solute and fluid exchange: tutorial

Question 1

what are the 3 types of capillaries

Answer 1

• continuous
  
  • fenestrated
• discontinuous

Question 2

What is passive diffusion, how is it regulated by Fick’s law?

Answer 2

Passive diffusion is the movement of a molecule from an area of high concentration to an area of low concentration. Passive diffusion doesn’t require ATP.
Putting together the properties of passive diffusion, the solute and the membrane leads you to Fick’s Law

Question 3

What are the roles of the intercellular cleft, glycocalyx and vesicle transport?

Answer 3

INTERCELLULAR CLEFT: it allows for the transport of fluids and solutes between cells, helping to equilibriate levels of molecules between the plasma and the interstitial fluid

VESICLE TRANSPORT: it's used for the transport of large solutes (such as proteins and lipophilic molecules)

GLYCOCALYX:

- it acts as a barrier (eg. from bacteria)
- it blocks solute permeation and access to transport mechanisms, so needs to be regulated - it affects the diffusion coefficient

Question 4

What controls solute diffusion rates?

Answer 4

BLOOD FLOW:
more blood brings more solutes

RECRUITMENT OF CAPILLARIES:
dilation of capillaries leads to more surface area, as well as shortening diffusion distance, increasing the diffusion rate

FALL IN INTERSTITIAL CONCENTRATION: during metabolism, more solute is used up therefore increasing the concentration difference

Question 5

What is Starling’s principle of fluid exchange?

Answer 5

Starling’s hypothesis states that the fluid movement due to filtration across the wall of the capillary is dependent on the balance between the hydrostatic pressure gradient and the oncotic pressure gradient across the capillary

Question 6

What are the different hydrostatic and oncotic pressures in a normal capillary bed – how do they change with hypovolemia?

Answer 6

The four pressures are:
- Pc: capillary blood pressure
- Pi: interstitial fluid pressure
- πp: plasma colloid osmotic proteins
- πi: interstitial colloid osmotic proteins
In the event of hypovolemia, all the pressures decrease.

Question 7

Do capillaries normally reabsorb or filter, and why?

Answer 7

Yes, they have to in order to distribute oxygen and nutrients.

The reabsorption maintains the blood volume. Filtration, however, dominates; well-perfused capillaries filter along their entire length.

Question 8

What are the lymphatics, and what are their role?

Answer 8

The lymphatic circulation returns excess tissue fluid/ solutes back to the CVS.
They have roles in fluid homeostasis, immunosurveillance and fat homeostasis.

Question 9

What is oedema, and how do changes in Pc , πP, and inflammation cause oedema?

Answer 9

Oedema is when there is excess fluid in the interstitial space.
It’s caused by increased capillary pressure (Pc), decreased plasma protein oncotic pressure (πp) and inflammation, all which lead to an increase in interstitial fluid.