)
Can be bacterial or viral; causes frequent, painful urination that is often bloody, fever/chills, nausea/vomiting
Cystitis: in the bladder; urethritis: in the urethra (inflammation)
Kidney Stones
Form from excess Ca2+ deposits in urine caused by frequent UTIs, dehydration, physical inactivity
Smaller stones eventually pass with urine; larger ones may need ultra-sound disintegration first, or surgical removal
Renal Insufficiency
General term for damaged nephrons (i.e. renal failure)
If damage is severe enough, patients need hemodialysis (artificial kidney), or peritoneal dialysis (using peritoneum as a kidney) to replace kidney function, or a transplant
Excretory System detailed
Excretion is the process of separating wastes from body fluids, then eliminating the wastes from the body.
Several body systems perform this function. The respiratory system excretes carbon dioxide and small amounts of other gases, including water vapour. The skin excretes water, salts, and some urea in perspiration. The digestive system excretes water, salts, lipids, and a variety of cellular chemicals. Note that the elimination of food residue—feces—is not considered to be a process of excretion.
Most metabolic wastes are dissolved or suspended in solution and are excreted by the excretory system (also called the urinary system).
Functions of the Excretory System:
The excretory system produces urine and conducts it to outside the body. As the kidneys produce urine, they carry out the following four functions that contribute to homeostasis:
* Excretion of Metabolic Wastes The kidneys excrete metabolic wastes, notably nitrogenous (nitrogen-containing) wastes. Nitrogenous wastes include ammonia, urea, and uric acid. Ammonia is highly toxic but is converted in the liver to the less toxic compound urea. Urea makes up the majority of nitrogenous waste in the body, and about half of it is eliminated in urine. Uric acid is present in much lower concentrations, and is contained in urine.
Maintenance of Water–Salt Balance Another important function of the kidneys is to maintain the appropriate balance of water and salt in the blood. Blood volume is closely tied to the salt balance of the body. By regulating salts in the blood, the kidneys are also involved in regulating blood pressure. The kidneys also help maintain the appropriate level of potassium (K+), bicarbonate (HCO3 –), and calcium (Ca2+) in the blood.
Maintenance of Acid–Base Balance The kidneys regulate the acid–base balance of the blood. The kidneys monitor and help keep the blood pH at about 7.4, mainly by excreting hydrogen ions (H+) and reabsorbing the bicarbonate ions (HCO3–) as needed. Human urine usually has a pH of 6 or lower because our diet often contains acidic foods.
Secretion of Hormones The kidneys assist the endocrine system in hormone secretion. The kidneys secrete two hormones: calcitriol and erythropoietin. Calcitriol is the active form of vitamin D. Vitamin D promotes calcium (Ca2+) absorption from the digestive tract. Erythropoietin, which stimulates the production of red blood cells, is released in response to increased oxygen demand or reduced oxygen-carrying capacity of the blood. The kidneys also secrete renin, a substance that leads to the secretion of the hormone aldosterone from the adrenal cortex.
Organs of the Excretory System:
As shown in Figure 10.2, the human excretory system consists of the kidneys and ureters, the urinary bladder, and the urethra.
Two fist-sized kidneys are located in the area of the lower back on each side of the spine. If you stand up and put your hands on your hips with your thumbs meeting over your spine, your kidneys lie just above your thumbs. A large cushion of fat usually surrounds the kidneys. This fat layer, along with the lower portion of the ribcage, offers some protection for these vital organs.
Although most people have two kidneys, humans are capable of functioning with only one. If one kidney ceases to work or if a single kidney is removed due to disease or because it is being donated to someone in need of a kidney, the single kidney increases in size to handle the increased workload.
The kidneys release urine into two muscular, 28-cm-long tubes called ureters. From the ureters, urine is moved by the peristaltic actions of smooth muscle tissue to the muscular urinary bladder where it is temporarily stored.
Drainage from the bladder is controlled by two rings of muscles called sphincters. Both sphincters must relax before urine can drain from the bladder. The innermost sphincter is involuntarily controlled by the brain. During childhood we learn to voluntarily control relaxation of the other sphincter.
Urine exits the bladder and the body through a tube called the urethra. In males, the urethra is approximately 20 cm long and merges with the ductus deferens of the reproductive tract to form a single passageway to the external environment. In females, the urethra is about 4 cm long and the reproductive and urinary tracts have separate openings.
The Kidneys:
As illustrated in Figure 10.3, the kidneys are bean shaped and reddish-brown in colour. The concave side of each kidney has a depression where a renal artery enters and a renal vein and a ureter exit the kidney. A lengthwise section of a kidney shows that many branches of the renal artery and renal vein reach inside a kidney.
