Tissue fluid

• Tissue fluid is the fluid that surrounds cells
• Tissue fluid is made up of the substances small enough to move out of the capillary. It also contains the waste products from cells
• Waste products from cells found in tissue fluid include carbon dioxide, water and urea
• Substances from capillaries found in tissue fluid include oxygen, water, glucose and mineral ions
• How much liquid leaves the capillaries to form tissue fluid depend on two opposing forces: hydrostatic pressure and water potential
• At the start of the capillary bed (the arteriole end), the hydrostatic pressure inside the capillaries is greater. Therefore the pressure forces fluid out of the capillaries and into the spaces around the cells
• Tissue fluid transfers molecules such as oxygen and glucose to the tissue cells. Waste molecules such as carbon dioxide pass from the tissue cells into the tissue fluid
• At the venous end of the capillary bed, hydrostatic pressure is much lower due to all the fluid that has been forced out of the blood. This means there is also a low water potential. This is because there are still plasma proteins in the blood (as they are too large to move out of the capillary). This causes water to move back into the blood by osmosis
• Around 90% of tissue fluid diffuses back into the blood
• The tissue fluid that does not diffuse back into the blood drains into a series of blind-ended vessels called lymph capillaries
• Lymph capillaries connect into larger lymph vessels and form the lymphatic system
• Eventually the lymph fluid returns to the bloodstream via blood vessels under the collar bone
• 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
• Plasma is a straw coloured liquid that constitutes about 55% of the blood
• Plasma is mostly composed of water. Because water is a good solvent, many substances can dissolve in it, allowing them to be transported around the body
• The composition of plasma and tissue fluid are very similar, but plasma contains many more proteins
• Lymph capillaries have closed ends and large pores that allow large molecules to pass through. Larger molecules not able to pass through the capillary wall enter the lymphatic system as lymph
• Lymph fluid moves along lymph vessels by compression caused by body movement. Sedentary people can experience swollen lower limbs
• Backflow of lymph is prevented by valves
• After digestion lipids are transported from the intestine to the bloodstream by the lymph system