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. Interstitial fluid makes up the 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.

  • Extracellular Fluid – Fluids that surround cells within the body.
  • Blood Plasma – The extracellular fluid that surrounds blood molecules in the vessels of the circulatory system.
  • Transcellular Fluid – Fluids contained within spaces that are “external” to the body, such as the urine, joint fluid, and ocular fluid.
  • Lymphatic System – A series of vessels that collects and redistributes interstitial fluid to the blood.


1. Oxygen binds to special proteins on the surface of red blood cells. When the blood cells reach an area in the capillaries of low oxygen concentration, the oxygen is releases. What role does interstitial fluid play in oxygen transportation?
A. None
B. It allows oxygen to diffuse to the cells
C. It actively binds oxygen and exchanges waste from the cells

Answer to Question #1
B is correct. Once oxygen is released in the capillaries, it must travel through the blood vessel wall and into the interstitial fluid. Oxygen is a polar molecule, and will diffuse naturally in a water-based solution. Interstitial fluid is present between most of the cells in the body, and allows the capillaries to only have to dump oxygen in specific areas, which can then be diffused through the tissue fluid to the cells.

2. What is the difference between lymph, extracellular fluid, plasma, and interstitial fluid?
A. There is none
B. The difference is in their location
C. The difference is their basic composition

Answer to Question #2
B is correct. These fluids are all similar in their basic composition. Water, small ions, and other small solutes cannot be excluded completely by the cell membranes of cells in the body. This means that over time, whether forced by pressure or not, a large volume of fluid would stray from where it is needed. This basic composition is added to by the vessels and location of each fluid. Lymph is in the lymph vessels, plasma in the blood vessels, and interstitial fluid between the cells. Extracellular fluid refers to all of the above, as any fluid that exists within an organism and outside of the cytoplasm.