In mammals, arteries branch to form smaller blood vessels called arterioles. Arterioles branch to form capillaries that supply blood to tissues. Fig. 4.1 shows transmission electron micrographs of cross-sections through an arteriole and a capillary. Fig. 4.2 shows a capillary network in a mammalian tissue. The arrows indicate the direction of flow of body fluids.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

1. 4(a) artery wall I narrow lumen to maintain high (blood) pressure I ref. to valves / ref. to inner lining being wrinkled or wavy 1 thick, walled / tunica media, to withstand high (blood) pressure / prevent bursting; 2 endothelium / endothelial cells / tunica intima, are smooth, little friction to blood flow / easy flow of blood / no eddies of blood flow/AW; 3 elastic, tissue / fibres, stretches to allow surges in blood flow / recoils to maintain blood pressure or force blood forward; 4 smooth muscle (contracts to), maintains / regulates / controls blood flow ; A smooth muscle distributes blood 5 collagen fibres, avoid rupturing / bursting;
2. 4(b)(i) red blood cells / erythrocytes ; R red and white blood cells one from biconcave (shape) ; no nucleus ; idea of uniform, cytoplasm / cell contents ; idea of rouleau / stacked cells ; I 'clumped' I size
3. 4(b)(ii) assuming arteriole unless told otherwise, accept alternative terminology for layers of wall of arteriole I ref. to folding 1 thicker wall / more than one layer of cells in wall / has tunica intima, tunica media and tunica adventitia whereas capillary has tunica intima; A endothelium for tunica intima 2 more cells forming, perimeter / tunica intima ; 3 wider (vessel) / wider lumen / AW; A actual width(s) 4 nucleus / nuclei, present in wall only in arteriole ; 5 cells lining lumen / endothelial cells, are thicker ; 6 lumen smaller, relative to the, thickness of the wall/overall width ; 7 more (red blood) cells (in lumen); 8 nuclei projecting inwards only in arteriole; 9 AVP; e.g. ref. to smooth muscle cells capillaries are surrounded by cells correct calculation of actual sizes using magnifications in Fig. 4.1 arteriole 20–35 µm and capillary 6–7 µm
4. 4(c)(i) 1 higher (hydrostatic) pressure of blood (at start of capillary); 2 (pressure / ultra) filtration of blood; 3 (causing) leakage / movement out / pushing out, of plasma ; R diffusion 4 either glucose / amino acids / salts, (filtered) out or (large) plasma proteins not (filtered) out ; 5 AVP; e.g. ref. to pores / fenestrations / gaps, within / between, endothelial / lining, cells e.g. molecules smaller than MM ~68 000 (g mol-1 / daltons) can pass out
5. 4(c)(ii) lymph; A lymphatic (fluid) one from I any cells / waste products / toxins / antibodies / fatty acids and glycerol no, named / large / plasma, proteins, e.g. albumen no/little/ less, oxygen / glucose higher concentration of / more, carbon dioxide AVP; e.g. higher concentration of/ more, fat / lipids / lipoproteins if fluid identified as tissue fluid give one mark as an ECF for a difference between plasma and tissue fluid e.g. no, named / large / plasma, proteins e.g. albumen