Homeostasis

Question 1

What is meant by the ‘dilution principle’?

Answer 1

C = m/V, therefore V = m/C

C = concentration, m = mass, V = volume

Question 2

Which processes are regulated in order to maintain H2O balance?

Answer 2

Input - regulated by thirst mechanism
Output - regulated by kidney function (i.e. urination)

Other processes are regulated, but their control is not oriented towards H2O balance, e.g. sweating is controlled as part of temperature regulation, so possible conflict between water and temperature regulation.

Question 3

Which two bodily compartments constitute the extracellular fluid?

Answer 3

Interstitial fluid (fluid between cells) and plasma (fluid component of blood)

Question 4

What are the three fluid compartments of the body and their respective ratios?

Answer 4

Plasma and interstitial fluid make the extracellular fluid - this accounts for 1/3 of the body’s total water. Intracellular fluid accounts for the other 2/3.

Question 5

Why is it important to know the proportion of body water/fat in a clinical situation? (Hint: think prescribing medications)

Answer 5

If treating with lipid or H2O soluble drugs, the proportion of body fat/water will influence the rate at which drug can be eliminated from the body.

Question 6

What is the only difference between the composition of the interstitial fluid and the blood plasma?

Answer 6

Interstitial fluid does not contain plasma proteins.

Question 7

Which bodily fluid compartment(s) can be directly sampled?

Answer 7

Only plasma

Question 8

Which bodily fluid compartment(s) can be directly measured (using the dilution principle)?

Answer 8

1. Plasma volume (PV)
   Since plasma proteins cannot cross the capillary walls, you can use dyes or radioactive labels that attach to plasma proteins.
2. Extracellular volume (ECF)
   You need something that freely crosses capillary walls, but cannot cross cell membranes e.g. insulin, sucrose, mannitol (which are all too large to cross the cell membrane), or Na+, Cl- (which are actively extruded from cells)
3. Total body water (TBW)
   There is no barrier to water in the body, so you can use a loading dose of heavy water/deuterated water (D2O)

Question 9

Which bodily fluid compartments can be sampled indirectly, and how is this done?

Answer 9

Interstitial fluid: ISF = ECF-PV

Intracellular fluid: ICF = TBW-ECF

Question 10

What is feedforward control?

Answer 10

Feedforward control can be likened to ‘learned anticipatory responses to known cues’, i.e. anticipatory regulation of heartbeat in advance of actual physical exertion