Circulatory system of mammals

• Why large organisms have a transport system


  • All organisms exchange materials between themselves and their environment
  • With increasing size, the surface area to volume ratio decreases
  • Specialist exchange surfaces are required to absorb nutrients and respiratory gases, and remove excretory products
  • A transport system is required to take materials from cells to exchange surfaces and from exchange surfaces to cells
  • As organisms have evolved into larger and more complex structures, the tissues and organs have become more specialised and dependent upon one another
• Surface area to volume ratio

  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
• Features of transport systems


  • A suitable medium in which to carry materials (e.g. blood, air)
  • A form of mass transport in which the transport medium is moved around in bulk over large distances - 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, achieved by muscular contraction or a specialised pumping organ like the heart
  • A mechanism to maintain the mass flow movement in one direction, e.g. valves
  • A means of controlling the flow of the transport medium to suit the changing needs of different parts of the organism
  • A mechanism for the mass flow of water or gases, e.g. intercostal muscles and diaphragm during breathing in mammals
• Circulatory systems in mammals


  • Closed, double circulatory system in which 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, its pressure is reduced, so it is returned to the heart to boost its pressure before being circulated to the rest of the tissues
  • 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
  • The vessels that make up the circulatory system are divided into three types: arteries, veins and capillaries
• 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 distances and there is a steep diffusion gradient
• Mass transport refers to the movement of materials within an organism or between different parts of it.
• The circulatory system is the network of blood vessels that transports oxygen, nutrients, hormones, and waste products throughout the body.
• Blood is a fluid connective tissue that carries out these functions.
• Arteries are thick-walled vessels with smooth muscle fibers that contract and relax to regulate blood flow.
• Arteries carry blood away from the heart under high pressure.
• Veins return blood back to the heart at lower pressures.
• Veins are thinner-walled vessels with less elasticity compared to arteries.
• Capillaries are tiny blood vessels where gas exchange occurs with surrounding tissues.
• Veins have thinner walls than arteries but still contain valves to prevent backflow of blood.
• Capillaries are tiny blood vessels where gas exchange occurs between the bloodstream and tissues.