The Cardiovascular System and Pathway of Blood
The Double-Circulatory System
The cardiovascular system uses a double-circulatory system. This means blood makes two separate loops before returning to its starting point. The pulmonary circulation carries deoxygenated blood from the right side of the heart to the lungs, where carbon dioxide is released and oxygen is collected, before returning the now-oxygenated blood to the left side of the heart. The systemic circulation then carries that oxygenated blood from the left side of the heart out to every organ and muscle in the body, returning deoxygenated blood to the right side of the heart via the veins. Separating these two loops prevents oxygen-rich and oxygen-poor blood from mixing, ensuring maximum efficiency of oxygen delivery to the working muscles.
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 prevent the backflow of blood. The bicuspid valve sits between the left atrium and left ventricle. The tricuspid valve sits between the right atrium and right ventricle. The semilunar valves are located at the exit of each ventricle, where the aorta leaves the left ventricle and where the pulmonary artery leaves the right ventricle. All three sets of valves open due to changes in blood pressure and close automatically to maintain one-directional blood flow.
Blood pathway: vena cava → right atrium → tricuspid valve → right ventricle → semilunar valve → pulmonary artery → lungs (gas exchange) → pulmonary vein → left atrium → bicuspid valve → left ventricle → semilunar valve → aorta → body. Note that the pulmonary artery carries deoxygenated blood — this is the opposite of most arteries.
The Pathway of Blood Through the Heart
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 the tricuspid valve into the right ventricle. The right ventricle contracts and forces blood through the semilunar valve into 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. The now-oxygenated blood returns to the heart via the pulmonary vein, entering the left atrium. It passes through the bicuspid valve into the left ventricle, which contracts powerfully, ejecting oxygenated blood through the semilunar valve into the aorta — from which it is distributed to every organ and muscle in the body.
The Role of Red Blood Cells
Red blood cells are the oxygen-carrying cells of the blood. They contain haemoglobin — a red iron-containing protein that binds readily with oxygen in the oxygen-rich environment of the lungs to form oxyhaemoglobin. When red blood cells reach the working muscles, where oxygen concentration is low, oxyhaemoglobin breaks down and releases its oxygen into the muscle tissue. The same red blood cells then pick up carbon dioxide — a waste product of respiration — and carry it back to the lungs to be exhaled. Without red blood cells in sufficient numbers, oxygen transport collapses and athletic performance is severely impaired.
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 ventricular contraction. The walls are elastic, allowing them to stretch and recoil with each heartbeat, smoothing out 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. 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.
One of the most common errors in cardiovascular questions is stating that the pulmonary artery carries oxygenated blood. It does not. The pulmonary artery is the only artery in the body that carries deoxygenated blood — it takes blood from the right ventricle to the lungs specifically so that oxygen can be collected. Similarly, the pulmonary vein is the only vein that carries oxygenated blood — returning fresh blood from the lungs to the left atrium. The rule “arteries carry oxygenated blood” has two exceptions that you must memorise: the pulmonary artery (deoxygenated away from heart) and the pulmonary vein (oxygenated toward heart).
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.
During exercise, the working muscles need much more oxygen and glucose than at rest. The body redistributes blood flow so that a greater proportion of cardiac output goes to the active muscles. Blood supply to the digestive system and other non-essential organs is reduced, while blood flow to the working skeletal muscles and heart muscle increases dramatically. This redistribution is an important short-term effect of exercise. It explains why exercising vigorously immediately after a large meal can cause discomfort — the digestive system is competing with the muscles for blood supply at a time when the muscles' demands are greatly elevated.