📚 Transport in Mammals | 哺乳动物体内的运输
Large multicellular mammals cannot rely on diffusion alone to supply oxygen and nutrients to deep tissues and remove waste products. The mammalian circulatory system is a closed, double circulation powered by a muscular heart. This article summarises the structures, mechanisms and transport functions needed for Cambridge A-Level Biology.
多细胞的大型哺乳动物不能仅靠扩散为深层组织供应氧气和营养并清除废物。哺乳动物的循环系统是由肌肉质心脏驱动的封闭式双循环系统。本文总结剑桥 A-Level 生物所需的有关结构、机制和运输功能的知识。
1. Overview of Mammalian Transport | 哺乳动物运输概览
Mammals have a closed blood system in which blood is confined to vessels and moves away from and back to the heart. This maintains high pressure and rapid delivery of materials.
哺乳动物具有封闭式血液系统，血液局限于血管内，从心脏流出再返回心脏。这维持了较高的压力并快速输送物质。
The blood transports oxygen, carbon dioxide, nutrients, hormones, antibodies and urea, and also distributes heat around the body.
血液运输氧气、二氧化碳、营养物质、激素、抗体和尿素，同时将热量分配到全身。
2. Double Circulation | 双循环
The mammalian heart is effectively two pumps. The right side pumps deoxygenated blood to the lungs in the pulmonary circulation; the left side pumps oxygenated blood to the rest of the body in the systemic circulation.
哺乳动物心脏实际上是两个泵。右心将缺氧血泵入肺循环到达肺部；左心将富氧血泵入体循环到达全身。
This double circulation prevents oxygenated and deoxygenated blood from mixing, and allows the systemic circuit to operate at high pressure while the pulmonary circuit operates at lower pressure.
这种双循环防止富氧血和缺氧血混合，并允许体循环在较高压力下运行，而肺循环在较低压力下运行。
The hepatic portal vein is an exception in the systemic circuit, carrying nutrient-rich blood from the gut to the liver before it returns to the heart.
肝门静脉是体循环中的一个例外，它将富含营养的血液从肠道输送到肝脏，然后才返回心脏。
3. Structure of the Heart | 心脏结构
The heart is a hollow muscular organ located in the thorax. It has four chambers: two thin-walled atria above and two thick-walled ventricles below.
心脏是位于胸腔的中空肌肉器官。它有四个腔室：上方两个薄壁心房和下方两个厚壁心室。
The left ventricle has a much thicker muscular wall than the right ventricle because it must generate sufficient pressure to pump blood around the whole body.
左心室的肌肉壁比右心室厚得多，因为它必须产生足够的压力将血液泵送到全身。
Atrioventricular valves separate the atria from the ventricles, and semilunar valves are found at the bases of the aorta and pulmonary artery. These valves prevent backflow.
房室瓣分隔心房和心室，半月瓣位于主动脉和肺动脉基部。这些瓣膜防止血液倒流。
The septum separates the left and right sides, keeping oxygenated and deoxygenated blood apart. The heart muscle itself is supplied with oxygenated blood by the coronary arteries, which branch from the aorta.
室间隔将左右两侧分开，使富氧血和缺氧血保持分离。心肌本身由从主动脉分支出来的冠状动脉供应富氧血。
4. Cardiac Cycle | 心动周期
The cardiac cycle is the sequence of events in one heartbeat. It includes atrial systole, ventricular systole and diastole.
心动周期是一次心跳中的事件序列，包括心房收缩、心室收缩和舒张。
During diastole, the atria and ventricles relax, blood enters the atria and flows passively into the ventricles. Atrial systole then forces the remaining blood into the ventricles.
在舒张期，心房和心室舒张，血液进入心房并被动流入心室。随后心房收缩，将剩余血液压入心室。
Ventricular systole follows, closing the atrioventricular valves and opening the semilunar valves so blood is ejected into the aorta and pulmonary artery. Pressure changes in the atria, ventricles and arteries can be recorded and interpreted.
