Water is distributed across various internal spaces, known as fluid compartments. The majority of this fluid is held inside the cells (intracellular fluid). Fluid outside the cells (extracellular fluid) is subdivided into plasma and a second fluid. This second fluid directly surrounds and bathes every cell, serving as the immediate environment for all cellular activity.
The Answer Interstitial Fluid
The term that describes the water residing between cells is Interstitial Fluid (IF). The word “interstitial” is derived from the Latin for “standing between,” reflecting its location. This fluid occupies the microscopic spaces, or interstitium, within tissues, outside of both the blood vessels and the lymphatic vessels. Interstitial fluid is a major subdivision of the extracellular fluid, making up about 75% of the total fluid found outside the cells.
What Interstitial Fluid Is Made Of
Interstitial fluid is fundamentally an ultrafiltrate of blood plasma. This process is driven by hydrostatic pressure, the force exerted by the blood against the capillary walls, which pushes fluid out into the tissue spaces. The composition includes electrolytes like sodium and chloride, along with nutrients such as glucose, amino acids, and fatty acids. Waste products like carbon dioxide and urea also enter the fluid from the surrounding cells. A distinguishing feature of interstitial fluid is its low protein concentration compared to plasma, because large proteins, such as albumin, are generally too big to easily pass through the narrow pores of the capillary walls and are retained in the bloodstream.
Essential Role in Cellular Exchange
The primary purpose of interstitial fluid is to act as the necessary transport medium between the blood and the individual cells. Cells cannot directly absorb substances from the blood, so they rely on the fluid that surrounds them. As blood flows through the capillaries, oxygen and nutrients diffuse out into the interstitial fluid. This fluid then delivers these substances across the cell membrane to sustain the cell’s metabolic needs. Concurrently, the fluid collects metabolic waste products, such as carbon dioxide, which diffuse out of the cell and into the interstitial space.
How Interstitial Fluid Becomes Lymph
Fluid dynamics ensure that slightly more fluid filters out of the blood vessels than is reabsorbed back into them. The circulatory system reabsorbs about 90% of the fluid, leaving an excess volume in the interstitial spaces; if this excess accumulated, it would cause tissue swelling, known as edema. The lymphatic system collects this remaining fluid, and once it enters the specialized lymphatic capillaries, it is renamed lymph. The lymphatic vessels have unique, overlapping endothelial cells that act as one-way valves, allowing fluid, proteins, and cellular debris to enter before the lymph is transported, filtered by lymph nodes, and eventually returned to the bloodstream.