生物哺乳动物知识点（A-level生物知识点整理）
生物哺乳动物知识点（A-level生物知识点整理）Capillaries are the site of the exchange of materials between the blood and tissues. Veins take blood back into the heart. 毛细血管是血液和组织之间物质交换的场所。静脉将血液带回心脏。Arteries carry blood away from the heart. 动脉将血液带离心脏。In mammals the pump is the heart. Substances are carried in a transport medium of the blood. The blood is contained within vessels with substances being released out of or into the blood as it
A recurring theme in biological systems is the surface area to volume ratio. All cells require nutrients and most require oxygen as well. Wastes also need to be removed.
With a small organism this demand can be met by simple diffusion over the body surface but larger or very active organisms need a transport system with a pump to ensure that the supply meets the demand of all cells even those deep within the body.
生物系统中一个反复出现的主题是表面积与体积的比率。所有的细胞都需要营养物质，而且大多数也需要氧气。废物也需要被清除。
对于一个小的生物体来说，这种需求可以通过在体表的简单扩散来满足，但较大的或非常活跃的生物体需要一个带有泵的运输系统，以确保供应满足所有细胞的需求，甚至那些在身体深处的细胞。
In mammals the pump is the heart. Substances are carried in a transport medium of the blood. The blood is contained within vessels with substances being released out of or into the blood as it flows through certain vessels called capillaries.
在哺乳动物中，这个泵就是心脏。物质在血液的运输介质中被携带。血液包含在血管内，当血液流经某些被称为毛细血管的血管时，物质被释放出来或进入血液。
Blood vessels 血管Blood is carried within a closed transport system that is made up of three types of vessel 血液是在一个封闭的运输系统内进行的，该系统由三种类型的血管组成:
- arteries 动脉
- capillaries 毛细血管
- veins. 静脉
Arteries carry blood away from the heart. 动脉将血液带离心脏。
Capillaries are the site of the exchange of materials between the blood and tissues. Veins take blood back into the heart. 毛细血管是血液和组织之间物质交换的场所。静脉将血液带回心脏。
Arteries 动脉
Blood pumped out of the heart is at a very high pressure so the structure of the arteries must be adapted to this. They can withstand high pressure by having very thick walls made up of elastic fibres and smooth muscle. These allow the wall to stretch as blood surges through them so that they don't burst or rupture.
It also means that as the artery increases in diameter the pressure is reduced a little. After they 'give' the elastic fibres recoil back inwards as the pressure falls. The artery decreases in diameter thus raising the pressure a little.
The lowering of the pressure when it is high and the raising of it when it is lower produces some smoothing out of the flow of the blood. It should be obvious though when you feel your neck or wrist that it is by no means complete - you can still feel the pulses of the flow some distance from your heart.
A large artery will split into smaller arterioles that then branch further into many tiny capillaries. Arterioles have walls with a similar structure to arteries but have a greater proportion of smooth muscle and less elastic tissue. They do not have to withstand as high a pressure as arteries and have the ability to contract because of the smooth muscle and regulate the flow of blood to a tissue.
从心脏泵出的血液处于非常高的压力下，因此动脉的结构必须适应这种情况。它们可以通过拥有由弹性纤维和平滑肌组成的非常厚的管壁来承受高压。这使得血管壁在血液涌入时能够伸展，从而不至于爆裂或破裂。
这也意味着，随着动脉直径的增加，压力会降低一些。弹性纤维在 "给力 "后，随着压力的下降向内回缩。动脉的直径减小，从而使压力升高了一些。
当压力较高时，压力降低，当压力较低时，压力升高，从而使血液流动变得更加顺畅。但当你感觉到你的脖子或手腕时，应该很明显，这绝不是完整的--你仍然可以感觉到离你的心脏有一段距离的血流脉冲。
一条大动脉会分裂成更小的动脉血管，然后进一步分支成许多微小的毛细血管。动脉小管的管壁结构与动脉相似，但平滑肌的比例较大，弹性组织较少。它们不必像动脉那样承受高压力，并且由于平滑肌的存在而具有收缩能力，可以调节血液流向组织。
Capillaries 毛细血管
To work efficiently the capillaries need to be small enough to be in close proximity with small groups of cells and their walls need to be thin enough to allow substances to move in and out of the blood.
To enable this there are tiny gaps between the cells making up the wall of the capillary. These allow substances to leave the blood and bathe the cells of the tissues. The fluid made up of plasma and dissolved substances is called tissue fluid.
