Topic 3 - Homeostasis

Question 1

What is homeostasis?

Answer 1

The maintenance of the same, optimum conditions within cells. The state of steady internal physical and chemical conditions maintained by living systems.

Question 2

What are tolerance limits?

Answer 2

Optimum zones, in which the cell functions most effectively, are called tolerance limits.

Question 3

What happens if conditions fall outside of tolerance limits for an organism or cell?

Answer 3

If the organism goes outside tolerance limits, homeostatic mechanisms might be able to shift the organism back into optimum range. If not, it will be unable to function properly and/or die.

Question 4

How do multicellular organism cells regulate internal environments?

Answer 4

Multicellular organism cells are surrounded by tissue fluid. This fluid maintains a very stable composition so it is easy for cells to maintain their optimum zones.

Question 5

List some examples of human homeostasis.

Answer 5

• blood glucose levels
• blood carbon dioxide levels
• water and solute balance
• body temperature

Question 6

What is a stimuli?

Answer 6

A change in environment that stimulates a compensation reaction. Organisms are selective in their response to stimuli.

Question 7

What are abiotic factors? + Examples

Answer 7

Abiotic factors are non-living factors that affect an environment or ecosystem:
- mineral composition
- light availability
- oxygen levels
- temperature
- wind
- pH

Question 8

What are tolerance factors in cells and how do they work?

Answer 8

Cells require particular (and chemically different) internal and external environments to survive. This is also considered a tolerance limit. Changing the chemical composition of a cell or the cell’s environment can result in cell death.

Question 9

What are some examples of tolerance factors in cells?

Answer 9

• how hypertonic or hypotonic tissue fluid is compared to the cell
• presence of minerals and nutrients such as salts for ions and glucose

Question 10

What is required to achieve homeostasis?

Answer 10

Communication between the nervous and endocrine (hormone) systems is vital to maintain homeostasis.

Question 11

How do the nervous and endocrine systems communicate?

Answer 11

Nerves and hormones are the life mechanisms by which these systems communicate.

Question 12

How does the stimulus-response model work?

Answer 12

A stimulus is detected by a sensory receptor. The receptor conveys the message through to a control centre (central nervous system). The control centre triggers a response by activating an effector. The effector removes the stimulus or initiates an action that negates the stimulus. This restores the status quo, achieving homeostasis.

Question 13

What are the major types of sensory receptors?

Answer 13

Photoreceptors, chemoreceptors, thermoreceptors, and mechanoreceptors.

Question 14

What are the characteristics of photoreceptors?

Answer 14

Vision.

Question 15

What are the characteristics of chemoreceptors?

Answer 15

Taste, smell, pain, blood oxygen, and blood pH.

Question 16

What are the characteristics of thermoreceptors?

Answer 16

Warmth (warm receptors), and cool (cold receptors).

Question 17

What are the characteristics of mechanoreceptors?

Answer 17

Blood pressure (baroreceptors), osmolarity of extracellular fluid (osmoreceptors), sound/balance and equilibrium (hair cells).

Question 18

What is a negative feedback loop?

Answer 18

A negative feedback loop is a response that results in the inhibition of a stimulus. This means that the output negates the input.

Question 19

How do negative feedback loops regulate the internal cellular environment?

Answer 19

Homeostasis uses homeostatic control mechanisms, in the form of negative feedback loops, to regulate the internal environment. The loop removes the stimulus by providing negative feedback (recognition of stimulus absence). These self regulating processes maintain the environment around a determined norm.

Question 20

Compare the action of the nervous and endocrine systems.

Answer 20

Communication: Nervous | Hormonal
Pathway: Direct via Axons of Nerve Cells | Indirect via Blood
Message: Electrochemical Impulse | Chemical
Site of Action: Highly Specific | Target Cells (can be widespread)
Speed of Action: Fast | Slow
Duration: Short Term | Long Term

Question 21

Describe how increased blood glucose levels are a negative feedback loop.

Answer 21

Blood glucose levels are controlled by insulin and glucagon. A rise in blood sugar (stimulus) triggers the complementary receptors which, in turn, convey the message to the control centre (CNS). This leads to the pancreas secreting more insulin into the blood (effector). Insulin increases the absorption of glucose from the bloodstream into liver and muscle cells. Once blood sugar levels reach homeostasis, the pancreas stops releasing insulin.

Question 22

Describe how decreased blood glucose levels are a negative feedback loop.

Answer 22

Blood glucose levels are controlled by insulin and glucagon. If the blood sugar levels are too low, the pancreas will release less insulin. As insulin controls the absorption of glucose from the bloodstream into liver and muscle cells, its decreased presence will increase blood glucose levels within the blood. Once blood sugar levels reach homeostasis, the pancreas stops releasing insulin.

