Transport Systems in Animals: Overview and Blood

Q1, Sample Band 6 response (7 marks), with marking criteria

In an open circulatory system, as seen in insects (e.g. a grasshopper), the transport fluid, haemolymph, is pumped by a tubular heart into the haemocoel (body cavity), where it directly bathes the organs and tissues. There are no blood vessels beyond the heart; haemolymph pools around organs and drains back through openings called ostia. Because the fluid is not contained, blood pressure is low. In insects, haemolymph does not carry oxygen, oxygen delivery is handled independently by the tracheal system.

In a closed circulatory system, as seen in mammals (e.g. humans), blood remains enclosed within a continuous network of vessels, arteries, capillaries, and veins, at all times. A chambered heart generates and maintains high blood pressure throughout this network. Exchange between blood and tissues occurs only at thin-walled capillaries.

Comparing the two systems on three criteria: (1) Fluid containmentopen: fluid leaves vessels and bathes tissues; closed: blood enclosed in vessels at all times. (2) Blood pressureopen: low, because no vessel walls contain the fluid; closed: high, sustained by the heart and vessel walls. (3) Speed and directedness of deliveryopen: slow, with no maintained pressure gradient; closed: fast and directed, reaching specific tissues rapidly.

One functional advantage of the closed system is that maintained high pressure enables O₂-rich blood to reach every tissue within seconds, supporting large body size and high metabolic rates such as sustained running or flight. The open system cannot achieve this because low pressure means slow, diffuse circulation.

However, the open system is adequate under the right conditions: insects are small, so diffusion distances within the haemocoel are short. The tracheal system compensates for the haemolymph's inability to carry O₂. For animals with low metabolic rates and small bodies, the open system is energetically cheaper and sufficient.

Marking criteria:

• 1 markCorrectly describes the structure of an open system: haemolymph leaves vessels, bathes organs in haemocoel, drains back through ostia. Named insect example.
• 1 markCorrectly describes the structure of a closed system: blood enclosed in vessels at all times; exchange only at capillaries. Named vertebrate example.
• 1 markCompares fluid containment criterion correctly (open: fluid uncontained; closed: enclosed in vessels).
• 1 markCompares blood pressure criterion correctly (open: low; closed: high, with mechanical explanation).
• 1 markCompares speed/efficiency or O₂ delivery mechanism (including tracheal system for insects).
• 1 markExplains one functional advantage of closed system correctly and specifically (not just "better", must link mechanism to outcome).
• 1 markAcknowledges open system adequacy in appropriate context (small body, low metabolic demand, tracheal compensation), avoids one-winner reasoning.

Q2, Sample Band 6 response (6 marks), with marking criteria

Red blood cells carry oxygen via haemoglobin, a protein packed into each cell. Haemoglobin binds O₂ at the lungs (high O₂ partial pressure), forming oxyhaemoglobin, and releases it at body tissues (low O₂ partial pressure). The biconcave disc shape of RBCs maximises the surface area to volume ratio, exposing more haemoglobin to oxygen at the cell membrane and minimising the diffusion distance from the membrane to internal haemoglobin, both adaptations accelerate O₂ loading and unloading.

A higher red blood cell count means more haemoglobin per litre of blood, so more O₂ is loaded onto the blood at the lungs per circuit. During endurance exercise, muscles consume O₂ rapidly; with more haemoglobin available per litre, more O₂ is unloaded at muscle capillaries per unit time. This delays the point at which muscles must switch to anaerobic respiration, directly extending the duration and intensity of aerobic performance.

Connecting this to IQ3 ("how does transport medium composition change as it moves around an organism?"): in a blood-doped athlete, the O₂ drop across active muscle capillaries would be greater (more O₂ is available to be delivered and used per circuit), and the CO₂ rise would also be correspondingly larger as more aerobic respiration occurs. The difference in composition between arterial and venous blood at active muscles is amplified compared to a normal athlete, a larger O₂ extraction and larger CO₂ addition per passage through the muscle.

Marking criteria:

• 1 markExplains haemoglobin binding O₂ at high partial pressure (lungs) and releasing it at low partial pressure (tissues).
• 1 markLinks biconcave disc structure to increased SA:V and faster O₂ loading/unloading.
• 1 markExplains mechanism: more RBCs = more haemoglobin per litre = more O₂ carried per circuit to muscles.
• 1 markConnects higher O₂ delivery to delaying the onset of anaerobic respiration and improving endurance performance.
• 1 markRelates to IQ3: correctly predicts that blood-doped athlete shows a greater O₂ drop and greater CO₂ rise across active muscle capillaries compared to a normal athlete.
• 1 markResponse demonstrates integrated understanding, links RBC structure, O₂-carrying function, and IQ3 composition change in a coherent chain of reasoning.

Q3, Sample Band 6 response (6 marks)

The claim contains three errors and one partially correct element.

Partially correct: It is true that oxygenated blood is bright red (due to oxyhaemoglobin) and deoxygenated blood is dark red/maroon (deoxyhaemoglobin). The distinction in colour is real.

Error 1, "arteries carry oxygenated blood; veins carry deoxygenated": Arteries are defined by directionthey carry blood away from the heart. Veins carry blood toward the heart. The pulmonary artery carries deoxygenated blood (heart to lungs), and the pulmonary vein carries oxygenated blood (lungs to heart). Correction: arteries and veins are classified by direction of flow, not O₂ content.

Error 2, "deoxygenated blood turns blue in veins": Deoxygenated blood is dark red/maroon, never blue inside a living body. Veins appear blue through the skin because tissue absorbs red wavelengths of light more strongly than blue wavelengths; the bluish appearance is an optical illusion, not the actual colour of the blood. Correction: deoxygenated blood is dark red; the blue appearance of veins through skin is due to the optical properties of tissue.

Error 3, "insect blood carries less oxygen because it lacks haemoglobin": Insect haemolymph carries no oxygen at allthis is not a quantitative difference (less oxygen) but a complete absence. Insects use a separate tracheal system to deliver O₂ directly to cells. Haemolymph transports nutrients and waste only. Correction: insect haemolymph does not carry oxygen; the tracheal system delivers O₂ directly to cells independently of haemolymph.

Corrected reformulation: "Arteries carry blood away from the heart, and veins carry blood toward the heart, both can carry oxygenated or deoxygenated blood. Deoxygenated blood is dark red/maroon, not blue; veins appear blue through skin because of light absorption by tissue. Insects do not transport oxygen in haemolymph at all, their tracheal system delivers O₂ directly to cells."

Marking criteria:

• 1 markStates an overall evaluative judgement (claim is largely incorrect with one correct element).
• 1 markCorrectly refutes arteries = oxygenated / veins = deoxygenated, stating the direction-based definition with the pulmonary artery as a counterexample.
• 1 markCorrectly refutes "blood turns blue", deoxygenated blood is dark red; blue appearance is an optical illusion caused by tissue light absorption.
• 1 markCorrectly refutes "insect blood carries less oxygen", haemolymph carries no oxygen; tracheal system delivers O₂ directly to cells (not a quantitative reduction but a complete structural difference).
• 1 markIdentifies the one correct element (oxygenated blood is bright red; deoxygenated is dark red, colour distinction is real).
• 1 markProvides a biologically accurate reformulation that correctly covers all three corrected points using precise terminology.