7.3 Circulatory System of a mammal

Closed, double circulatory system
1. blood is confined to vessels and passes twice through the heart for each complete circuit of the body
When blood is passed through the lungs, pressure is reduced
1. If it were to pass immediately to the rest of the body its low pressure would make circulation very slow. Blood is therefore returned to the heart to boost its pressure.
  1. As a result, substances are delivered to the rest of the body quickly, which is necessary as mammals have a high body temperature and hence a high rate of metabolism.
Although a transport system is used to move substances longer distances, the final part of the journey to cells is by diffusion. The final exchange from blood vessels into cells is rapid because it takes place over a large surface area, across short distance and there is a steep diffusion gradient
A transport system is required in large organisms to take materials from cells to exchange surfaces and from exchange surfaces to cells. Materials have to ne transported between exchange surfaces and the environment. They also need to be transported between different parts of the organism
Whether or not there is a specialised transport medium, and whether or not it is circulated by a pump, depends on two factors...
1. ... the surface area to volume ratio
2. ... how active the organism is
3. The lower the surface area to volume ratio, and the more active the organism, the greater is the need for a specialised transport system with a pump

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