Topic 3: Homeostasis

Question 1

What are tolerance limits? What do they include?

Answer 1

Tolerance limits are the range of conditions required for survival of an organism.
These limits include physical and chemical environmental factors that can be internal and external to the organism

Question 2

What will happen if environmental conditions fall outside an organism’s tolerance limits?

Answer 2

If environmental conditions fall outside an organism’s tolerance limits, then it will not survive unless it has mechanisms to maintain its own internal environment

Question 3

Why do unicellular organisms have limited ability to maintain their internal composition? Are the exceptions?

Answer 3

Because they are surrounded by their external environment
Some bacteria can form ‘spores’ in response to lack of water, and can survive in this form for years

Question 4

Why are the cells of multicellular organisms able to maintain a constant composition easily?

Answer 4

They are surrounded by tissue fluid, which maintains a constant composition, enabling cells to maintain the conditions needed for their survival more easily so that they can function normally

Question 5

What are some factors organisms must maintain within their tolerance limits to function normally?

Answer 5

Body temperature
Blood glucose levels
Carbon dioxide and oxygen in blood
Water balance
Ion levels (such as sodium, magnesium and potassium)

Question 6

What are abiotic factors?

Answer 6

Abiotic factors are non-living factors that affect organisms, including temperature and oxygen level

Question 7

What are biotic factors?

Answer 7

Biotic factors are living factors that affect organisms, such as predators

Question 8

Why must the body water of organisms maintain specific solute concentration levels?

Answer 8

Body water of organisms must maintain specific solute concentration levels to allow for optimal osmosis levels

Question 9

Where is CO2 found in the body and in what concentration? What is it needed for?

Answer 9

The CO2 concentration in the blood is about 5-10%
It is carried in blood plasma and red blood cells
It is critical in maintaining the pH of the blood between 7.35-7.45

Question 10

What processes are involved in maintaining stable CO2 and O2 levels?

Answer 10

Blood circulation and breathing

Question 11

What is respiratory acidosis?

Answer 11

A state where hypoventilation occurs, and so there is a buildup of CO2, which is acidic, so the pH is decreased (more H+ ions, becoming acidic)

Question 12

What is respiratory alkalosis?

Answer 12

A state where hyperventilation occurs, and so too much CO2 is lost, resulting in a higher blood pH (less H+ ions, becoming alkaline or basic)

Question 13

What are the normal blood glucose levels?

Answer 13

Normally between 75-110mg/dL

Question 14

What controls blood glucose levels?

Answer 14

Controlled by pancreatic hormones insulin and glucagon
Insulin is released when there are high blood glucose levels to lower them
Glucagon is released when there are low blood glucose levels (glucose is gone) to raise them

Question 15

What is homeostasis? What does it involve?

Answer 15

Homeostasis is the maintenance of a relatively stable internal environment
It involves regulation by a number of processes to do so

Question 16

The body is not ___, but rather…

Answer 16

The body is not static, but rather revolves around the optimum conditions (within the tolerance limits)

Question 17

When the external environment changes, what must organisms (and cells) do?

Answer 17

Maintain their internal environment in the face of changes to ensure optimum conditions are maintained for proper functioning

Question 18

Describe what makes up the internal environment and what aspects of them must be regulated

Answer 18

Cells (duh)
The fluid surrounding the cells (interstitial fluid)
The fluid inside cells (intracellular fluid)
The blood plasma (liquid part of the blood)
The physical and chemical aspects of these fluids (e.g. composition, pH, concentration of ions, temperature, etc.) must be regulated

Question 19

How does the body maintain homeostasis?

Answer 19

Using the stimulus-response model

Question 20

What are the five main aspects of the stimulus-response model?

Answer 20

Stimulus - any change in the external or internal environment that can be detected
Receptor - detects a stimulus
Message - transmitted by the receptor via the endocrine or nervous system
Effector - receives the transmitted message to bring about a
Response - a change in the organism due to the stimulus

Question 21

What is negative feedback?

