54) Homeostasis

Question 1

What is homeostasis?

Answer 1

• Maintenance of a steady state

Question 2

What are the two types of feedback control?

Answer 2

• Negative feedback: Increase in control variable causes a decrease in control variable. They try to bring thing back to balance and keep control variables within a set point (but may oscillate slightly around the set point).
• Positive feedback: Increase in control variable causes an increase in control variable. It moves things further and further away from the point of equilibrium. They are often involved in dramatic events (e.g. blood clotting and child birth)

Question 3

What is a challenge?

Answer 3

• Something that changes the levels of different aspects/ factors that keep us alive

Question 4

What is the function of homeostatic mechanisms?

Answer 4

• To oppose challenges and bring levels of different aspects back to normal range

Question 5

What are the different components of a feedback system?

Answer 5

• Regulated factor/control variable: (e.g. blood glucose level) Variables that operate around a normal range of values
• Sensor: Tells the body when things are operating outside of the normal range and are called the afferent pathway
• Control centre/ Comparator: Determines set point of variable and maintains these variables at the set point. We can have intrinsic control centres that act locally (e.g. autoregulation in tissues and cells) or we can have extrinsic control centres (e.g. endocrine and nervous system)
• Effector: Returns variables back to their set point (efferent path)
• Response

Question 6

What is the error signal?

Answer 6

• Error signal = Value of controlled variable - set point

- It is affected by the amount that the controlled variable is outside its range

Question 7

How do values of a controlled variable vary during homeostatic control?

Answer 7

• It oscillates around a set point because of the time delay between sensing a change and its correction

Question 8

What type of variables are controlled within the body?

Answer 8

• Physical entities (e.g. blood pressure and core temperature)
• Circulating circulation of chemical substances (e.g. ions, nutrients and hormones)

Question 9

What physiological changes take place in our body in order to maintain core body temperature?

Answer 9

• If it is too cold shivering and vasoconstriction takes place which reduces blood flow to the surface of the skin and helps maintain core temperatures. There may also be a longer term response such as increased metabolism which helps generate heat
• If it is too hot/warm there is vasodilation taking place which increases blood flow to the surface of the skin and so allows heat to be lost by convection. There is also sweating and evaporation that occurs which helps to remove the heat from the body

Question 10

How does the homeostatic control of core body temperatures occur?

Answer 10

• The core body temperature is detected by Hypothalamic temperature receptors located in the brain or the skin
• They feed into the hypothalamus where there is a predetermined set point for body temperature
• If temperature varies from the set point we turn on one of the effector pathways.
• If temperature is too low we need to heat up the body via the heat gain pathway (i.e. shivering, vasoconstriction, increased metabolism)
• If temperature is too high we have to cool down the body via the heat loss pathway (i.e. sweating, vasodilation)
• These pathways cause a change in body temperatures which feeds back into the hypothalamus

Question 11

How does our body temperature change during infection?

Answer 11

• Pyrogens are molecules that are produced by bacterial or viral infections (e.g. bacterial endotoxins.
• They cause the set point for body temperature to increase, which occurs in the hypothalamus of the brain, resulting in a fever
• The temperature is increased by shifting blood flow to the core regions of the body to conserve heat, an increased muscle activity (due to shivering).
• The chills stop when we reach a high temp
• Our body returns back to normal set point after the infection in which we experience a period of sweating and vasodilation to loose heat

Question 12

Why does our body change core body temperature during an infection?

Answer 12

• Inhibit bacterial growth
• Speed up metabolic reactions to combat the infection
• Increase delivery of white blood cell to the site of the infection

Question 13

How does the homeostatic control of blood pressure occur?

Answer 13

• Our blood pressure is capped at a set point which is determined by the medulla in the brain
• If the blood pressure varies too much from this set point it is detected by baroreceptors/stretch receptors around the body (important baroreceptors found in the aortic arch and in the carotid sinus)
• These receptors signal back to the brain to alter heart rate and peripheral resistance (TPR)

Question 14

What pathologies increase blood pressure?

Answer 14

• Genetics
• Environmental
• Age
• Hypotension

Question 15

How do pathologies increase blood pressure?

Answer 15

• Pathologies that increase blood pressure do so by resetting sensory feedback of stretch receptors which causes set point to slip
• This raises blood pressure

Question 16

How is secretion of vasopressin (ADH) increased?

Answer 16

• Sympathetic stimulation
• Decreased atrial receptor firing
• Hyperosmolarity
• Angiotensin II

Question 17

How does ADH control blood pressure?

