Homeostasis

Question 1

Define homeostasis.

Answer 1

The maintenance of a constant condition in the internal environment.

Question 2

Describe the core features of the extracellular fluid, including ion concentrations, osmolarity, temperature and pH.

Answer 2

1/3 of body fluid
Na+ = 145mM 
K+ = 4mM
Osmolarity ~ 300mosm/L 
Temp = 37.1 °C
pH = 7.4

Question 3

Name the 4 general components of a homeostatic system.

Answer 3

Variable, sensor, integrating centre, effector.

Question 4

What is the principle effector mechanism used to return a variable to the set-point?

Answer 4

Negative feedback

Question 5

Define negative feedback.

Answer 5

Negative feedback is the principle effector mechanism that returns a variable to its set-point, within a closed-loop system. This is achieved by producing an effect that is proportional in magnitude to the initial change.

Question 6

Name 2 examples of feed-forward “anticipatory” mechanisms.

Answer 6

1. The incretin effect - readying GI tract in anticipation of glucose.
2. We get thirsty when eating salty food, prompting fluid consumption before NaCl levels in the blood have time to change.

Note: these are not true homeostatic mechanisms, but may be learned behaviours.

Question 7

What are dual effectors? Name 2 examples.

Answer 7

Pairs of effectors that have antagonistic effects on a variable. E.g. insulin/glucagon for blood glucose, shivering/sweating for temp.

Question 8

What is positive feedback? Name an example.

Answer 8

Not a homeostatic mechanism. Tends to de-stabilise a system and accelerate the transition between different states, by amplifying changes in a variable.

E.g. contractions of the uterus during labour, in response to pressure from baby’s head on cervix.

Question 9

Receptor function is affected principally by which 4 factors?

Answer 9

1. Density
2. Location
3. Affinity
4. Ligand concentration

Question 10

Explain the differences between whole-body, local and cellular homeostasis, providing examples.

Answer 10

Whole-body - integrated responses involving a joint action of ANS, endocrine system and behaviour, e.g. ionic balance, BP, blood volume.

Local - organ-specific responses - e.g. in the lungs, blood vessels in areas of low oxygen constrict to shift blood flow to areas where oxygen uptake can be maximised.

Cellular - after depolarisation of a neuron, opening of K+ channels restores the RMP.