Answer 21

Negative feedback is when the response inhibits the initial stimulus that brings the body outside the optimal range

Question 22

What is positive feedback?

Answer 22

Positive feedback encourages and intensifies a change in the body’s physiological condition, driving it farther out of the optimal range
The cycle will continue until there is an endpoint

Question 23

What are the six main types of receptors, what do they detect, what do they make up, and where are they found?

Answer 23

Olfactory receptor neurons detect chemicals and make up the sense of smell, found in the nose
Photoreceptor cells detect visible light and make up the sense of sight, found in the eyes
Mechanoreceptors detect pressure and make up the senses or hearing and touch, found in the ears and under the skin
Chemoreceptors detect chemicals and make up the senses of smell and taste, found in the nose and under the skin
Thermoreceptors detect temperature, different receptors for hot and cold, found under the skin (and in the thermoregulatory centre of the hypothalamus)
Nociceptors detect pain and are found all over the body

Question 24

What is the role of the nervous system? What can it be divided into?

Answer 24

To detect stimuli, process information, and elicit a response
it can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS)

Question 25

What are nerve impulses?

Answer 25

Electrochemical pulses that travel along nerve cells, neurons, to send signals to and from various parts of the body

Question 26

What makes up the CNS and what is it involved in?

Answer 26

Consists of the brain and spinal cord
It is mainly involved in storing, responding to, and coordinating information

Question 27

What makes up the PNS and what is involved in?

Answer 27

Consists of nerves that lie outside the brain and spinal cord: sensory and motor neurons
The motor neurons are divided into voluntary and involuntary
Voluntary neurons control voluntary movements, and make up the somatic nervous system (SNS)
The autonomic nervous system (ANS) contains involuntary nerves, controlling involuntary responses
The autonomic nervous system can also be divided into the sympathetic and parasympathetic nervous systems, involved in the fight or flight and rest or digest responses
The PNS is mainly involved in transmitting signals to and from the CNS

Question 28

What are neurons? Describe their structure.

Answer 28

Neurons (nerve cells) are the cells that transmit nerve impulses
They are made up of:
A cell body (soma) - contains the nucleus and other organelles
Dendrites - projections that branch off the cell body of the neurons. They receive messages from other neurons
An axon - a long extension that carries the electrical impulses away from the cell body
Axon terminals - at the end of the axon
The axon is usually surrounded by a myelin sheath, an insulating layer which increases the speed of transmission and is made up of Schwann cells

Question 29

How are nerve impulses transmitted?

Answer 29

Along a nerve pathway consisting of several neurons with tiny gaps between them called synapses
The impulse travels along the axon of the neuron as an electric current
Then, a chemical called a neurotransmitter crosses the gap and transfers the impulse to the next neuron
These impulses can only travel in one direction along a neuron, from the dendrite ‘end’ towards the axon terminal

Question 30

How can neurons be classified?

Answer 30

By morphology, (size), and function

Question 31

How can neurons be classified by morphology?

Answer 31

Unipolar - the soma has only one ‘process’ extending from it
Bipolar - the soma has two processes extending from it
Multipolar - the soma has many processes extending from it

Question 32

Describe sensory neurons. AKA?

Answer 32

Detect stimuli with specialised nerve endings called receptors, triggering a nerve impulse that is carried towards interneurons in the CNS
Most are unipolar
AKA afferent neurons

Question 33

Describe interneurons.

Answer 33

Located in the brain and spinal cord
Receive signals from sensory neurons and transmit them to motor neurons
Most are multipolar

Question 34

Describe motor neurons. AKA?

Answer 34

Carry nerve impulses from the CNS to effectors such as muscles or glands, creating a response such as movement or secretion
Most are multipolar
AKA efferent neurons

Question 35

Describe the structure of a nerve pathway from receptor to effector (stimulus-response NS version)

Answer 35

Stimulus is detected by a receptor, triggering a nerve impulse which travels along a sensory neuron towards the spinal cord. Transmitted along interneurons in the spinal cord to the brain, where it is processed, sending a nerve impulse down the spinal cord along a motor neuron.
The motor neuron carries the impulse to an effector (muscle or gland), resulting in a response

Question 36

What is a reflex response? Explain.

