How Much Water Do Humans Need?
Water is a unique substance upon which all life depends. It is essential both inside cells—where it provides the medium in which all chemical reactions take place—and outside cells, where it is necessary for the transport of nutrients and other materials, and for the removal of wastes.
On land, plants and animals must conserve water within their bodies. Animals lose water through evaporation from lung surfaces and the outer body surface, and through elimination in feces and excretion in urine. The water that is lost must be replaced.
Most land animals are adapted to minimize water loss through excretion and elimination. Our kidneys, for example, are extremely efficient in their use of water. While approximately 170 liters of water are cycled through a human’s kidneys each day, almost all of this water is reabsorbed. Water used during the digestion of food also is reabsorbed by the body. This process occurs in the large intestine.
The threat of water loss is especially significant for animals living in dry environments. Most of these animals have evolved special strategies to conserve water. Kangaroo rats living in deserts, for example, hardly ever drink water. They obtain almost all of the water they need from the chemical breakdown of the grains they eat. To reduce water loss, the rats are inactive during the hottest parts of the day, produce very dry feces, and release extremely concentrated urine.
An average human doing light work in a temperate climate loses nearly 6 pints (3 liters) of water daily. This water must be replaced to keep the body functioning optimally.
Healthy human beings show the effects of water deprivation (dehydration) after about three days. Death is likely when the body loses about 20% of its total volume of water. This equals approximately 2.75 gallons (22 pints, or 10.5 liters) in a medium-sized adult. On the other hand, as long as water is available, it is possible to survive for up to two months without food (and lose up to half of the body’s weight).
- Using the 2,000-mL handled beaker, measure or have students in each group measure 3,000 mL of water into a large dishpan (or tub). This is the amount of water that enters the body in food and liquid during a typical day.
- Ask students, What happens to the water in our bodies? Where does it go? Have students take turns moving the following quantities of water from the tub into the milk jug. Students should use a funnel when pouring water into the jug.
150 mL - Water eliminated by the intestines
600 mL - Water lost as vapor during breathing
1,500 mL - Water eliminated as urine
750 mL - Water lost as perspiration
- Ask the students to record the amount of water left in the first container. (It all will be gone!) Ask, What would happen if no water entered the body?
- Ask students to identify different ways the body’s water supply could be replenished. Have each group create plans or strategies to replace the 3,000 mL of water needed by the body each day to survive. Note that about half of the water we need can come from food, and that about 300 mL of water per day is produced inside the body, as energy is released from food. Have students share their ideas with the rest of the class.
Keywords: body | cells | chromatography | dehydrate | dehydration | evaporation | excretion | health | kidney | life science | transpiration | water | lesson
- Moreno, N., Tharp, B., and Dresden, J. (2011) The Science of Water Teacher’s Guide. Baylor College of Medicine: Houston.
- Photo © Minghong. Licensed for use.
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My Health My World: National Dissemination
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The Environment as a Context for Opportunities in Schools
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Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education