Capillary

Question 1

Other than causing slow blood flow, explain one advantage of capillaries being narrow.

Answer 1

Short pathway / short distance between blood and outside of capillary;

Reference to blood and cells required

2. Large surface area (of blood) in contact with walls of capillaries;

Idea is per unit volume of blood but candidates need not say this

3. Fast exchange / fast diffusion / fast osmosis.

Must relate to increased speed

Question 2

Describe and explain four ways in which the structure of a capillary adapts it for the exchange of substances between blood and the surrounding tissue.

Answer 2

permeable capillary wall / membrane;

2. single cell thick / thin walls, reduces diffusion distance;
3. flattened (endothelial) cells, reduces diffusion distance;
4. fenestrations, allows large molecules through;
5. small diameter / narrow, gives a large surface area to volume / short diffusion distance;
6. narrow lumen, reduces flow rate giving more time for diffusion;
7. red blood cells in contact with wall / pass singly, gives short diffusion distance / more time for diffusion;