Answer 36

An automatic response to a stimulus
The brain is not directly involved as the signal from the receptor travels along a sensory neuron to the spinal cord, then along an interneuron to a motor neuron that signals the muscles (effector) to respond
Protects the organism by providing a rapid response to the stimulus

Question 37

How can the brain modify reflex responses?

Answer 37

A nerve impulse is also sent up to the brain at the same time, so it can increase or decrease the reflex, even though it is not directly involved.

Question 38

Describe the role of synapses and neurotransmitters

Answer 38

In between each neuron is a junction called a synapse, with a gap called a synaptic cleft
As the nerve impulse reaches the axon terminals of the pre-synaptic neuron, vesicles containing containing chemicals called neurotransmitters secrete them (exocytosis) into the synaptic cleft
The neurotransmitters then attach to receptors on the dendrites of the post-synaptic neurons to pass on the message

Question 39

Describe the different types of neurotransmitters

Answer 39

Some neurotransmitters are called excitatory because they stimulate the next neuron in the pathway
Other are called inhibitory because they block the nerve impulses

Question 40

What happens after a neurotransmitter is released into the synaptic cleft and used?

Answer 40

It is reabsorbed by the pre-synaptic neuron via reuptake, destroyed by an enzyme, or it diffuses out of the cleft
If a neurotransmitter remained the synaptic cleft, it would cause continual stimulation of the next neuron, leading to unregulated responses/overstimulation

Question 41

What is the endocrine system?

Answer 41

The group of endocrine glands (tissues which secrete substances)
They are ductless glands that secrete hormones directly into the bloodstream

Question 42

Compare different types of glands

Answer 42

Exocrine glands secrete substances through openings (ducts) onto your body surfaces, such as tears and sweat
Endocrine glands are ductless and secrete hormones directly into the bloodstream

Question 43

What are hormones? What do they effect?

Answer 43

Chemical messengers secreted into and carried in the blood
A particular hormone will only produce an effect when it reaches target cells that are ‘tuned in’ to it by having specific receptor molecules
These target cells make up target tissues and target organs
Once they are used they are broken down and excreted

Question 44

Describe the four different types of hormones, include examples

Answer 44

Amine (amino acid derivative) - made up of amino acids, e.g. adrenaline
Peptide - short chains of amino acids, e.g. ADH/vasopressin
Protein - longer chains of amino acids, e.g. insulin
These amine/peptide hormones are mostly polar and so do not enter the cell, but rather bind to specific receptors on the cell membrane
The receptor then changes shape, causing an internal cascade of reactions in the cytoplasm that alter the cell’s metabolism
Steroid - lipids, made from cholesterol
Are able to move directly through the phospholipid bilayer
They bind to specific receptors in the cytoplasm to form a hormone-receptor complex which goes into the nucleus and affects transcription
E.g. testosterone

Question 45

What type of hormone has a faster effect? Why?

Answer 45

The effect of amine/peptide hormones is faster than steroid hormones as binding to a receptor on the cell membrane causes a Signal Transduction pathway, affecting many relay molecules with the message so the response is quicker

Question 46

What are examples of processes and the hormone that controls them and what structures they target

Answer 46

Metabolism (TSH activates thyroxine/every cell in the body)
Adult development (FSH and LH trigger puberty/multiple organs and tissues)
Reproduction (FSH and LH control menstruation/many organs)
Growth (growth hormone promotes growth/bones, muscles, fat, tissue)
Equilibrium = homeostasis (e.g., ADH and water balance/cells in the kidney)

Question 47

What can hormonal responses be stimulated by?

