Circulatory System

Question 1 (10 marks)

(a) At altitude the partial pressure of O₂ in inspired air is low (1), so less O₂ is available to bind haemoglobin in the lungs (1); haemoglobin becomes less saturated, so each unit of blood carries less O₂ and tissue delivery falls (1). (3)

(b) More RBCs means more haemoglobin per unit volume (1), increasing the total oxygen-carrying capacity of the blood to compensate for lower saturation (1). (2)

(c) Any two, each consequence (1) + justification (1):

• Heart must work harder / higher workload because thicker (more viscous) blood is harder to pump (1)+(1).
• Increased risk of clots / sluggish flow / poor capillary perfusion because viscous blood flows more slowly (1)+(1).
• Raised blood pressure due to increased resistance (accept). (4)

(d) Any: platelets — role in clotting; WBCs — role in immune defence; plasma proteins — transport/osmotic balance. (1 for component + role) (1)

Question 2 (12 marks)

(a) Left atrium and left ventricle (2); left side of the heart (1). (3)

(b) During ventricular systole (contraction) (1). Normally the rising ventricular pressure forces the bicuspid valve shut (1), preventing backflow into the atrium so all blood is ejected into the aorta (1). (3)

(c) Correct order (award ~0.5 per correct element, round to 4): Left ventricle → aortic (semilunar) valve → aorta → systemic arteries/body/capillaries → venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary (semilunar) valve → pulmonary artery → lungs (capillaries) → pulmonary veins → left atrium → bicuspid valve → left ventricle. (4)

(d) Blood leaks back into the left atrium, so the ventricle handles extra volume each cycle (1); the left atrium becomes overfilled and enlarges/dilates over time (1). (2)

Question 3 (10 marks)

(a) Type O Rh− (1). O red cells have neither A nor B antigens, and Rh− has no D antigen (1), so there are no antigens on the donor cells for any recipient antibodies to attack (1) — universal donor, safe regardless of the victim's group (1). (4)

(b) The victim is Type B, so their plasma contains anti-A antibodies (1). AB donor cells carry the A antigen (1), so anti-A would bind and agglutinate the transfused cells, causing a haemolytic reaction (1). (3)

(c) In the first pregnancy the Rh− mother is usually not yet sensitised, so no anti-Rh (anti-D) antibodies are present at first (1). Fetal Rh+ blood mixing at birth causes her to produce anti-D antibodies (1). In a second Rh+ pregnancy these antibodies cross the placenta and destroy fetal RBCs (haemolytic disease of the newborn) (1). (3)

Question 4 (10 marks)

(a) 120 = systolic pressure, during ventricular contraction/systole (1)+(1 event); 80 = diastolic pressure, when ventricles relax/diastole (1). (3)

(b) Pulse pressure = 120 − 80 = 40 mmHg (1). Cardiac cycle duration = 60 s ÷ 72 = 0.833 s (≈0.83 s) (2, working shown). (3)

(c) On standing, blood pools in the legs, cardiac output and BP drop (1). Baroreceptors in the carotid sinus/aortic arch detect reduced stretch (1) and send signals to the medulla (cardiovascular centre) (1). Sympathetic activity increases → heart rate and force rise and vessels constrict, restoring blood pressure (1). (4)

Question 5 (8 marks)

(a) Any two (1 each): open — blood (haemolymph) bathes tissues directly in body cavity vs closed — blood stays within vessels; open — low pressure vs closed — higher pressure; closed has capillaries for exchange, open does not; closed allows faster/more directed delivery. (2)

(b) The insect uses a tracheal system: air enters through spiracles and travels via trachea/tracheoles directly to tissues (1)+(1), so O₂ delivery does not depend on blood (1); the open system only needs to distribute nutrients/hormones, for which low pressure suffices. (3)

(c) In double circulation blood is re-pressurised by the heart between the lungs and body (1), maintaining high pressure to the systemic tissues (1), and oxygenated and deoxygenated blood are kept separate for more efficient O₂ delivery (1). (3)

[
  {"claim": "Pulse pressure = 120 - 80 = 40 mmHg", "code": "result = (120 - 80 == 40)"},
  {"claim": "Cardiac cycle duration = 60/72 ≈ 0.833 s", "code": "result = abs(Rational(60,72) - Rational(5,6)) == 0 and abs(float(Rational(60,72)) - 0.8333) < 0.01"},
  {"claim": "Heart rate 72 bpm gives 0.833 s per beat, so ~1.2 beats per second", "code": "result = abs(72/60 - 1.2) < 1e-9"}
]