Transport in animals — OCR A-Level Biology

Multicellular animals have a small surface area to volume ratio, so diffusion across the body surface alone cannot supply enough oxygen and nutrients to every cell or remove enough carbon dioxide and other wastes quickly enough. Diffusion distance from the surface to deep tissues is large, and metabolic demand is high because many cells respire actively. A transport system, typically blood circulated by a heart through vessels, carries substances close to every cell so that short diffusion distances remain at exchange surfaces. Learners should be able to explain why large, active animals need a circulatory system, linking surface area to volume ratio, diffusion distance, metabolic rate and the need to maintain concentration gradients, and to apply this reasoning to unfamiliar organisms.

Circulatory systems can be classified by whether blood stays within vessels and by how many circuits exist. In a closed system blood is confined to vessels, whereas in an open system blood leaves vessels and bathes the tissues in a haemocoel, as in many insects. Systems also differ in the number of circuits: a single circulation has blood passing through the heart once per complete circuit, as in fish, while a double circulation passes through the heart twice, as in mammals, with a pulmonary circuit to the lungs and a systemic circuit to the body. Learners should compare these types, describe the advantages of a double circulation in maintaining pressure and rapid flow to the body, and recognise examples of each type.

Blood vessels are adapted to their position in the circulatory system. Arteries carry blood away from the heart at high pressure: thick muscular walls with elastic fibres stretch during systole and recoil during diastole, smoothing flow, while a narrow lumen maintains pressure. Arterioles are small arteries whose smooth muscle controls lumen diameter; vasoconstriction and vasodilation redistribute blood to capillary beds. Capillaries have a single endothelial cell layer and narrow lumen, giving a short diffusion distance and large surface area for exchange; their walls are permeable to plasma and tissue fluid. Venules collect blood from capillaries and merge into veins. Veins return blood at low pressure: thin walls, a wide lumen and valves prevent backflow, aided by skeletal muscle action during movement.

At the arterial end of a capillary, hydrostatic pressure from the heart's pumping is higher than the opposing osmotic pull of plasma proteins. This net outward force pushes water and small dissolved solutes out through the capillary wall into the surrounding spaces, forming tissue fluid. Large plasma proteins and blood cells are held back because they are too big to pass through the capillary wall, so tissue fluid contains no red blood cells and few, if any, large plasma proteins, though it can carry white blood cells that have left the blood. At the venous end, hydrostatic pressure has fallen and the osmotic effect of the retained plasma proteins draws most fluid back into the capillary; the remainder drains away as lymph.

You must be able to describe the external and internal structure of the mammalian heart, so learn it as a labelled three-dimensional organ rather than a flat diagram. Externally, identify the four chambers' surface features, the coronary vessels running in the grooves, the aorta and pulmonary artery emerging at the top, and the atria as thinner-walled receiving chambers. Internally, trace the route of blood: vena cava → right atrium → right ventricle → pulmonary artery; pulmonary vein → left atrium → left ventricle → aorta. Note the thicker left ventricular wall, the atrioventricular and semilunar valves, the septum separating the sides, and the tendinous cords and papillary muscles anchoring the AV valves. Link each structure to its function: valves prevent backflow, the thick left wall generates systemic pressure, and the septum keeps oxygenated and deoxygenated blood separate.

This practical requires you to dissect a mammalian heart safely, examine its external and internal structure, and produce an accurate labelled drawing. Work systematically: identify the outside vessels and coronary vessels, then cut along the line between the ventricles to expose the chambers, valves and septum. Use a scalpel carefully with a cutting board, keep the specimen moist, and record what you actually observe rather than an idealised diagram. Your drawing should be large, in pencil, with clear continuous lines, correct proportions and labels with straight ruled lines. Explain how each observed feature, such as the thick left ventricular wall or the valve flaps, relates to function. Assessment judges your technique, the accuracy of your observations and the quality of your drawing and annotations.

The cardiac cycle is the sequence of events in one complete heartbeat, described through the coordinated contraction (systole) and relaxation (diastole) of the atria and ventricles. Teach it as pressure-driven flow: blood moves from higher to lower pressure, and valves open and close passively in response to pressure differences. Atrial systole follows ventricular diastole, pushing the remaining blood through the open atrioventricular valves; ventricular systole then raises ventricular pressure, closing the atrioventricular valves (first heart sound) and opening the semilunar valves once ventricular pressure exceeds arterial pressure. Ventricular diastole lowers ventricular pressure, closing the semilunar valves (second heart sound) and reopening the atrioventricular valves.

Heart action is initiated and coordinated by the heart's own conduction system, which generates and spreads electrical activity so contraction is rhythmic and coordinated. The sinoatrial node (SAN) in the right atrial wall acts as the pacemaker, spontaneously generating impulses that spread across the atria, causing atrial contraction. A layer of non-conducting tissue between atria and ventricles prevents impulses passing directly, so the atrioventricular node (AVN) receives the impulse and introduces a delay, allowing the atria to finish emptying before the ventricles contract. The impulse then travels down the bundle of His and Purkyne fibres to the ventricular apex and up the ventricle walls, causing coordinated ventricular contraction from the apex upwards.

An ECG records the electrical activity of the heart as a trace of voltage against time, and its use is to show the rhythm and coordination of the cardiac cycle. A normal trace shows a P wave (atrial depolarisation spreading from the sinoatrial node, causing atrial systole), a QRS complex (ventricular depolarisation, triggering ventricular systole, which hides atrial repolarisation) and a T wave (ventricular repolarisation). Interpretation means reading the trace: counting the number of QRS complexes in a time interval to find heart rate, measuring the distance between successive R waves to judge rhythm regularity, and identifying abnormal patterns such as missing P waves, widened QRS complexes or extra beats. For example, a trace with no clear P waves and an irregular distance between R waves indicates an abnormal rhythm rather than a normal coordinated cycle.

Haemoglobin is a quaternary protein with four haem groups, each containing an iron ion (Fe²⁺) that binds one oxygen molecule, so a fully loaded molecule carries four oxygen molecules. In the lungs, a high partial pressure of oxygen (pO₂) causes oxygen to associate with haemoglobin, forming oxyhaemoglobin; in respiring tissues, a lower pO₂ causes oxygen to dissociate and be released. The affinity changes with pO₂, giving the S-shaped oxygen dissociation curve, and a high partial pressure of carbon dioxide lowers affinity further (the Bohr shift), increasing oxygen release where respiration is greatest. For carbon dioxide, most is transported as hydrogencarbonate ions (HCO₃⁻) formed in red blood cells by carbonic anhydrase, with some carried bound to haemoglobin as carbaminohaemoglobin and a small fraction dissolved in plasma.

You must interpret and compare the oxygen dissociation curves of adult and fetal human haemoglobin. The curve plots percentage oxygen saturation of haemoglobin against partial pressure of oxygen (pO₂). Its sigmoid (S) shape arises because binding the first oxygen molecule changes the haemoglobin's shape, making further binding easier (cooperative binding). At low pO₂ the curve is steep; at high pO₂ it plateaus as haemoglobin becomes fully saturated. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its curve lies to the left of the adult curve. This means that at any given pO₂, fetal haemoglobin is more saturated. In the placenta, where maternal and fetal blood are close, fetal haemoglobin can therefore take up oxygen from maternal blood.