Answer 47

The nervous system, e.g. adrenaline released by the adrenal gland
Other Hormonal messages: hormonal messages that stimulate hormonal responses are from the pituitary gland, which releases hormones which, in turn, affect other hormones. For example, TSH is released from the pituitary gland to stimulate the release of thyroxine by the thyroid gland

Question 48

Describe the hypothalamus and pituitary gland.

Answer 48

The hypothalamus is a region of the brain and is made of nerve cells
Below it is the pituitary gland (master gland)
It has 2 lobes called the anterior pituitary and the posterior pituitary
The posterior releases hormones which are made in the hypothalamus
The anterior receives messages from the hypothalamus to release other hormones which it makes

Question 49

What is the fight or flight response. How is it induced?

Answer 49

The fight or flight reflex response prepares the body to react in an emergency, to fight or fly
If you receive a fright, are stressed, or threatened by danger, the brain sends a signal via the sympathetic nervous system to the adrenal glands which secrete adrenaline

Question 50

What structures does adrenaline affect (fight or flight)?

Answer 50

Smooth muscle around blood vessels of skeletal muscles dilates, increasing blood flow
Smooth muscle around blood vessels of digestive system constricts, redirecting blood flow to muscles
Heart rate and cardiac output are increased, raising blood pressure and blood flow
Pancreas stimulated to secrete glucagon, initiating the release of glucose in the liver into the blood

Question 51

Describe the role of TSH in the production of thyroxine

Answer 51

The hypothalamus releases thyroid-releasing hormone (TRH) and this causes the anterior pituitary to secrete thyroid stimulating hormone (TSH)
The TSH then triggers the production and secretion of thyroxine by the thyroid gland
Increased thyroxine then goes to inhibit the secretion of TRH, a negative feedback loop

Question 52

Compare the action of the nervous and endocrine systems

Answer 52

FEATURE: NERVOUS SYSTEM; ENDOCRINE SYSTEM
Signal pathway: direct via neurons; indirect via blood
Message: electrochemical pulses; chemical
Site of action; highly specific; target cells - can be widespread
Speed of action; fast - nerve impulse travels across the body in a fraction of a second; slow - limited by speed of blood flow
Duration of action; short term; long term

Question 53

Why is the speed of hormonal responses slower than the speed of nerve impulses?

Answer 53

The speed of hormones is slower and the hormones are not sent directly to the target tissue, but travel in the blood to all parts of the body

Question 54

Describe adrenaline

Answer 54

Adrenal gland; most cells (mainly muscle); overall increase cell metabolism to increase energy (ATP) production

Question 55

Describe noradrenaline

Answer 55

(This isn’t in textbooks but anyways:)
Neurotransmitter secreted by the brain, causing an increase in metabolism in muscles in preparation for adrenaline stimulating the longer-lasting fight of flight response.

Question 56

Describe TSH

Answer 56

(Thyroid stimulating hormone) anterior pituitary; thyroid; stimulates production of thyroxine by thyroid gland

Question 57

Describe thyroxine

Answer 57

Thyroid gland; most cells; increases oxidative metabolism

Question 58

Describe antidiuretic hormone (ADH)/Vasopressin

Answer 58

Hypothalamus (via posterior pituitary); kidneys; increased reabsorption of water by kidneys

Question 59

Describe insulin

Answer 59

Pancreas; mainly liver and muscle cells; lowers blood sugar level, increases glycogen storage

Question 60

Describe glucagon

Answer 60

Pancreas; most cells, particularly liver; stimulates breakdown of glycogen to glucose, increases blood sugar level

Question 61

Describe melatonin

Answer 61

Pineal gland; eyes; helps with internal clock and sleep rhythms

Question 62

Describe aldosterone

Answer 62

Adrenal gland; kidneys; increases sodium and water reabsorption in kidneys, increases blood pressure

Question 63

How does the hypothalamus ‘bridge’ the nervous and endocrine systems?

Answer 63

-It receives signals via afferent nerves, sends nerve impulses via autonomic nerves, and also secretes hormones via the pituitary gland which control the secretion of other hormones

Question 64

What regulates internal body temperature?

