The Circulatory System

• Plasma


  A straw-coloured liquid that constitutes around 55% of the blood
• Plasma


  • Largely composed of water (95%)
  • Many substances can dissolve in it, allowing them to be transported around the body
• Plasma leakage


  1. Blood passes through capillaries

  2. Some plasma leaks out through gaps in the walls of the capillary

  3. Surrounds the cells of the body

  4. Tissue fluid formation
• Tissue fluid


  Composition is virtually the same as plasma, although it contains far fewer proteins
• Proteins are too large to fit through gaps in the capillary walls and so remain in the blood</b>
• Tissue fluid bathes almost all the cells of the body outside of the circulatory system
• Exchange of substances between cells and the blood occurs via the tissue fluid
• Hydrostatic pressure


  Pressure that fluid exerts when pushing against the side of a vessel
• Oncotic pressure

  Pressure created by the osmotic effects of the solutes
• Fluid exchange between plasma and tissue fluid


  1. At arteriole end of capillary, hydrostatic pressure pushes molecules out of capillary

  2. Proteins remain in blood, increasing protein content and creating water potential difference

  3. At venous end, oncotic pressure greater than hydrostatic pressure, water flows back into capillary

  4. 90% of fluid lost at arterial end reabsorbed at venous end, 10% becomes tissue fluid
• Excess tissue fluid is drained by the lymphatic capillaries
• Lymphatic system


  1. Lymph capillaries collect fluid and return it to the circulatory system as lymph vessels

  2. Larger molecules that can't pass through capillary wall enter lymphatic system as lymph

  3. Liquid moves along larger lymphatic vessels by compression caused by body movement
• If blood pressure is high (hypertension)


  More fluid pushed out of capillary, fluid accumulates around tissues (oedema)
• Plasma proteins


  Exert oncotic pressure within a blood vessel
• Plasma proteins lower the water potential within the blood vessel


  Causing water to move into the blood vessel by osmosis
• Water potential of the capillaries becomes more negative


  Causes water to move down the water potential gradient, from the tissue fluid to blood via osmosis
• At the venous end of the capillary


  Oncotic pressure becomes greater than hydrostatic pressure, leading to water flow back into the capillary from the tissue fluid
• Roughly 90% of the fluid lost at the arterial end is reabsorbed at the venous end, while the remaining 10% becomes tissue fluid
• Single closed circulatory system

  Consists of one circuit from the heart, the blood passes through the heart once per cycle and there is only one circuit that the blood takes
• Single closed circulatory system

  • Fish
• Single closed circulatory system in fish


  1. Blood passes through two sets of capillaries immediately after being pumped out of the heart

  2. Blood flows through capillaries in the gills to become oxygenated

  3. Blood flows through capillaries delivering the blood to the body before returning it back to the heart
• Single closed circulatory system would not enable efficient gas exchange for mammals but it does work for fish because they have that counter current flow mechanism
• Double closed circulatory system

  The blood passes through the heart twice per cycle and there are two separate circuits the blood would take
• Circuits in double closed circulatory system


  • Blood vessels carrying blood from the heart to the lungs for gas exchange (pulmonary circuit)
  • Blood vessels carrying the blood from the heart to the rest of the body to deliver oxygen, nutrients and collect waste (systemic circuit)
• Closed circulatory system


  The blood stays within blood vessels, kept separate from body tissues and doesn't come directly in contact with cells, the transport medium is blood and it always remains inside of blood vessels
• Closed circulatory system


  • Gas and small molecules can leave the blood by diffusion or due to high hydrostatic pressure
  • The blood is pumped under pressure and molecules leave through walls of the blood vessels
  • The amount of blood flowing can be adjusted by widening or narrowing the blood vessels
• Open circulatory system


  The transport medium, the hemolymph or blood, is usually pumped directly to the body cavity called the haemocoll from the heart, there are few transport vessels so the transport mediums are pumped at low pressure, food and nitrogenous waste will be transported but not gases which are instead transported via their tracheal system, once exchange has taken place at the cells and tissues the transport medium returns the heart through an open-ended vessel
• Open circulatory system


  • Invertebrates like insects
• Arteries


  Carry blood away from the heart to the rest of the body
• Arteries


  • Outer layer made of connective tissue with fibres of collagen and elastin
  • Elastin fibres allow arteries to stretch and recoil as the heart beats
  • Inner lining (endothelium) is folded
• Artery structure


  • Elastic tissue in the walls
  • Thick muscle layer
  • Smaller lumen (space in centre)
  • Folded endothelium
• Arterioles


  Smaller vessels that distribute blood from arteries to capillaries
• Arterioles


  • Muscles inside contract to restrict blood flow or relax to allow full blood flow
  • Rings of smooth muscle in the arteriole wall
• Capillaries


  Smallest blood vessels that are the site of molecule exchange between blood and tissue fluid
• Capillaries


  • Narrow diameter of 7µm
  • One cell thick
• Capillaries have leaky walls that allow plasma and dissolved substances to leave the blood
• Venules

  Allow deoxygenated blood to return from capillary beds to larger blood vessels called veins
• Venules


  • Range from 8 to 100μm in diameter
  • Formed when capillaries come together
  • Wall consists of thin layers of muscle, elastin and collagen
• Veins


  Take blood back to the heart under low pressure
• Veins


  • Wider lumen with little elastic or muscle tissue
  • Outer layer of connective tissue with fibres of collagen
  • Thin wall with little elastic fibres and smooth muscle
  • Blood flow helped by contraction of body muscles
  • Have pocket valves that prevent backflow