Fig. 4.1 shows drawings of the transverse sections of two blood vessels, labelled A and B. Both drawings were made from the same specimen at the same magnif...

This question tests careful measurement from drawings, a ratio, a magnification calculation, and how artery and vein structure relate to their functions.

(a) Measuring the wall thickness of vessel A along the marked line gives a value in the region of 20 to 32 mm (accept your own careful reading) [1].

(b) Measuring the wall thickness of vessel B gives about 6 to 12 mm [1].

(c) Measuring the lumen diameter of vessel B gives about 55 to 65 mm [1].

(d) Record your three measurements in the table [1] and calculate the ratio of wall thickness to lumen diameter for vessel B as \( \dfrac{\text{wall thickness}}{\text{lumen diameter}} \) [1]; for example \( \dfrac{9}{60} = 0.15 \) [1]. A worked example table is:

(e) The artery is vessel A [1], because it has the thicker wall relative to its lumen, which is needed to withstand the high pressure of blood pumped from the heart [1].

(f) Real size = image size ÷ magnification, so the real wall thickness of A is \( \dfrac{\text{measured value}}{50} \); for example \( \dfrac{26}{50} = 0.52\ \text{mm} \) [1 working, 1 answer].

(g) Vessel A suits carrying high-pressure blood because its thick, muscular wall withstands the high pressure [1]; elastic tissue lets the wall stretch and recoil to smooth the flow [1]; and the relatively narrow lumen helps keep the pressure high [1].

(h) Vessel B returns blood to the heart because its wide lumen offers little resistance, so blood at low pressure flows back easily [1]; its valves stop the blood flowing backwards [1]; together these help return the blood to the heart, even against gravity [1].

(i) Both drawings were made at the same magnification so that the sizes of the two vessels can be compared fairly (directly) [1].

(j) Any two features used to tell the vessels apart: A has a thicker wall; A has a smaller lumen; A has a more muscular wall [2].

(k) The tissue in the wall of A that allows it to recoil after stretching is elastic tissue [1].