Capillary Solute and Fluid Exchange I

Question 1

Explain the difference between diffusion and electrochemical flux.

Answer 1

Diffusion - movement down concentration gradient.

Electrochemical flux - movement down concentration and electrical gradient.

Question 2

Capillaries are one cell thick and are composed of only one tissue type. What is it?

Answer 2

Endothelium

Question 3

Capillaries connect what vessels together?

Answer 3

Terminal arterioles to venules.

Question 4

Where are the highest densities of capillaries found?

Answer 4

Highly active tissues - brain, heart, muscles, liver, kidneys.

Question 5

Why is passive diffusion only useful over very short distances?

Answer 5

Time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with the distance squared:

t = x^2 / 2D, where D = diffusion coefficient.

Question 6

Recall Fick’s law.

Answer 6

Js (solute movement) = - D A ΔC/x

D = diffusion coefficient, A = area, ΔC/x = concentration gradient over distance x.

Higher value of D means easier for solute to move through solvent.

Question 7

Compare and contrast the 3 types of capillaries.

Answer 7

Continuous capillaries:

• Moderate permeability; tight gaps between neighbouring cells; constant basement membrane.
• Blood-brain barrier, muscle, skin, fat, connective tissue.

Fenestrated capillaries:

• High water permeability, fenestration structures, modest disruption of membrane.
• ‘High water turnover’ tissues; salivary glands, kidney, synovial joints, anterior eye, choroid plexus (cerebrospinal fluid), gut mucosa.

Discontinuous capillaries:

• Very large fenestration structures, disrupted membrane.
• When movement of cells is required; RBCs in liver, spleen, bone marrow.

Question 8

Describe the 3 structural features of the capillary wall that influence solute transport.

Answer 8

Intercellular cleft - 10-20nm wide.

Glycocalyx - covers endothelium, -ve charged material, blocking solute permeation and access to transport mechanisms, highly regulated.

Caveolae and vesicles - large pores system.

Question 9

Define permeability and explain the modification of Fick’s law for a porous membrane.

Answer 9

Permeability - the rate of solute transfer by diffusion across unit area of membrane per unit concentration difference - “how freely a solute crosses a membrane.”

Js = - P Am ΔC, where Am = surface area of capillary involved in transport.

Question 10

How much glucose transport is accounted for by diffusion and filtration?

Answer 10

Diffusion (facilitated) via GLUTs - 98%.

Filtration - 2%.

Question 11

What 3 main factors control diffusion rate?

Answer 11

1. Increased blood flow.
2. Fall in interstitial concentration.
3. Recruitment of capillaries.