A kidney has three regions. The renal cortex is an outer layer that dips down into an inner layer called the renal medulla. As shown in Figure 10.3, the renal medulla contains cone-shaped tissue masses. The renal pelvis is a central space, or cavity, that is continuous with the ureter.
Embedded within the renal cortex and extending into the renal medulla are more than one million microscopic structures called nephrons. Closely associated with these nephrons is a network of blood vessels. The nephrons are responsible for filtering various substances from blood, transforming it into urine. To perform this function, each nephron is organized into three main regions: a filter, a tubule, and a collecting duct. These regions are highlighted in Figure 10.4 and discussed in further detail below the figure.
1. A Filter: The filtration structure at the top of each nephron is a cap-like formation called the Bowman’s capsule. Within each capsule, the renal artery enters and splits into a fine network of capillaries called a glomerulus [pronounced glow-MEER-you-lus] (the term means “little ball” in Latin). The walls of the glomerulus act as a filtration device. They are impermeable to proteins, other large molecules, and red blood cells, so these remain within the blood. Water, small molecules, ions, and urea—the main waste products of metabolism—pass through the walls and proceed further into the nephron. The filtered fluid that proceeds from the glomerulus into the Bowman’s capsule of the nephron is referred to as filtrate.
2. A Tubule: The Bowman’s capsule is connected to a small, long, narrow tubule that is twisted back on itself to form a loop. This long hairpin loop is a reabsorption device. The tubule has three sections: the proximal tubule, the loop of Henle, and the distal tubule. Like the small intestine, this tubule absorbs substances that are useful to the body, such as glucose and a variety of ions, from the filtrate passing through it. Unlike the small intestine, this tubule also secretes substances into the tissues surrounding it. You will find out more about these twin processes of reabsorption and secretion in Section 10.2.
3. A Duct: The tubule empties into a larger pipe-like channel called a collecting duct. The collecting duct functions as a water-conservation device, reclaiming water from the filtrate passing through it so that very little water is lost from the body. The filtrate that remains in the collecting duct is a suspension of water and various solutes and particles. It is now called urine. Its composition is distinctly different from the fluid that entered the Bowman’s capsule. The solutes and water reclaimed during reabsorption are returned to the body via the renal vein.
Note that the upper portions of each nephron are located in the renal cortex of the kidney, while the lower portions are located in the renal medulla of the kidney. Note also the presence of vessels of the circulatory system in association with the nephrons.
These details indicate that nephrons are surrounded by the tissues of the renal cortex and the renal medulla. Nephrons are also closely associated with a network of blood vessels that spreads throughout this surrounding tissue. Thus, any substances that are secreted from the nephrons enter the surrounding tissues of the kidney. Most of these substances return to the bloodstream through the network of blood vessels. The remainder leave the body in the form of urine.
How Urine Forms:
Four processes are crucial to the formation of urine. These processes are outlined below. * Glomerular filtration moves water and solutes, except proteins, from blood plasma into the nephron. Recall that this filtered fluid is called filtrate. * Tubular reabsorption removes useful substances such as sodium from the filtrate and returns them into the blood for reuse by body systems. * Tubular secretion moves additional wastes and excess substances from the blood into the filtrate. * Water reabsorption removes water from the filtrate and returns it to the blood for reuse by body systems.
Glomerular Filtration Filters Blood:
The formation of urine starts with glomerular filtration. This process forces some of the water and dissolved substances in blood plasma from the glomerulus, shown in Figure 10.5, into the Bowman’s capsule.
Keep in mind that this process is occurring in millions of nephrons all at the same time. Here, you are focussing your attention on only a single nephron.
Two factors contribute to this filtration. One factor is the permeability of the capillaries of the glomerulus. Unlike capillaries in other parts of the body, capillaries of the glomerulus have many pores in their tissue walls. These pores are large enough to allow water and most dissolved substances in the blood plasma to pass easily through the capillaries and into the Bowman’s capsule. On the other hand, the pores are small enough to prevent proteins and blood cells from entering.
The other factor is blood pressure. Blood pressure within the glomerulus is about four times greater than it is in capillaries elsewhere in the body. The great rush of blood through the glomerulus provides the force for filtration.
The process of glomerular filtration, as well as the remaining processes that form urine, are summarized in Figure 10.6, on the next page.
Each day, 1600 L to 2000 L of blood pass through your kidneys, producing about 180 L of glomerular filtrate. This filtrate is chemically very similar to blood plasma, as you can see in Table 10.1. Essentially, the filtrate is identical to blood plasma, minus proteins and blood cells. If the composition of urine were the same as that of the glomerular filtrate, the body would continually lose water, salts, and nutrients. Therefore, the composition of the filtrate must change as this fluid passes through the remainder of the tubule.
Tubular Reabsorption; recovery of substances in the proximal tubule:
About 65 percent of the filtrate that passes through the entire length of the proximal tubule (including the loop of Henle) is reabsorbed and returned to the body.
