9.2: Water’s Importance to Vitality

Water is Essential for Life

You get up in the morning, flush waste down the toilet, take a shower, brush your teeth, drink, eat, drive, wash the grime from your windshield, get to work, and drink coffee. Next to a fountain, you eat lunch and down it with a glass of water; you use the toilet again and again, drive home, and prepare dinner. Add all the ways you use water every day, and you will still not come close to the countless uses water has in the human body.

Of all the nutrients, water is the most critical, as its absence proves lethal within a few days. In fact, your body is mostly water. The average adult person contains 60-70% total body water, but the amount of water or hydration in the human body varies depending on age, sex, and body composition. In this chapter, we will discuss how our bodies use and regulate the amount of water in our cells and tissues. We begin with the functions of water in the human body.

Figure \(\PageIndex{1}\): Water is the foundation of all life—the surface of the earth is 70% water; the volume of water in humans is about 60%.  "The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972 by the crew of the Apollo 17 spacecraft en route to the Moon. (Public Domain; NASA)

 

Water As a Transportation Vehicle

Water is called the “universal solvent” because more substances dissolve in it than any other fluid. It is helpful to imagine mixing up a glass of salt water. When you add a teaspoon of salt to water, the salt will eventually dissolve into the water. When this happens, we call the salt molecules solute and water the solvent. Molecules like salt and sugar dissolve in water because of water's ability to loosely bond with other molecules. Water molecules consist of oxygen and two hydrogen atoms, H₂O, that readily surround solute molecules to dissolve them. The solvent action of water allows for substances to be more readily transported. A pile of undissolved salt crystals would be difficult to move throughout tissues, as would a bubble of gas or a glob of fat. Blood, the primary transport fluid in the body, is about 78% water. Dissolved substances or solutes in the blood include proteins, lipoproteins, glucose, electrolytes, and metabolic waste products, such as carbon dioxide and urea, all of which are either dissolved in the surrounding blood to be transported to cells to support basic functions or are removed from cells to prevent waste buildup and toxicity. Blood is not just the primary vehicle of transport in the body. Blood also acts as a fluid tissue that structurally supports blood vessels, which would collapse without water. Similarly, the brain is 75% water, with the water in the brain providing structure.

Figure \(\PageIndex{2}\): Water is the universal solvent. Water molecules, shown in red and white, can surround and dissolve many molecules, such as chloride (green) and sodium (purple) ions. In this case, water is the solvent, and the salt ions are solutes. "Water is the universal solvent" from Biology, 2e by OpenStax is licensed under CC BY 4.0.

Water As a Medium for Chemical Reactions

Water is required for even the most basic chemical reactions. Previously, you learned that enzymes are proteins, and proteins fold into their functional shape based on how their amino-acid sequences react with water. Once formed, enzymes must conduct their specific chemical reactions in a medium, which in all organisms is water. Water is an ideal medium for chemical reactions as it can store a large amount of heat, is electrically neutral, and has a pH of 7.0, meaning it is not acidic or basic. Additionally, water is involved in many enzymatic reactions, including those in chemical digestion. Water acts as an agent to break chemical bonds or is removed from other molecules to form chemical bonds.

Water As a Lubricant/Shock Absorber

Many may view the slimy products of a sneeze as gross, but sneezing is essential for removing irritants and cannot occur without water. Mucus, which is not only essential to discharge nasal irritants but also required for breathing, transportation of nutrients along the gastrointestinal tract, and elimination of waste materials through the rectum, is composed of more than 90 percent water. Mucus is a front-line defense against injury and foreign invaders. It protects tissues from irritants, entraps pathogens, and contains immune-system cells that destroy pathogens. Water is the main component of the lubricating fluid between joints and eases the movement of articulated bones.

The aqueous and vitreous humors, which fill the extra space in the eyes and the cerebrospinal fluid surrounding the brain and spinal cord, are primarily water and buffer these organs against sudden environmental changes. Watery fluids surrounding organs provide both chemical and mechanical protection. Just two weeks after fertilization, water fills the amniotic sac in a pregnant woman, providing a cushion of protection for the developing embryo.

Water As a Temperature Regulator

Another homeostatic function of the body, termed thermoregulation, balances heat gain with heat loss. Water in the body plays an important role in accomplishing this. Like blood glucose levels, our body temperature is tightly regulated. Human life is supported within a narrow range of temperatures, with the temperature set point or homeostasis of the body being 98.6°F (37°C). Too low or too high a temperature causes enzymes to stop functioning, and metabolism is halted. At 82.4°F (28°C), muscle failure occurs, and hypothermia sets in. At the opposite extreme of 111.2°F (44°C) the central nervous system fails and death results.

Water is good at storing heat, an attribute referred to as heat capacity, and thus helps maintain the temperature set point of the body despite changes in the surrounding environment. The hypothalamus in the brain regulates body temperature in several ways. In response to cold temperatures, the hypothalamus signals to smooth muscle tissue surrounding blood vessels, causing them to constrict and reduce blood flow. This reduces heat lost to the environment. When body temperature rises, as occurs during exercise, the hypothalamus sends a signal to the smooth muscle of blood vessels, causing them to dilate, which can cause flushed skin as the blood vessels get larger. This also supports water loss in the form of sweat, which cools the body temperature back to its set point.

Attributions

• Zimmerman, "An Introduction to Nutrition (Zimmerman)", CC BY-NC-SA 3.0. Some text was removed, and remaining text was updated.