Mass Transport

Mass transport = efficient supply of materials over larger distances.

As the size of an organism increases the SA:Vol ratio decreases. So specialist exchange surface needed to absorb nutrients, repiratory gases and remove excretory products.

Whether or not there is a transport medium or whther it is circulated by a pump depends on:

> The SA:Vol ratio > How active the organism is

> Suitable medium in which to carry materials (usually liquid based on water because it readily dissolves substances and is moved around easily).

> Form of mass transport - transport medium is moved in bulk over large distances

> Closed system of tubular vessels - contain transport medium and branches to distribute to all parts of the organism.

> Mechanism for moving transport medium - requires pressure differences between one part of the system and another:

   > Animals: muscular contraction of body cells / pumping organ (heart)

   > Plants: rely on passive natural processes such as evaporation of water

> Mechanism to maintain mass flow movement in one direction (valves)

> Means of controlling flow of transport medium to suit changing needs of the different parts of the organism

Closed blood systems - blood confined to vessels > Heart circulates blood around body. Mammals have a double circulatory system - blood passes through the heart twice in every complete circuit of the body. Due to loss of pressure when blood passes through the lungs. Pressure boosted at heart before being transported to rest of tissues. Necessary because mammals have a high rate of metabolism. Three types of vessels: veins, arteries, capillaries.

Trnsport system used for mass transport around the body but final part of journey to cells is by diffusion. Final exhcnage between blood vessels and cells rapid because: large SA, Short diffusion distance & steep diffusion gradient.

Arteries - carry blood away from heart into arterioles

Arterioles - control blood flow from arteries to capillaries

Capillaries - link arterioloes to veins. Veins - carry blood from capillaries to heart

Layers outside in

Tough outer layer - resists pressure changes from within an outside

Muscle layer - can contract to control flow of blood

Elastic layer - helps mainatin blood pressure by stretching and springing back

Thin inner layer - (endothelium) smooth to prevent friction and tin to allow diffusion

Lumen - central cavity of blood vessel through which blood flows

Artery structure

Thick muscle layer (compared to veins) - smaller arteries can be constricted and dilated to control volume of blood passing through them.

Relatively thick elastic layer - blood pressure must be kept high to get blood to extremities. Elastic wall stretched at each heart beat (systole) > then springs back at diastole. Maintains high BP and smooth pressure surges created by th heart beating.

Large overall thickness of wall - resists the vessel bursting under pressure.

No valves - (except in arteries leaving the heart) because blood under constant high pressure, does not tend to flow backwards.

Arteriole structure

Relatively thick muscle layer - allows constriction of the lumen - restricts the flow of blood as to control its movement into the capillaries.

Relatively thin elastic layer - blood pressure is lower.

Vein structure

Relatively thin muscle layer - carries blood away from tissues so constriction and dilation will not control the flow of blood to the tissues.

Relatively thin elastic layer - low pressure of veins does not cause them to burst or recoil.

Overall thickness is small - no risk of bursting. Can be flattened easily aiding flow of blood within them.

Valves throughout - ensures no backflow of blood. When body muscles contract the veins are compressed, pressurising the blood within them - directs blood towards the heart.

Capillary structure

Walls consist only of lining layer - extremely thin = short diffusion distance = rapid diffusion

Numerous and highly branched - large SA for diffusion

Narrow diameter - permeate tissues and no tissue far from a capillary

Narrow lumen - red blood cells squeezed flat against capillary wall - closer to cells they supply oxygen to. Reduces diffusion distance.

Spaces between endothelial cells - white blood cells can escape to deal with infection within tissues.

Final journey of metabolic materials is through the Tissue Fluid.