11.1 Circulatory system
- The pulmonary circulation takes blood from the heart to the lungs and back. It is a low-pressure circulation, so that the delicate capillaries in the lungs are not damaged and the blood flows slowly enough to pick up oxygen.
- The systemic circulation takes blood from the heart to the body tissues and back. It is a high-pressure circulation, so that the blood is delivered quickly to all parts of the body.
11.2 Heart
Structure of the heart
| Structure | Function |
|---|---|
| muscular wall (cardiac muscle) | contracts to pump the blood |
| septum | separates the left and right sides, so oxygenated and deoxygenated blood do not mix |
| right atrium | receives deoxygenated blood from the body (vena cava) |
| right ventricle | pumps deoxygenated blood to the lungs (pulmonary artery) |
| left atrium | receives oxygenated blood from the lungs (pulmonary vein) |
| left ventricle | pumps oxygenated blood to the body (aorta) |
| atrioventricular valves | between each atrium and ventricle; prevent blood flowing back into the atria |
| semilunar valves | at the base of the aorta and pulmonary artery; prevent blood flowing back into the ventricles |
| coronary arteries | supply the heart muscle with oxygen and glucose |
Why the walls differ in thickness.
- The left ventricle has a thicker muscle wall than the right ventricle, because it pumps blood at high pressure around the whole body, while the right ventricle only pumps blood a short distance to the lungs.
- The atria have thinner walls than the ventricles, because they only pump blood into the ventricles next to them, while the ventricles pump it out of the heart.
A heartbeat
- Blood flows into the atria.
- The atria contract, pushing blood through the atrioventricular valves into the ventricles.
- The ventricles contract; the atrioventricular valves close (making the "lub" sound), and blood is forced out through the semilunar valves into the arteries.
- The semilunar valves close (the "dub" sound) to stop blood going back.
- The heart muscle relaxes, and the cycle repeats.
Blood is pumped away from the heart in arteries and returns to the heart in veins.
Monitoring the heart
The activity of the heart can be monitored by an electrocardiogram (ECG) (which records the heart's electrical activity), by the pulse rate (felt at the wrist or neck) and by listening to the sounds of the valves closing with a stethoscope.
Heart rate and exercise
During exercise, the muscles need more oxygen and glucose, and have to get rid of more carbon dioxide, so the heart rate increases (and so does the stroke volume), to deliver blood faster. After exercise it takes time to return to the resting rate. The effect can be investigated by measuring the pulse at rest and after exercise.
Coronary heart disease
Risk factors: diet (high in saturated fat and salt), a sedentary lifestyle (lack of exercise), stress, smoking, genetic predisposition (a family history), age (higher risk with age) and gender (higher in men).
Reducing the risk. A diet low in saturated fat and salt and high in fibre, fruit and vegetables lowers the build-up of fatty deposits and blood pressure. Regular exercise strengthens the heart, helps control weight and lowers blood pressure. Not smoking also reduces the risk.
11.3 Blood vessels
The main blood vessels
| Vessel | Carries blood |
|---|---|
| aorta | from the left ventricle to the body |
| vena cava | from the body to the right atrium |
| pulmonary artery | from the right ventricle to the lungs |
| pulmonary vein | from the lungs to the left atrium |
| hepatic artery | to the liver |
| hepatic vein | from the liver |
| hepatic portal vein | from the small intestine to the liver |
| renal artery | to the kidneys |
| renal vein | from the kidneys |
Arteries, veins and capillaries
| Artery | Vein | Capillary | |
|---|---|---|---|
| direction | away from the heart | towards the heart | between arteries and veins |
| blood pressure | high | low | falling |
| wall | thick, with muscle and elastic tissue | thin, with little muscle or elastic tissue | one cell thick |
| lumen | narrow | wide | very narrow |
| valves | no (except at the heart) | yes, to prevent backflow | no |
- Arteries carry blood at high pressure, so they have thick, strong walls. The elastic tissue lets the wall stretch and spring back, which helps to smooth out the pulse and maintain the pressure.
- Veins carry blood at low pressure, so they need valves to stop the blood flowing backwards, and a wide lumen to reduce resistance.
- Capillaries have walls one cell thick, which lets substances diffuse quickly between the blood and the cells.
11.4 Blood
Components of blood
| Component | Function |
|---|---|
| red blood cells | carry oxygen (as oxyhaemoglobin) |
| white blood cells | defend against disease: lymphocytes make antibodies; phagocytes engulf pathogens |
| platelets | help blood to clot: they cause soluble fibrinogen to be changed into insoluble fibrin, which forms a mesh and prevents blood loss and the entry of pathogens |
| plasma | the liquid part, which transports blood cells, ions, glucose, amino acids, hormones, carbon dioxide, urea, vitamins and plasma proteins |
Identifying cells. Red blood cells are small, round and have no nucleus (they appear as pink discs). Phagocytes have a large, lobed nucleus and granular cytoplasm. Lymphocytes have a large, round nucleus that takes up most of the cell.
Tissue fluid
Blood at the capillaries is under pressure, so some plasma is forced out through the thin capillary walls. This forms tissue fluid, which surrounds the body cells. Oxygen and glucose diffuse from the tissue fluid into the cells, and carbon dioxide and urea diffuse out of the cells into the tissue fluid and then back into the blood. Red blood cells and the large plasma proteins stay inside the capillaries.
Platelets cause soluble fibrinogen to be changed into insoluble fibrin threads, which form a mesh that traps blood cells and seals the wound. This stops blood loss and prevents pathogens from entering.
Practise this chapter
Every past-paper question on these topics, with the mark scheme.