Multiple Choice
1. C The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs at high pressure. Pulmonary vein carries oxygenated blood back to the heart; the aorta carries oxygenated blood at high pressure away from the left ventricle; the vena cava is a vein carrying blood toward the heart.
2. D Insects' tracheal system delivers O₂ directly to cells independently of haemolymph. This means haemolymph only needs to transport nutrients and waste, tasks achievable at low pressure. Large vertebrates cannot do this because tracheal diffusion becomes too slow over large body distances.
3. A The primary benefit is maximised haemoglobin content, and the direct trade-off is no DNA → no cell division → fixed ~120-day lifespan. Flexibility comes from the biconcave disc shape and elastic cell membrane, not from nucleus absence.
4. B Post-meal, glucose absorbed from the small intestine enriches the hepatic portal vein → higher glucose. Intestinal cells are metabolically active during absorption, consuming O₂ and producing CO₂ → lower O₂ and higher CO₂ than the aorta.
5. A Both open and closed systems have a pumping organ: a tubular heart in insects, a chambered heart in mammals. Haemoglobin is absent from insect open systems; closed vessels are only in closed systems; open systems cannot supply large active bodies.
Q6, Model Answer
As body size increases, volume increases proportionally faster than surface area, causing the SA:V ratio to decrease. This means the outer body surface area relative to the volume of cells needing supply becomes progressively smaller.
Diffusion over distances greater than a few millimetres is extremely slow, diffusing across 1 metre would take approximately 11 days. Interior cells of a large organism would receive oxygen and nutrients far too slowly to sustain life by diffusion from the body surface alone.
A specialised transport system solves this using bulk flow, the heart pumps blood under pressure through a vessel network, delivering oxygen and nutrients to within diffusion distance of every cell in seconds rather than days.
Q7, Model Answer
Feature 1, Biconcave disc shape: RBCs have a flattened biconcave disc shape, depressed on both faces. This increases the surface area to volume ratio compared to a sphere of equivalent volume, exposing more haemoglobin molecules to O₂ at the cell membrane and reducing the maximum diffusion distance from membrane to any interior haemoglobin molecule. Both effects accelerate O₂ loading at the lungs and unloading at body tissues.
Feature 2, No nucleus at maturity: Mature RBCs eject their nucleus during development, freeing approximately 40% more internal volume for haemoglobin. Each mature RBC contains approximately 280 million haemoglobin molecules, maximising O₂-carrying capacity per cell. The trade-off is that without a nucleus, the cell cannot produce proteins, divide, or repair damage, limiting its lifespan to approximately 120 days before breakdown and replacement by new cells from bone marrow.
Q8, Model Answer
The pulmonary artery carries blood from the right ventricle to the lungs, it contains low O₂ (haemoglobin largely unloaded from previous systemic circulation) and high CO₂ (accumulated from cellular respiration throughout the body).
The pulmonary vein carries blood from the lungs back to the left atrium, it contains high O₂ (haemoglobin fully saturated) and low CO₂.
These changes occur because in the alveolar capillaries, O₂ diffuses from alveolar air (high O₂ partial pressure) into the blood (low O₂ partial pressure), binding to haemoglobin. Simultaneously, CO₂ diffuses from blood (high CO₂ partial pressure) into alveolar air (low CO₂ partial pressure) and is exhaled. Both movements are driven by concentration gradients maintained by continuous ventilation refreshing the alveolar air.