As much as 70% of a human’s body weight is water. This water is contained both within the cells and between the cells that make up tissues and organs. Its several roles make water indispensable to human function.

Part 1. Biological Importance of Water

Water as a Lubricant and Cushion

Water is a major component of many of the body’s lubricating fluids. Just as oil lubricates the hinges on a door, water in synovial fluid lubricates the actions of body joints, and water in pleural fluid helps the lungs expand and recoil during breathing. Watery fluids help keep food flowing through the digestive tract and ensure that the movement of adjacent abdominal organs is frictionless.

Water also protects cells and organs from physical trauma. Water is a major component of CSF (Cerebrospinal Fluid), which cushions the brain within the skull. Water also helps protect the delicate nerve tissue of the eyes and cushions a developing fetus in the mother’s womb.

Water as a Heat Sink

A heat sink is a substance or object that absorbs and dissipates heat but does not experience a large corresponding increase in temperature. Because of its ability to form numerous hydrogen bonds, water naturally resists large changes in temperature. Consider a pot of water that has been heated on a stove for 3–5 minutes. The pot is already too hot to safely touch, while the water inside the pot has barely increased its temperature. Compared to many other liquids, water requires an extraordinary amount of heat energy to be absorbed before changing its temperature. This makes water an excellent heat sink within the human body, as it is able to absorb the heat generated by chemical reactions without greatly increasing our body temperature. Moreover, when environmental temperature soars, the water stored in the body helps keep the body cool. This cooling effect happens as warm blood from the body’s core flows through blood vessels just under the skin and is released out to the environment as radiant heat. At the same time, sweat glands secrete warm water in sweat. For evaporation of this water to occur, the hydrogen bonds between the individual water molecules must be broken, requiring a relatively high amount of energy that in part includes heat. This removal of heat by evaporation results in a cooling of the blood in the body’s periphery, near the surface of the skin, which then circulates back to the body core and cools the body. As the hottest sweat molecules evaporate from the skin’s surface, this leaves behind only the cooler sweat molecules, thus decreasing the overall temperature of the skin’s surface.

Water as a Component of Liquid Mixtures

A mixture is a combination of two or more substances, each of which maintains its own chemical identity. In other words, the constituent substances are not chemically bonded into a new, larger chemical compound. This concept is easy to imagine if you think of powdery substances such as flour and sugar. When stirred together in a bowl, they obviously do not bond to form a new compound. The air you breathe is a gaseous mixture, containing the element argon, molecules of nitrogen and oxygen, and one compound—carbon dioxide.

For cells in the body to survive, they must be kept moist in a water-based liquid called a solution. In chemistry, a liquid solution consists of a solvent that dissolves a substance called a solute. An important characteristic of solutions is that they are homogeneous; that is, the solute molecules are distributed evenly throughout the solution.

The Role of Water in Chemical Reactions

Two types of chemical reactions involve the creation or the consumption of water: dehydration synthesis and hydrolysis.

• During dehydration synthesis, one monomer donates an atom of hydrogen, and another monomer donates a hydroxyl group (OH). These monomers then form a covalent bond between themselves. In the formation of their covalent bond, a molecule of water is released as a by-product (Figure 1). This process is also sometimes referred to as a condensation reaction.
• During hydrolysis, a molecule of water disrupts a compound, breaking its covalent bond. The water is itself split into H⁺ and OH⁻. One portion of the severed compound then bonds with the hydrogen atom, and the other portion bonds with the hydroxyl group.

Both of these reactions are reversible and play an important role in the chemistry of organic compounds (which will be discussed shortly).

Watch this Crash Course video to learn more about the importance of water and its chemical properties. Direct link: https://youtu.be/HVT3Y3_gHGg

Watch this Amoeba Sisters video to learn more about the properties of water! Direct link: https://youtu.be/3jwAGWky98c

Part 2. Fluid Compartments in the Human Body

Body Water Content

The human body is mostly water, ranging from about 75% of body mass in infants to as low as 45% in elderly individuals. In adults, the average percent of body mass in women is 50%, whereas in men, the average is 60%. The percent of body water changes with development, because the proportions of the body given over to each organ and to muscles, fat, bone, and other tissues change from infancy to adulthood (Figure 2).

Fluid Compartments

Body fluids can be discussed in terms of their specific fluid compartment, a location that is largely separate from another compartment by some physical barrier. The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. Extracellular fluid (ECF) surrounds all cells in the body. This fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not located in the blood (Figure 3).

1. Intracellular Fluid: The intracellular fluid lies within cells and is the principal component of the cytosol. The intracellular fluid makes up approximately 60% of the total water in the human body, and in an average-size adult male, the intracellular fluid accounts for about 25 liters (seven gallons) of fluid (Figure 4). This fluid volume tends to be very stable, because the amount of water in living cells is closely regulated. If the amount of water inside a cell falls to a value that is too low, the cytosol becomes too concentrated with solutes to carry on normal cellular activities. If too much water enters a cell, the cell may burst.

2. Extracellular Fluid: The extracellular fluid accounts for the other approximately 40% of the body’s water content. About half of the extracellular fluid is found in plasma. Plasma travels through the body in blood vessels and transports a range of materials, including blood cells, proteins (including clotting factors and antibodies), electrolytes, nutrients, gases, and wastes. Gases, nutrients, and waste materials travel between capillaries and cells through the interstitial fluid. Cells are separated from the interstitial fluid by a selectively permeable cell membrane that helps regulate the passage of materials between the interstitial fluid and the interior of the cell.

The body has other water-based extracellular fluids. These include the cerebrospinal fluid that bathes the brain and spinal cord, lymph, synovial fluid in joints, pleural fluid in the pleural cavities, pericardial fluid in the cardiac sac, peritoneal fluid in the peritoneal cavity, and aqueous humor of the eye. Because these fluids are outside cells, these fluids are also considered components of the extracellular fluid compartment.