Tissue Fluid

• Cells are bathed in tissue fluid.
• Substances are exchanged between tissue fluid and cells.
• Tissue fluid is constantly being produced by the capillaries, which filter out substances from blood plasma.
• At the arteriole end of a capillary, there is a higher blood pressure (due to contraction of left ventricle) which means small molecules (water + glucose + oxygen) leak out of the capillary.
• Blood pressure drops along the length of the capillary due to friction of red blood cells against capillary wall, which slows the blood down.
• There is a lower water potential outside of the capillary as water leaks out, leaving proteins and blood cells.
• At the venule end of the capillary, there is a lower blood pressure.
• Most small molecules return to the capillary after leaking out due to osmosis/diffusion as there is a higher water potential/concentration in the tissue fluid, it travels down the concentration/water potential gradient.
• Not all leaked out tissue fluid is reabsorbed into the capillary - it is the job of the Lymphatic System to drain it and take it back to the circulatory system.
• Explain how a low protein diet or starvation results in swelling.


  Less protein in the blood will increase the water potential in the capillary, so this decreases the water potential gradient between the tissue fluid and the capillary, so less water is absorbed.
• Explain how a high blood pressure results in oedema (swelling).

  High blood pressure causes more water to leak out of the capillary at the arterial end and the pressure is higher at the venule end, so tissue cannot return to the capillary as the pressure gradient is reduced.