BioEd Online

Teacher Background

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).

Objectives and Standards

Concepts

• Water is essential for survival.

• Under normal conditions, our bodies take in and release balanced amounts of water.

Science, Health and Math Skills

• Making and recording observations

• Calculating values based on observations

• Measuring

Materials and Setup

Teacher Materials (see Setup)

• Beaker with handle, or a pitcher, 2,000-mL

Materials per Student Group

• Beaker, 1,000-mL cap (or clear plastic cup marked in mL)

• Dishpan, 15-qt (or tub with a minimum capacity of 3 liters)

• Funnel, plastic, 2 3/4-in.

• Plastic milk jug, gal size

• Water, about 3 liters

Setup

1. Ask students to bring clean, empty one-gallon milk or juice jugs from home. Each group of students will need one jug.

2. Use beakers or graduated cylinders, or make your own (or have students make their own) by calibrating clear plastic cups ahead of time.

3. Place materials in a central location for Materials Managers to collect. Conduct this activity in groups of four students.

Procedure and Extensions

1. 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.

2. 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

3. 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?

4. 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.

Extensions

• Students can explore the volume of water filtered by the kidneys by calculating the number of 2-liter bottles of water that would be processed each day. (The kidneys process approximately 170 liters of water each day.)

• This activity also can be conducted as a demonstration by the teacher, using pre-measured and colored amounts of water to represent water loss through urine (yellow), perspiration (clear), feces (brown) and breathing (blue).

• Desert organisms have had to adopt special strategies to save water. Have students use resources in the library or on the Internet to investigate some of the unique characteristics of desert dwellers.

• Aquatic organisms (plants and animals that live in water) have another problem: too much water. Have students research strategies used by aquatic organisms to survive while submerged.

Handouts and Downloads