哺乳动物体内的运输

Mammals have a closed double circulation. "Closed" means the blood stays inside blood vessels 血管 the whole time. "Double" means the blood passes through the heart 心脏 twice for each full trip around the body. This gives two linked loops, so we call it a double circulation 双循环 and the whole thing a circulatory system 循环系统:

• the pulmonary circulation 肺循环 carries blood from the heart to the lungs and back.
• the systemic circulation 体循环 carries blood from the heart to the rest of the body and back.

Blood travels in this order: arteries 动脉 → arterioles 小动脉 → capillaries 毛细血管 → venules 小静脉 → veins 静脉.

The main vessels are:

• the pulmonary artery 肺动脉 — carries blood low in oxygen 氧气 from the heart to the lungs.
• the pulmonary vein 肺静脉 — carries oxygen-rich blood from the lungs back to the heart.
• the aorta 主动脉 — the big artery that carries oxygen-rich blood from the heart to the body.
• the vena cava 腔静脉 — the big vein that returns oxygen-poor blood from the body to the heart.

How the vessels suit their jobs

Blood cells

You should recognise: red blood cells 红细胞 (which carry oxygen), and three white blood cells — monocytes 单核细胞, neutrophils 中性粒细胞 and lymphocytes 淋巴细胞.

Water, plasma and tissue fluid

Water is the main part of blood. It is a good solvent 溶剂, so it carries dissolved substances, and it has a high specific heat capacity 比热容, so the blood's temperature stays steady.

At the start of a capillary, the high blood pressure pushes liquid (but not the cells or large proteins) out of the plasma 血浆 and through the capillary wall. This liquid around the cells is tissue fluid 组织液. It supplies the cells with oxygen and glucose and carries waste away. Most of it returns to the capillary at the far end, where the pressure is lower.

Carrying oxygen

Oxygen is carried by haemoglobin 血红蛋白 in the red blood cells. We show how well haemoglobin holds oxygen with the oxygen dissociation curve 氧解离曲线. This S-shaped graph plots the saturation 饱和度 (how full of oxygen the haemoglobin is) against the partial pressure 分压 of oxygen:

• where the partial pressure of oxygen is high (in the lungs), haemoglobin loads up and becomes almost fully saturated.
• where it is low (in respiring tissues), haemoglobin unloads its oxygen for the cells to use.

The Bohr shift

When tissues are very active, they release more carbon dioxide 二氧化碳, which lowers the pH. This makes haemoglobin release oxygen more easily, so the curve moves to the right. This helpful change is the Bohr shift 波尔位移: oxygen is given up exactly where it is most needed.

Carrying carbon dioxide

A little carbon dioxide dissolves straight into the plasma, but most is carried after a reaction inside the red blood cells:

1. the enzyme 酶 carbonic anhydrase 碳酸酐酶 speeds up the reaction of carbon dioxide with water to make carbonic acid.
2. the carbonic acid splits into hydrogen ions and hydrogencarbonate ions 碳酸氢根离子.
3. the hydrogencarbonate ions move out into the plasma. This is the main way carbon dioxide is carried.
4. to keep the charge balanced, chloride ions 氯离子 move into the red blood cells. This movement is the chloride shift 氯转移.
5. the hydrogen ions join haemoglobin to form haemoglobinic acid 血红蛋白酸; this mops up the hydrogen ions and keeps the pH steady.

Some carbon dioxide also joins haemoglobin directly to form carbaminohaemoglobin 氨甲酰血红蛋白.

Worked example. At the lungs the partial pressure of oxygen is about $12\ \text{kPa}$ and haemoglobin is about 97% saturated; in an exercising muscle it is about $3\ \text{kPa}$ and saturation falls to about 39%. How much oxygen is unloaded, and why is the curve S-shaped? Subtract the saturations: $97 - 39 =$ 58% of the haemoglobin's oxygen is released in the muscle. The S shape comes from cooperative binding - the first oxygen to bind changes haemoglobin's shape so the next ones bind more easily, which is why the middle of the curve is so steep. That steepness is the point: a small fall in partial pressure in a respiring tissue causes a large release of oxygen. The extra carbon dioxide in an exercising muscle lowers the pH and shifts the curve right, so even more is unloaded at the same partial pressure. On the graph the shifted curve gives about 21% at $3\ \text{kPa}$, so the haemoglobin now gives up $97 - 21 = 76\%$ of its oxygen instead of 58%. (In the lungs carbon dioxide is low again, so the blood is back on the normal curve and loads to about 97%.) That is the Bohr shift doing its job.

