Final answer:
The extracellular fluid outside the vascular system, comprising about 80% of all ECF, is known as interstitial fluid. This fluid surrounds cells that are not blood cells and plays a vital role in the exchange of nutrients and waste between cells and the bloodstream.
Step-by-step explanation:
The extracellular fluid (ECF) that exists outside the vascular system and comprises approximately 80% of the ECF is called interstitial fluid.
The extracellular fluid is a crucial component of the body's fluids and accounts for one-third of the body's water content. The ECF is divided into two main components, the interstitial fluid and the plasma. The plasma is about 20 percent of the ECF, and it functions as a transporter within blood vessels, delivering cells, proteins, electrolytes, nutrients, gases, and wastes throughout the body. In contrast, the interstitial fluid (IF), which makes up approximately 80% of the ECF, surrounds cells that are not blood cells, providing a medium for the exchange of substances between cells and the blood. It contains various important ions and nutrients necessary for cellular function, and its composition is regulated to maintain homeostasis within the body. The interstitial fluid is separated from cells by a selectively permeable membrane, allowing for the controlled movement of materials.