Tissue Fluid

• 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
• Formation of Tissue Fluid

  1. Blood passes through capillaries

  2. Plasma leaks out through gaps in the walls of the capillary

  3. Surrounds the cells of the body

  4. Results in the formation of tissue fluid
• Tissue Fluid


  The composition of plasma and tissue fluid are very similar, although tissue fluid 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 that are outside the circulatory system
• Exchange of substances between cells and the blood

  1. Occurs via the tissue fluid

  2. Carbon dioxide produced in aerobic respiration will leave a cell, dissolve into the tissue fluid surrounding it, and then move into the capillary
• Tissue Fluid Formation


  • The volume of liquid that leaves the plasma to form tissue fluid depends on two opposing forces: Hydrostatic pressure and Oncotic pressure
• Hydrostatic Pressure


  The pressure exerted by a fluid, e.g. blood pressure generated by the contraction of the heart muscle
• Oncotic Pressure

  The osmotic pressure exerted by plasma proteins within a blood vessel, which lowers the water potential within the blood vessel, causing water to move into the blood vessel by osmosis
• Tissue Fluid Formation at Arterial End

  1. Hydrostatic pressure is greater than osmotic pressure, so net movement of water is out of the capillaries into the tissue fluid

  2. Proteins remain in the blood as they are too large to pass through the pores in the capillary wall

  3. Increased protein content creates a water potential gradient between the capillary and the tissue fluid
• Tissue Fluid Formation at Venous End

  1. Hydrostatic pressure is reduced due to increased distance from the heart and slowing of blood flow

  2. Osmotic pressure is greater than hydrostatic pressure, so water begins to flow back into the capillary from the tissue fluid

  3. Roughly 90% of the fluid lost at the arterial end is reabsorbed at the venous end
• The other 10 % of the fluid lost at the arterial end remains as tissue fluid and is eventually collected by lymph vessels and returned to the circulatory system
• If blood pressure is high (hypertension)


  More fluid is pushed out of the capillary, causing fluid to accumulate around the tissues (oedema)
• Formation of Lymph


  1. Some tissue fluid reenters the capillaries while some enters the lymph vessels

  2. Lymph vessels have closed ends and large pores that allow large molecules to pass through

  3. Larger molecules that are not able to pass through the capillary wall enter the lymphatic system as lymph

  4. Liquid moves along the larger vessels of this system by compression caused by body movement, with valves preventing backflow

  5. Lymph eventually reenters the bloodstream through veins located close to the heart
• Any plasma proteins that have escaped from the blood are returned to the blood via the lymph capillaries
• If plasma proteins were not removed from tissue fluid they could lower the water potential (of the tissue fluid) and prevent the reabsorption of water into the blood in the capillaries
• After digestion, lipids are transported from the intestines to the bloodstream by the lymph system