Blood, tissue fluid and lymph

• Plasma

  Carries dissolved glucose, amino acids, mineral ions, hormones and large plasma proteins albumin (maintaining osmotic potential), fibrinogen (for blood clotting) and globulins (transport and the immune system) Red blood cells, white blood cells, platelets.
• Functions of the blood

  Transport of:

  • Oxygen to and carbon dioxide from the respiring cells.
  • Digested food from small intestine.
  • Nitrogenous waste products from cells to excretory organs.
  • Hormones.
  • Food molecules from storage compounds to cells.
  • Platelets to damaged areas.
  • Cell and antibodies.
• Oncotic pressure

  Substances dissolved in plasma can pass through the fenestrations in the capillary walls, except large plasma proteins.

  • The plasma proteins have an osmotic effect.
  • They give blood in capillaries a relatively high solute potential compared with surrounding fluid.
  • So water has a tendancy to move into the blood in capillaries from surround fluid by osmosis= oncotic pressure.
• Hydrostatic pressure

  = As blood flows through arterioles into capillaries it's still under pressure from surge of blood from contractions.

  • At arteriole end capillary the hydrostatic pressure forcing fluid out of the capillaries is relatively high.
  • It is higher than the oncotic pressure attracting water in, so fluid is squeezed out capillaries.
  • This fluid fills the spaces between the cells and is called tissue fluid.
  • Has same composition as plasma but without red blood cells and plasma proteins.
  • Diffusion takes place between blood and cells through tissue fluid.
• Tisue fluid

  As blood moves through capillaries towards venous system the balance of forces changes.

  • Hydrostatic pressure falls in the vessels as fluid has moved out and the pulse is lost.
  • The oncotic pressure is stronger than the hydrostatic pressure, so water moves back into capillaries by osmosis as it approaches venous end of the capillaries.
• Lymph

  Some of the tissue fluid does not return to capillaries, 10% of the liquid that leaves the blood vessels drains into a system of blind-ended tubes called lymph capillaries= lymph.

  • Lymph has a similar composition to tissue fluid, has less oxygen. It contains fatty acids, which have been absorbed into lymph from small intestine.
  • The fluid is transported through larger vessels by the squeezing of body muscles.
  • One way valves prevents backflow of lymph.
  • Lymph then returns to the blood, into right and left subclavian veins.
• Lymph nodes

  Lymphocytes build up in the lymph nodes to produce antibodies, which are then passed into the blood.

  Also intercept bacteria and other debris from lymph, which are ingested by phagocytes.