The Cardiovascular System and Exercise
The Heart and Blood: Delivering Oxygen to Working Muscles
The cardiovascular system is the body’s transport network. It delivers oxygen and nutrients to every working cell and removes waste products including carbon dioxide and heat. During exercise, the demand for oxygen in the muscles multiplies, and the cardiovascular system responds with a series of rapid adjustments to maintain performance. Understanding these structures and responses is central to this topic.
Functions of the Cardiovascular System
The cardiovascular system has four key functions in physical activity. First, transport of oxygen: red blood cells carry oxygen from the lungs to working muscles, enabling aerobic energy production. Second, transport of carbon dioxide: blood carries CO2 away from muscles back to the lungs for exhalation. Third, transport of nutrients: glucose and fatty acids travel in the plasma to fuel muscle contractions. Fourth, regulation of body temperature: blood carries heat from working muscles to the skin surface, where it can be dissipated through sweating and radiation.
Additionally, blood contributes to clotting of open wounds via platelets — protecting athletes from blood loss after contact injuries — and to immune defence via white blood cells.
Structure of the Heart
The heart is a double pump, divided into left and right sides by the septum. Each side has an upper chamber (an atrium) and a lower chamber (a ventricle). The right side deals with deoxygenated blood; the left side deals with oxygenated blood.
Blood flows through the heart in a precise sequence. Deoxygenated blood from the body enters the right atrium via the vena cava (the large vein returning blood from the upper and lower body). It passes through the tricuspid valve into the right ventricle. From there it is pumped out through the pulmonary artery to the lungs to pick up oxygen. Oxygenated blood returns from the lungs via the pulmonary vein into the left atrium. It passes through the bicuspid (mitral) valve into the left ventricle, which pumps it out through the aorta to the rest of the body.
The semi-lunar valves sit at the exits of both ventricles — one at the base of the pulmonary artery (pulmonary semi-lunar valve) and one at the base of the aorta (aortic semi-lunar valve). Their job is to prevent blood flowing back into the ventricles after each contraction. The “lub-dub” sound of the heartbeat is the sound of these valves — and the atrioventricular valves (tricuspid and bicuspid) — snapping shut.
Blood pathway through the heart: vena cava → right atrium → tricuspid valve → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → bicuspid valve → left ventricle → aorta → body.
Blood Vessels: Arteries, Capillaries and Veins
Arteries carry blood away from the heart under high pressure. Their thick, muscular and elastic walls withstand and smooth out the surges of pressure produced by each heartbeat. The aorta is the largest artery, branching into progressively smaller arteries as it distributes blood to all regions of the body.
Capillaries are the smallest vessels, with walls only one cell thick. This ultra-thin wall allows oxygen, glucose and carbon dioxide to diffuse freely between the blood and the muscle fibres. A dense network of capillaries surrounds every active muscle fibre, and endurance training increases the density of this network — one of the key long-term adaptations covered in Lesson 9.
Veins return blood to the heart under low pressure. Their walls are thinner than arteries because the pressure is much lower. Veins contain valves that prevent blood from flowing backwards (particularly important in the legs where blood must travel upward against gravity).
Cardiac Output, Stroke Volume and Heart Rate
Cardiac output (Q) is the total amount of blood pumped by the heart per minute. It is calculated by the formula:
$$Q = \text{stroke volume} \times \text{heart rate}$$
Stroke volume (SV) is the amount of blood pumped out with each beat. At rest a typical adult has a stroke volume of around 70 ml. Heart rate (HR) at rest is typically 60–80 bpm. So resting cardiac output is approximately 70 × 70 = 4,900 ml/min (roughly 5 litres per minute).
During maximal exercise, heart rate can reach 200 bpm and stroke volume can increase to 110–130 ml in a fit person, giving a cardiac output of over 20 litres per minute — a four-fold increase. This increased delivery of oxygenated blood to muscles is the central cardiovascular response to exercise.
Cardiac output (Q) = stroke volume (SV) × heart rate (HR). At rest: ~70 ml × 70 bpm = ~4,900 ml/min. During maximal exercise: cardiac output can exceed 20,000 ml/min (20 L/min) as both SV and HR increase.
Vascular shunting is the process by which blood is redirected during exercise. At rest, organs such as the gut, liver and kidneys receive a large proportion of cardiac output. During intense exercise, the arterioles supplying these organs vasoconstrict (their smooth muscle contracts, narrowing the vessel), while the arterioles supplying the working muscles vasodilate (relax and widen), directing a much greater fraction of cardiac output to the muscles that need it. This redistribution can shift up to 80% of cardiac output to working muscles during maximal exercise, compared with around 15–20% at rest.
Blood is a mixture of cells and fluid. Red blood cells (erythrocytes) contain haemoglobin, which binds oxygen in the lungs and releases it in the muscles. They have no nucleus, maximising their oxygen-carrying capacity. White blood cells (leucocytes) form part of the immune system, destroying pathogens. Platelets (thrombocytes) trigger blood clotting when a vessel is damaged. Plasma is the fluid component (about 55% of blood volume) that transports cells, dissolved gases, nutrients, hormones and waste products. Plasma also distributes heat generated by working muscles to the skin for cooling.