9.3: Water Balance in the Body

Osmoregulation: Achieving water balance

As you eat a bite of food, the salivary glands secrete saliva. As the food enters your stomach, gastric juice is secreted. As it enters the small intestine, pancreatic juice is secreted. Each of these fluids contains a great deal of water. How is that water replaced in these organs? What happens to the water now in the intestines? The processes that control the exchange of water in cells and tissues is referred to as osmoregulation. Osmoregulation involves complex communication between the brain, kidneys, and endocrine system to achieve a balance in water intake, primarily through the diet, and water loss through excretion. Through osmoregulation, a stable set point or homeostasis for water balance is achieved.

Regulation of Daily Water Input

Each day, the average person puts out approximately 2.5 liters of water through their breath, sweat, urine, and feces. This must be balanced with water input so that the body does not lose water or gain water over the day (Figure \(\PageIndex{1}\)). Our tissues produce around 0.3 liters of water per day through metabolic processes. The remaining water output must be balanced by drinking fluids and eating solid foods. The average daily fluid consumption is 1.5 liters, and water gained from solid foods approximates 0.7 liters.¹

Figure \(\PageIndex{1}\): Water balance in the body. Water output or loss through the skin, lungs, and excretion must be balanced by water inputs from food, drink, and metabolism. "Water balance in the body" from An Introduction to Nutrition (Zimmerman) is licensed under CC BY-NC-SA 3.0.

Recommendations for Water Intake

The Adequate Intake (AI) for water is a recommendation based on the average water intake observed in a survey of Americans. It will prevent detrimental effects of dehydration, or the state of having too little water in the body. For adult males, the AI for water is 3.7 liters (15.6 cups), and adult females should consume 2.7 liters (11 cups).¹ It is important to note that the AI for total water intake includes water from all dietary sources: that is, water coming from food as well as beverages. In America, approximately 20 percent of dietary water comes from solid foods. See Table \(\PageIndex{1}\) for the range of water contents for selected food items.

Table \(\PageIndex{1}\): Water Content in Foods

Percentage	Food Item
90–99%	Nonfat milk, cantaloupe, strawberries, watermelon, lettuce, cabbage, celery, spinach, squash
80–89%	Fruit juice, yogurt, apples, grapes, oranges, carrots, broccoli, pears, pineapple
70–79%	Bananas, avocados, cottage cheese, ricotta cheese, baked potato, shrimp
60–69%	Pasta, legumes, salmon, chicken breast
50–59%	Ground beef, hot dogs, steak, feta cheese
40–49%	Pizza
30–39%	Cheddar cheese, bagels, bread
20–29%	Pepperoni, cake, biscuits
10–19%	Butter, margarine, raisins
1–9%	Walnuts, dry-roasted peanuts, crackers, cereals, pretzels, peanut butter
0%	Oils, sugars

Source: USDA Agricultural Research Service FoodData Central. https://fdc.nal.usda.gov/

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. The amount of hydration a person needs actually varies based on the climate a person lives in, as well as their age, physical activity level, and kidney function. Situations such as fevers, diarrhea, skin burns, increased physical activity, and breastfeeding will increase water needs. Because our bodies can efficiently excrete excess water and maintain water balance in most cases, there is no recommended upper limit for water intake.

Thirst Mechanism: Why Do We Drink?

When we get dehydrated and need to increase water intake, the thirst mechanism kicks in. Thirst is a part of osmoregulation that aims to increase water input. The thirst mechanism is activated in response to changes in water volume in the blood (Figure \(\PageIndex{2}\)). When blood volume decreases, blood pressure drops and hormones such as angiotensin signal the brain that a person is thirsty. The thirst mechanism is also triggered by changes in the concentration of molecules dissolved in the blood, known as blood osmolality. Blood osmolality is primarily driven by the concentration of sodium in the blood. Specific proteins called osmoreceptors sense blood osmolality in the hypothalamus, a portion of the brain that lies just above the brain stem. The hypothalamus is ultimately responsible for stimulating the thirst you feel when you are dehydrated. In older people, the thirst mechanism is not as responsive, and as we age, there is a higher risk for dehydration.

The physiological control of thirst is the backup mechanism to increase water input. Fluid intake occurs primarily through conscious eating and drinking habits dependent on social and cultural influences, as well as other dimensions of wellness. For example, you might have a habit of drinking a glass of orange juice and eating a bowl of cereal every morning before school or work. Most American adolescents have a daily habit of soft drink consumption. Demographics, access to soft drinks, social impacts, and personal behavior all influence soft drink consumption.

Figure \(\PageIndex{2}\): Thirst mechanism. When hydration levels drop, there is a decrease in blood volume and an increase in blood osmolality. This triggers a response in the brain's hypothalamus to increase thirst and promote water input. "Thirst mechanism" from Anatomy & Physiology, 2e by OpenStax is licensed under CC BY 4.0.

Regulation of Daily Water Output

As mentioned previously, there is no upper limit for total water intake because our bodies can efficiently excrete excess water in most cases. There are two types of water outputs: insensible water loss and sensible water loss. Insensible water loss is not easily measured and occurs primarily through the skin and breath. In most cases, this water output accounts for about 40-50% of the total water output daily. The body loses about 0.25 liters of water through exhalation each day, but this output will increase if a person is breathing more rapidly because of increased physical activity, for example. Another 0.5 liters is lost through our skin, but increased temperatures will increase this output. The remaining water output occurs through sensible water loss, which includes urine and feces. Urine accounts for about 1.5 liters of water output, and feces account for roughly 0.1 liters of water output. Regulating urine output is a primary function of the kidneys and involves communication with the brain and endocrine system.

The Kidneys and Hypothalamus Regulate Water Output

The kidneys are two bean-shaped organs, each about the size of a fist and located on either side of the spine just below the rib cage. On average, the kidneys filter about 190 liters of blood and produce 1.5 liters of urine per day. Urine is mostly water, but it also contains electrolytes and waste products, such as urea. The color of urine can be an indicator for hydration status (Figure \(\PageIndex{3}\)). The amount of water filtered from the blood and excreted as urine depends on the water input and electrolyte levels in the blood.

Kidneys have protein sensors that detect blood volume from the pressure, or stretch, in the blood vessels of the kidneys. When blood volume is low, kidney cells detect decreased pressure and secrete the enzyme that activates the hormone angiotensin. Angiotensin targets three different organs (the adrenal glands, the hypothalamus, and the muscle tissue surrounding the arteries) to rapidly restore blood volume and, consequently, pressure. In response to high sodium levels that occur during dehydration, the hypothalamus activates the thirst mechanism, as discussed above. The hypothalamus can also limit urine output by stimulating the release of antidiuretic hormone, which signals the kidneys to decrease water output.

Figure \(\PageIndex{3}\): Urine color and hydration status. Urine is a primary output of water and is produced when the kidneys filter the blood. The color of urine can indicate hydration status. "Urine Color and Hydration Status" from Anatomy & Physiology, 2e by OpenStax is licensed under CC BY 4.0.

Attributions:

• Zimmerman, "An Introduction to Nutrition (Zimmerman)," CC BY-NC-SA 3.0. Text was updated and edited for clarity and flow of information. Additional figures were added.

References

1. Stallings VA, Quirk M, Oria M, eds. Dietary Reference Intakes for Sodium and Potassium: A Consensus Study Report of The National Academies of Sciences, Engineering, Medicine. Washington, DC: The National Academies Press; 2019. https://nap.nationalacademies.org/re...5353/chapter/1. Accessed October 2, 2023.