The Cardiovascular System and Exercise
The Heart: Structure and Function
The heart is a hollow, muscular organ roughly the size of a fist, located in the chest slightly left of centre behind the sternum. It is divided into four chambers by a central wall called the septum. The two upper chambers are called atria (singular: atrium) — the right atrium receives deoxygenated blood returning from the body, and the left atrium receives oxygenated blood returning from the lungs. The two lower chambers are called ventricles — the right ventricle pumps deoxygenated blood to the lungs, and the left ventricle pumps oxygenated blood to the rest of the body. The left ventricle has a thicker, more muscular wall than the right because it must generate much higher pressure to push blood all the way around the body.
Valves inside the heart open due to changes in blood pressure and close automatically to prevent backflow of blood. The right side of the heart and the left side each have valves between the atrium and the ventricle (atrioventricular valves) and between the ventricle and the outgoing vessel (semilunar valves). You do not need to name individual valves, but you must know they prevent backflow.
The Cardiac Cycle: How the Heart Beats
The heart works through a two-phase cycle. During diastole, the chambers relax and fill with blood — the atria fill with incoming blood and then push it through to the ventricles. During systole, the ventricles contract and eject blood out of the heart: the right ventricle forces blood into the pulmonary artery (to the lungs) and the left ventricle forces blood into the aorta (to the body). This cycle repeats every beat, which at rest typically occurs 60–80 times per minute.
The Pathway of Blood Through the Heart and Lungs
Following the pathway of blood precisely is a common exam demand. Deoxygenated blood from the body enters the right atrium via the vena cava. From the right atrium it passes through a valve into the right ventricle. The right ventricle then contracts and forces the blood through the pulmonary artery to the lungs. In the lungs, carbon dioxide is released and oxygen is picked up by haemoglobin in the red blood cells to form oxyhaemoglobin. The now-oxygenated blood returns to the heart via the pulmonary vein, entering the left atrium. It passes through a valve into the left ventricle, which contracts powerfully and ejects the oxygenated blood into the aorta — from which it is distributed to every organ and muscle in the body.
Blood pathway: vena cava → right atrium → right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary vein → left atrium → left ventricle → aorta → body. Note that the pulmonary artery carries deoxygenated blood — this is the opposite of most arteries.
Blood Vessels: Arteries, Veins and Capillaries
Arteries carry blood away from the heart. They have thick, muscular walls because they must withstand the high pressure generated by the ventricular contraction. The walls are elastic, allowing them to stretch and recoil with each heartbeat, smoothing out the pulsed blood flow. All arteries (except the pulmonary artery) carry oxygenated blood.
Veins carry blood back towards the heart. They have thinner walls than arteries because blood pressure is much lower by the time it reaches the venous system. Veins contain valves that prevent backflow, which is especially important in the legs where blood must travel upward against gravity. All veins (except the pulmonary vein) carry deoxygenated blood.
Capillaries are the microscopic vessels that form the link between arteries and veins in the tissues. Their walls are just one cell thick, allowing oxygen and nutrients to diffuse into the muscle cells and carbon dioxide and waste products to diffuse back into the blood. Every working muscle is surrounded by a dense network of capillaries. Long-term aerobic training increases capillary density in muscles, improving oxygen delivery.
During exercise, the working muscles need much more oxygen and glucose than at rest. The body redirects blood flow using two mechanisms. Vasoconstriction reduces the diameter of blood vessels supplying areas that are not active during exercise (e.g. the digestive system), reducing blood flow to those areas. Vasodilation increases the diameter of the blood vessels supplying the active muscles, dramatically increasing blood flow to where it is needed most. The redistribution of blood during exercise through vasoconstriction and vasodilation is called the ‘vascular shunt’. This is why it is inadvisable to exercise vigorously immediately after a large meal — the digestive system is competing with muscles for blood supply.
Cardiac Output, Stroke Volume and Heart Rate
Three linked measurements describe how efficiently the heart pumps blood. Stroke volume is the amount of blood pumped out of the heart with each beat. At rest, stroke volume is approximately 70 ml. Heart rate is the number of beats per minute (bpm). A typical resting heart rate is 60–80 bpm (trained endurance athletes may have a resting heart rate as low as 40 bpm — a long-term adaptation called bradycardia). Cardiac output (Q) is the product of the two:
$$Q = \text{stroke volume} \times \text{heart rate}$$
At rest: Q = 70 ml × 72 bpm = 5,040 ml/min (approximately 5 litres per minute). During intense exercise, heart rate can rise to 200 bpm or more and stroke volume increases to around 110–130 ml, so cardiac output can reach 20–25 litres per minute in a trained athlete.
Cardiac output (Q) = stroke volume × heart rate. Always show the formula, substitute the values, and include the unit (ml/min or L/min). Example: Q = 70 ml × 72 bpm = 5,040 ml/min. At maximum exercise this can rise to over 20,000 ml/min in a trained athlete.
A heart rate graph during exercise typically shows four phases. First, an anticipatory rise — heart rate climbs before exercise even begins, driven by the release of adrenaline as the performer prepares mentally and physically. This is why athletes’ hearts are already beating faster as they stand in the starting blocks. Second, heart rate rises steeply at the onset of exercise as oxygen demand increases. Third, a plateau occurs at a steady exercise intensity as supply and demand equalise. Fourth, a recovery curve drops steeply at first then more gradually after exercise stops, as the body repays its oxygen debt. A trained performer recovers faster (steeper drop) because their cardiovascular system is more efficient.