Circulatory Systems.

As organisms began to develop specialised cells and grow larger, the cells that made them up needed to obtain nutrients, gases, and other metabolites. Hence the need for a transporting system

Circulatory system need the following:

1. A pump
2. A circulating fluid
3. Vessels to carry the fluid
4. Valves to ensure 1 way flow of materials 

Circulatory systems can be classified in 2 ways, whether they are open or closed or by how may times they go through the heart.  In the case of an open circulatory system, the blood leaving the heart is eventually deposited on the tissues.  In the case of a closed system, the circulating fluid is wholly contained within blood vessels. 

Insects have an open circulatory system.  This is not as efficient as the organism gets larger.  As a result, the size of insects is restricted.

In vertebrates, the circulatory system is closed, but the type of heart can vary.  Fish have a single pump heart.  This means that the blood goes through the heart once in a complete circuit.  By going into the small blood vessels of the gills before entering body tissues, the blood pressure in fish is generally low.  This is compensated by having a comparatively low metabolic rate and using water to provide body support, further lowering energy needs.

Amphibians and reptiles have a double pump system which means that blood enters the heart twice as part of a circuit through the body.  The ventricle of the heart is incompletely divided and so there is mixing of the oxygenated and deoxygenated blood. While the double pump system increases blood pressure, the mixing of blood in the heart means that the efficiency of oxygen transport is generally lowered.  As a result, most reptiles are incapable of sustained periods of high energy activity such as running etc.

In mammals and birds, the circulatory system is a double pump system, but the oxygenated and deoxygenated blood are kept separate.  This means that the body receives well oxygenated blood under high pressure, which allows mammals and birds to undertake activities that are high in energy use like flying, running and maintaining constant body temperature.

Once blood flows through the capillaries (finely branched to enhance SA:vol ratio), blood pressure is lost.  

To ensure that blood flows in the one direction, apart from having valves in the heart, veins also have valves to ensure that blood cannot move backwards.  Circulation in the veins is enhanced by muscle movement.

The red blood cells are made up of haemoglobin, which has unique properties. In areas of high oxygen (the lungs), oxygen readily binds to it.  In areas of low oxygen levels (tissues in the body) haemoglobin will release oxygen to the environment.  As a result oxygen can be captured in the lungs and be supplied to the tissues.