HSC Exam Practice

Sample response. The four heart valves are: (1) Tricuspid valvebetween the right atrium and right ventricle. (2) Pulmonary (semilunar) valvebetween the right ventricle and the pulmonary artery. (3) Bicuspid (mitral) valvebetween the left atrium and left ventricle. (4) Aortic (semilunar) valvebetween the left ventricle and the aorta.

Marking notes. 1 mark per valve: correct name AND correct location stated (the two structures it separates). Partial credit (0.5 mark, rounding down) if name is correct but one structure is misidentified. Total 4 marks.

Sample response. The left ventricle pumps blood into the aorta and around the entire systemic circuit, which includes every organ in the body from brain to feet [1]. This circuit is much longer and has higher total resistance than the short, low-resistance pulmonary circuit supplied by the right ventricle [1]. Consequently, the left ventricle must generate approximately three times the pressure of the right (~120 mmHg vs ~25 mmHg), requiring a significantly thicker and more muscular wall to produce that force [1].

Marking notes. 1 mark for identifying the destination (systemic circuit / whole body); 1 mark for identifying the longer distance or higher resistance compared to the pulmonary circuit; 1 mark for explicitly linking higher pressure requirement to thicker wall. Total 3 marks.

Sample response. The general rule is that arteries carry blood away from the heart and veins carry blood toward the heart. The pulmonary artery is an exception because, despite being named an artery (it carries blood away from the right ventricle), it carries deoxygenated blood, it is the only artery in the body that does so [1]. The pulmonary vein is an exception because, despite being named a vein (it carries blood toward the left atrium), it carries oxygenated blood from the lungs, it is the only vein in the body that does so [1]. The reason for these exceptions is that the pulmonary circuit transports blood to and from the lungs for gas exchange, reversing the usual oxygenation status associated with the vessel type [1].

Marking notes. 1 mark for correctly identifying pulmonary artery as carrying deoxygenated blood (exception to artery rule); 1 mark for correctly identifying pulmonary vein as carrying oxygenated blood (exception to vein rule); 1 mark for explaining that the naming rule relates to direction (away/toward heart), not oxygenation status. Total 3 marks.

Sample response. Feature 1: One-cell-thick wall (endothelium only, ~0.5–1 μm). This minimises the diffusion distance between blood and surrounding tissue cells. According to Fick’s principle, the rate of diffusion is inversely proportional to distance, so the thinner the wall, the faster O₂, glucose, CO₂ and metabolic waste can be exchanged. Arteries and veins have walls many cell layers thick, making exchange impossibly slow at those sites. [2 marks: 1 feature + 1 functional link]

Feature 2: Lumen diameter ~5–10 μm (forces RBCs to pass single file). Red blood cells are ~7–8 μm in diameter, so they are physically pressed against the capillary wall as they pass through. This maximises the surface area of each RBC in contact with the thin exchange surface, prolonging the exposure time available for gas exchange between haemoglobin and the surrounding tissue. [2 marks: 1 feature + 1 functional link]

Marking notes. 2 marks per structural feature correctly described and functionally explained (1 mark for feature, 1 mark for function). Accept any two valid capillary structural features (e.g. also acceptable: short diffusion path through endothelium; high surface area to volume ratio of the network; lack of muscle/connective tissue reducing diffusion barrier). Total 4 marks.

Sample response (a). Blood leaving the small intestine has a significantly higher glucose concentration than entering (4.5 → 7.8 mmol/L, an increase of 3.3 mmol/L). This rise occurs because nutrients absorbed from digested food, including glucose, enter the blood of the intestinal villi and pass into the hepatic portal vein. [1] Blood entering the liver via the hepatic artery has the same elevated glucose (7.8 mmol/L, corresponding to portal blood arriving from the intestine). Blood leaving the liver via the hepatic vein shows a large fall in glucose (7.8 → 4.9 mmol/L). This decrease occurs because hepatocytes remove excess glucose from the blood and convert it to glycogen (glycogenesis), storing it in the liver as a buffer against post-meal hyperglycaemia and restoring blood glucose toward the normal fasting range. [2]

Sample response (b). Urea concentration falls sharply from 5.8 to 1.9 mmol/L across the kidneys (a decrease of 3.9 mmol/L). [1] The kidneys filter blood under pressure in the glomerulus, removing urea into the renal filtrate. Urea is not reabsorbed to a significant extent by the tubules, so it remains in the filtrate and is excreted in urine, reducing the concentration remaining in the blood leaving the kidneys. [1] Urea entering the kidneys is elevated (5.8 mmol/L) compared to blood entering the small intestine (4.0 mmol/L) because blood reaching the kidneys has already passed through the liver, where amino acid deamination produced urea (urea rose from 4.0 to 5.8 mmol/L across the liver). The liver is the only site of urea synthesis, so blood accumulates urea as it passes through the liver before being cleared by the kidneys. [1]

Marking notes. Part (a): 1 mark for describing glucose rise at small intestine with values and reason (absorption of digested carbohydrate); 1 mark for identifying liver as the site of glucose removal; 1 mark for naming glycogenesis (or glycogen synthesis) as the mechanism. Part (b): 1 mark for describing the fall in urea across kidneys with values; 1 mark for explaining the mechanism (glomerular filtration + excretion in urine); 1 mark for explaining why entering urea is high (urea produced by liver deamination of amino acids and accumulated in blood before reaching kidneys). Total 6 marks.

Sample response. Starting at the right atrium (blood is deoxygenated), blood is pushed through the tricuspid valve into the right ventricle. Ventricular contraction drives blood through the pulmonary (semilunar) valve into the pulmonary artery, which carries deoxygenated blood to the lungs. At the alveolar capillaries, O₂ diffuses from alveolar air (higher partial pressure) into the blood, binding to haemoglobin to form oxyhaemoglobin; CO₂ diffuses from the blood into the alveolar air and is exhaled. Blood is now oxygenated. It returns via the pulmonary veins into the left atrium, then passes through the bicuspid (mitral) valve into the left ventricle. The left ventricle generates high pressure (~120 mmHg) and ejects blood through the aortic (semilunar) valve into the aorta. Blood distributes through systemic arteries, arterioles, and systemic capillaries. At the capillaries, O₂ and glucose diffuse from blood into tissue cells down their concentration gradients; CO₂ and metabolic waste diffuse from tissue cells into the blood. Blood is now deoxygenated. It drains through venules and systemic veins into the superior and inferior vena cava, returning to the right atrium to complete one full circuit.

Marking criteria:

• 1 markAll four heart chambers named in correct sequence (right atrium → right ventricle → left atrium → left ventricle).
• 1 markAll four valves named in correct sequence (tricuspid → pulmonary → bicuspid/mitral → aortic) and positioned between the correct structures.
• 1 markMajor vessels named in order: pulmonary artery, pulmonary veins, aorta, systemic arteries/arterioles/capillaries, venules/veins, vena cava; oxy/deoxy status stated at each major stage.
• 1 markGas exchange at lung capillaries described: O₂ loaded onto haemoglobin (diffusion from alveolar air into blood); CO₂ unloaded (diffusion from blood into alveolar air).
• 1 markExchange at systemic capillaries described: O₂ and glucose diffuse from blood to tissues; CO₂ and waste diffuse from tissues into blood.
• 1 markOverall response is coherent, correctly closed (ends back at right atrium), with no structural transpositions.