Topic 7: Blood circulation

Circulatory system of a mammal 

A transport system is required to take materials from cells to exchange surfaces and from exchange surfaces to cells. The need for a transport system depends on the surface area to volume ratio and activity of the organism. The lower the surface area to volume ratio, and the more active the organism, the greater its need for a specialised transport system with a pump. 

Features of transport systems: 

• A suitable medium in which to carry materials, usually a liquid based on water because it readily dissolves substances and can be moved around easily, e.g. blood
• A form of mass transport in which the transport medium is  moved around in bulk over large distances. This is more rapid than diffusion
• A closed system of tubular vessels that contains the transport medium and forms a branching network to distribute it to all parts of the organism. 
• A mechanism for moving the transport medium within vessels, e.g animals use muscular contractions of body muscles or specialised organs (the heart); plants rely on passive processes such as evaporation of water. This requires a pressure difference between one part of the system and another. 
• A mechanism to maintain mass flow movement in one direction, e.g. valves
• A way to control the flow of the transport medium to suit the changing needs of different parts of the organism. 
• A mechanism for mass flow of water or gases, e.g. intercostal muscles and diaphragm during breathing in mammals. 

The structure of the heart 

Mammals have a closed (=blood is confined to vessel), double circulatory system(=blood passes twice through the heat for each complete circuit of the body). It is necessary to return blood to the heart before pumping it to the rest of the body because blood pressure has to be increased (it has to pass through tiny capillaries in the lungs in order to have a large surface area for the exchange of gases so pressure is decreased). 

The heart has four chambers:  the left and right atria (thin-walled, elastic and stretch as they collect blood), and the left and right ventricles (thicker muscular wall as they have to contract strongly to pump blood a great distance- more so for the left ventricle). Between each atrium and ventricle are two valves: the left atrioventricular (bicuspid) valve and the right atrioventricular (tricuspid) valve. These prevent backflow when contraction of ventricles causes ventricular pressure to exceed atrial pressure. Closing these valves ensures that blood does not flow back into the atria. The atria receive blood from the veins, whilst the ventricles pump blood away from the heart and into arteries. The pulmonary vessels are: 

• Pulmonary vein- connected to the left atrium and brings oxygenated blood back from the lungs. It is the only vein to carry oxygenated blood. 
• Aorta- connected to the right ventricle and carries oxygenated blood to all parts of the body except the lungs. 
• Vena cava- connected to the right atrium and brings deoxygenated blood…