The heart, blood vessels and haemoglobin: exam questions

Answer: C – surface area to volume ratio is small, so diffusion alone is too slow

• A: surface area = 6 × 2² = 24 mm², volume = 2³ = 8 mm³, ratio = 3 : 1 (1)
• B: surface area = 6 × 8² = 384 mm², volume = 8³ = 512 mm³, ratio = 0.75 : 1 (1)
• B has a much smaller surface area to volume ratio / longer diffusion distance to the centre (1)
• so diffusion alone is too slow to supply all of its cells, whereas in A diffusion is fast enough (1)

• thick wall / collagen, to withstand high pressure / prevent bursting (1)
• elastic fibres stretch and recoil to even out the pressure surge / keep blood flowing smoothly (1)
• smooth muscle can contract to narrow the lumen / control blood flow (1)
• narrow lumen helps maintain high pressure (1)
• smooth endothelium reduces friction (1)
• Max 4

• wall is one cell thick / a single layer of endothelial cells, so short diffusion distance (1)
• very narrow lumen so red blood cells are squeezed close to the wall / flow is slow, giving time for exchange (1)
• large total surface area / dense network close to every cell (1)
• gaps between cells allow small molecules and fluid to pass (1)
• Max 3

• (i) open, as atrial pressure (1.1) is higher than ventricular pressure (0.9) (1)
• (ii) closed, as ventricular pressure (16.0) is higher than atrial pressure (0.6) (1)
• (iii) open, as ventricular pressure (16.0) is higher than aortic pressure (15.5) (1)
• (iv) closed, as aortic pressure (12.5) is higher than ventricular pressure (11.0) (1)

• time for one cycle = 60 ÷ 90 (1)
• = 0.67 s (1)
• percentage = 0.3 ÷ 0.667 × 100 = 45 % (1)

• septum separates the left and right sides / oxygenated and deoxygenated blood (1)
• atrioventricular valves prevent backflow from ventricles to atria (1)
• tendinous cords stop the valves turning inside out / being forced back (1)
• semilunar valves in the aorta and pulmonary artery prevent backflow into the ventricles (1)
• valves open when pressure behind is higher than in front / shut when pressure in front is higher (1)
• Max 4

• some dissolved in plasma (1)
• some combines with haemoglobin to form carbaminohaemoglobin (1)
• most converted to hydrogencarbonate ions in red blood cells, catalysed by carbonic anhydrase (1)
• hydrogencarbonate ions diffuse out into the plasma, with chloride ions moving in (1)
• H⁺ ions produced bind to haemoglobin, acting as a buffer (1)
• Max 4

Answer: B – the curve shifts right (Bohr effect), so more oxygen is released

• (i) fetal haemoglobin is more saturated at every pO₂ (1)
• the difference is greatest at low pO₂ / 4 kPa (19 percentage points) and smaller at high pO₂ (1)
• (ii) fetal haemoglobin has a higher affinity for oxygen / curve is to the left (1)
• in the placenta pO₂ is low, so adult (maternal) haemoglobin unloads oxygen while fetal haemoglobin is still highly saturated / loads it (1)
• so oxygen passes from the mother’s blood to the fetus (1)
• Max 5

• at low pO₂ the first oxygen molecule binds with difficulty, so the curve is shallow (1)
• binding of the first oxygen changes the shape of haemoglobin, making it easier for the next ones to bind / cooperative binding, so the curve is steep (1)
• at high pO₂ haemoglobin is almost saturated / few haem groups remain free, so the curve levels off (1)