Transport in mammals

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.

Double circulation: the pulmonary loop goes to the lungs, the systemic loop to the body; blood passes through the heart twice

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

An artery has a thick wall and narrow lumen; a vein a thin wall, wide lumen and valves; a capillary is one cell thick

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.

A stained blood smear: many small red cells, plus a lymphocyte (centre) and neutrophils (lobed nucleus) — the white cells are larger and have a nucleus

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.

High pressure at the arterial end pushes fluid out to form tissue fluid; most returns at the venous end

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.

Haemoglobin loads oxygen in the lungs and unloads it in the tissues; the Bohr shift moves the curve right so more is released

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.

Most CO₂ travels as hydrogencarbonate ions; the chloride shift keeps the charge balanced and haemoglobin mops up the H⁺

Worked example. At the lungs the partial pressure of oxygen is about $12\ \text{kPa}$ and haemoglobin is about 98% saturated; in an exercising muscle it is about $3\ \text{kPa}$ and saturation falls to about 40%. How much oxygen is unloaded, and why is the curve S-shaped? Subtract the saturations: $98 - 40 =$ 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 - 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 four chambers, the valves and the main vessels; the left ventricle wall is the thickest

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.

The ventricle pressure spikes in systole and pushes blood into the aorta; a valve opens or closes whenever two pressure curves cross

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.

The SAN sets the rhythm; the wave passes to the AVN, then the Purkyne tissue makes the ventricles contract bottom-up

• 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).