Tissue Fluid and Lymph

• Plasma is a straw-coloured liquid that constitutes around 55% of the blood.
• Plasma is largely composed of water (95%). 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.
• Plasma and tissue fluid are both extracellular fluids that contain water, ions and solutes. Tissue fluid contains far fewer proteins because proteins are too large to fit through gaps in the capillary walls.
• Tissue fluid bathes almost all the cells of the body outside of the circulatory system.
• The exchange of substances between the blood and cells occurs via tissue fluid. For example, carbon dioxide produced in aerobic respiration will leave a cell, dissolve into the tissue fluid surrounding it, and then diffuse into a capillary.
• The amount of liquid that leaves the plasma to form tissue fluid depends on two opposing forces:

  1. Hydrostatic pressure - this is the pressure exerted by a fluid (blood)
  2. Oncotic pressure - this is the osmotic pressure exerted by plasma proteins within a blood vessel. Osmotic pressure usually pulls water into the circulatory system
• When blood is at the arterial end of a capillary, the hydrostatic pressure is great enough to force fluid out of the capillary into the interstitial spaces between cells. 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. The high 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 of the capillary, the hydrostatic pressure within the capillary is reduced. The water potential gradient between the capillary and the tissue fluid remains the same as at the arterial end, so water begins to flow back into the capillary from the tissue fluid. Overall, more fluid leaves the capillary than returns, leaving tissue fluid behind to bathe cells. 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.
• In plasma there is:

  • A higher concentration of glucose
  • A higher concentration of glycerol and fatty acids
  • A higher concentration of amino acids
  • A higher concentration of plasma proteins
  • A lower water potential
  • A higher oxygen concentration
  • A lower carbon dioxide concentration

  In tissue fluid there is:

  A higher concentration of the substances secreted by cells e.g. insulin
• 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.
• The lymph capillaries are separate from the circulatory system. They have closed ends and large pores that allow large molecules to pass through.
• Lymph travels along along lymphatic vessels by compression caused by body movement (any backflow is prevented by valves). This is why people who sit down for long periods of time can experience swollen lower limbs.
• Lymph eventually re-enters 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. This is an important step - if plasma proteins are not removed from tissue fluid they can lower the water potential of the tissue fluid and prevent the reabsorption of water into the blood in the capillaries.