Figure 10.7 shows that this process of reabsorption involves both active and passive transport mechanisms.
The cells of the proximal tubule contain many mitochondria, which use the energy- releasing power of ATP to drive the active transport of sodium ions (Na+), glucose, and other solutes back into the blood. Negatively charged ions tag along passively, attracted by the electrical charge on the transported substances. Water follows the ions by osmosis, so it, too, is reabsorbed into the blood flowing through the capillaries.
Focussing on the Loop of Henle in the Proximal Tubule:
The function of the loop of Henle is to reabsorb water and ions from the glomerular filtrate.
As the descending limb of the loop of Henle plunges deeper into the medulla region, it encounters an increasingly salty environment. The cells of the descending limb are permeable to water and only slightly permeable to ions. As a result of the salty environment of the medulla and permeability of the descending limb, water diffuses from the filtrate to the capillaries by osmosis, as shown in Figure 10.8A.
As water moving through the descending limb leaves the filtrate, the concentration of sodium ions (Na+) inside the tubule increases, reaching its maximum concentration at the bottom of the loop.
As the filtrate continues around the bend of the loop of Henle and into the ascending limb, the permeability of the nephron tubule changes. Near the bend, the thin portion of the ascending tubule is now impermeable to water and slightly permeable to solutes. Sodium ions diffuse from the filtrate along their concentration gradient and pass into nearby blood vessels, as shown in Figure 10.8B.
At the thick-walled portion of the ascending limb of the loop of Henle, sodium ions are moved out of the filtrate by active transport, as illustrated in Figure 10.8C. This transport of Na+ out of the filtrate has two consequences: * First, it helps replenish the salty environment of the medulla, which aids in the absorption of water from filtrate in the descending limb. * Second, the removal of sodium ions from the filtrate in the thick-walled portion of the tubule makes the filtrate less concentrated than the tissues and blood in the surrounding cortex tissue.
By now, about two thirds of the Na+ and water from the filtrate has been reabsorbed.
Tubular Reabsorption and Secretion in the Distal Tubule:
The active reabsorption of sodium ions from the filtrate into the capillaries depends on the needs of the body.
Passive reabsorption of negative ions such as chloride occurs by electrical attraction. The reabsorption of ions decreases the concentration of the filtrate, which causes water to be reabsorbed by osmosis, as shown in Figure 10.9.
Potassium ions (K+) are actively secreted into the distal tubule from the bloodstream in the capillaries. Hydrogen ions (H+) are also actively secreted from the blood into the distal tubule as necessary in order to maintain the pH of the blood.
Other substances that are not normally part of the body, such as penicillin and other medications, are secreted from the blood into the distal tubule.
Reabsorption and secretion in the distal tubule are under the control of hormones, as you will see in Section 10.3.
Reabsorption from the Collecting Duct:
The filtrate entering the collecting duct still contains a lot of water.
Because the collecting duct extends deep into the medulla, the concentration of ions along its length increases. This concentration of ions is the result of the active transport of ions from the ascending limb of the loop of Henle. This causes the passive reabsorption of water from the filtrate in the collecting duct by osmosis.
If blood plasma is too concentrated (for example, if a person is dehydrated), the permeability to water in the distal tubule and the collecting duct is increased. This causes more water to be reabsorbed into the surrounding capillaries in order to conserve water in the body.
In the collecting duct, as in the distal tubule, hormones control reabsorption and secretion.
The reabsorption of water in the collecting duct causes the filtrate to become about four times as concentrated by the time it exits the duct. This filtrate—which is approximately 1 percent of the original filtrate volume—is now called urine.
