Water (H2O) is the most abundant compound in living material and accounts for about two-thirds of the weight of an adult human. It is the major component of blood and other body fluids, including those within cells.

When substances dissolve in water, the polar water molecules cause molecules of the substance to separate from each other, or even to break up into ions. These particles are much more likely to take part in chemical reactions. Consequently, most metabolic reactions occur in water.

Water also plays an important role in transporting chemicals within the body. Blood, which is mostly water, carries many vital substances, such as oxygen, sugars, salts, and vitamins, from organs of the digestive and respiratory systems to cells. Blood also carries waste materials, such as carbon dioxide and urea, from these cells to the lungs and kidneys, respectively, which remove them from the blood and release them outside the body.

In addition, water can absorb and transport heat. Blood carries heat released from muscle cells during exercise from deeper parts of the body to the surface. At the same time, water released by skin cells in the form of perspiration can carry heat away by evaporation.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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