接着心室收缩，关闭房室瓣并打开半月瓣，使血液射入主动脉和肺动脉。可以记录并解释心房、心室和动脉中的压力变化。
Cardiac output is the volume of blood pumped by one ventricle per minute, calculated as heart rate multiplied by stroke volume.
心输出量是一个心室每分钟泵出的血液体积，用心率乘以每搏输出量来计算。
5. Coordination of the Heartbeat | 心跳的协调
Cardiac muscle is myogenic, meaning it can contract without nervous stimulation. The heartbeat is initiated by the sinoatrial node (SAN) in the wall of the right atrium.
心肌是肌源性的，即无需神经刺激即可收缩。心跳由右心房壁内的窦房结（SAN）发起。
The SAN acts as the pacemaker, sending waves of electrical excitation across the atria, causing atrial contraction. The excitation reaches the atrioventricular node (AVN), where there is a slight delay.
窦房结起起搏器的作用，向心房发送电兴奋波，引起心房收缩。兴奋到达房室结（AVN），在此有短暂延迟。
This delay allows the ventricles to fill before they contract. The impulse then travels along the bundle of His and Purkyne tissue to the apex and up the ventricle walls, producing ventricular contraction from the apex upwards.
这种延迟使得心室在收缩前得以充盈。冲动随后沿希氏束和浦肯野组织传导至心尖并向上传至心室壁，使心室从心尖向上收缩。
6. Blood Vessels | 血管
Arteries carry blood away from the heart. They have thick muscular walls with elastic tissue to withstand and smooth out high pressure.
动脉将血液运离心脏。它们有厚实的肌肉壁和弹性组织，以承受并缓冲高压。
Veins carry blood back to the heart. They have thinner walls, a wider lumen and valves to prevent backflow of low-pressure blood.
静脉将血液送回心脏。它们的壁较薄，管腔较宽，并有瓣膜防止低压血液倒流。
Capillaries are the site of exchange between blood and tissues. Their walls are one cell thick, giving a short diffusion distance. Arterioles and venules connect arteries and veins to capillaries; contraction of smooth muscle in arterioles controls blood flow to tissues.
毛细血管是血液与组织之间物质交换的场所。其管壁仅一层细胞厚，扩散距离短。小动脉和小静脉将动脉和静脉与毛细血管连接起来；小动脉平滑肌的收缩控制流向组织的血流量。
7. Blood Components | 血液成分
Blood consists of plasma, red blood cells, white blood cells and platelets. Plasma transports soluble substances such as glucose, amino acids, hormones, urea and carbon dioxide.
血液由血浆、红细胞、白细胞和血小板组成。血浆运输葡萄糖、氨基酸、激素、尿素和二氧化碳等可溶性物质。
Red blood cells (erythrocytes) are biconcave discs with no nucleus in mammals, increasing surface area and space for haemoglobin.
哺乳动物的红细胞（红血球）呈双凹圆盘状且无细胞核，这增加了表面积和容纳血红蛋白的空间。
White blood cells defend the body against infection, and platelets are involved in blood clotting.
白细胞保护身体免受感染，血小板参与血液凝固。
8. Tissue Fluid and Lymph | 组织液与淋巴
At the arteriolar end of a capillary, hydrostatic pressure forces water and small solutes out of the blood to form tissue fluid. Large plasma proteins remain in the capillary.
在毛细血管的小动脉端，静水压将水和小分子溶质压出血液，形成组织液。大分子血浆蛋白留在毛细血管内。
At the venular end, the hydrostatic pressure is lower and the osmotic pull of plasma proteins draws water back into the capillary.
在小静脉端，静水压较低，血浆蛋白的渗透吸引力将水拉回毛细血管。
Excess tissue fluid drains into lymph capillaries and is returned to the blood via the lymphatic system.
多余的组织液排入淋巴毛细管，并通过淋巴系统返回血液。
9. Transport of Oxygen | 氧气的运输
Oxygen is transported mainly by binding reversibly to haemoglobin in red blood cells. Each haemoglobin molecule can bind up to four oxygen molecules. The reaction can be written as:
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