Why do humans need water?

Maintaining cell volume and shape

About 70 to 80 percent of the volume of animal cells is composed of water. Water moves in and out of cells by osmosis, generally from areas with more water to areas with less water, through specialized channels in the cell membrane called aquaporins. This movement balances cellular fluid levels and maintains cell volume and shape, preventing the cell from shrinking or swelling. Maintaining a correct cell volume is crucial for nerve impulse conduction; cellular processes including the cell cycle, cell migration, and cell death; and transport of nutrients and minerals across surface epithelial tissues. When dehydrated, not only are these vital cellular processes impaired, but the cell itself may be physically damaged because of the loss of volume.

Enabling molecular and ionic transport

As a polar solvent, water dissolves a range of substances including inorganic salts, ions, sugars, amino acids, and several gases, which have vital roles in cellular metabolism and respiration. These substances are transported dissolved in blood plasma, 90 to 92 percent of which is water, to organs and tissues. On reaching tissues, the substances leave circulation and may cross cell membranes through diffusion or specific membrane transport mechanisms. Within cells, many of these substances enter the aqueous cytosol, where numerous metabolic reactions take place. As an essential component of the body’s transportation system, water also helps to move around excretory and waste products for the body to expel them. In the absence of this transport, human body cells would starve and metabolic byproducts would accumulate in the body, often causing toxicity.

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Providing a medium and reactant for biochemical reactions

Several properties of water make it an ideal medium for biochemical and metabolic reactions. Water is amphoteric—meaning it can act as a base by acquiring a proton or as an acid by giving up a proton, depending on the substances with which it interacts. Since water is a polar molecule, it can interact with and dissolve many polar and charged substances. It has a high dielectric constant, which means that it weakens the electrostatic attraction between positively and negatively charged ions. Water molecules also surround and interact with individual ions, which stabilizes them and keeps them separated in solution. Once dissolved, these substances can move through the aqueous environment of cells, come into contact with other molecules, and participate in biochemical reactions including those involved in energy production and waste processing.

Water also interacts closely with proteins, including enzymes. Water molecules form dynamic hydrogen-bonding networks with proteins—networks of bonds that form, rearrange, and break easily. These interactions help proteins fold into and maintain the proper shapes they need to function. They also allow them to change shape when necessary. Enzymes, on the other hand, use such interactions to recognize the molecules on which they act—called substrates—and carry out biochemical reactions that are fundamental for survival, including digestion, respiration, and even blood clotting.

In addition to facilitating biochemical reactions, water can directly participate in them. Most notable among such reactions is hydrolysis—the process in which digestive enzymes with the help of water break nutrients such as starches and proteins into smaller units that the intestine can absorb. Water is also involved in biological electron-transfer processes, in which electrons move from one cell to another, driving energy production for processes such as muscle contraction, neural signaling, and thermoregulation.

Regulating body temperature

The average human body temperature is 98.6 °F (37 °C). Body temperatures above 105.8 °F (41 °C) and below 95 °F (35 °C) are detrimental to normal body functions. Water has a high heat capacity—it can store a large amount of heat without undergoing extreme temperature increase—and a high heat of vaporization. These properties allow water to provide the body with a thermal buffer against extreme temperature variations. Water is also an ideal medium for redistributing body heat as a component of blood plasma. In response to signals from the brain’s thermoregulatory center, the blood either carries heat away from the inner organs to be dissipated via the skin or conserves internal body heat away from the skin, thereby controlling body temperature. Evaporation of water in the form of sweat also helps to cool the body down. In the absence of water’s thermoregulatory effects, muscles and the central nervous system, among others, would cease to function properly.

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Supporting lubrication, mechanical protection, and innate immune defense

Water is the main component of several body fluids. Mucus—the slimy gel-like substance lining the stomach, intestine, lungs, eyes, and urinary and genital tracts—is chemically more than 90 percent water. A vital function of mucus is to lubricate the surfaces it covers; depending on the organ, this function helps in the smooth movement of nutrients or expulsion of waste products. As a lining of the nose, mucus helps to trap external irritants. It also contains a host of molecules with immune functions as well as some immune cells, and, as an integral component of the innate immune system, acts as the body’s front-line defense against invading pathogens.

The synovial fluid found between the movable joints of human bones is also largely water. It reduces friction and helps in smooth movement of the joints. Saliva, about 99 percent of which is water, also helps in lubrication during swallowing and speech production. Water is also the principal component of the cerebrospinal fluid of the brain and spinal cord, the vitreous humor of the eyes, and the fluid of the amniotic sac surrounding a growing fetus. Mechanical shock absorption is one of the many functions of these fluids—they cushion the organs (or fetus) and protect them from trauma and injuries.