Circulatory system of mammals

• Diffusion is fast enough and adequate for short distances, however larger organisms require a transport system.
• Whether or not there is a circulatory system and transport system depends on:

  The surface area to volume ratio

  How active the organism is
• Transport systems have the following features:

  A suitable medium in which to carry materials

  A form of mass transport in which the transport medium is moved around in bulk

  A closed system of vessels.

  Pressure differences for moving the transport medium within vessels
• Mammals have a double, closed circulatory system.

  Blood stays in blood vessels and passes twice through the heart through each circuit around the body.
• There is a high-pressure circuit to the lungs and back and a low-pressure circuit to the rest of the body and back.
• The high-pressure circulatory pumping of blood to the body tissues is needed because mammals have a high body temperature and therefore a high rate of metabolism.
• The blood vessels in a mammal’s circulatory system are arteries, veins, and capillaries.
• Although a transport system is used to move substances long distances around the body, the final part of transport is by diffusion.

  Substances diffuse from the blood in to surrounding tissues in areas where there is a large surface area, short diffusion distances and there is a steep concentration gradient.