The Role of Water in Human Biology

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  • 0:03 What Exactly Is Water?
  • 1:03 Water in Human Biology
  • 1:35 Cell Life & Transport
  • 2:07 Chemical Reactions
  • 2:30 Body Temperature &…
  • 3:14 Lesson Summary
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Lesson Transcript
David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Expert Contributor
Jillian Conte

Dr.Conte has taught biology and forensic science courses. She holds a PhD in Cell & Molecular Biology, a MS in Forensic Science, and a BS in Biology.

After completing this lesson, you will be able to explain why water is so important, and describe the five main functions of water in the human body. A short quiz will follow.

What Exactly Is Water?

Water is the most important natural resource in the world. Without it, we humans wouldn't exist. Not only can we not go more than a few days without water, but life on Earth most likely would never have evolved at all. And then there are all the more trivial uses: cooking, bathing, washing clothes, brushing teeth, flushing toilets, and watering plants. Humans use a lot of water.

Water is a compound containing molecules, each of which is made up of two atoms of hydrogen and one atom of oxygen bonded together. Across a large proportion of the Earth it's in liquid form, which is convenient because humans need to drink it. Though at the poles it is a solid called ice, it can also evaporate to form water vapor. Water vapor then rises to create clouds, which eventually cause rain. On Earth, water literally falls from the skies. This little fact is something we're so used to that it may seem trivial, but it's a pretty big deal.

Water in Human Biology

Water is vital to human biology, and indeed, all biology on Earth. We currently believe that the presence of liquid water is an important requirement for the development of life, to the point that it's the main thing we're searching for in our attempts to find life elsewhere in the solar system.

But why exactly is water important to human biology? There are five main functions of water in the human body:

  • Cell life
  • Transport
  • Chemical reactions
  • Temperature regulation
  • Waste excretion

Cell life is based on the fact that cells have water inside them. Water is used to transport nutrients to cells and within cells (intracellular). If cells don't get nutrients including vitamins, minerals, and sugars, they die.

As mentioned, water is needed for transport. It's how oxygen and food make it through our bodies, and how blood flows through the veins and arteries to take those nutrients to where they're needed. Blood is, after all, 92% water. In fact, 60% of your entire body is water!

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Additional Activities

Discussion Topics

This discussion will help us brainstorm and hypothesize what happens when water is not available and the functions that water helps facilitate in the human body are dysfunctional.

For each of the following roles of water in the human body, use your scientific knowledge to support your hypotheses for what the outcome would be.

Cell Life

The process of osmosis is the movement of water from areas of low concentration of solutes to high concentration of solutes. Due to an increase in salt ions outside the cell, water leaves the cell. What effect does this have on the cell's shape? What effect does this have on the movement of vitamins within the cell?

Chemical Reactions

Two examples in the lesson about chemical reactions that take place in our body is how our food is broken down to give us energy and how our waste is handled. These examples go hand in hand: we eat food and harness energy and then excrete waste material from the food. Think about a time when you ate a meal and did not have any water before, during, and after. How did you feel in your abdominal area? How did you feel during excretion the next day? How would this experience be different if water was consumed before, during, and after the eating period?

Temperature Regulation

One of the great characteristics of water is its high heat capacity. Having a high heat capacity means water can better regulate temperature, and minimal changes in temperature take place. Hypothesize what would happen if water had a low heat capacity. What would happen if water had an even higher heat capacity?


For 'Cell Life', students may discuss the cell shrinking in size because of the water loss, and movement of vitamins inside the cell slow down. They may even see the link between this slowing of transport and decrease in chemical reactions within the cell. For 'Chemical Reactions', students may describe feelings of bloating, and then delayed or painful bowel movements after not having water. For 'Temperature Regulation', with a low heat capacity students may hypothesize that we get fevers more often or drops in temperature; with a high heat capacity, less fevers would be anticipated. Linking a low heat capacity to changes in chemical reactions is a great way to tie together the roles water plays in the human body.

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