Interstitial Fluid Definition
Interstitial fluid, or simply tissue fluid, is a mixture of water, ions, and small solutes that are forced out of the blood plasma by the systolic pressure created when the heart pumps. Plasma is a mixture of water and many other constituents, which carry blood cells and oxygen to various parts of the body.
The interstitial fluid makes up a large part of the extracellular fluid in organisms. As the heart enters the systole, or contracting phase it exerts a large pressure on the arteries in the circulatory system. These arteries, all the way down to the smallest capillaries, swell with pressure. Because the vessels are created by a series of cells, there exist small gaps between the many cells that make up a vessel, and some water and solutes can leak out.
Between the cells of the body, this fluid is known collectively as interstitial fluid. If there were no mechanism to remove it, parts of the body would swell up with pressure. Luckily, the lymphatic system is a network of vessels and tissues that actively removes the tissue fluid from tissues, and returns it to the blood plasma.
Once in the lymph vessels, the fluid contains many other cells and substances which aid in the immune response, by allowing white blood cells to find and digest harmful bacteria and virus-infected cells. Many cells also remove their metabolic wastes into the interstitial fluid, and the wastes are cleaned through the lymphatic system.