What Does Water Do Inside Us? The Function of Water

The Mechanics of Hydration and Dehydration

By Roberta H. Anding, MS, Baylor College of Medicine and Texas Children’s Hospital
Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

Water is great on a hot summer day, but what does it actually do for us? Professor Anding explains how water is an essential component to our overall health and well-being.

Woman drinking glass of water
Daily water intake replenishes water volume in our bodies, which is essential for maintaining our health and well-being. Photo by wavebreakmedia / Shutterstock

Water’s Function

The function of water goes beyond satisfying your thirst. Certainly, if you’re thirsty, it’s going to refresh you, but it also plays a vital role in your internal functions.

Water serves as a transport vehicle for digestion and the transport of nutrients to the cells within the blood. Maintaining this adequate blood volume enhances nutrient delivery and helps us feel healthy and well.

Have you ever gone from a sitting to a standing position quickly? If you jump out of your chair and stand up, all of a sudden, you get a little lightheaded and dizzy. 

This dizziness occurs because you don’t have enough blood volume. You’re not delivering enough carbohydrates to your central nervous system, and your brain goes into a holding mode. We’ve all had that experience, but this is a great example of water serving as a transport vehicle.

Fluid can have mechanical functions. For example, the synovial fluid in your knees lubricates joints and allows for ease of movement. Additionally, the fluid in tears helps to clear out any debris or dirt that you might have in your eyes. 

For most of us, a major function of water is to dissipate, or get rid of, heat and regulate heat loss. Water is a really good medium for holding and eliminating heat. 

Blood, which is mostly made up of water, moves toward your organs when you need to conserve heat. Water also helps to cool off your body after strenuous physical activity through the process of sweating and then evaporating.

Problems with Water

Your body mostly stores water around the cells and in the bloodstream, but nutritional factors and the state of your health can influence where your body water is located. For example, if you have low protein in your blood, some of the fluid seeps out of your blood vessels and actually goes into the tissues. 

Edema, or swelling, can occur with congestive heart failure, where the heart weakens and does not pump blood as effectively. You’re unable to move that body water, and it leaks into the tissues, where it gets stuck and cannot be urinated out.

If our bodies are 60 to 75 percent water, fluctuations in water will affect your scale weight. Overhydration, in the case of edema, will cause your weight to rise, while dehydration will cause you to lose weight. 

In both cases, though, it is only water weight, and not fat, that accounts for this change. This is something to keep in mind when taking certain weight loss products such as “slimming” teas, which may be leading you to lose weight, but it’s typically only water weight from dehydration.

Daily Fluid Loss

The average adult will eliminate somewhere in the range of 2.5 liters, or about 84 ounces, of water a day. About 1.5 of those liters comes through urine, and the rest is through sweating, breathing, and bowel movements. 

However, depending on your level of physical activity, the amount that you eliminate through sweat could be significant. If you run or do anything aerobic, you breathe in and out more times per minute, and the amount of fluid that you lose is greater.

Although we don’t think of our breath as being a source of fluid loss, it is 100 percent humidified. It’s got a lot of water in it, and any time you breathe in and out more often, you’re going to increase your body water loss.

If you have a fever, your heart rate, body temperature, and respiration rate go up. Thus, the amount of fluid that you lose would increase. 

If you’ve ever had diarrhea, you can lose a significant amount of fluid by an increase in bowel movements, as well as an increase in the amount of water in the bowel movements. Insensible losses are considered fluid losses through the skin and, again, sweat is going to be an example of that.

Most of us don’t think about our food consumption as a source of fluid, but the food that we consume is, depending on our food choices, equivalent to about 17 ounces or a half a liter per day. High water-volume foods include fruits, vegetables, milk, and yogurt, which are about 80 to 90 percent water.

For individuals who really struggle with hydration and can’t drink any more voluntary fluid because their losses are so significant, food oftentimes becomes the magic bullet. For some individuals, foods high in water volume can be the tipping point between optimal hydration and dehydration.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.

About Kate Findley 283 Articles
Kate is a writer, novelist, and blogger living in Los Angeles. She has been writing for The Great Courses since 2017. It incorporates her two favorite things: writing and learning.