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.
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.
Blood is carried within a closed transport system that is made up of three types of vessel:
Arteries carry blood away from the heart.
Capillaries are the site of the exchange of materials between the blood and tissues. Veins take blood back into the heart.
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.
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.
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.
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.
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.
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.
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.
This diagram shows the formation of tissue fluid:
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.