Capillaries & Tissue Fluid (AQA AS Biology)

• Capillaries are another type of blood vessel present in the circulatory system
• They have thin walls which are “leaky”, allowing substances to leave the blood to reach the body’s tissues
• They can form networks called capillary beds which are very important exchange surfaces within the circulatory system

Structure and function of capillaries

• They have a very small diameter (lumen)
  • This forces the blood to travel slowly which provides more opportunity for diffusion to occur

• Capillaries branch between cells
  • Substances can diffuse between the blood and cells quickly as there is a short diffusion distance

• The wall of the capillary is made solely from a single layer of endothelial cells (this layer is also found lining the lumen in arteries and veins)
  • The wall is only one cell thick – this reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body
  • The cells of the wall have gaps called pores which allow blood plasma to leak out and form tissue fluid

The structure of a capillary

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 virtually the same, 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 outside of 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 diffuse into the capillary

Formation of tissue fluid

• How much liquid leaves the plasma to form tissue fluid depends on two opposing forces
• When blood is at the arteriole end of a capillary, the hydrostatic pressure is great enough to push molecules out of the capillary
• Proteins remain in the blood; the increased protein content creates a water potential between the capillary and the tissue fluid
  • However, overall movement of water is out from the capillaries into the tissue fluid

• At the venule end of the capillary, less fluid is pushed out of the capillary as pressure within the capillary is reduced
• The water potential gradient between the capillary and the tissue fluid remains the same as at the arteriole 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 arteriole end is even greater
  • This pushes more fluid out of the capillary and fluid begins to accumulate around the tissues. This is called oedema

Formation of tissue fluid

Formation of lymph

• Some tissue fluid reenters the capillaries while some enters the lymph capillaries
• The lymph capillaries 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