exchange at the capillaries

• Blood plasma + Fluid 1

  • consists of a liquid, plasma containing many red blood cells
  • containing many dissolved substances including minerals, oxygen, carbon dioxide, glucose, amino acids + plasma protein
  • the cells include the erythrocytes, leucocytes + fragments of platelets
• Blood plasma + fluid 2

  • tissue fluid doesn’t contain most cells found in plasma + neither plasma protein
  • tissue fluid is formed by blood plasma as it leaks from capillaries

  →it surrounds cells tissues resourcing them with sufficient oxygen + nutrients

  →as it leaks it transports also dissolved substances into tissue fluid

  (movement is mass flow rather than diffusion)

  →waste products from cell metabolism will be carried back to capillary as some of the tissue fluid returns too
• Formation of tissue fluid 1

  • when an artery reaches the tissues, it branches into arterioles + network capillaries

  →eventually link up with venules to carry blood back to veins

  therefore blood flowing into organ or tissue is contained in capillaries

  • at the arterial end of the capillary, there is high (hydrostatic) pressure

  →this tends to push the blood out of capillary through capillary walls

  →the fluid can leave through tiny gaps between cells
• Formation of tissue fluid 2

  • fluid that leaves blood consists of plasma, nutrients + oxygen

  →all red blood cells + most white blood cells remain in blood, including plasma protein, as they’re too large to be pushed out

  • tissue fluid surrounds cells so exchange of gases + nutrients can occur across plasma membrane

  →exchange occurs by diffusion, facilitated diffusion + active uptake (oxygen + nutrients in, carbon dioxide + waste out)
• Returning to blood

  • the blood pressure at the venous end of capillary is low

  →allowing some of tissue fluid carrying carbon dioxide + waste to enter blood

  Not all tissue fluid go into blood

  • some is directed to another tubular system called lymphatic system
  • fluid in lymphatic system is lymph + is similar to the composition of tissue fluid

  →it will contain more lymphocytes as they’re produced in lymph nodes
• lymph - excess fluid
• lymph nodes - swelling found at intervals along the lymphatifc system + has an important part to play in the immune system

  } if a tissue is infected, the walls of capillary become more leaky so it’s directed to lymphatic system - helping direct bacteria into lymph nodes