7.3 Circulatory system of a mammal

• Whether or not there is a specialised transport medium and whether or not it is circulated by a pump depends on:

  1. surface area to volume ratio
  2. How active the organism is (the metabolic rate)
• • low surface area to volume ratio indicates that the organism is likely to have a specialised transport medium or a pump circulation system
• high metabolic rate indicates that the organism is likely to have a specialised transport medium or a pump circulation system
• Features of transport systems:

  1. Suitable medium in which to carry materials
  2. A form of mass transport in which the transport medium is moved around in bulk over large distances
  3. A closed system of tubular vessels that contains the transport medium
  4. A mechanism for moving the transport medium within vessels
• • The suitable medium is normally a liquid based on water e.g blood
  • This is because water readily dissolves substances and can be moved around easily
  • Can be a gas such as air
• A closed system of tubular vessels forms a branching network to distribute the transport medium to all parts of the organism
• A mechanism for moving the transport medium requires a pressure difference between one part of the system and another
• Mammals have a closed, double circulatory system in which blood is confined to vessels and passes twice through the heart for each complete circuit of the blody
• Mammals have a closed, double circulatory system because:

  • When blood passes through the lungs, its pressure is reduced
  • If it were passed immediately to the rest of the body, its low pressure would make circulation very slow
• Advantage of closed, double circulatory system is:

  • Blood is therefore returned to the heart to increase pressure before being circulated to the rest of the tissues
  • So, the rate of blood flow to tissues increases since substances are delivered to the rest of the body quickly
  • This is necessary since mammals have a high body temperature and so a high rate of metabolism
• The vessels that make up the circulatory system of a mammal:

  • Arteries
  • Veins
  • Capillaries
• The final exchange from blood vessels into cells is by diffusion

  • This is rapid because it takes place over a large surface area, across short distances and there is a steep diffusion gradient