Endocrine System + Reproductive system
Endocrine gland: secretes hormones directly into bloodstream without using ducts (i.e. ductless)
Some organs do more than make hormones (e.g. pancreas, testes, ovaries, thymus, hypothalamus)
Some are exclusively endocrine glands (e.g. pineal, pituitary, adrenal, thyroid, parathyroid)
Hormone: chemical messengers sent to targeted cells via bloodstream to produce specific effect
Together, endocrine glands and their hormones make up the endocrine system, which works with the nervous system to maintain homeostasis
lipid-based steroid hormones can easily diffuse through the cell membrane and into the cell; once inside, they bind to nucleus receptors. E.g. testosterone, estrogen, cortisol
Water-soluble hormones bind to receptor proteins on the cell membrane to trigger a reaction cascade that ultimately produces the targeted effect. E.g. epinephrine, human growth H, thyroxin, insulin
in receptor-mediated endocytosis, the cell membrane contains receptor proteins that recognize and bind specific molecules
Similarly, target cells contain receptor proteins that recognize and bind specific hormones
- in steroid harmone it will diffuse through the membrane and bind to recpector inside the nuclues. the harmone receptor complex activates gene and synthesis of specific mRNA molecule and which moves to ribosomes and protein synthesis happens
- in water soluble harmone it binds to a receptor in membrane and binding leads to activation of an enzyme that changes ATP to cAMP and cAMP activates an enzyme cascade and things will enter the blood stream
Tropic hormones: Hormones that regulate activity of other hormones. hypothalamus releases harmone that target pituitary gland which releases a stimulating harmone which targets gland and target gland secretes harmone that feedback inhibits the release of hormones from hypothalamus and pituitary gland
The pituitary gland:
Has two lobes that function as two separate glands:
- Posterior Pituitary: Part of the nervous system. Does not produce hormones, but stores and releases ADH and oxytocin produced in the hypothalamus
- Anterior Pituitary: Produces and releases six major hormones
thyroid-stimulating (TSH), adrenocorticotropic (ACTH), prolactin (PRL), human growth (hGH), follicle-stimulating (FSH), lutenizing (LH)
A portal system delivers releasing hormones from the hypothalamus, which either stimulate or inhibit the release of specific hormones
Human Growth Hormone: Affects almost every body tissue regulating growth, development, and metabolism. Some effects via direct stimulation; majority are tropic. Stimulates liver to secrete growth factor hormones, which work with hGH to increase protein synthesis, mitosis, cell growth, metabolism and release of fats. Stimulates growth of muscles, connective tissue, and growth plates at the ends of long bones 🡪 elongation during childhood/adolescence; decreases in adults
Thyroid and Parathyroid Glands: Secretes thyroxin (T4) to increase metabolism of fats, proteins, and carbohydrates for energy by stimulating heart, muscles, liver, kidneys to increase respiration. Hypothyroidism: thyroid produces insufficient levels of T4 🡪 leads to slow pulse, fatigue, stunted & stocky growth, mental development delays (e.g. cretinism). Hyperthyroidism: thyroid overproduces T4 🡪 leads to anxiety, insomnia, heat intolerance, weight-loss, irregular heartbeat (e.g. Graves’ disease). T4 is controlled by negative feedback under thyroid-stimulating hormone (TSH) 🡪 increases T4, which returns to hypothalamus and anterior pituitary. Parathyroid is made of four small glands that secrete parathyroid hormone (PTH) in response to low calcium
- The hypothalamus secretes a releasing hormone that stimulates the anterior pituitary.
- The anterior pituitary releases TSH into the bloodstream.
- TSH targets the thyroid gland.
- TSH causes the thyroid to secrete T4 into the bloodstream, stimulating increased cellular respiration in target cells throughout the body.
- High levels of T4 cause negative feedback on the pituitary and hypothalamus, shutting down production of TSH.
The Adrenal Glands:
- Function as a pair of glands, the L&R adrenal glands, one each found atop the two L&R kidneys
- Each gland has an inner layer, the adrenal medulla; and an outer layer, the adrenal cortex
- Each layer produces different hormones:
Adrenal medulla produces epinephrine and norepinephrine in times of short-term stress (heart rate, breathing, blood pressure increase)
RECALL: Adrenal cortex stimulated by ACTH from anterior pituitary
produces corticoids 🡪 gluco-, mineralo-, gonado-, to sustain responses to long-term stress (blood pressure/glucose increase)
Examples of Corticoids:
Glucocorticoid: Cortisol
Steroid hormone, synthesized from cholesterol. Anterior pituitary secretes ACTH when brain detect danger, targeting adrenal cortex to release cortisol. Often works with epinephrine as the longer-lasting team member in regulating the body during stress. Raises blood sugar by metabolizing proteins, fats. Sustained high cortisol immuno-suppresses body
Mineralocorticoid: Aldosterone
Stimulates kidneys to increase Na+ absorption in blood, which increases water reabsorption and bp
Pancreatic Hormones:
Clustered group of cells, the islets of Langerhans in the pancreas, secrete insulin and glucagon
Beta cells 🡪insulin, decreases blood glucose
Alpha cells 🡪glucagon, increases blood glucose
Therefore, insulin and glucagon are antagonistic
Both hormones regulated via negative feedback
Eating increases blood glucose, triggers beta cells to secrete insulin until blood glucose is normal
Exercising or fasting decreases blood glucose, triggers alpha cells to secrete glucagon, which then converts stored glycogen into glucose
Male Structures and Functions:
Testes: function as a pair, kept outside the body in a “pouch”, or scrotum, fostering ideal sperm production at a few degrees cooler than normal body temperature (~35oC). Composed of long, coiled, seminiferous tubules and interstitial cells, which secrete testosterone. Sperm leave the testes via a duct, the epididymis, to mature and wait to be moved out of the penis through the ductus (or vas) deferens and ejaculatory duct
Penis: Transfers sperm to the female vagina from the male to the female’s reproductive tract during sexual intercourse → internal fertilization. Consists of a shaft with an enlarged tip, the glans penis, which is covered by foreskin for protection
Seminal Fluid: Not a structure, but a mix of fluids from the seminal vesicles, prostate gland, and Cowper’s gland. Seminal fluid + sperm cells = semen→ exits penis via the urethra from the ductus deferens as a result of para/sympathetic and somatic NS interactions
Male Sex Hormone Regulation:
Sexual maturation starts at puberty, ages 10-13 years, and the hypothalamus increases gonadotropin-releasing hormone (GnRH) production
GnRH makes A. pituitary release follicle-stimulating hormone (FSH) & lutenizing hormone (LH), causing sperm and testosterone production in the testes
Sperm production is regulated by negative feedback involving FSH, LH, and inhibin, a hormone released by seminiferous tubules that reduces FSH
Testosterone production is also regulated via negative feedback
LH causes release of testosterone in testes, producing secondary sex characteristics (e.g. facial hair, muscle mass, deep voice, etc.)
