Capillary Exchange - Lecture 14

Question 1

Why are capillaries good for exchange?

Answer 1

Short Diffusion distance:
○ Thin walls (1um)
○ Small diameter (8um)
○ Close proximity to cells
Blood flows slowly:
○ Large cross-sectional area of capillary network
Large surface area for exchange:
○ 10+ billion capillaries
○ ~600m^2

Question 2

What are the three types of capillaries and where are they found?

Answer 2

○ Continuous: most common
○ Fenestrated: endocrine organs, kidneys, intestines
○ Sinusoid: endocrine organs, liver, bone marrow, spleen

Question 3

What transport mechanisms are involved in capillary exchange?

Answer 3

○ Diffusion: down a concentration gradient
○ Bulk flow: down a pressure gradient
○ Transcytosis: by vesicular transport

Question 4

How does diffusion occur?

Answer 4

○ Through endothelial membrane: lipid soluble gases & molecules e.g. O2, CO2 and fatty acids
○ Through channels: Ions e.g. K+ Na+ Cl- Ca2+
○ Between endothelial cells: small water-soluble moleculues e.g. glucose, urea, amino acids

Question 5

How does transcytosis occur?

Answer 5

In vesicles: macromolecules e.g. glycoproteins

Question 6

How does bulk flow occur?

Answer 6

○ Through clefts: small water-soluble molecules
○ Through pores: large water-soluble molecules

Question 7

How is bulk flow determined?

Answer 7

Net filtration pressure

Question 8

What are the four forces that influence net filtration pressure?

Answer 8

○ Capillary hydrostatic pressure: pushing fluid into ISF
○ Intersitial fluid hydrostatic pressure: pushing fluid into capillaries
○ Blood colloid osmotic pressure: pulling fluid into capillaries
○ Interstitial fluid colloid osmotic pressure: pulling fluid into ISF

Question 9

What pressures are negligible?

Answer 9

○ Intersitial fluid hydrostatic pressure
○ Interstitial fluid colloid osmotic pressure

Question 10

What is the equation for net filtration pressure?

Answer 10

Net hydrostatic pressure - net osmotic pressure

Question 11

What is filtration and what influences it?

Answer 11

○ Pushing fluid from capillaries into ISF
○ Capillary hydrostatic pressure (decreases along capillary)
○ Happens if net filtration is positive

Question 12

What is reabsorption and what influences it?

Answer 12

○ Pulling fluid into the capillaries from ISF
○ Blood colloid osmotic pressure (affected by blood volume)
○ Happens if net filtration negative

Question 13

What are the dynamics of capillary exchange?

Answer 13

○ Max filtration pressure is greater than max absorption pressure
○ Transition point is located towards the venous end
○ More filtration takes place along the capillary than absorption

Question 14

What happens to net filtration pressure in someone with hypertension?

Answer 14

○ Capillary hydrostatic pressure increases
○ Net filtration pressure increases and is more positive
○ Increased filtration
○ Fluid collects in extremities leading to systemic oedema

Question 15

What happens to net filtration pressure after severe haemorrhage?

Answer 15

○ Capillary hydrostatic pressure decreases
○ Net filtration pressure decreases and is more negative
○ Increased reabsorption
○ Fluid recalled from tissue into bloodstream which increases blood pressure and cardiac output

Question 16

What happens to net filtration pressure when you are dehydrated?

Answer 16

○ Blood colloid osmotic pressure increases
○ Net filtration pressure decreases and becomes more negative
○ Increased reabsorption
○ Fluid recalled from tissue into bloodstream
○ Delays onset of symptons

Question 17

What happens to net filtration pressure when you have tissue damage?

Answer 17

○ Interstitial fluid colloid osmotic pressure increases
○ Net filtration pressure increases and becomes positive
○ Increased filtration
○ Plasma proteins leak into ISF increasing ISFCOP
○ Local swelling (oedema)

Question 18

Still have a few more to add :(