The high blood pressure at the arterial end of a capillary network results in some components of blood leaving the capillaries and forming tissue fluid. At the venous end, the presence of plasma proteins allows movement of water by osmosis back into the capillaries. Fig. 2.1 is a diagram showing a capillary network. The lymph vessels and the blood vessels at the arterial and venous ends of the network are also shown. [Figure 2.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

1. [2] too large / large size ; A not small enough
2. cannot pass through endothelial, pores / gaps I holes A fenestrations / AW
3. or
4. cannot, pass across / cross the membranes of, endothelial cells ; A epithelial cells in context of entering an endothelial cell and exiting to the tissue fluid
5. [2] biuret (solution / reagent);
6. A copper sulfate solution and, sodium / potassium, hydroxide
7. (change from light) blue to, lilac / purple / violet / mauve ;
8. [3] accept Hb / hb for haemoglobin
9. any three from:
10. 1 (at altitude) lower, partial pressure of oxygen / pO2 (in, atmosphere / inhaled air / inspired air / alveolar air) ; I low pO2 in blood R if stated as high ppCO2 and low pp O2 R low, volume / saturation, of oxygen
11. 2 percentage saturation of haemoglobin (with oxygen), lower / decreased ; A haemoglobin is less saturated A fewer molecules of/less, oxygen, combine / associate, with haemoglobin I absorbed / taken up by R if in context of Bohr effect but then allow ecf for overall max 2
12. 3 (as) haemoglobin has lower affinity for oxygen (than at sea level) ;
13. 4 more haemoglobin (synthesised / required / provided) ;
14. 5 compensation / described; e.g. helps to transport the same quantity of oxygen as at sea level
15. 6 AVP ; e.g. idea of more rbc through pulmonary capillaries per unit time ref. to EPO / erythropoietin (secreted to increase red blood cell production)
16. [3] any three from:
17. A for water potential A plasma for blood
18. mp1 and mp2 = consequence in blood of lower blood albumin than normal
19. 1 idea that low blood albumin means, less / AW, solute;
20. 2 (so) higher water potential (in blood); must be in context of blood context can be anywhere along the capillary A high water potential if ref. to low albumin is made A steeper water potential gradient (than normally, at arterial end)
21. mp3 and mp4 = return of water to blood from tissue fluid (capillary venous end)
22. 3 less steep water potential gradient (than normal) / little difference in water potentials;
23. 4 less / little, water returns to, blood / capillary (from tissue fluid); A (tissue) fluid for water R plasma for water
24. additional ideas
25. 5 idea that more, water / plasma / fluid, enters tissue fluid (than normal) R tissue fluid for fluid A more, water / fluid, enters tissue fluid than exits (to the capillary or lymph system) ; (compared to normal)
26. 6 AVP ; e.g. water can enter tissue fluid for a longer time than normal less albumin than normal to act as an osmotic force for return of water / AW too much excess, water / fluid / tissue fluid, to be taken up into lymph, capillaries / vessels / system
27. [1] (cell surface) receptor(s) ;