Homeostasis

Question 1

Define homeostasis

Answer 1

The ability to maintain a relatively stable
internal environment

Question 2

What do most homeostatic processes rely on?

Answer 2

Negative feedback

Question 3

What happens during negative feedback?

Answer 3

Any deviations from the set point are
resisted and maintained within a healthy normal range

Question 4

Explain the role of insulin

Answer 4

Released when blood glucose levels rise e.g. following a carb-rich meal, and promotes glucose uptake into cells

Question 5

Explain the role of glucagon

Answer 5

Released when blood glucose levels fall e.g. when sleeping or between meals and stimulates the release of glucose from the liver

Question 6

What variables are controlled by negative feedback?

Answer 6

Temperature
Blood pressure
Heart rate
Ion balance
Blood sugars
pH

Question 7

What are the four requirements for negative feedback?

Answer 7

Stimulus
Sensor
Control centre
Effectors

Question 8

Explain stimulus as a requirement for negative feedback

Answer 8

This is the deviation from the set point, for example an increase in body temperature due to exercise

Question 9

Explain sensor as a requirement for negative feedback

Answer 9

This is required to detect the deviation from the set point e.g. the thermoreceptors in the skin and viscera which detect changes in body
temperature

Question 10

Explain the control centre as a requirement for negative feedback

Answer 10

To decide what to do e.g. the hypothalamus is the location of the thermoregulatory centre

Question 11

Explain effectors as a requirement for negative feedback

Answer 11

Structures which return the variable towards its set point, e.g. sweat glands in the skin secrete a
watery sweat which evaporates to cool the body and help normalise the body temperature

Question 12

What is an example of a single hormone which contributes to homeostatic control?

Answer 12

Aldosterone regulates ionic sodium
and potassium levels in the blood
and as a result also influences water
balance and blood pressure
Aldosterone is a mineralocorticoid
steroid produced by the adrenal
cortex

Question 13

Sodium homeostasis

Answer 13

• Normal plasma sodium levels are typically between 135 - 145 mmol/litre
• Aldosterone is released when plasma sodium levels fall below the normal range, this is termed hyponatraemia
• Common causes of hyponatraemia include not eating enough salt or sweating too much
• Aldosterone promotes sodium re-absorption in the kidney, returning sodium levels to within the normal range
• Too much sodium in the blood is termed hypernatraemia and common in high salt diets in the UK, here aldosterone secretion is inhibited and sodium is rapidly lost in the urine

Question 14

Potassium homeostasis

Answer 14

• Normal blood potassium levels are 3.5 - 5.0 mmol/litre
• Aldosterone is released when plasma potassium levels rise above the normal range, this is termed hyperkalaemia
• Hyperkalaemia can be caused by an excess of potassium in the diet or following a physical injury where potassium is released from inured
  cells
• Aldosterone promotes the elimination of potassium by the kidney returning the potassium levels to within the normal range
• Low blood potassium is termed hypokalaemia and is commonly caused dietary potassium deficiency or prolonged used of certain diuretic
  medications. In hypokalaemia, aldosterone secretion is inhibited resulting in potassium re-absorption by the kidney

Question 15

What is positive feedback?

Answer 15

• Any deviation from the set point is amplified and made larger
• In human physiology childbirth (parturition) is a good example of positive feedback

Question 16

Parturition and positive feedback

Answer 16

• Childbirth is initiated through the release of the hormone oxytocin from the pituitary gland
• Oxytocin stimulates contraction of the myometrium (muscular layer of the uterus)
• Contraction of the uterus stimulates more oxytocin release from the pituitary which initiates further uterine contractions
• This mechanism leads to progressively more powerful uterine contractions which (in most cases) ensures the quickest possible delivery
• Oxytocin is often referred to as the
  “love hormone” and also promotes
  maternal and newborn bonding

Question 17

What is the difference between negative feedback and positive feedback?

Answer 17

Negative feedback opposes a change to keep a system stable, like blood sugar regulation, while positive feedback amplifies a change to push a process to completion