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7.2: Functions of Water

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    Learning Objectives

    • Describe the major functions of water in the human body.

    Of all the nutrients, water is the most critical as its absence proves lethal within a few days. Water's importance in the human body can be loosely categorized into four basic functions: transportation vehicle, medium for chemical reactions, lubricant/shock absorber, and temperature regulator.

    Water As a Transportation Vehicle

    Water is called the “universal solvent” because more substances dissolve in it than any other fluid. The dissolved substances (solutes) include many materials including ions, sugars, amino acids, vitamins, and minerals. Molecules dissolve in water because of the hydrogen and oxygen molecules ability to loosely bond with other molecules. Molecules of water (H2O) surround substances, suspending them in a sea of water molecules. The solvent action of water allows for substances to be more readily transported. Blood, the primary transport fluid in the body, is mostly made up of water. Dissolved substances in blood include proteins, lipoproteins, glucose, electrolytes, and metabolic waste products, such as carbon dioxide and urea, all of which are either dissolved in the watery surrounding of blood to be transported to cells to support basic functions or are removed from cells to prevent waste build-up and toxicity.

    Water As a Medium for Chemical Reactions

    Water is required for even the most basic chemical reactions. Previously, you learned that enzymes are proteins that conduct specific chemical reactions. Enzmyes conduct these reactions in a medium (environment); in the human body the medium 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 as an agent to break bonds or, by its removal from a molecule, to form bonds.

    Water As a Lubricant and Shock Absorber

    Water is the main component of the fluids that protect and lubricate tissues. For example:

    • cerebrospinal fluid surrounding the brain and spinal cord protects these organs against sudden changes in the environment
    • amniotic fluid provides a cushion of protection for a pregnant woman's developing baby
    • pleural fluid lubricates the lungs to make breathing easier
    • digestive secretions allow for easy passage of material through the gastrointestinal tract
    • mucus lines the walls of the intestines which eases the movement of food along the gastrointestinal tract
    • synovial fluid lubricates joints and eases the movement

    Water As a Temperature Regulator

    Water plays an important role in thermoregulation (temperature regulation). Human life is supported within a narrow range of temperature, with the temperature set point of the body being 98.6°F (37°C). Too low or too high of 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. Water has a high heat capacity which means that it takes alot of external energy to raise water's temperature. This ability helps maintain the temperature set point of the body despite changes in the surrounding environment (e.g., high temperatures outside).

    There are several mechanisms in place that move body water from place to place as a method to distribute heat in the body and equalize body temperature. The hypothalamus in the brain is the thermoregulatory center (Figure \(\PageIndex{1}\)). The hypothalamus contains special protein sensors that detect blood temperature. The skin also contains temperature sensors that respond quickly to changes in immediate surroundings.

    When the surrounding temperature is cold, sensors in the skin send a signal to the hypothalamus, which then sends a signal to smooth muscle tissue surrounding blood vessels causing them to constrict and reduce blood flow. This reduces heat lost to the environment. The hypothalamus also sends signals to muscles to erect hairs and shiver and to endocrine glands like the thyroid to secrete hormones capable of ramping up metabolism. These actions increase heat conservation and stimulate its production in the body in response to cooling temperatures.

    Image of homeostatic control driven by the hypothalamus including regulation of external temperature, blood temperature, sweating, and metabolic rate.
    Figure \(\PageIndex{1}\): Thermoregulation is the ability of an organism to maintain body temperature despite changing environmental temperatures. (CC BY 4.0; by Lauren Groth)

    When body temperature rises (for example, during exercise), the hypothalamus detects an increase in blood temperature. In response, the hypothalamus sends a signal to the smooth muscle of blood vessels causing them to dilate so that more blood flows to the body’s periphery. Once on the skin, extra heat exits the body via perspiration (sweat), which is 98 to 99 percent water. Water on the skin’s surface evaporates, a process that uses energy and results in the loss of heat, thereby cooling the body. Perspiration is a process that intertwines temperature regulation with fluid and electrolyte balance. Water and electrolytes lost in sweat need to be replenished in order to maintain fluid and electrolyte balance.

    Key Takeaways

    • Uses of water in the human body can be loosely categorized into four basic functions: transportation vehicle, medium for chemical reactions, lubricant/shock absorber, and temperature regulator.
    • Water has been termed the “universal solvent” because more substances dissolve in it than any other fluid. Blood is the primary transport fluid in the body.
    • Water serves as a medium for chemical reactions.
    • Water, as a component of body fluids, acts as a lubricant and shock absorber.
    • Water is good at storing heat and buffers the body against extreme variations in temperature.

    7.2: Functions of Water is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by LibreTexts.