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7.3: Regulation of Water Balance

  • Page ID
    21144
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    Learning Objectives

    • Describe water balance including sources of water intake and water output.
    • Describe the thirst mechanism.

    When the body is in 'water balance', the intake and output of water are equal. One estimate of the body's total water intake and output can be seen in Table \(\PageIndex{1}\).

    Table \(\PageIndex{1}\): Water Balance
    Water Intake Water Output
    Fluids: 1,200 ml/day Insensible: 800 ml/day
    Foods: 1,000 ml/day Urine: 1,500 ml/day
    Metabolism: 300 ml/day Feces: 200 ml/day
    TOTAL INTAKE: 2,500 ml/day TOTAL OUTPUT: 2,500 ml/day

    Water Intake

    Most of the water we gain each day comes from drinking fluids and eating solid foods; however, there is a small amount of water produced by our tissues through various metabolic processes. The National Academies of Sciences, Engineering, and Medicine determined that the Adequate Intake (AI) for total fluid intake is:

    • About 15.5 cups (3.7 liters) of fluids for men, which includes ~3.0 liters (13 cups) as beverages, including water
    • About 11.5 cups (2.7 liters) of fluids a day for women, which includes ~2.2 liters (9 cups) as beverages, including water1

    It is important to note that the AI for fluids includes fluid intake from all dietary sources; that is, fluids coming from food as well as beverages (including water, tea, coffee, sugar-sweetened beverages, and juices). People are not expected to consume 15.5 or 11.5 cups of pure water per day. In America, approximately 20 percent of total water intake comes from solid foods.1 Even foods that seem dry (like crackers, cereal, bread, and other grains) still contain some water. Fluid is also obtained from metabolic reactions. Metabolic water is a product of many chemical reactions in the body and contributes 10-14% of the body’s needs.

    There is some debate over the amount of water required to maintain health because there is no consistent scientific evidence proving that drinking a particular amount of water improves health or reduces the risk of disease. In fact, kidney-stone prevention seems to be the only premise for water-consumption recommendations. You may be surprised to find out that the commonly held belief that people need to drink eight 8-ounce glasses of water per day isn’t an official recommendation and isn’t based on any scientific evidence! The amount of water/fluids a person should consume every day is actually variable and should be based on the climate a person lives in, as well as their age, physical activity level, and kidney function. Your fluid intake is probably adequate if you rarely feel thirsty and your urine is colorless or light yellow. No maximum for water intake has been set.

    Thirst Mechanism - One Way to Increase Water Intake

    Thirst is a regulatory mechanism to increase water input. The thirst mechanism is activated in response to changes in water volume in the blood and the concentration of sodium in the blood. The urge to drink results from a complex interplay of hormones and nervous system responses that coordinate to increase water input and contribute toward fluid balance and composition in the body. The “thirst center” is contained within the hypothalamus. The thirst mechanism is not always sufficient and has limitations. For example, in older people the thirst mechanism is not as responsive so as we age there is a higher risk for dehydration. Thirst happens in the following sequence of physiological events:

    1. Receptor proteins in the kidney, heart, and hypothalamus detect decreased fluid volume or increased sodium concentration in the blood.
    2. Hormonal and neural messages are relayed to the brain’s thirst center in the hypothalamus.
    3. The hypothalamus sends signals to higher sensory areas in the cortex of the brain, stimulating the conscious thought to drink.
    4. Fluids are consumed.
    5. Receptors in the mouth and stomach detect mechanical movements involved with fluid ingestion.
    6. Neural signals are sent to the brain and the thirst mechanism is shut off.

    The physiological control of thirst is the back-up mechanism to increase water input. Fluid intake is controlled primarily by conscious eating and drinking habits. For example, you might have a habit of drinking a beverage with meals.

    Water Output

    The kidneys are the organ primarily responsible for maintaining fluid balance. They constantly help maintain fluid balance by either excreting or retaining the fluid we consume. As shown in Table \(\PageIndex{1}\), daily water output averages 2,500 milliliters. There are two types of outputs:

    • insensible water loss: occurs without us being aware of it and includes water lost through breathing (exhalation) and evaporation from the skin
    • sensible water loss: noticeable water loss through sweating as well as excretion of urine and feces

    Certain situations can cause a significant loss of fluid from the body:

    • Illnesses
    • Blood loss
    • Exercise
    • Environmental conditions
    • Pregnancy and breastfeeding
    • Diuretics (increase fluid loss via urine), including:
      • certain medications
      • alcohol

    Key Takeaways

    • When the body is in 'water balance', the intake and output of water are equal.
    • Most of the water we gain each day comes from drinking fluids and eating solid foods; however, there is a small amount of water produced by our tissues through various metabolic processes.
    • The amount of water a person should consume everyday is variable and should be based on the climate a person lives in, as well as their age, physical activity level, and kidney function.
    • Thirst results from a complex interaction of hormones and nervous system responses that coordinate to achieve fluid balance and composition in the body.
    • Water output consists of both insensible and sensible water loss.

    References

    1. Panel on Dietary Reference Intakes for Electrolytes and Water. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: The National Academies of Science, Engineering, and Medicine; 2005. https://www.nap.edu/read/10925/chapter/6. Accessed June 22, 2020.

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

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