Transport Systems in Animals | 动物的运输系统

📚 Transport Systems in Animals | 动物的运输系统

All living cells need a continuous supply of oxygen and nutrients, and they must remove waste products such as carbon dioxide and urea. In large multicellular animals, diffusion alone is far too slow to meet these needs, so a specialised transport system is essential for moving materials rapidly over long distances.

所有活细胞都需要持续供应氧气和营养物质，并清除二氧化碳和尿素等代谢废物。在大型多细胞动物中，仅靠扩散速度太慢，无法满足这些需求，因此必须依靠专门的运输系统，将物质快速输送到身体各处。

1. Why Animals Need Transport Systems | 为什么动物需要运输系统

As an organism increases in size, its surface area to volume ratio falls. Diffusion becomes insufficient over distances greater than about 1 mm because the time taken for diffusion is proportional to the square of the distance travelled. A mass transport system overcomes this problem by using a pump to move fluid rapidly through vessels.

随着生物体体积增大，其表面积与体积之比会下降。当距离超过约 1 mm 时，扩散就不再高效，因为扩散所需时间与移动距离的平方成正比。物质运输系统利用泵将流体快速压入血管，从而克服这一难题。

In mammals, the circulatory system carries oxygen from the lungs to respiring tissues, transports digested food from the gut to the liver and other organs, and delivers metabolic wastes to the kidneys and lungs for excretion. It also distributes hormones and helps regulate body temperature by redistributing heat.

在哺乳动物中，循环系统将氧气从肺部运送到呼吸组织，将消化后的营养物质从肠道运送到肝脏和其他器官，并将代谢废物运送到肾脏和肺进行排泄。它还能分配激素，并通过重新分配热量来帮助调节体温。

2. Open and Closed Circulatory Systems | 开放与封闭式循环系统

In an open circulatory system, such as that found in insects, a simple tubular heart pumps haemolymph into large open spaces called the haemocoel. The fluid bathes the tissues directly, but blood pressure is low and flow is slow. This limits the rate of oxygen delivery, which is acceptable for less active animals but cannot support high metabolic rates.

在开放式循环系统中，例如昆虫体内，简单的管状心脏将血淋巴泵入称为血腔的大型开放空间。血淋巴直接浸泡组织，但血压较低、流速较慢。这限制了氧气输送速率，对活动量较小的动物来说可以接受，但无法支持较高代谢率。

In a closed circulatory system, blood is confined within vessels and is pumped by a muscular heart at high pressure. This allows faster and more directed delivery of oxygen and nutrients, which is essential for the high metabolic demands of vertebrates. Closed systems also allow the formation of tissue fluid and lymph, which are separate from blood.

在封闭式循环系统中，血液被限制在血管内，由肌肉质心脏以较高压力泵送。这样可以更快、更有方向性地输送氧气和营养物质，满足脊椎动物较高的代谢需求。封闭系统还能形成与血液分开的组织液和淋巴。

3. Single and Double Circulation | 单循环与双循环

Fish have a single circulatory system: the heart pumps deoxygenated blood to the gills, where gas exchange occurs, and then the oxygenated blood flows to the rest of the body before returning to the heart. However, blood pressure drops significantly as blood passes through the narrow gill capillaries, so oxygen delivery to the body tissues is relatively slow.

鱼类具有单循环系统：心脏将缺氧血泵送到鳃，在鳃部完成气体交换后，含氧血液流向全身，再回到心脏。但是，血液流经狭窄的鳃毛细血管后血压会显著下降，因此向身体组织输送氧气的速度相对较慢。

Mammals have a double circulatory system: the right side of the heart pumps blood to the lungs in the pulmonary circulation, while the left side pumps oxygenated blood to the rest of the body in the systemic circulation. This separation allows the systemic circuit to operate at high pressure and prevents oxygenated and deoxygenated blood from mixing, which improves oxygen delivery to highly active tissues.

哺乳动物具有双循环系统：右心将血液泵送到肺，形成肺循环；左心将含氧血泵送到全身，形成体循环。这种分离使体循环能够在较高压力下运行，并防止含氧血与缺氧血混合，从而提高对高活性组织的氧气供应效率。

4. Structure of the Mammalian Heart | 哺乳动物心脏的结构

The mammalian heart has four chambers: the right atrium, right ventricle, left atrium and left ventricle. The right atrium receives deoxygenated blood from the vena cavae, and the right ventricle pumps it to the lungs through the pulmonary artery. The left atrium receives oxygenated blood from the pulmonary veins, and the thick-walled left ventricle pumps it to the whole body through the aorta.

哺乳动物心脏有四个腔室：右心房、右心室、左心房和左心室。右心房接受来自腔静脉的缺氧血，右心室将血液经肺动脉泵送到肺。左心房接受来自肺静脉的含氧血，壁厚的左心室将血液经主动脉泵送到全身。

Valves prevent backflow and ensure one-way flow through the heart. The tricuspid valve lies between the right atrium and right ventricle, and the bicuspid (mitral) valve lies between the left atrium and left ventricle. Semilunar valves guard the entrances to the pulmonary artery and aorta. The muscular septum separates the left and right sides, keeping oxygenated and deoxygenated blood apart.

瓣膜防止血液倒流，确保血液在心脏内单向流动。三尖瓣位于右心房和右心室之间，二尖瓣位于左心房和左心室之间。半月瓣守护肺动脉和主动脉的入口。肌性室间隔将心脏左右两侧分开，防止含氧血与缺氧血混合。

5. The Cardiac Cycle | 心动周期

The cardiac cycle consists of three phases: atrial systole, ventricular systole and diastole. During atrial systole, the atria contract, pushing blood into the ventricles through the open atrioventricular valves. During ventricular systole, the ventricles contract; the atrioventricular valves close and the semilunar valves open, ejecting blood into the pulmonary artery and aorta.

心动周期包括三个阶段：心房收缩期、心室收缩期和舒张期。在心房收缩期，心房收缩，将血液通过开放着的房室瓣推入心室。在心室收缩期，心室收缩，房室瓣关闭，半月瓣打开，将血液射入肺动脉和主动脉。

During diastole, the heart muscle relaxes, the semilunar valves close, and the atria refill with blood from the veins. As atrial pressure exceeds ventricular pressure, the atrioventricular valves open and blood flows passively into the ventricles. A typical resting cycle lasts about 0.8 seconds, with diastole occupying the longest portion.

在舒张期，心肌舒张，半月瓣关闭，心房从静脉重新充满血液。当心房压力超过心室压力时，房室瓣打开，血液被动流入心室。典型静息心动周期约为 0.8 秒，其中舒张期所占时间最长。

6. Control of the Heartbeat | 心跳的控制

Cardiac muscle is myogenic, meaning it can contract and relax without nervous stimulation. The sinoatrial node (SAN), located in the wall of the right atrium, acts as the pacemaker. It initiates a wave of electrical excitation that spreads across both atria, causing them to contract simultaneously.

心肌具有自主节律性，即无需

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