When testosterone levels reach a certain threshold, it acts on the A. pituitary to inhibit further LH
Female Structures and Functions:
Ovaries: Female version of testes, but are internally supported by ligaments in the abdominal cavity, and produce larger, non-motile ova approximately once a month. Have follicles, where ovum matures, rupturing and releasing them into oviduct during ovulation
Uterus: Muscular organ that grows during fetal development. Endometrial lining thickens during ovulation, which then exits through the vagina as menses or nourishes the developing zygote and fetus after fertilization
Female Sex Harmone Regulation:
Like males, sexual maturation starts at puberty with increased GnRH, and then LH & FSH stimulate ovaries to produce estrogen and progesterone
Menstrual cycle is actually two distinct but connected cycles, ovarian(in ovaries) and uterine (in uterus), and together they regulate monthly production and release of ova
Ovarian cycle: 2 stages. Follicular stage: FSH causes follicle to mature, then a series of hormones work together to cause ovulation. Luteal stage: LH develops empty follicle into corpus luteum, increases progesterone, and then degenerates
Uterine Cycle: Prepares uterus for fertilized egg as ovulation occurs
Degeneration of corpus luteum in ovarian cycle occurs on day 1 of menses, thinning endometrium for 5 days. After menses endometrium thickens for 20 days and thins again if fertilization does not occur
Hormonal Regulation of Reproductive System
The human reproductive system is adapted to unite a single reproductive cell from a female parent with a single reproductive cell from a male parent.
To achieve this outcome, the male and female reproductive systems have different structures, functions, and hormones.
The two systems also have many features in common. Both the male and female reproductive systems include a pair of gonads. The gonads (testes and ovaries) are the organs that produce reproductive cells: sperm in males and eggs in females. The male and female reproductive cells are also called gametes. The gonads also produce sex hormones. Sex hormones are the chemical compounds that control the development and function of the reproductive system.
Structures and Functions of the Male Reproductive System:
The male reproductive system includes organs that produce and store large numbers of sperm cells and organs that help to deposit these sperm cells within the female reproductive tract. Some of the male reproductive structures are located outside the body, and others are located inside the body.
The two male gonads are called the testes. The testes are held outside the body in a pouch of skin called the scrotum. The scrotum regulates the temperature of the testes.
In humans, sperm production is most successful at temperatures around 35°C, which is a few degrees cooler than normal body temperature. In cold conditions, the scrotum draws close to the body, so the testicles stay warm. In hot conditions, the scrotum holds the testicles more loosely, allowing them to remain cooler than the body.
As shown in Figure 9.24, the testes are composed of long, coiled tubes, called seminiferous tubules, as well as hormone-secreting cells, called interstitial cells, that lie between the seminiferous tubules. The interstitial cells secrete the male hormone testosterone. The seminiferous tubules are where sperm are produced. Each testis contains more than 250 m of seminiferous tubules and can produce more than 100 million sperm each day. From each testis, sperm are transported to a nearby duct called the epididymis. Within each epididymis, the sperm mature and become motile. The epididymis is connected to a storage duct called the ductus deferens (plural: ductus deferentia), which leads to the penis via the ejaculatory duct.
The penis is the male organ for sexual intercourse. Its primary reproductive function is to transfer sperm from the male to the female reproductive tract. The penis has a variable-length shaft with an enlarged tip called the glans penis. A sheath of skin called the foreskin surrounds and protects the glans penis. The foreskin does not have any reproductive function. Circumcision, the surgical removal of the foreskin, is a common practice in some cultures and families. During sexual arousal, the flow of blood increases to specialized erectile tissues in the penis. This causes the erectile tissues to expand. At the same time, the veins that carry blood away from the penis become compressed. As a result, the penis engorges with blood and becomes erect. Sperm cells move out of each epididymis though the ductus deferens.
