Every one of your cells needs a constant supply of oxygen and glucose, and a way to get rid of waste — yet most cells are buried deep inside the body, far from the air and the food you eat. The circulatory system is the delivery network that links them all: a muscular pump, a vast set of pipes, and a liquid that flows through every tissue. In this topic you’ll see how the heart, blood vessels and blood work together to keep you alive.
The double circulation
Humans have a double circulation: blood passes through the heart twice for each complete circuit of the body.
- The pulmonary circulation carries blood from the heart to the lungs and back, picking up oxygen.
- The systemic circulation carries oxygenated blood from the heart to the rest of the body and back.
The advantage is that blood can be pumped at higher pressure to the body after being topped up with oxygen at the lungs, so oxygen is delivered quickly and efficiently. This supports the high energy needs of mammals.
The heart
The heart is a muscular pump made of four chambers: two thin-walled atria at the top and two thick-walled ventricles below.
- The right side receives deoxygenated blood and pumps it to the lungs.
- The left side receives oxygenated blood and pumps it around the body.
- The left ventricle wall is thicker than the right because it must generate enough pressure to reach the whole body.
- Valves between the atria and ventricles, and at the start of the main arteries, stop blood flowing backwards.
The muscle of the heart is supplied with oxygen by the coronary arteries. If these become blocked, the result is a heart attack. Physical activity increases heart rate to deliver oxygen faster to working muscles.
Blood vessels
There are three types of blood vessel, each adapted to its job.
- Arteries carry blood away from the heart at high pressure. They have thick, muscular, elastic walls and a narrow lumen.
- Veins carry blood back to the heart at low pressure. They have thinner walls, a wide lumen, and valves to stop backflow.
- Capillaries are tiny vessels with walls only one cell thick. This lets substances such as oxygen, glucose and carbon dioxide diffuse quickly between the blood and the tissues.
Blood and its functions
Blood is a tissue made of cells suspended in a liquid called plasma.
- Red blood cells carry oxygen. They contain haemoglobin, which combines with oxygen to form oxyhaemoglobin. They have no nucleus and a biconcave shape to carry more oxygen.
- White blood cells defend the body — phagocytes engulf pathogens, and lymphocytes make antibodies.
- Platelets are cell fragments that help blood to clot, sealing wounds and keeping out pathogens.
- Plasma is the straw-coloured liquid that transports dissolved substances such as carbon dioxide, glucose, hormones, urea and heat.