Question 23

Describe the stimulus-response model.

Answer 23

A stimulus produces a change to the conditions of the cell. A receptor then recognises that something has changed and passes the message along. The control centre receives the message and signals for a response. An effector then makes a change that restores the cellular homeostasis.

Question 24

What are the characteristics of nerves?

Answer 24

Electrochemical signals travel in pulses along nerve cells from one part of the body to another. These electrochemical signals are called nerve impulse or action potentials. Nerves are very selective about the target tissue/cells that they enervate (as opposed to hormones that will act on any tissue with complementary receptors). Nerve impulses are also short-lived.

Question 25

What are the different types of nerves?

Answer 25

Sensory neurons, interneurons and motor neurons.

Question 26

What do sensory neurons do?

Answer 26

Sensory neurons carry information from the site of stimulus towards the central nervous system (CNS). These signals stimulate receptors which trigger action potentials towards the CNS.

Question 26

How are sensory neurons structured?

Answer 26

Sensory neurons have a receptor cell attached, an axon covered in myelin sheath, a cell body on the axon, and dendrites at the other end.

Question 27

What do interneurons do?

Answer 27

Interneurons receive signals from sensory neurons and convey them to motor neurons. They are located within the CNS.

Question 28

How are interneurons structured?

Answer 28

Interneurons have a reduced cell body that is located near the dendrites, an axon with no myelin sheath, and dendrites at the opposite end with pre-synaptic terminals.

Question 29

What do motor neurons do?

Answer 29

Motor neurons carry signals from the central nervous system to the effectors.

Question 30

How are motor neurons structured?

Answer 30

Motor neurons have a large cell body near the dendrites, an axon contained within myelin sheath, and synaptic terminals on the opposite end to the cell body.

Question 31

What is the nervous system composed of?

Answer 31

The nervous system is composed of the central nervous system (CNS) and the peripheral nervous system (PNS).

Question 32

What is the CNS composed of?

Answer 32

Brain and spinal cord.

Question 33

What is the PNS composed of?

Answer 33

Nerves outside the CNS.

Question 34

How is the PNS broken down?

Answer 34

Within the PNS, there is the somatic nervous system (SNS), which is the voluntary nervous system, and the autonomic nervous system (ANS), which contains involuntary nerves that regulate all the automatic functions like breathing and heart rate. The ANS is further broken down into the sympathetic (fight or flight, and homeostasis) and the parasympathetic (rest and digest) nervous systems.

Question 35

How does the stimulus-response model incorporate nerves?

Answer 35

A stimulus is detected by a receptor. The receptor conveys the message through to a control centre (CNS) by a sensory neuron. Interneurons within the control centre trigger a response by activating an effector. This is done by sending an impulse along a motor neuron. The effector removes the stimulus or initiates an action that negates the stimulus. This restores homeostasis.

Question 36

How does the reflex response work?

Answer 36

The reflex response is an automatic response which the brain is not directly involved in. It provides a rapid response to a stimulus. However, consciousness can override or modify some reflexes. Sensory neurons detect the stimulus and convey the message to relay neurons within the spinal cord. These convey the message to motor neurons which express it to an effector.

Question 37

What are some examples of reflexes?

Answer 37

Knee jerk, pupil contraction/dilation, and muscle contraction in the inner ear are all types of reflexes.

Question 38

What is the function of the endocrine system?

Answer 38

The endocrine system is responsible for the production and use of hormones. Hormones are chemicals that are secreted by endocrine glands located in various places around the body. These chemicals are released directly into the bloodstream.

Question 39

How does the endocrine system work with the circulatory system for homeostasis?

Answer 39

In order to get substances like hormones to the target cells/organs and the wastes from the cell, blood vessels must access every part of the body.

Question 40

How do hormones work for homeostasis?

Answer 40

Blood transports hormones all around the body through the circulatory system, however, the hormone will only act upon the cells that have complementary receptors to the hormone (target cells). Water-soluble hormones act upon membrane receptors and lipid-soluble steroid hormones act upon cytoplasmic receptors. These target cells, which make up target tissue and target organs, are stimulated to evoke the desired response.

Question 41

What are amino acid derived hormones and their characteristics?

Answer 41

Amino acid derived hormones are either amines, peptides or protein/glycoproteins. These hormones are water-soluble, meaning they act upon receptors on the cell membrane.

Question 42

What are lipid/steroid hormones and their characteristics?

