S1: Homeostasis

Question 1

List the components of a feedback system

Answer 1

Regulated factors: have set point, operating range, ‘error signals’

Detector/sensor: Afferent path

Comparator/control centre: Determines the set point of variable. Intrinsic (local cell or tissue) and extrinsic (endocrine, NS, hypothalamus)

Effector: Returns variable to set point

Question 2

What are error signals in homeostasis?

Answer 2

The value of a controlled variable will oscillate around the set point -> Because of time delay between sensing a change and correction.

The deviation is the error signals.

Question 3

What increases body temperature?

Answer 3

Shivering
Vasoconstriction
Increased metabolism

Question 4

What decreases body temperature?

Answer 4

Vasodilation

Sweating

Question 5

Explain the feedback mechanism for temperature

Answer 5

Variable = core body temp

The temperature is detected by cutaneous temperature receptors or hypothalamic temperature receptors which will relay back to the hypothalamus which can then use effectors to increase or decrease body temeperature

Question 6

What are pyrogens?

Answer 6

Bacterial or viral infections that change to set point to a higher level resulting in fever

Question 7

What are the benefits of a higher temperature?

Answer 7

• Inhibits bacterial growth
• Speed of metabolic reactions
• Increases delivery of white blood cells to infection sites

Question 8

What does pyrexia mean?

Answer 8

Fever

Question 9

Describe the baroreceptor reflex

Answer 9

BP= TPR x CO
so if CO or TPR increases, BP will increase

If blood pressure is increased, baroreceptors will detect this and relay it to the medulla. Heart rate will decrease and peripheral resistance will decrease.

Increased paraympathetic input and decreased sympathetic input decrease CO
Decreased SI decreses TPR

Question 10

What structures does the baroreceptor reflex include?

Answer 10

Vagus nerve
Aortic arch receptors

They help increase CO and TPR.

Question 11

What NS will be activated if blood pressure falls?

Answer 11

Activation of sympathetic output increases CO and TPR which increases BP.

Question 12

What is hypertension?

Answer 12

High BP

There can be resetting of the sensitivity of baroreceptors which will ultimately alter BP

Question 13

Explain ADH origin in the hypothalamic pituitary axis and anterior pituitary gland

Answer 13

• Neurones in the hypothalamus synthesise and release hormones into the posterior pituitary e.g.ADH through neurosecretory cells
• There is also anterior pituitary (glandular tissue), other neurosecretory cells in the hypothalamus release their hormones into the portal capillaries in which they are transported directly to endocrine cells of the anterior pituitary gland.

Question 14

What is another name for ADH?

Answer 14

Vasopressin

Question 15

Explain the control of extracellular fluid

Answer 15

• When osmolarity increases (due to increased Na+), osmoreceptor cells detect this in the hypothalamus by shrinking
• AP generated which signals posterior pituitary gland to release ADH
• ADH is transported to the kidneys which increases water permeability of distal and collecting tubules resulting in more water reabsorption into the blood and more concentrated urine

Question 16

Explain the role of ADH in the control of blood pressure

Answer 16

ADH has an effect on the vasoconstriction of vessels which causes an increase in arterial pressure
This increases blood volume

Question 17

What reflexes would our body use after haemorrhage?

Answer 17

After a haemorrhagebloodvolumeand hencebloodpressureare reduced. To help restore blood pressure several homeostatic control systems are activated, these include:

Thebaroreceptorreflexto increasecardiacoutputandtotalperipheralresistance
Stimulation ofADHsecretionto increasebloodvolume

Question 18

How is cortisol released?

Answer 18

CRH (corticotrophin-releasing hormone) is released by hypothalamus

This stimulates ACTH (adrenocorticotrophic hormone) to be released from the anterior pituitary gland

This stimulates cortisol secretion from the adrenal cortex

Question 19

Explain negative feedback on cortisol release

Answer 19

If there is too much cortisol, there will be–ve feedback on the anterior pituitary and hypothalamus. If cortisol is too low, there will be an inhibition of–ve feedback, so these hormones will increase.

Cortisol is released according to ourbodyclock, cortisol is high in themorningand gradually drops down.Stresscan affect this cycle by keeping cortisol levels high.

Question 20

What do thyroid hormones control?

Answer 20

Rate of metabolism

Question 21

Explain negative feedback of Thyroid hormones

Answer 21

When T3and T4levels are low, there is a lack of negative feedback causing TRH secretion from hypothalamus.

TSH acts on the thyroid gland to release T3(tri-iodothyronine) and T4(thyroxine)

Question 22

How are thyroid hormones released?

Answer 22

TRH (thyroid releasing hormone) is released from the hypothalamus]

This acts on the anterior pituitary to release TSH (thyroid stimulating hormone)

TSH acts on the thyroid gland to release thyroid hormone

Question 23

What is negative feedback?

Answer 23

An increase in a controlled variable results in a decrease in a controlled variable

Question 24

What is positive feedback?

Answer 24

An increase in a controlled variable results in a increase in a controlled variable

Question 25

Explain the control of uterine contractions in labour by oxytoxin

Answer 25

1. In labour oxytocin stimulates contraction of uterine muscles
2. Cervix dilates and activates stretch receptors
3. Action potentials signal to hypothalamus
4. Stimulates further release of oxytocin
5. This stops once birth has occurred

Question 26

Give an example of positive feedback with nerves

Answer 26

Another example is the generation of action potentials, through the influx of Na+, as a few voltage gated Na+channels opening triggers the opening of many others.
The voltage and time dependent Na+channels then quickly inactive which stops the +ve loop.