Circulatory systemIB MYP Biology: Revision notes

The human heart is a muscular pump with four chambers: two atria (upper) and two ventricles (lower). The left side and the right side are separated by a wall, so oxygenated and deoxygenated blood do not mix. Valves between the atria and ventricles, and at the exits, stop blood flowing backwards.

Humans have a double circulation:

• Right side: deoxygenated blood from the body enters the right atrium (via the vena cava), passes to the right ventricle and is pumped through the pulmonary artery to the lungs.
• Left side: oxygenated blood from the lungs returns via the pulmonary vein to the left atrium, passes to the left ventricle and is pumped out through the aorta to the body.

The left ventricle has a thicker wall than the right because it pumps blood all around the body.

• Arteries carry blood away from the heart at high pressure. They have thick, muscular, elastic walls and a narrow lumen.
• Veins carry blood to the heart at low pressure. They have thinner walls, a wide lumen and valves to prevent backflow.
• Capillaries are tiny vessels with walls one cell thick. They form networks through the tissues so that oxygen, glucose and other substances can pass between blood and cells.

The coronary arteries supply the heart muscle itself.

Blood has four main components:

• Plasma: a yellow liquid that carries dissolved glucose, carbon dioxide, urea, hormones and heat.
• Red blood cells: contain haemoglobin, which binds oxygen to form oxyhaemoglobin. They have no nucleus and are a biconcave shape.
• White blood cells: defend the body against pathogens by making antibodies or engulfing microbes.
• Platelets: cell fragments that help blood clot.

Transport: oxygen by haemoglobin in red cells; carbon dioxide dissolved in plasma to the lungs; glucose dissolved in plasma from the gut to cells; urea dissolved in plasma from the liver to the kidneys.

When a blood vessel is cut, platelets help form a clot (a scab) by making a mesh of fibres that traps red blood cells. A clot stops blood loss and stops microbes entering the wound.

In coronary heart disease, fatty deposits narrow the coronary arteries. Less blood reaches the heart muscle, which gets less oxygen. A blood clot can block an artery completely, causing a heart attack. Risk factors include smoking, a high-fat diet, lack of exercise and stress. Treatments include medicines, stents and bypass surgery.

Exercise increases the pulse rate because muscles respire faster and need more oxygen and glucose, so the heart pumps faster. Pulse rate is measured in beats per minute, e.g. beats in 30 s multiplied by 2. After exercise, the pulse returns to normal more quickly in a fit person.