Answer 64

Blood temperature is monitored by the thermoregulatory centre via thermoreceptors in the hypothalamus and any change from 37oC will cause several negative feedback mechanisms (stimulus-response model) that may involve the nervous/endocrine systems

Question 65

What detects a fall in body temperature (below 37oC) ?

Answer 65

Detected by thermoreceptors in the thermoregulatory centre and stimulates it to:
Send nerve messages to the skeletal muscles to shiver (repeatedly relax and contract), making muscles respire, generating heat;
Send nerve messages to smooth muscles around blood vessels in the skin, constricting the flow of blood to the surface, directing it mainly to essential core organs, helping conserve heat energy;
Send nerve messages to muscles of hair follicles, causing them to stand up to trap heat;
Release TSH into the blood via the pituitary, stimulating the thyroid gland to secrete thyroxine, stimulating cell metabolism and causes an increase in temperature;
These mechanisms increase heat production and restrict heat loss, leading to an increase in body temperature - negative feedback

Question 66

What does a rise in body temperature (above 37oC) stimulate?

Answer 66

Detected by thermoreceptors in the thermoregulatory centre and stimulates it to:
Send nerve impulses to the sweat glands, causing them to secrete sweat onto the body’s surface. Evaporation of the sweat removes heat from the skin;
Send fewer nerve messages to smooth muscles around blood vessels in the skin, causing them to relax. This is vasodilation and it increases blood flow to the surface of the skin, increasing the rate of heat loss from the body;
Decrease the secretion of TSH into the blood by the pituitary, lowering the amount of thyroxine released by the thyroid, decreasing cell metabolism;
These mechanisms decrease heat production and increase heat loss, leading to a decrease in body temperature - negative feedback

Question 67

Where else (other than blood temperature) does the thermoregulatory centre receive stimuli from?

Answer 67

It also receives nerve impulses from thermoreceptors in the skin
The stimuli received in this way result in voluntary actions such as moving to a warmer/cooler place, adding/removing clothing, etc.

Question 68

What is osmoregulation and what does it mainly involve?

Answer 68

Refers to the maintenance of water and solute balance in the body, mainly involving the endocrine system and kidneys

Question 69

What can change the water content of the blood?

Answer 69

External and internal changes such as water intake, amount of sweating, and intake of salts

Question 70

What does osmolarity refer to?

Answer 70

Osmolarity refers to the blood solute concentration. High osmolarity means an increase in solute concentration (dehydration), and low osmolarity means a decrease in solute concentration (overhydration)

Question 71

What detects changes in osmolarity?

Answer 71

Osmoreceptors in the hypothalamus

Question 72

Where does ADH specifically target?

Answer 72

Targets the cells of the collecting ducts in the kidneys, increasing the number of aquaporins in the cell membrane to increase water reabsorption

Question 73

What happens when osmolarity increases?

Answer 73

Osmoreceptors in the hypothalamus detect the change, producing ADH and secreting it into the blood via the posterior pituitary.
The ADH then binds to receptors on the cells of collecting ducts in the kidneys, increasing the number of aquaporins inserted into their cell membranes, increasing water reabsorption (more concentrated urine)
This decreases osmolarity

Question 74

What happens when osmolarity decreases?

Answer 74

Osmoreceptors in the hypothalamus detect the change, decreasing the production and secretion of ADH via the posterior pituitary
Less cells in the collecting ducts bind to ADH, so there are less aquaporins in their membranes, decreasing water reabsorption (more dilute urine)
This increases the osmolarity

Question 75

What do baroreceptors detect?

Answer 75

Changes in blood pressure
Decreased blood pressure stimulates them, causing the same response as osmoreceptors detecting high osmolarity, increasing water reabsorption and vice versa

Question 76

What does increased water content result in?