Answer 42

Lipid/steroid hormones are derived from cholesterol. They are fat/lipid soluble hormones, meaning they act upon receptors within the cytoplasm (cytoplasmic receptors). These hormones are typically associated with the adrenal medulla or sex glands. They tend to end in “one” or “ol”.

Question 43

What are the characteristics of hormones?

Answer 43

The effects of hormones tend to be long lasting, and take more time to have an impact (exception is adrenaline). Therefore, hormones are involved in long-term regulation such as growth, reproduction and blood glucose levels.

Question 44

What is diabetes?

Answer 44

Diabetes is a disorder that means body’s ability to produce and/or respond to the hormone insulin is impaired.

Question 45

What does insulin do?

Answer 45

Insulin enables all cells (except brain cells) to uptake glucose from the bloodstream.

Question 46

What are the two types of diabetes?

Answer 46

Type 1 Diabetes mellitus (insulin-dependent diabetes) - autoimmune disorder where the immune system attacks the cells of the pancreas.
Type 2 Diabetes mellitus (non insulin-dependent diabetes) - caused by either a deficiency of insulin or more commonly by a reduced responsiveness by target cells to insulin caused by changes to the insulin receptors.

Question 47

What are endotherms?

Answer 47

Organisms that generate their own heat through metabolic processes are called endotherms.

Question 48

What are ectotherms?

Answer 48

Organisms that require heat from external sources are called ectotherms.

Question 49

How does homeostasis counteract a decreased blood glucose level?

Answer 49

Decreased blood glucose levels are detected in the pancreas. Glucagon is then released from alpha cells within the pancreas. The liver breaks down glycogen into glucose, and the glucose is released into the bloodstream. Thus, the blood glucose levels are increased to reach homeostasis.

Question 50

How does homeostasis counteract an increased blood glucose level?

Answer 50

Increased blood glucose levels are detected in the pancreas. Insulin is then released from beta cells within the pancreas. This presence of insulin increases the absorption of glucose from the bloodstream into liver and muscle cells. Thus, the blood glucose levels are lowered to reach homeostasis.

Question 51

How does thermoregulation function?

Answer 51

If body temperature is too low, detected at the hypothalamus, an increased release of thyroid releasing-hormone (TRH) will occur from the pituitary. This is complementary in shape to receptors on the pituitary that stimulate thyroid-stimulating hormone (TSH), releasing more of the chemical. This is complementary in shape to receptors on the thyroid, that stimulate the release of thyroxine. An increased concentration of thyroxine causes the rate of metabolic processes to increase, releasing more heat energy and increasing body temperature. REVERSE FOR DECREASED BODY TEMPERATURE.

Question 52

How does thermoregulation function?

Answer 52

Thermoreceptors in the hypothalamus detect changes in blood temperature. For an increase in blood temperature, the hypothalamus releases thyroid-releasing hormone (TRH), which stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH). TSH triggers the thyroid gland to release thyroxine, increasing the rate of cell metabolism and thus the heat production. This increase of heat triggers the thermoreceptors in the hypothalamus to decrease the release of TRH.

Question 53

What is osmoregulation and where does it occur?

Answer 53

Osmoregulation is the maintenance of water and solutes in the body. It occurs within nephrons in the kidneys.

Question 54

What are nephrons and how are they structured?

Answer 54

The nephron is the location where osmoregulation occurs. Like other exchange surfaces, the kidneys have a large blood supply. Blood arrives to the bundle of capillaries called glomerulus via the afferent blood vessel. It exits via the efferent blood vessel that leads to a network of capillaries surrounding the nephron tubule. At the glomerulus, filtration occurs due to the pressure to remove water, ions, glucose and urea.

Question 55

Afferent VS efferent blood vessel.

Answer 55

The afferent blood vessel is the method in which the blood arrives to the bundle of capillaries called the glomerulus. Whereas, the efferent blood vessel is the method in which the blood exits the glomerulus.

Question 56

How are nephrons involved in osmoregulation?

Answer 56

Materials such as ions, water, urea, amino acids and glucose are filtered from the glomerulus and into the Bowman’s capsule. This is collectively called the filtrate. Proteins and blood cells do not filter into the nephron as they are too big. Useful materials in the filtrate are reabsorbed as they pass through the nephrons, specifically water and ions.

Question 57

How do nephrons maximise surface area?

Answer 57

There are approximately one million nephrons in each kidney with microvilli lining the epithelial layer of the nephron along the proximal convoluted tubule, loop of henle and distal convoluted tubule. This maximises the surface area of the nephrons.

Question 58

How does an increased salt concentration impact osmoregulation?