Answer 17

• Neurosecretory cells within the hypothalamus synthesise and release hormones
• These hormones travel to the posterior pituitary and stimulate them which releases vasopressin (ADH) into circulation
• The vasopressin secreted causes vasoconstriction and increases renal fluid reabsorption (which increases blood volume)
• Both these factors cause increased arterial pressure

Question 18

How does the body attempt to restore loss of blood pressure?

Answer 18

• After a haemorrhage blood volume is lost and hence blood pressure is reduced
• To help restore blood pressure several homeostatic control systems are activated
• Firstly the baroreceptor reflex is triggered which increases cardiac output and TPR
• Secondly ADH secretion is stimulated to increase blood volume
• Overall we have an increase in blood pressure back to the normal range

Question 19

How is cortisol regulated?

Answer 19

• Cortisol is regulated by the hypothalamic-pituitary axis
• It is regulated by the hormone Corticotropin-Releasing Hormone (CRH) which is released by the hypothalamus from the neurosecretory cells into the blood vessels perfusing the anterior pituitary lobe
• This stimulates the anterior pituitary to secrete Adrenocorticotropic Hormone (ACTH) which circulates in the blood and stimulates cortisol synthesis and secretion from the adrenal gland

Question 20

Explain the feedback loop in cortisol regulation?

Answer 20

• There is a set point of cortisol which is set by the hypothalamus
• When cortisol levels drop too low the hypothalamus releases CRH which acts on the anterior pituitary gland to secrete ACTH
• ACTH circulates in the body which acts on receptors in the adrenal cortex to produce and synthesise cortisol
• This causes cortisol levels to rise which causes a negative feedback loop to switch off the CRH secretion in the hypothalamus

Question 21

What factors affect cortisol regulation?

Answer 21

• Circadian rhythm (body clock): Cortisol levels high in the morning and low during the night
• Stress: Increased stress causes increased cortisol release

Question 22

Explain the negative feedback loop in sex steroid concentration

Answer 22

• There is a set point of sex steroid which is set by the hypothalamus
• When sex steroid levels drop too low the hypothalamus releases Gonadotrophin-Releasing Hormone (GnRH) which acts on the anterior pituitary gland to secrete LH and FSH
• LH and FSH circulates in the body which acts on receptors in the gonads to produce and synthesise sex steroids (such as oestrogen and testosterone)
• This causes sex steroids levels to rise which causes a negative feedback loop to switch off the GnRH secretion in the hypothalamus

Question 23

How is blood glucose levels controlled within the body?

Answer 23

• Blood glucose is tightly controlled within the body
• After a meal blood glucose levels rise rapidly
• The Beta cells within the pancreas will sense the increase in blood sugar and will secrete insulin
• Insulin circulates in the blood and will cause fat (adipose), muscle and liver tissue to take up and store glucose
• This removes it from the bloodstream and so causes blood sugar levels to drop
• After prolonged fasting our blood sugar levels fall
• The Alpha cells within the pancreas secrete glucagon
• Glucagon stimulates gluconeogenesis (making of glucose from precursor molecules) and also triggers the release of glucose from the liver
• Overall there is an increase in blood sugar levels

Question 24

Explain the negative feedback loop in controlling blood sugar levels

Answer 24

• There is a set point of blood sugar levels which is monitored by the pancreas
• When blood glucose levels rise too high (e.g. after eating a meal) the pancreas will detect this and secretes insulin
• Insulin stimulates glucose uptake into muscle, fat and liver tissues and store it
• This decreases blood glucose level which is detected by the pancreas and causes a negative feedback loop to switch off insulin secretion
• When blood glucose levels fall too low (e.g. after fasting) the pancreas detects this and will secretes glucagon
• Glucagon stimulates the liver to undergo gluconeogenesis and to release glucose molecules from stores in the liver
• This raises blood glucose level which is detected by the pancreas and causes a negative feedback loop to switch off glucagon secretion

Question 25

Explain the positive feedback loop of blood clotting

Answer 25

• When there is a tear in the blood vessel tissue factor will be released which activates thrombin
• Thrombin activates fibrinogen (converting it to fibrin) which will plug up the hole in the vasculature
• This causes platelets in the hole to activate which releases a chemical which causes more platelets to activate until the wound is clotted (switching off the positive feedback loop)

Question 26

Explain the positive feedback in uterine contractions in childbirth

Answer 26

• In labour oxytocin (made by the pituitary gland) stimulates contraction in the uterus which will activate stretch receptors in the cervix
• This will help fire signals to the brain which triggers oxytocin release
• The oxytocin released circulates in the blood to cause more and more uterine contractions
• This continues until the baby has been delivered (which switches off the positive feedback loop)