Answer 76

Increased water content increases blood pressure & volume, and also decreases solute concentration (low osmolarity), so osmoregulation is closely linked to blood pressure, blood volume, and solute concentration

Question 77

What does the control of blood sugar level mainly involve? What is it an example of?

Answer 77

Mainly involves the endocrine system
Control of blood sugar is an example of a homeostatic control mechanism, involving negative feedback

Question 78

What is the blood sugar level of a healthy human?

Answer 78

After a meal it is 4.0-7.8mmol/L
After fasting it is 4.0-5.5mmol/L

Question 79

What are the receptors involved in detecting blood sugar levels?

Answer 79

Beta and alpha cells, which are found in the islets of Langerhans in the pancreas

Question 80

Describe what happens when blood sugar levels increase

Answer 80

An increase in blood sugar level is detected by the beta cells in the pancreas, stimulating it to secrete insulin into the blood
Insulin targets liver cells, causing them to store glucose in the form of glycogen
Insulin targets muscle cells, causing them to increase consumption of glucose
Insulin targets fat tissue cells, causing them to store glucose in the form of glycogen
These responses result in a decrease in blood sugar level, negative feedback

Question 81

Describe what happens when blood sugar levels decrease

Answer 81

A decrease in blood sugar level is detected by the alpha cells in the pancreas, stimulating it to secrete glucagon into the blood
Glucagon targets mainly the liver cells, causing them to break down glycogen into glucose and secrete it into the blood
This results in an increase in blood sugar level, negative feedback

Question 82

What does diabetes refer to?

Answer 82

The inability to maintain blood sugar level in the normal range

Question 83

Describe Type I diabetes

Answer 83

Type I diabetes is when the body’s immune cells attack and destroy beta cells, so no insulin is made
The cause is genetic but may also be from bacterial/viral infection
Treatment requires injections of insulin and diet

Question 84

Describe Type II diabetes

Answer 84

Type II diabetes results from the body becoming resistant to insulin
The insulin is made but receptors do not respond
Cause is mainly sedentary lifestyle, aging, but mostly obesity
Treatment is diet, exercise, weight loss and injections of insulin

Question 85

What does monitoring pH mainly involve?

Answer 85

Mainly involves the nervous system

Question 86

What happens when humans respire?

Answer 86

Humans respire to produce energy, which also produces carbon dioxide
Some of the carbon dioxide dissolves in the tissue fluid, but most of it reacts with water to form carbonic acid, which dissociates to form bicarbonate and hydrogen ions in the blood
This increase in hydrogen ions lowers the pH of the blood

Question 87

What is pH monitored by? How?

Answer 87

pH of the blood is monitored by chemoreceptors in the respiratory centre of the brain.
However, charged particles such as hydrogen cannot diffuse through the brain blood barrier, but uncharged particles can.
So, carbon dioxide diffuses through capillary walls into the cerebrospinal fluid (CSF), where it reacts with water to form carbonic acid, which then dissociates to form bicarbonate and hydrogen ions in the CSF, lowering its pH

Question 88

Describe what happens when carbon dioxide levels in the blood increase.

Answer 88

The carbon dioxide diffuses into the CSF, reacting with water to form carbonic acid, which then dissociates to form bicarbonate and hydrogen ions, decreasing the pH.
The increased hydrogen ions are detected by chemoreceptors in the respiratory centre. This results in more nerve impulses from the respiratory centre to the chest muscles and diaphragm to increase breathing rate.
This lowers carbon dioxide levels in the blood and increases pH.

Question 89

Describe what happens when carbon dioxide levels in the blood decrease.

Answer 89

Decreased carbon dioxide levels means that less carbon dioxide diffuses into the CSF and reacting with water to form carbonic acid, which dissociates to from bicarbonate and hydrogen ions.
This decrease in hydrogen ions and hence increased pH is detected by chemoreceptors in the respiratory centre, resulting in fewer nerve impulses being sent from the to the chest muscles and diaphragm, decreasing breathing rate
This increases carbon dioxide in the blood, decreasing pH