[Medical Q&A] Is the “water” in the human body the “water” you imagine?

Planner: Chinese Medical Association

Reviewer: Yu Jianchun, Chief Physician and Professor of Peking Union Medical College Hospital

Water is the source of life. Jellyfish in nature contain more than 95% water in their bodies, and they look like they are made of water. Humans do not look like they are made of water, but about 60% of their body weight is "water".

The "water" here actually refers to body fluids. In the body fluids of a normal adult, about 2/3 (about 40% of body weight) is distributed in cells, which is intracellular fluid; about 1/3 (about 20% of body weight) is distributed outside cells, which is extracellular fluid. In the extracellular fluid, about 3/4 is distributed in the intercellular space, which is interstitial fluid; about 1/4 circulates continuously in the blood vessels, which is plasma. In addition, body fluids also contain a small amount of lymph fluid, cerebrospinal fluid, joint fluid, etc. Different body fluids in the human body are separated from each other, but at the same time they communicate with each other. For example: the cell membrane is a barrier between the intracellular fluid and the interstitial fluid, and at the same time, it is a channel for the two to communicate with each other. Some substances can freely pass through the lipid bilayer structure in the cell membrane, but some substances must pass through the special proteins on the cell membrane to transfer from one side of the membrane to the other side; the capillary wall is a barrier between the plasma and the interstitial fluid, and at the same time, it is a bridge for the two to communicate with each other; plasma is an important medium for communicating between various parts of body fluids and exchanging substances with the external environment, so it is also the most active of all parts of body fluids. Body fluids are an important part of maintaining life activities and undertake multiple physiological functions, such as maintaining body temperature, transporting nutrients, maintaining cell morphology, participating in metabolism, etc. Maintaining water balance through reasonable drinking and diet is crucial to human health.