动物体内的运输

The circulatory system 循环系统 carries blood around the body. It has three parts: a pump 泵 (the heart 心脏), a network of blood vessels 血管, and valves 瓣膜 that keep the blood flowing one way only.

Single and double circulation (Supplement)

• A fish has a single circulation 单循环: the blood passes through the heart once on each trip around the body.
• A mammal 哺乳动物 has a double circulation 双循环: the blood passes through the heart twice on each full trip — once on the way to the lungs, and once on the way to the rest of the body.
• The advantage of a double circulation: the blood can be pumped again at high pressure 压力 before going to the body, so it travels faster and delivers oxygen 氧气 quickly.

The heart is made of muscle 肌肉. A wall called the septum 隔膜 divides it into a left side and a right side. On each side there is an upper chamber, the atrium 心房, and a lower chamber, the ventricle 心室. Valves between the chambers stop the blood flowing backwards. The heart muscle is fed with blood by the coronary arteries 冠状动脉.

(Supplement) The wall of the left ventricle is thicker than the right, because it must pump blood all the way around the body; the right side only pumps blood to the nearby lungs. The atria have thin walls, as they only push blood into the ventricles just below them.

The septum keeps oxygenated blood 含氧血 on the left side apart from deoxygenated blood 缺氧血 on the right side, so the two never mix.

(Supplement) The valves between each atrium and ventricle are the atrioventricular valves 房室瓣. The valves at the exit of each ventricle are the semilunar valves 半月瓣.

How the heart beats (Supplement)

1. The atria contract 收缩 and push blood down into the ventricles.
2. The ventricles contract and force blood out into the arteries; the valves snap shut so blood cannot flow back.
3. The muscle relaxes and the heart fills with blood again.

Blood is pumped away from the heart in arteries and returns to the heart in veins.

Heart rate and exercise

You can check the heart's activity with an ECG, by feeling the pulse 脉搏 in an artery, or by listening to the valves closing. During exercise the heart rate 心率 goes up, so blood reaches the muscles faster, bringing the extra oxygen and glucose they need.

Worked example. A student counts 18 pulse beats in 15 seconds at rest, and 33 beats in 15 seconds straight after exercise. Give both heart rates in beats per minute. A minute holds 60 ÷ 15 = 4 lots of 15 seconds, so multiply by 4. At rest: 18 × 4 = 72 beats per minute. After exercise: 33 × 4 = 132 beats per minute. The rise delivers more oxygen and glucose to the muscles. Writing the 15-second count straight down as the heart rate is the commonest mistake - always scale it up to a full minute.

Coronary heart disease

Coronary heart disease 冠心病 happens when the coronary arteries become narrow or blocked, so the heart muscle cannot get enough oxygen. Risk factors 风险因素 include a diet high in fat, lack of exercise, stress, smoking, family history (genetics), older age, and being male. A healthy diet and regular exercise lower the risk.

There are three kinds of blood vessel. The space inside a vessel is its lumen 管腔.

(Supplement) Arteries have thick elastic walls to cope with the high pressure from the heart. Veins have valves because their blood is at low pressure and could otherwise flow backwards. Capillaries are very thin (one cell thick) and very narrow, giving a large surface area 表面积 and a short distance for fast exchange.

Main blood vessels

(Supplement) The liver is served by the hepatic artery 肝动脉, the hepatic vein 肝静脉, and the hepatic portal vein 肝门静脉, which brings blood from the gut to the liver.

Blood has four parts:

Defending the body, and clotting

White blood cells fight pathogens 病原体 (the microbes that cause disease) in two ways:

• phagocytes 吞噬细胞 carry out phagocytosis 吞噬作用 — they surround and digest the pathogens.
• Some lymphocytes 淋巴细胞 make antibodies 抗体, which bind to specific antigens and help neutralise pathogens or mark them for destruction.

When you cut yourself, the blood clots to seal the wound. This stops blood loss and stops pathogens getting in. (Supplement) During clotting, a soluble protein called fibrinogen 纤维蛋白原 is changed into threads of fibrin 纤维蛋白. The threads form a net that traps blood cells and makes a solid clot.

Common mistakes.

• Saying arteries carry oxygenated blood and veins carry deoxygenated blood. Arteries carry blood away from the heart and veins carry it back. The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein brings oxygenated blood back.
• Saying the left ventricle has a thicker wall because it holds more blood. Both ventricles pump the same volume; the left one pumps the blood all round the body, so it needs a higher pressure.
• Saying veins have valves because of high pressure. Blood in veins is at low pressure, and the valves stop it flowing backwards.
• Saying capillary walls are thin so blood flows fast. Blood flows slowly in capillaries; the thin wall gives a short distance for oxygen and glucose to diffuse into the cells.
• (Supplement) Saying blood in a double circulation collects oxygen twice. It collects oxygen once, in the lungs; the second pass through the heart raises its pressure again.

• Circulatory system = pump (heart) + vessels + valves for one-way flow.
• Double circulation (mammals): blood passes through the heart twice. The left ventricle wall is thickest because it pumps to the whole body.
• Arteries carry blood away (thick, high pressure); veins carry it back (valves, low pressure); capillaries are for exchange (one cell thick).
• Blood = red cells (oxygen, haemoglobin) + white cells (defence) + platelets (clotting) + plasma (transport).
• Phagocytes engulf pathogens; lymphocytes make antibodies.