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.
生物系統中一個反複出現的主題是表面積與體積的比率。所有的細胞都需要營養物質，而且大多數也需要氧氣。廢物也需要被清除。
對于一個小的生物體來說，這種需求可以通過在體表的簡單擴散來滿足，但較大的或非常活躍的生物體需要一個帶有泵的運輸系統，以確定供應滿足所有細胞的需求，甚至那些在身體深處的細胞。
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.
在哺乳動物中，這個泵就是心髒。物質在血液的運輸媒體中被攜帶。血液包含在血管内，當血液流經某些被稱為毛細血管的血管時，物質被釋放出來或進入血液。
Blood vessels 血管
Blood is carried within a closed transport system that is made up of three types of vessel 血液是在一個封閉的運輸系統内進行的，該系統由三種類型的血管組成:
- arteries 動脈
- capillaries 毛細血管
- veins. 靜脈
Arteries carry blood away from the heart. 動脈将血液帶離心髒。
Capillaries are the site of the exchange of materials between the blood and tissues. Veins take blood back into the heart. 毛細血管是血液群組織之間物質交換的場所。靜脈将血液帶回心髒。
Arteries 動脈
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.
從心髒泵出的血液處于非常高的壓力下，是以動脈的結構必須适應這種情況。它們可以通過擁有由彈性纖維和平滑肌組成的非常厚的管壁來承受高壓。這使得血管壁在血液湧入時能夠伸展，進而不至于爆裂或破裂。
這也意味着，随着動脈直徑的增加，壓力會降低一些。彈性纖維在 "給力 "後，随着壓力的下降向内回縮。動脈的直徑減小，進而使壓力升高了一些。
當壓力較高時，壓力降低，當壓力較低時，壓力升高，進而使血液流動變得更加順暢。但當你感覺到你的脖子或手腕時，應該很明顯，這絕不是完整的--你仍然可以感覺到離你的心髒有一段距離的血流脈沖。
一條大動脈會分裂成更小的動脈血管，然後進一步分支成許多微小的毛細血管。動脈小管的管壁結構與動脈相似，但平滑肌的比例較大，彈性組織較少。它們不必像動脈那樣承受高壓力，并且由于平滑肌的存在而具有收縮能力，可以調節血液流向組織。
Capillaries 毛細血管
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.
為了有效地工作，毛細血管需要足夠小，以便與小群細胞緊密相連，而且其壁需要足夠薄，以允許物質在血液中進出。
為了實作這一點，在構成毛細血管壁的細胞之間有微小的縫隙。這些縫隙允許物質離開血液，沐浴在組織細胞中。由血漿和溶解物質組成的液體被稱為組織液。
Tissue fluid 組織液
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.
組織液的形成是由于血液在毛細血管的動靜脈末端的高靜水壓将液體從血液中擠出。
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.
血液中含有血漿蛋白，使血液具有相對較高的溶質潛能（是以水潛能較低），傾向于将水吸入血液。由于靜水壓力的影響大于動脈血管末端的溶質電位，是以淨影響是液體離開毛細血管。沒有血細胞或大的蛋白質離開，因為它們太大，無法通過縫隙。
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.
在毛細血管的靜脈端，由于液體已經流失，血液的靜水壓較低，溶質電位較高。正因為如此，液體排回了血液中。在這個階段，有用的物質如氨基酸和葡萄糖已經被細胞吸收了，而組織液現在含有廢物，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.
從毛細血管滲出的液體中約有90%滲回，剩下的10%由淋巴系統傳回血液，被稱為淋巴。該系統由許多盲端淋巴管組成，允許組織液通過單程閥流入其中。這些閥門大到足以讓太大而無法進入毛細血管的蛋白質進入淋巴管。如果組織液積聚而不是被淋巴系統送回血液，就會造成腹脹或水腫。
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.
血液由沐浴在液體血漿中的細胞組成。當這種血漿從毛細血管中漏出時，它被稱為組織液。它的成分幾乎與血漿相同，但含有較少的蛋白質分子，沒有紅細胞。白細胞可以從毛細血管漏出，進入組織液。淋巴與組織液的成分幾乎相同，隻是由于其位置不同而有不同的名稱。
This diagram shows the formation of tissue fluid 這張圖顯示了組織液的形成:
Veins 靜脈
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.
然後毛細血管連接配接形成更大的靜脈，靜脈本身又連接配接形成靜脈。
由于在這個階段，血液的壓力很低，是以需要 "鼓勵 "血液流回心髒。為了防止血液倒流（如果血液逆向流動尤其重要），靜脈中設有閥門。此外，靜脈穿過或非常接近肌肉。當肌肉活躍地收縮和放松時，對靜脈的擠壓會使血液流動，但由于閥門的存在，血液隻能流向心髒。
由于靜脈中的壓力比動脈中的低得多，是以幾乎不需要血管壁上的彈性纖維和平滑肌。