Answer 58

Osmoregulators in the hypothalamus detect an increase in salt concentration within the blood. This stimulates the pituitary to release an increased concentration of antidiuretic hormone (ADH). ADH travels via the blood to receptors on the cell walls of the collecting ducts. This increases the permeability of the duct walls by increasing the number of aquaporins on the filtrate side of the duct. Thus, this increases the reabsorption of water into the blood via osmosis, reducing the concentration of salt and increasing both blood pressure and volume. This means that less urine is produced as the water is present more within the blood than as waste.

Question 59

What does a decrease in blood pressure/volume do to the ADH released?

Answer 59

Decreased blood pressure/volume means there is less water presence. Thus, the solute concentration is increased and the release of ADH is increased.

Question 60

What are the examples of fight or flight responses?

Answer 60

brain - signal to adrenal glands
lungs - fast breathing
liver - converts glycogen to glucose
eye - tunnel vision and pupil dilation
heart - acceleration
muscles - tense
adrenal glands - produces hormones
stomach - slow digestion
bladder - relaxation
hair - erection

Question 61

What is the fight or flight response?

Answer 61

The flight or fight response is the body’s response to an acute stress situation when a threat is perceived.

Question 62

How does the fight or flight response work?

Answer 62

Nervous signals from receptors travel to the amygdala, which triggers the hypothalamus, which triggers the pituitary gland to release Adrenocorticotrophic hormone (ACTH). Simultaneously, the adrenal medulla (located on the top of the kidney) releases adrenaline (called epinephrine in the USA). Cortisol is also produced, which increases blood pressure, sugar and suppresses the immune system. The purpose of these mechanisms is to free energy up for immediate physical action.

Question 63

How does adrenaline impact muscles?

Answer 63

Nervous signals from receptors travel to the adrenal medulla, which stimulates the release of adrenalin. Adrenalin acts on smooth muscle around the blood vessels of the skeletal muscle causing them to dilate, which allows more blood flow. Whereas, the blood vessels around the intestines contract, making blood flow available to the periphery (for musculoskeletal action).

Question 64

How does carbon dioxide reduce the pH of tissue fluid?

Answer 64

Carbon dioxide is produced as a waste product of aerobic respiration. The carbon dioxide dissolves in tissue fluid but mostly reacts with water to form carbonic acid. This ionises to produce hydrogencarbonate and hydrogen ions (H+ ions reduce pH and make the blood more acidic). Carbon dioxide also forms these ions in the cerebospinal fluid around the brain and spinal cord.

Question 65

How does reduced pH from carbon dioxide initiate a nervous response?

Answer 65

The respiratory centre in the brain recognises the decrease in pH through chemoreceptors. This triggers a nervous response to increase the speed and depth of breathing.

Question 66

How does the acidity impact breathing in a negative feedback loop?

Answer 66

Breathing faster and deeper means that more carbon dioxide diffuses out of the blood in the alveoli of the lungs. This decreases the amount of carbon dioxide in the blood and hence in the cerebrospinal fluid.

Question 67

How do work neurotransmitters?

Answer 67

Neurotransmitters are the chemicals released from the presynaptic cell and passed onto the postsynaptic cell. The change in charge of the neurotransmitters as they are passed along the nerve cell causing them to be packaged by the Golgi body. These vesicles containing the neurotransmitters are taken from the Golgi body to the cell membrane using the cytoskeleton. The vesicles then fuse to the cell membrane, expelling the contents out of the presynaptic cell. The neurotransmitters pass along the synaptic cleft, then binds to a complementary receptor on the postsynaptic nerve cell. This stimulates the ion exchange, leading to the de-polarisation of the membrane.

Question 68

What are the two types of neurotransmitters?

Answer 68

Excitatory - stimulate the next neuron in the pathway.
Inhibitory - block the nerve impulse from continuing.

Question 69

What are some examples of excitatory neurotransmitters?

Answer 69

Dopamine, serotonin, glutamate, and acetylcholine.

Question 70

What are some examples of inhibitory neurotransmitters?

Answer 70

GABA, glycine, and dopamine.

Question 71

How does vasodilation work?

Answer 71

Vasodilation occurs when adrenaline binds in a complementary manner to specific receptors on smooth muscle around blood vessels of skeletal muscle, causing them to dilate. This allows more blood flow near the surface of the muscle, meaning the blood can cool much more quickly and body temperature can be lowered.

Question 72

How does vasoconstriction work?

Answer 72

Vasoconstriction occurs when adrenaline binds in a complementary manner to specific receptors on smooth muscle around blood vessels of skeletal muscle, causing them to narrow. This reduces blood flow near the surface of the muscle, meaning the blood can conserve much more heat and body temperature can be increased.