为了有效地工作，毛细血管需要足够小，以便与小群细胞紧密相连，而且其壁需要足够薄，以允许物质在血液中进出。
为了实现这一点，在构成毛细血管壁的细胞之间有微小的缝隙。这些缝隙允许物质离开血液，沐浴在组织细胞中。由血浆和溶解物质组成的液体被称为组织液。
Tissue fluid 组织液Tissue fluid is formed because of the high hydrostatic pressure of the blood at the arteriole end of the capillary that pushes fluid out of the blood.
组织液的形成是由于血液在毛细血管的动静脉末端的高静水压将液体从血液中挤出。
The blood contains plasma proteins giving the blood a relatively high solute potential (and therefore a low water potential) tending to draw water into the blood. Since the hydrostatic pressure has a greater effect than the solute potential at the arteriole end the net effect is that fluid leaves the capillary. No blood cells or large proteins leave as they are too big to fit through the gaps.
血液中含有血浆蛋白，使血液具有相对较高的溶质潜能（因此水潜能较低），倾向于将水吸入血液。由于静水压力的影响大于动脉血管末端的溶质电位，所以净影响是液体离开毛细血管。没有血细胞或大的蛋白质离开，因为它们太大，无法通过缝隙。
At the venule end of the capillary since fluid has been lost the hydrostatic pressure of the blood is lower and the solute potential is higher. Because of this fluid drains back into the blood. At this stage the useful materials such as amino acids and glucose will have been taken up by the cells and the tissue fluid will now contain waste substances such as carbon dioxide and urea.
在毛细血管的静脉端，由于液体已经流失，血液的静水压较低，溶质电位较高。正因为如此，液体排回了血液中。在这个阶段，有用的物质如氨基酸和葡萄糖已经被细胞吸收了，而组织液现在含有废物，如二氧化碳和尿素。
About 90% of the fluid which leaks out of the capillaries seeps back in the remaining 10% is returned to the blood by the lymphatic system and is called lymph. This system is made up of many blind-ending lymph vessels which allow tissue fluid to flow into them via one way valves. These valves are large enough to allow proteins which are too big to get into the capillaries into the lymph vessels. If tissue fluid accumulates rather than be returned to the blood by the lymphatic system bloating or oedema is the result.
从毛细血管渗出的液体中约有90%渗回，剩下的10%由淋巴系统返回血液，被称为淋巴。该系统由许多盲端淋巴管组成，允许组织液通过单程阀流入其中。这些阀门大到足以让太大而无法进入毛细血管的蛋白质进入淋巴管。如果组织液积聚而不是被淋巴系统送回血液，就会造成腹胀或水肿。
Blood consists of cells bathed in a liquid plasma. When this plasma leaks out of the capillaries it is called tissue fluid. This is almost identical in composition to plasma but contains less protein molecules and no red blood cells. White blood cells can escape the capillaries into the tissue fluid. Lymph is virtually identical in composition to tissue fluid and just has a different name due to its different location.
血液由沐浴在液体血浆中的细胞组成。当这种血浆从毛细血管中漏出时，它被称为组织液。它的成分几乎与血浆相同，但含有较少的蛋白质分子，没有红细胞。白细胞可以从毛细血管漏出，进入组织液。淋巴与组织液的成分几乎相同，只是由于其位置不同而有不同的名称。
This diagram shows the formation of tissue fluid 这张图显示了组织液的形成:
The capillaries then join to form larger venules which themselves then join to form veins.
Since at this stage the pressure of the blood is low blood needs to be 'encouraged' to flow back to the heart. To prevent any backflow of the blood (particularly important if blood is flowing against gravity) there are valves in the veins. Also the veins pass through or very close to muscles. When the muscles are active in contracting and relaxing the squeezing on the veins moves blood along but due to the valves only ever towards the heart.
As the pressure is so much lower in the veins than in the arteries there is little need for the elastic fibres and smooth muscle in the walls.
然后毛细血管连接形成更大的静脉，静脉本身又连接形成静脉。
由于在这个阶段，血液的压力很低，所以需要 "鼓励 "血液流回心脏。为了防止血液倒流（如果血液逆向流动尤其重要），静脉中设有阀门。此外，静脉穿过或非常接近肌肉。当肌肉活跃地收缩和放松时，对静脉的挤压会使血液流动，但由于阀门的存在，血液只能流向心脏。
由于静脉中的压力比动脉中的低得多，所以几乎不需要血管壁上的弹性纤维和平滑肌。
