1.1.d. The cardiovascular and respiratory systems - Structure and function of the cardiovascular system — OCR GCSE Physical Education

A double-circulatory system means blood passes through the heart twice for each complete circuit of the body. In the pulmonary circuit, the right side of the heart pumps deoxygenated blood to the lungs, where it picks up oxygen and returns oxygenated to the left side of the heart. In the systemic circuit, the left side pumps oxygenated blood around the body, and deoxygenated blood returns to the right side. The two circuits are separate but linked in series, so blood flows heart → lungs → heart → body → heart. This arrangement keeps oxygenated and deoxygenated blood largely separate and maintains pressure for the body circuit. A useful check: name the side of the heart, the destination and the oxygen state at each stage.

Three blood vessel types carry blood around the body, and each structure suits its function. Arteries carry blood away from the heart at high pressure; their thick muscular, elastic walls stretch and recoil with each heartbeat, and a narrow lumen maintains pressure. Veins carry blood back to the heart at low pressure; their thinner walls and wider lumen suit this, and valves prevent backflow so blood travels one way towards the heart. Capillaries are tiny, one-cell-thick vessels that link arteries to veins; their thin walls and large total surface area allow rapid exchange of oxygen, carbon dioxide and nutrients between blood and tissues. A useful check: match each vessel to direction of flow, wall thickness, lumen size and any valves.

Trace deoxygenated blood arriving from the body through the vena cava into the right atrium, then through the tricuspid valve into the right ventricle. Contraction pushes it through the pulmonary valve into the pulmonary artery to the lungs. Oxygenated blood returns via the pulmonary vein to the left atrium, passes the bicuspid (mitral) valve into the left ventricle, and is ejected through the aortic valve into the aorta to the body. The septum separates the two sides so oxygenated and deoxygenated blood do not mix. Remember the valves open and close to keep flow one-way, and that the pulmonary artery carries deoxygenated blood while the pulmonary vein carries oxygenated blood.

Heart rate is the number of times the heart beats per minute, usually written in beats per minute (bpm). Stroke volume is the volume of blood ejected from a ventricle in a single beat, measured in millilitres (cm³ or ml). Cardiac output is the total volume of blood pumped by the heart per minute, measured in litres per minute, and is calculated as cardiac output = heart rate × stroke volume. For example, a heart rate of 70 bpm with a stroke volume of 70 cm³ gives a cardiac output of 4900 cm³ per minute, which is 4.9 litres per minute. During exercise all three can rise, with cardiac output increasing because both heart rate and stroke volume increase.

Red blood cells are the oxygen carriers of the blood. They are packed with haemoglobin, a red protein that binds oxygen in the lungs, where oxygen concentration is high, and releases it in the tissues, where oxygen concentration is low. This makes red blood cells central to aerobic respiration and therefore to sustained muscular work during exercise. They have no nucleus, which leaves more room for haemoglobin, and a biconcave disc shape that gives a large surface area for gas exchange. A useful method is to link structure to function: haemoglobin binds oxygen, the cell transports it, and the tissue receives it. Learners should also recognise that red blood cells carry some carbon dioxide, though most carbon dioxide travels dissolved in plasma.