As the sperm cells pass through the ductus deferentia, they are mixed with fluids from a series of glands (the seminal vesicles, the prostate gland, and Cowper’s gland). The combination of sperm cells and fluids is called semen. If sexual arousal continues, semen enters the urethra from the ductus deferentia. The urethra is a duct that carries fluid through the penis. The movement of semen is the result of a series of interactions between the sympathetic, parasympathetic, and somatic nervous systems. Sensory stimulation, arousal, and coordinated muscular contractions combine to trigger the release, or ejaculation, of semen from the penis. The semen is deposited inside the vagina.
Sex Hormones and the Male Reproductive System:
The development of the male sex organs begins before birth. In embryos that are genetically male, the Y chromosome carries a gene called the testis-determining factor (TDF) gene. The action of this gene triggers the production of the male sex hormones. The male sex hormones are also known as androgens. The prefix andro- comes from a Greek word that means “man” or “male.” The presence of androgens initiates the development of male sex organs and ducts in the fetus. As the reproductive structures develop, they migrate within the body to their final locations. For example, the testes first develop in the abdominal cavity. During the third month of fetal development, the testes begin to descend toward the scrotum. This process is not complete until shortly before birth.
A boy’s genitalia are visible at birth, but his reproductive system will not be mature until puberty. Puberty is the period in which the reproductive system completes its development and becomes fully functional. Most boys enter puberty between 10 and 13 years of age, although the age of onset varies greatly. At puberty, a series of hormonal events lead to gradual physical changes in the body. These changes include the final development of the sex organs, as well as the development of the secondary sex characteristics. Puberty begins when the hypothalamus increases its production of gonadotropin- releasing hormone (GnRH). GnRH acts on the anterior pituitary gland, causing it to release two different sex hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In males, these hormones cause the testes to begin producing sperm and to release testosterone. Testosterone acts on various tissues to complete the development of the sex organs and sexual characteristics.
From the end of puberty, the male reproductive system is usually capable of producing millions of sperm every hour of the day, seven days a week until death. The same hormones that trigger the events of puberty also regulate the mature male reproductive system over a person’s lifetime. Hormone feedback mechanisms control the process of sperm production, and they maintain the secondary sexual characteristics.
Refer to Figure 9.25, on the next page, as you read the following paragraphs. As shown, the release of GnRH from the hypothalamus triggers the release of FSH and LH from the anterior pituitary. FSH causes the seminiferous tubules in the testes to produce sperm. At the same time, FSH causes cells in the seminiferous tubules to release a hormone called inhibin. Inhibin acts on the anterior pituitary to inhibit the production of FSH. The result is a negative feedback loop. As the level of FSH drops, the testes release less inhibin. A decrease in the level of inhibin causes the anterior pituitary to release more FSH. This feedback loop keeps the level of sperm production relatively constant over time.
A similar feedback loop maintains the secondary sex characteristics. LH causes the interstitial cells in the testes to release testosterone, which promotes changes such as muscle development and the formation of facial hair. As well, testosterone acts on the anterior pituitary to inhibit the release of LH. This feedback loop keeps the testosterone level relatively constant in the body. Reproductive function and secondary sex characteristics both depend on the continued presence of male sex hormones. Substances that interfere with the hormonal feedback system can cause changes in the reproductive system. For example, anabolic steroids mimic the action of testosterone in promoting muscle development. For this reason, some athletes illegally use steroids to increase their speed or strength. Steroids, however, also disrupt the reproductive hormone feedback systems. The side effects of steroid use in men may include shrinking testicles, low sperm count, and the development of breasts.
A man in good health can remain fertile for his entire life. Even so, most men experience a gradual decline in their testosterone level beginning around age 40. This condition is called andropause. In some men, the hormonal change may be linked to symptoms such as fatigue, depression, loss of muscle and bone mass, and a drop in sperm production. However, some studies suggest that low doses of testosterone can help to counter the symptoms of andropause. Because not all men experience symptoms of andropause, and because the symptoms can vary widely, this condition is difficult to diagnose accurately. Other hormonal changes associated with aging can also affect the male reproductive system. For example, the prostate gland often begins gradually to grow in men over age 40. This can lead to discomfort and urinary difficulties, because the prostate squeezes on the urethra as it grows. Older men have an increased risk of cancer of the prostate gland, as well. Surgery may be used to provide relief and to reduce the cancer risk.
Structures and Functions of the Female Reproductive System:
In contrast to the male reproductive system, the female reproductive system does not mass-produce large numbers of gametes. The two female gonads, or ovaries, produce only a limited number of gametes. The female gametes are called eggs, or ova (singular: ovum). The other female sexual organs are adapted to provide a safe environment for fertilization, for supporting and nourishing a developing fetus, and for allowing the birth of a baby. Most of the structures of the female reproductive system are located inside the body.
