Tissue Fluid

The means by which materials are exchanged between blood and cells.

It is the immediate environment of all cells. Formed from blood plasma whose composition is controlled by various homeostatic systems. Provides a mostly constant environment for the cells it surrounds.

Tissue fluid = Watery liquid containing glucose, amino acids, fatty acids, salts and oxygen - Supplies substances to tissues & receives Carbon dioxide and waste materials from tissues.

Since blood is pumped through arteries, then narrower arterioles, then even narrower capillaries, hydrostatic pressure is created at the arterial end of capillaries. This forces tissue fluid out of the blood plasma. The outward pressure is opposed by two other forces....

  > Hydrostatic pressure of TF outside the capillaries - prevents outward movement of liquid.

  > Lower Water Potential of the blood - due to plasma proteins - pulls water back into the blood within the capillaries.

The combined effect is to create an overall pressure that pushes TF out of the capillaries. Pressure is only enough to force small molecules out of the capillaries - leaving all cells and proteins in the blood = filtration under pressure = ultrafiltration

Once it has exchanged metabolic materials with the cells, TF must be returned to the circulatory system. Most TF returns directly to the blood plasma via capillaries, as follows...

  > Loss of TF from capillaries reduces hydrostatic pressure inside them

  > By the time blood has reached the venous end of capillary network - its hydrostatic pressure is less than that of TF outside it.

  > TF forced back into the capillaries by the higher hydrostatic pressure outside

  > Osmotic forces resulting from the proteins in blood plasma - pull water back into the capillaries

TF has lost most of its oxygen and nutrients but gained carbon dioxide and waste materials. 

Not all the TF can return to the capillaries - some returns via the lymphatic system = system of small vessels beginning in tissues that merge into larger vessels that drain their contents back into the bloodstream via two ducts that join veins close to the heart.

Contents of lymphatic system moved by...

  > Hydrostatic pressure of TF that has left the capillaries

  > Contraction of body muscles - squeeze lympth vessels - valves ensure fluid moves away from tissues towards heart.