Transport systems and stuff.

Why do some organisms need a transport system?

• Large organism: Small surface area to volume ratio, needs can't be met by the surfaace of the body.
  
• Diffusion is inadequate over large distances.
  
• Transport system required to take materials from cells to exchange surface and vice versa.

The lower the SA/V ratio, and the more active the organism, the greater the need for a specialised system with a pump.

Main function of a blood system is to transport substances around the body. Allows organism to move substances around in bulk, quickly, over large distances. Blood vessels, a closed system of tubular vessels contain blood and transport it. The heart is the mechanism that pumps. Valves maintain this flow in one direction.

Arteries take blood away from the heart into ARTERIOLES, thinner branches of arteries in organs. CAPILLARIES- mass of very narrow vessels which penetrate tissues. Link ARTERIOLES to VENULES. VENULES, blood flows back from capillaries into veins, which transports blood back to the heart.

Structure of blood vessels:
Arteries, arterioles, and veins have the same basic layered structure. Relative proportions of each layer differ between each type of blood vessel:-

• Tough outer layer- resists pressure changes, inside and out, (OUTER LAYER)
  
• Muscle layer, can contract and control the flow of blood.
  
• Elastic layer, helps maintain pressure by stretching and recoiling.
  
• Endothelium- thin inner lining, smooth to prevent friction, and thin to allow diffusion. INNER LAYER.
  
• Lumen - central cavity of blood vessel.
  

Have different proportions of these features in different types.

Arteries- thick walls to maintain pressure, thin to absorb it and keep it constant.

Arteries Have a smooth endothelium, inner layer, cells are flat. This ensures the blood flows freely and doesn't stick to the walls. It has a layer of elastic fibresm, which allows it to expandand recoil each time the heart beats, smoothing out pressure changes. The have a realtively thick muscle layer, so that smaller arteries can be constricted to control the volume of blood passing through them. There is a tought outer layer of protein fibres, which allows the wall to resist bursting under pressure.

Arteriole's muscle layers are relatively thicker then that of the arteries. This allows contraction of the muscle allowing constriction of the lumen of the arteriole, restricting bloodflow and controlling it. Also the elastic layer is thinner then that in the arteries, as the blood pressure is lower.

Veins have a realtively thin muscle layer, as they carry blood away from the tissues, and therefore, cannot control blood flow to the tissues. The elastic layer is thin, becuase there is a low blood pressure, and the recoil action cannot be made. The walls are thinner then the arteries walls are. This is because blood pressure is lower and pressure is resisted less. There are valves throughout (the body muscles contract and compress them), this ensures that blood flows one way.

Capillaries are one cell thick, and their function is to exchange metabolic material. The flow of blood is slow in capillaries, which allows more time for exchange of materials. Also, the walls are extremely thin, which makes for a short diffusion distance. There are numerous capillaries, which provides a large SA for diffusion. They are narrow in diameter, so they can penetrate tissues.