Tissue Fluid (HL) (HL IB Biology)

• 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 is forced 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 fewer proteins and cells
  • Proteins are too large to fit through gaps in the capillary walls and so remain in the blood
  • Red blood cells and platelets are not present in the tissue fluid as they are also too large to leave the capillaries
• 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 back into the capillary

The effect of blood pressure on tissue fluid

Tissue fluid formation at the arterial end

• When blood is at the arterial end of a capillary the blood pressure is high enough to force fluid out of the capillary
• The process by which tissue fluid is forced out of the capillaries at high pressure is known as pressure filtration

Reuptake of tissue fluid at the venous end

• At the venous end of the capillary the blood 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
• Tissue fluid drains back into the capillaries from the tissues
  • 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 vessels known as lymph vessels
    • From the lymph vessels the fluid is eventually returned to the circulatory system

The effect of solute concentration on tissue fluid

• Pressure filtration occurs due to the outward force exerted by high blood pressure at the arterial end of capillaries, but high blood pressure isn't the only factor that affects tissue fluid: 
  • After pressure filtration has occurred, proteins remain in the blood as they are too large to pass through the pores in the capillary wall
  • The increased protein concentration of the capillary creates a solute gradient between the capillary and the tissue fluid
    • The solutes in this case are the proteins in the blood
  • The solute gradient exerts an inward pull on the tissue fluid which draws water back into the capillary
    • This is due to osmosis, which is the movement of water from a dilute solution (the tissue fluid) to a solution with a higher solute concentration (the capillary)
• At the arterial end the outward force generated by the high blood pressure is greater than the inward force of the solute gradient so the net movement of water is out of the capillaries into the tissue fluid
• At the venous end the inward force of the solute gradient is greater than the outward force of the blood pressure and fluid moves back into the capillaries

At the arterial end the hydrostatic pressure (blood pressure) has a greater effect than the solute gradient so the tissue fluid is forced out of the capillaries

At the venous end the hydrostatic pressure has less effect than the solute gradient so most of the fluid is drawn back into the capillary

• Blood plasma and tissue fluid are similar in their composition, although there are some differences between them
  • Water and solutes form an important part of both plasma and tissue fluid
  • Substances that are too large to filter out through the capillary wall, such as plasma proteins and red blood cells will not be found in tissue fluid
  • Some white blood cells, which are very flexible cells, are able to squeeze out of the capillaries, so are present in the tissue fluid
  • Substances such as glucose, amino acids and oxygen are absorbed by cells, so tissue fluid will contain less of these substances
    • Glucose and oxygen are requirements of aerobic respiration while amino acids are needed for protein synthesis
  • Cells produce carbon dioxide as a waste product during cell respiration which will diffuse into the tissue fluid

Composition of blood plasma and tissue fluid table

Formation of lymph

• Some tissue fluid re-enters the capillaries while some enters the lymph capillaries
• The lymph capillaries are separate from the circulatory system and form part of the lymphatic system
  • They have closed ends and large pores that allow large molecules to pass through
  • They contain fluid known as lymph
• Larger molecules that are not able to pass through the capillary wall enter the lymphatic system
  • Small valves in the vessel walls are the entry point to the lymphatic system
• Lymph fluid moves along the larger vessels of this system by compression caused by body movement. Any backflow is prevented by valves
  • These larger vessels are known as lymph ducts 
  • People who have been sedentary on planes may experience swollen lower limbs due to the lack of movement which causes lymph to accumulate in these parts
• Lymph nodes are present in the lymph ducts
  • These contain macrophages which will engulf bacteria present in the lymph
  • Other cells of the immune system can also be found in the lymph nodes
• The 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
  • If plasma proteins are not removed from tissue fluid they could increase the solute concentration 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, so lymph fluid contains lipids

Tissue fluid and lymph vessels diagram

Some components of tissue fluid drain into the lymph vessels