3.2

• Multicellular animals need a transport system
• All living animal cells need a supply of oxygen and nutrients to grow and survive. They also need to remove waste products so that these do not build up and become toxic.
• Very small animals do not need a separate transport system, because all their cells are surrounded by the environment in which they live. Diffusion will supply enough oxygen and nutrients to keep the cell alive.
• In larger animals the diffusion distance becomes too long, and diffusion alone will be too slow to supply all the requirements.
• Factors that influence the need for a transport system

  • Size
  • Surface area to volume ratio
  • Level of metabolic activity
• Features of a good transport system


  • A fluid to carry nutrients, oxygen and wastes around the body (blood)
  • A pump to create pressure that will push the fluid around the body (heart)
  • Exchange surfaces that enable substances to enter the blood and leave it again where they are needed (capillaries)
• Additional features of an efficient transport system

  • Tubes or vessels to carry the blood by mass flow
  • Two circuits - one to pick up oxygen and another to deliver oxygen to the tissues
• Single circulatory system


  One in which the blood flows through the heart once for each circuit of the body
• Fish have a single circulatory system
• Double circulatory system


  One in which the blood flows through the heart twice for each circuit of the body
• Mammals have a double circulatory system
• Circuits in a double circulatory system

  • Pulmonary circulation (carries blood to the lungs to pick up oxygen)
  • Systemic circulation (carries the oxygen and nutrients around the body to the tissues)
• Advantages of a double circulation


  • Efficient delivery of oxygen and nutrients to where they are needed
  • Ability to increase blood pressure created by the heart to improve flow
• Closed circulatory system


  The blood is enclosed inside blood vessels
• Advantages of a closed circulatory system

  • Higher pressure, so that blood flows more quickly
  • More rapid delivery of oxygen and nutrients
  • More rapid removal of carbon dioxide and other wastes
  • Transport is independent of body movements
• Open circulatory system


  Blood isn't enclosed in blood vessels all the time, it flows freely through the body cavity
• Features of an open circulatory system


  • The heart is segmented and contracts in a wave, pumping blood into a single main artery
  • The artery opens up into the body cavity
  • The blood flows around the insect's organs, gradually making its way back to the heart, supplying the insect's cells with nutrients (not oxygen)
• Types of blood vessels

  • Arteries
  • Arterioles
  • Capillaries
  • Venules
  • Veins
• Arteries


  • Carry blood away from the heart
  • Blood is at high pressure, so the artery wall must be thick to withstand that pressure
  • The lumen is relatively small in order to maintain high pressure
  • Inner wall is folded to allow the lumen to expand as blood flow increases
  • Arteries branch into arterioles
  • Carry oxygenated blood except pulmonary artery
• Arterioles


  • Smaller than arteries
  • Arteriole walls contain a layer of smooth muscle, allowing them to expand or contract thus controlling the amount of blood flowing to tissue
  • Arterioles branch into capillaries
• Capillaries


  • Have very thin walls
  • Allow exchange of materials between the blood and tissue fluid
  • The lumen is very narrow - diameter about the same as a red blood cell
  • The walls consist of a single layer of flattened endothelial cells, reducing the diffusion distance
  • Capillaries connect to venules
• Venules


  • The venule wall consists of thin layers of muscle and elastic tissue outside the endothelium, and a thin outer layer of collagen
  • Venules join together to form veins
• Veins


  • Take blood back to the heart
  • The blood is at low pressure and the walls do not need to be thick
  • The lumen is relatively large, in order to ease the flow of blood
  • The walls have thinner layers of collagen, smooth muscle and elastic tissue than in artery walls
  • Contain valves that stop the backflow of blood and by contraction of the surrounding skeletal muscle applies pressure to the blood, forcing the blood to move along in a direction determined by the valves
  • Carry oxygenated blood except pulmonary veins
• Components of blood


  • Plasma (55%)
  • Cells (45% - red blood cells, white blood cells, platelets)
• Tissue fluid


  Similar to blood plasma but it doesn't contain the cells found in blood, and neither does it contain plasma proteins as they are too large
• Formation of tissue fluid

  1. At the arterial end of the capillary bed, the blood has high hydrostatic pressure compared to the hydrostatic pressure in the tissue fluid, forcing fluid out of the capillary and into spaces around the cells

  2. As fluid leaves, the hydrostatic pressure reduces in the capillary, allowing some tissue fluid to return to the capillary containing waste products

  3. At the venules end of the capillary bed, the water potential in the capillaries is lower than the water potential in tissue fluid due to fluid loss from the capillaries and the high oncotic pressure, allowing some water to re-enter the capillaries from the tissue fluid by osmosis
• Hydrostatic pressure


  The pressure that a fluid exerts when pushing against the sides of a vessel
• Oncotic pressure


  The pressure created by the osmotic effects of the solutes
• The net result of the pressure gradients creates a pressure gradient to push fluid out of the capillary at the arterial end and into the capillary at the venule end.
• Not all of the tissue fluid re-enters the blood. The excess tissue fluid eventually returns to the blood through the lymphatic system - a kind of drainage system made from lymph vessels.
• If a tissue is infected, then the capillaries become more leaky and more fluid is directed into the lymph system — this helps direct bacteria towards the Lymph nodes.
• Lymph vessels have valves to stop lymph going backwards.
• Lymph gradually moves towards the main lymph vessels in the thorax (chest cavity) and returns to the blood.
• Differences between blood, tissue fluid and lymph


  • Red blood cells (too big to get through capillary walls into tissue fluid)
  • White blood cells (most are in lymph system, only enter tissue fluid when there's an infection)
  • Platelets (only present in tissue fluid if capillaries are damaged)
  • Proteins (plasma proteins are too big to get through capillary walls)
  • Water (tissue fluid/lymph have a higher water potential than blood)
  • Dissolved substances (can move freely between each)
• Blood plasma has high hydrostatic pressure and more negative oncotic pressure than tissue fluid.
• Tissue fluid has low hydrostatic pressure and less negative oncotic pressure than blood plasma.
• Lymph has low hydrostatic pressure and less negative oncotic pressure than blood plasma.
• The mammalian heart is a muscular pump. On both sides, the heart squeezes the blood, putting it under pressure. This pressure forces the blood along the arteries and through the circulatory system.
• The heart is divided into two sides. The right side pumps the deoxygenated blood to the lungs to be oxygenated. The left side pumps oxygenated blood to the rest of the body.
• External features of the heart

  • The heart lies just off-centre towards the left of the chest cavity
  • The main part of the heart consists of firm, dark-red muscle called cardiac muscle
  • There are two main pumping chambers - the ventricles. Above the ventricles are two thin-walled chambers - the atria
  • Lying over the surface of the heart are coronary arteries that supply oxygenated blood to the heart muscle
  • At the top of the heart are a number of tubular blood vessels - the veins that carry blood into the atria and the arteries that carry blood away from the heart