Capillaries: Solute Movement

Question 1

Why do we need solute and fluid transport?

Where does solute and fluid exchange occur?

Answer 1

• For metabolism

- At capillaries, which are semi-permeable, thin and have a small diameter.

Question 2

What is passive transport?

Answer 2

• Movement of molecules from high to low concentration until equilibrium is reached - doesn’t require ATP.

Question 3

What are the 4 properties that control the rate of solute transport?

Answer 3

1. Properties of passive diffusion:
   Great for transport of lipid-soluble solutes very short distances e.g. O2, CO2
2. Properties of solutes:
   Concentration gradient, Solute size and Lipid solubility
3. Properties of membranes:
   Membrane thickness/composition - can have aqueous pores and carrier-mediated transporters
4. Properties of capillaries

Question 4

What are the 3 types of capillaries and what are their structures? Where are they found?

LOOK AT PICTURES!

Answer 4

1. Continuous capillaries:
• Moderate permeability
• Tight gaps between cells
• Constant basement membrane
e.g. BBB so solutes can’t move easily between brain and blood

2. Fenestrated capillaries:
   • High WATER permeability
   • Fenestrations - protein that sits in plasma membrane and acts like a sieve
   • Bit of disruption of basement membrane.
   e.g. In ‘high water turnover’ tissues like salivary glands, kidneys
3. Discontinuous capillaries:
   • Large fenestrations
   • Lots of disruption of basement membrane
   e.g. In places where movement of cells is required like RBCs in liver, spleen, bone marrow.

Question 5

Describe the 3 other structural features of the capillary walls?

LOOK AT PICTURES!

Answer 5

• Intracellular cleft - for fluid
• Glycocalyx - covers endothelium and is -ve charged. They block solute permeation and access to transport mechanisms - for protein
• Caveolae and vesicles - large pores in cell - for protein

Question 6

What is Fick’s Law?

What is the equation?

LOOK IN NOTES!

Answer 6

How much of a substance is transported per unit of time

Solute movement = D x A x (C1 - C2) / X

D - Diffusion coefficient
A - Area
C1 - C2 - Concentration gradient
X - Diffusion distance

Question 7

What is permeability?

What’s the main way in which glucose is transported into cells?

Answer 7

• Permeability is the rate of solute transfer by diffusion across unit area of membrane per unit concentration difference. It’s all the factors that control diffusion rate together.
• Most of glucose transport is done via GLUT transporters (passive). Only a small amount is filtered through the fenestrations.

Question 8

What controls Diffusion rate? How?

Answer 8

1. ↑Blood flow:
   ↑[solutes] in capillaries. ↑O2/CO2 exchange can occur in lungs because there’s LESS TIME FOR EQUILIBRIUM TO OCCUR.
2. ↓Interstitial concentration (more solute used up in metabolism):
   ↑Concentration difference. Also, ↑Metabolism = ↑Blood flow. This is called Metabolic hyperaemia - ↑O2 delivery to active tissues.
3. Recruitment of capillaries:
   Dilation of arterioles = ↑Capillaries are perfused = ↑Surface area for diffusion and ↓Diffusion distance.