Structure

The heart has four chambers. The two upper chambers are the atria 心房 (singular: atrium); they have thin walls because they only push blood down into the chambers below. The two lower chambers are the ventricles 心室; they have thick muscular walls because they pump blood out of the heart.

The left ventricle wall is thicker than the right ventricle wall, because the left side must pump blood all the way round the body, while the right side only pumps to the nearby lungs.

The cardiac cycle

One heartbeat is the cardiac cycle 心动周期. It has two parts: systole 收缩期 (when heart muscle contracts) and diastole 舒张期 (when it relaxes and fills). When a chamber contracts, the pressure inside rises; this pressure change opens and closes the valves so that blood flows one way only.

A valve opens when the pressure behind it is higher than the pressure in front of it, and shuts when the pressure in front becomes higher. So read the graph at the four places where the ventricle curve crosses another curve:

1. The AV valve 房室瓣 closes: the ventricle starts to contract and its pressure rises above the atrium's. This stops blood flowing back into the atrium.
2. The semilunar valve 半月瓣 opens: the ventricle pressure rises above the aorta's. Blood is pushed into the aorta, so the two pressures now rise and fall together.
3. The semilunar valve closes: the ventricle relaxes and its pressure falls below the aorta's. The small notch in the aorta curve is the blood bouncing back against the closed valve.
4. The AV valve opens: the ventricle pressure falls below the atrium's, and the ventricle starts to fill again.

Between 1 and 2, and between 3 and 4, both valves are shut. The volume of blood in the ventricle cannot change, so the pressure changes very fast.

Worked example. Use the graph. (a) For how long is blood leaving the left ventricle? (b) What is the heart rate? (c) Why does the aorta pressure never fall to zero?

• (a) Blood leaves only while the semilunar valve is open, from point 2 (about $0.16\ \text{s}$) to point 3 (about $0.34\ \text{s}$): $0.34 - 0.16 = 0.18\ \text{s}$.
• (b) One cycle takes $0.8\ \text{s}$, so the heart rate is $60 \div 0.8 = 75$ beats per minute.
• (c) The elastic wall of the aorta stretches when blood is pushed in, then recoils in diastole, keeping the blood under pressure and moving.

Common mistakes.

• Saying a valve is opened or closed by muscles. Valves move only because of the pressure difference across them.
• Saying the ventricle pressure rises far above the aorta's. Once the semilunar valve is open, the two are almost the same.
• Mixing up the sides. On a graph for the left side, the peak is about $16\ \text{kPa}$; the right ventricle reaches only about a fifth of that, because the lungs are close.
• Reading the AV valve events off the aorta curve. The AV valve is between the atrium and the ventricle, so use those two curves.

Controlling the heartbeat

The heart sets its own rhythm:

• the sinoatrial node 窦房结 in the right atrium is the pacemaker. It sends out a wave of electrical excitation that spreads across the atria and makes them contract.
• the atrioventricular node 房室结 picks up the wave, holds it back for a moment (so the atria empty first), then passes it on.
• the Purkyne tissue 浦肯野组织 carries the wave down and through the ventricle walls, so the ventricles contract from the bottom upwards and push blood out.

• Trace the double circulation and name the chambers, valves and vessels; the left ventricle wall is thicker (pumps further at higher pressure).
• Read the cardiac-cycle pressure graph: a valve opens/closes when the pressures across it cross over.
• Explain the oxygen dissociation curve (S-shaped) and the Bohr shift (more CO2 shifts it right, unloading more O2 to active tissue).
• Most CO2 is carried as hydrogencarbonate (via the chloride shift).