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From blood to cells: Do you know how extracellular fluid works?
In cell biology, extracellular fluid (ECF) refers to the body fluid outside the cells of all multicellular organisms. In healthy adults, water accounts for approximately 50-60% of total body weight (ranging from 45% to 75%), with women and obese people generally having a lower ratio than lean men. Extracellular fluid accounts for about one-third of the body's fluid, and the remaining two-thirds is intracellular fluid. The main component of extracellular fluid is the interstitial fluid that surrounds the cells.
Extracellular fluid is considered the internal environment of all multicellular animals. For animals with a circulatory system, this fluid also includes plasma.
Plasma and interstitial fluid constitute at least 97% of the extracellular fluid. Lymphatic fluid makes up a small portion of the interstitial fluid. The remaining small amount of extracellular fluid includes about 2.5% of transcellular fluid. The extracellular fluid can also be viewed as having two parts: plasma and lymph, which supply the system, and the interstitial fluid, which is used to exchange water and solutes with the cells.
Interstitial fluid, the extracellular fluid, forms the body's internal environment, bathes all cells and is essential for their normal function. The composition of the extracellular fluid must be regulated by a variety of homeostatic mechanisms, including negative feedback mechanisms. These homeostatic processes regulate the pH, sodium, potassium, and calcium concentrations of the extracellular fluid, while also stabilizing body fluid volume, blood glucose, oxygen, and carbon dioxide levels.
Components of extracellular fluid
The main component of extracellular fluid is interstitial fluid, also known as tissue fluid, which is located between blood vessels and cells. Another major component of extracellular fluid is plasma, which is the fluid of the circulatory system. A small amount of extracellular fluid includes transcellular fluid. These components are often referred to as "fluid compartments".
In a 70 kg young male, the extracellular fluid accounts for about 20% of the body weight, which is about fourteen liters. Of these fluids, interstitial fluid accounts for 11 liters and plasma accounts for 3 liters.
Function of interstitial fluid
Interstitial fluid is similar in nature to plasma and makes up the majority of extracellular fluid. Its composition varies with different tissues and exchanges between cells in biological tissues. The contents of the interstitial fluid are subject to a variety of influences, including the diffusion of nutrients and the discharge of waste products resulting from cellular metabolism.
Transcellular fluid
On the other hand, transcellular fluid is formed by the transport activities of the cell and is the smallest component of the extracellular fluid. It includes cerebrospinal fluid, aqueous humor in the eyes, serous fluid in the body cavity, and anterior ear fluid and inner ear fluid in the inner ear. The composition of the transcellular fluid varies dramatically depending on its location.
Function of extracellular fluid
Extracellular fluid provides a medium for the exchange of substances, which can be dissolved, mixed and transported in the liquid medium. Substances in the extracellular fluid include dissolved gases, nutrients, and electrolytes, which are necessary to sustain life.
In addition, the extracellular fluid contains substances secreted from the cells that exist in dissolved form but quickly condense into fibers or precipitate in solid or semisolid form.
Oxygen Exchange
One of the main functions of extracellular fluid is to facilitate the exchange of molecular oxygen from the blood to tissue cells, and the return of carbon dioxide produced by cell mitochondria to the blood. Carbon dioxide is about 20 times more soluble in water than oxygen, so it can diffuse between cells and blood relatively easily.
Regulation of Extracellular FluidThe stability of the internal environment comes from the operation of the homeostatic mechanism. Many complex homeostatic mechanisms regulate and maintain the composition of the extracellular fluid. The sodium-potassium pump in the cell membrane pushes sodium ions out of the cell and lets potassium ions in. This difference in ion concentration is essential for maintaining a stable cell volume and for some cells to generate action potentials.
Interactions between plasma, interstitial fluid and lymph
Arterial plasma, interstitial fluid, and lymph interact at the level of the vascular capillaries. The permeability of capillaries allows water to flow in and out freely. At the arterial end of the capillaries, blood pressure is higher than the hydrostatic pressure of the tissues, so water leaks from the capillaries into the interstitial fluid.
This series of processes ensures that the fluid environment in which the body's cells are located is always close to ideal conditions, and these exchanges will continue as long as the homeostatic mechanisms are functioning properly.
The composition and function of extracellular fluid is not just the simple flow of body fluids. Behind it is the organism's deep understanding of physiological homeostasis and countless global and biochemical regulatory processes. Ultimately, it's all about keeping your cells and the entire organism functioning healthily. Have you ever thought deeply about the role that extracellular fluid plays in your body?
