3.2.4 Tissue Fluid

• Plasma is a straw-coloured liquid that constitutes around 55 % of the blood
• Plasma is largely composed of water (95 %) and because water is a good solvent many substances can dissolve in it, allowing them to be transported around the body
• As blood passes through capillaries some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body
  • This results in the formation of 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
• Tissue fluid bathes almost all the cells of the body that are outside the circulatory system
• Exchange of substances between cells and the blood occurs via the tissue fluid
  • For example, 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
    • This is the pressure exerted by a fluid, e.g. blood
    • The hydrostatic pressure in this example is the blood pressure, generated by the contraction of the heart muscle
  • Oncotic pressure
    • This is the osmotic pressure exerted by plasma proteins within a blood vessel
      • Plasma proteins lower the water potential within the blood vessel, causing water to move into the blood vessel by osmosis

At the arterial end

• When blood is at the arterial end of a capillary the hydrostatic pressure is great enough to force fluid out of the capillary
• Proteins remain in the blood as they are too large to pass through the pores in the capillary wall
• The increased protein content creates a water potential gradient (osmotic pressure) between the capillary and the tissue fluid
• At the arterial end the hydrostatic pressure is greater than the osmotic pressure so the net movement of water is out of the capillaries into the tissue fluid

At the venous end

• At the venous end of the capillary the hydrostatic pressure within the capillary is reduced due to increased distance from the heart and the slowing of blood flow as it passes through the capillaries
• The water potential gradient between the capillary and the tissue fluid remains the same as at the arterial end
• At the venous end the osmotic pressure is greater than the hydrostatic pressure and water begins to flow back into the capillary from the tissue fluid
  • Roughly 90 % of the fluid lost at the arterial end of the capillary is reabsorbed at the venous end
  • The other 10 % remains as tissue fluid and is eventually collected by lymph vessels and returned to the circulatory system
• If blood pressure is high (hypertension) then the pressure at the arterial end is even greater
  • This pushes more fluid out of the capillary and fluid begins to accumulate around the tissues. This is called oedema

At the arterial end the hydrostatic pressure is higher than the osmotic pressure so the tissue fluid is forced out of the capillaries. At the venous end the hydrostatic pressure drops below the osmotic pressure so most of the fluid is drawn back into the capillary.

Formation of lymph

• Some tissue fluid reenters the capillaries while some enters the lymph vessels
• The lymph vessels are separate from the circulatory system
  • They have closed ends and large pores that allow large molecules to pass through

• Larger molecules that are not able to pass through the capillary wall enter the lymphatic system as lymph
  • Small valves in the vessel walls are the entry point to the lymphatic system

• The liquid moves along the larger vessels of this system by compression caused by body movement. Any backflow is prevented by valves
  • This is why people who have been sedentary on planes can experience swollen lower limbs

• The 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

     Any three of the following:

• In plasma there is:
  • A higher concentration of glucose; [1 mark]
  • A higher concentration of glycerol and fatty acids; [1 mark]
  • A higher concentration of amino acids; [1 mark]
  • A higher concentration of plasma proteins; [1 mark]
  • A lower water potential; [1 mark]
  • A higher oxygen concentration; [1 mark]
  • A lower carbon dioxide concentration; [1 mark]

• In tissue fluid there is:
  • A higher concentration of the substances secreted by cells e.g. insulin; [1 mark]

      [Total: 3 marks]