HSC Exam Practice

Sample response. A closed circulatory system is a transport system in which blood remains enclosed within a continuous network of blood vessels, arteries, capillaries, and veins, at all times, never leaving to bathe tissues directly. Exchange of substances between blood and body cells occurs only at thin-walled capillaries.

Marking notes. 1 mark for blood enclosed in vessels at all times (never leaves vessels / never bathes tissues directly); 1 mark for exchange occurring at capillaries OR mention of continuous vessel network.

Sample response. Arteries carry blood away from the heart; veins carry blood toward the heart. O₂ content does not define the type: the pulmonary artery is an artery but carries deoxygenated blood (right ventricle to lungs), while the pulmonary vein is a vein but carries oxygenated blood (lungs to left atrium).

Marking notes. 1 mark for correct direction definition of arteries (away from heart); 1 mark for correct direction definition of veins (toward heart); 1 mark for a specific counterexample to the oxygen misconception, pulmonary artery (deoxygenated artery) or pulmonary vein (oxygenated vein), named correctly.

Sample response. Feature 1, Biconcave disc shape: this increases the surface area to volume ratio compared to a sphere, exposing more haemoglobin molecules to oxygen at the cell membrane and reducing the diffusion distance to any internal haemoglobin, both adaptations accelerate O₂ loading at the lungs and unloading at tissues. Feature 2, No nucleus at maturity: ejecting the nucleus during development frees internal volume for approximately 280 million haemoglobin molecules per cell, maximising O₂-carrying capacity per cell; the trade-off is a fixed ~120-day lifespan.

Marking notes. 2 marks per feature: 1 for identifying the structural feature correctly; 1 for a specific and mechanistic explanation of how it enhances O₂ transport (not just "it's useful").

Sample response. As body volume increases, surface area grows more slowly (SA:V ratio decreases). Interior cells are therefore further from the outer body surface, and diffusion across large distances is extremely slow, diffusion over 1 metre takes approximately 11 days. Interior cells would receive oxygen far too slowly to sustain cellular respiration. A circulatory system uses bulk flow driven by the heart to deliver oxygen to within diffusion distance of every cell in seconds.

Marking notes. 1 mark for SA:V ratio decreasing with increasing body size; 1 mark for diffusion being too slow over large distances (quantitative example accepted but not required); 1 mark for explaining that transport system replaces diffusion with fast bulk flow to reach interior cells.

Sample response. B lymphocytes are a specific subtype of white blood cell that produces antibodies (immunoglobulins). They do this by recognising pathogens and secreting specific antibody proteins that bind to and help neutralise or mark those pathogens for destruction. The statement "white blood cells produce antibodies" is incorrect because it overgeneralises, only B lymphocytes produce antibodies; other WBC subtypes (neutrophils, monocytes, T lymphocytes) use different immune mechanisms such as phagocytosis or cell-mediated killing, not antibody production.

Marking notes. 1 mark for stating B lymphocytes specifically (not all WBCs) produce antibodies; 1 mark for explaining that other WBC subtypes use different mechanisms (phagocytosis / cell-mediated immunity) and do not produce antibodies.

Sample response. The pulmonary artery shows very low O₂ and very high CO₂; the pulmonary vein shows the opposite, very high O₂ and very low CO₂. This reversal occurs because blood passes through the alveolar capillaries of the lungs between these two vessels: O₂ diffuses from alveolar air (high partial pressure) into the blood (low O₂ partial pressure), binding to haemoglobin; CO₂ diffuses in the opposite direction (from blood to alveolar air), driven by the concentration gradient maintained by continuous ventilation.

Marking notes. 1 mark for correctly describing the trend (O₂ low in pulmonary artery, high in pulmonary vein; CO₂ high in pulmonary artery, low in pulmonary vein, or equivalent); 1 mark for identifying diffusion of O₂ into blood at alveoli; 1 mark for identifying CO₂ diffusion out of blood at alveoli, driven by concentration gradient.

Sample response. After a meal, digested carbohydrates are absorbed as glucose across the small intestinal epithelium into capillaries, which drain directly into the hepatic portal vein, this is why glucose is very high in that vessel. After the blood passes through the liver, glucose concentration would be lower (returning toward the normal regulated range) because the liver removes excess glucose from the blood and stores it as glycogen (glycogenesis). The liver acts as the blood glucose regulator.

Marking notes. 1 mark for identifying intestinal glucose absorption post-meal as the cause of high glucose in the hepatic portal vein; 1 mark for predicting lower glucose in the hepatic vein; 1 mark for identifying liver glycogen storage (glycogenesis) as the mechanism of reduction.

Sample response. In an open circulatory system, as seen in insects such as a grasshopper, the transport fluid (haemolymph) is pumped by a simple tubular heart into the haemocoel (body cavity), where it directly bathes organs. There are no blood vessels beyond the heart, haemolymph drains back through openings (ostia). Because fluid is uncontained, blood pressure is very low. In insects, haemolymph does not carry oxygen, oxygen is delivered directly to tissues by the tracheal system. The open system is adequate because insects are small (short diffusion distances) and have relatively low metabolic demands.

In a closed circulatory system, as seen in mammals such as humans, blood remains enclosed within a continuous vessel network (arteries, capillaries, veins) at all times. A chambered heart generates and sustains high blood pressure. Exchange between blood and tissue cells occurs only at thin-walled capillaries.

Structurally, the key differences are: fluid containment (open: fluid leaves vessels; closed: blood enclosed always), vessel network (open: none; closed: complete arteries-capillaries-veins network), and blood pressure (open: low; closed: high). These structural differences lead directly to functional differences: the closed system's high pressure drives rapid, directed flow to every tissue simultaneously, supporting large body sizes and high metabolic rates. The open system's low pressure means slow, diffuse circulation, adequate only for small bodies with low metabolic demands or with an independent O₂-delivery system (tracheal system).

Marking notes. 1 mark, correctly describes open system structure (haemolymph, haemocoel, ostia, no vessels beyond heart) with named insect example. 1 mark, correctly describes closed system structure (blood enclosed in continuous vessel network, exchange only at capillaries) with named vertebrate example. 1 mark, compares fluid containment (open: leaves vessels; closed: enclosed). 1 mark, compares blood pressure (open: low; closed: high, maintained by heart). 1 mark, compares O₂ delivery mechanism correctly (open/insect: tracheal system; closed: haemoglobin in blood vessels). 1 mark, explicitly links structural difference to functional consequence (e.g. high pressure enables fast directed delivery, supporting large body size / high metabolic rate). 1 mark, acknowledges condition under which open system is adequate (small body, low metabolic rate, tracheal compensation), avoids one-winner reasoning. Award maximum 7.