The two ovaries are suspended by ligaments within the abdominal cavity. The ovaries are the site of oogenesis—the production of an ovum. Oogenesis comes from two Greek words that mean “egg-creation.” Ova are also called oocytes. In contrast to the male reproductive system, in which both testes function at the same time, the ovaries usually alternate so that only one produces an egg each month. The ovary contains specialized cell structures called follicles. A single ovum develops within each follicle. Each month, a single follicle matures and then ruptures, releasing the ovum into the oviduct. This event is called ovulation. Thread-like projections called fimbriae continually sweep over the ovary. When an ovum is released, it is swept by the fimbriae into a cilia-lined tube about 10 cm long called an oviduct. The oviduct carries the ovum from the ovary to the uterus. Within the oviduct, the beating cilia create a current that moves the ovum toward the uterus. A mature ovum is a non-motile, sphere-shaped cell approximately 0.1 mm in diameter (that is, over 20 times larger than the head of a sperm cell). The ovum contains a large quantity of cytoplasm, which contains nutrients for the first days of development after fertilization. The ovum is encased in a thick membrane that must be penetrated by a sperm cell before fertilization can take place.
The uterus is a muscular organ that holds and nourishes a developing fetus. The uterus is normally about the size and shape of a pear, but it expands to many times its size as the fetus develops. The lining of the uterus, called the endometrium, is richly supplied with blood vessels to provide nutrients for the fetus. At its upper end, the uterus connects to the oviducts. At its base, the uterus forms a narrow opening called the cervix. The cervix, in turn, connects to the vagina. The vagina serves as an entrance for an erect penis to deposit sperm during sexual intercourse. The vagina also serves as an exit for the fetus during childbirth. The ovum survives in the oviduct for up to 24 hours after ovulation. If a living egg encounters sperm in the oviduct, fertilization may take place. The fertilized egg, now called a zygote, continues to move through the oviduct for several days before reaching the uterus. During this time, the endometrium thickens as it prepares to receive the zygote. The zygote implants itself in the endometrium, and development of the embryo begins. If the egg is not fertilized, it does not implant in the endometrium. The endometrium disintegrates, and it’s tissues and blood flow out the vagina in a process known as menstruation. The vagina opens into the female external genital organs, known together as the vulva. The vulva includes the labia majora and labia minora, which are two pairs of skin folds that protect the vaginal opening. The vulva also includes the glans clitoris.
Sex Harmones and the Female Reproductive System:
Our understanding of the specific factors that trigger the development of female sex organs in a genetically female embryo is incomplete. Until recently, scientists assumed that the development of female sex organs was a “default” pattern—that is, if there is no Y chromosome, then female organs will develop. Researchers now suspect that the processes of female sex development are more complex and that specific hormonal triggers cause female sex organs to develop. Like a baby boy, a baby girl has a complete but immature set of reproductive organs at birth. North American girls usually begin puberty between 9 and 13 years of age. The basic hormones and hormonal processes of female puberty are similar to those of male puberty. A girl begins puberty when the hypothalamus increases its production of GnRH. This hormone acts on the anterior pituitary to trigger the release of LH and FSH. In girls, FSH and LH act on the ovaries to produce the female sex hormones estrogen and progesterone. These hormones stimulate the development of the female secondary sex characteristics and launch a reproductive cycle that will continue until about middle age.
Hormonal Regulation of the Female Reproductive System:
In humans, female reproductive function follows a cyclical pattern known as the menstrual cycle. The menstrual cycle ensures that an ovum is released at the same time as the uterus is most receptive to a fertilized egg. The menstrual cycle is usually about 28 days long, although it may vary considerably from one woman to the next, and even from one cycle to the next in the same woman. By convention, the cycle is said to begin with menstruation and end with the start of the next menstrual period. The menstrual cycle is actually two separate but interconnected cycles of events. One cycle takes place in the ovaries and is known as the ovarian cycle, shown in Figure 9.27. The other cycle takes place in the uterus and is known as the uterine cycle. Both cycles are controlled by the female sex hormones estrogen and progesterone, which are produced by the ovaries.
