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 is forced out (under pressure) 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
- The 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
- This is how all substances are exchanged between cells and their surroundings in large, multicellular organisms
Formation of Tissue Fluid Diagram
Formation of tissue fluid
Examiner Tip
It is important to realise that blood does not bathe cells. Blood stays within the blood vessels. A part of the blood's plasma can fit through the narrow gaps between cells and into the surrounding tissue. There, it bathes the cells, and an exchange of substances between the cell and the tissue fluid can take place.