The Ovarian Cycle:
The ovary contains cellular structures called follicles. Each follicle contains a single immature ovum. At birth, a baby girl has more than 2 million follicles. Many degenerate, leaving up to about 400 000 by puberty. During her lifetime, only approximately 400 of these follicles will mature to release an ovum. In a single ovarian cycle, one follicle matures, releases an ovum, and then develops into a yellowish, gland-like structure known as a corpus luteum. The corpus luteum then degenerates. Figure 9.27 illustrates the ovarian cycle, and Figure 9.28 illustrates the hormone systems that control this cycle. The ovarian cycle can be roughly divided into two stages. The first stage is known as the follicular stage. It begins with an increase in the level of FSH released by the anterior pituitary gland. FSH stimulates one follicle to mature. As the follicle matures, it releases estrogen and some progesterone. The rising level of estrogen in the blood acts on the anterior pituitary to inhibit the release of FSH. At the same time, the estrogen triggers a sudden release of GnRH from the hypothalamus. This leads to a sharp increase in LH production by the anterior pituitary triggering ovulation—the follicle bursts, releasing its ovum. Ovulation marks the end of the follicular stage and the beginning of the second stage. The second stage is called the luteal stage. Once the ovum has been released, LH causes the follicle to develop into a corpus luteum. The corpus luteum secretes progesterone and some estrogen. As the levels of these hormones rise in the blood, they act on the anterior pituitary to inhibit FSH and LH production. The corpus luteum degenerates, leading to a decrease in the levels of estrogen and progesterone. The low levels of these sex hormones in the blood cause the anterior pituitary to increase its secretion of FSH, and the cycle begins again. If the ovum is fertilized and implants in the endometrium, blood hormone levels of progesterone and estrogen remain high under stimulus of hormones released by embryo-supporting membranes. The continued presence of progesterone maintains the endometrium to support the developing fetus. The continued presence of estrogen stops the ovarian cycle so no additional follicles mature.
The uterine cycle is closely linked to the ovarian cycle. As you have seen, ovulation takes place about halfway through the ovarian cycle, around day 14. The ovum survives for up to 24 h after ovulation. If fertilization occurs, the fertilized egg completes the passage through the oviduct and arrives at the uterus a few days later. The timing of the uterine cycle ensures that the uterus is prepared to receive and nurture a new life. The events of the uterine cycle cause a build-up of blood vessels and tissues in the endometrium. If fertilization does not occur, the endometrium disintegrates and menstruation begins. The uterine cycle begins on the first day of menstruation (which is also the first day of the ovarian cycle). On this day, the corpus luteum has degenerated and the levels of the sex hormones in the blood are low, as shown in Figure 9.29. Menstruation lasts for the first 5 days of the uterine cycle and by the end, the endometrium is very thin, also shown in Figure 9.29. As a new follicle begins to mature and release estrogen, the level of estrogen in the blood gradually increases.
Beginning around the sixth day of the uterine cycle, the estrogen level is high enough to cause the endometrium to begin thickening, also shown in Figure 9.29. After ovulation, the release of progesterone by the corpus luteum causes a more rapid thickening of the endometrium. Between days 15 and 23 of the cycle, the thickness of the endometrium may double or even triple. If fertilization does not occur, the corpus luteum degenerates. The levels of the sex hormones drop, the endometrium breaks down, and menstruation begins again. You have seen that the menstrual cycle involves a number of different hormones, each of which triggers different events in the body. Figure 9.29 summarizes the hormonal and physical changes that occur in the body throughout the menstrual cycle.
Aging and the Menstural cycle:
After puberty, the male reproductive system can continue to produce viable sperm for a lifetime. In contrast, the number of functioning follicles in the female reproductive system decreases with age. This, in turn, leads to a gradual overall decline in the amount of estrogen and progesterone in the blood. As hormone levels drop, a woman’s menstrual cycle becomes irregular. Within a few years, it stops altogether. The end of the menstrual cycle is known as menopause. Among North American women, the average age of menopause is approximately 50, but menopause can begin earlier or later. A woman who has completed menopause no longer produces ova, so she is no longer fertile. As well, the decrease in the sex hormones disrupts the homeostasis of a number of hormone systems. This has a range of effects on the body. During menopause, blood vessels alternately constrict and dilate, resulting in uncomfortable sensations for some women known as “hot flashes.” Some women also experience variable changes in mood. Over the longer term, menopause is associated with rising cholesterol levels, diminishing bone mass, and increased risk of uterine cancer, breast cancer, and heart disease. For these reasons, many women consider hormone replacement therapy during or following menopause.
Hormone replacement therapy (HRT) is a prescription of low levels of estrogen with or without progesterone. However, while this therapy can ease some symptoms of menopause, the treatment also carries a number of health risks. In recent studies, hormone replacement therapy has been linked to * an increased risk of coronary heart disease, strokes, and blood clots * an increased risk of breast cancer and colorectal cancer * an increased risk of dementia. For this reason, Health Canada advises that a woman should not begin hormone replacement therapy without a thorough medical evaluation and a careful assessment of her own particular needs, health, and medical history. In some cases, the benefits of the therapy may outweigh the risks. In other cases, the reverse is true. Scientists continue to search for other ways to alleviate the symptoms and long-term health effects of menopause.
