Body Water Basics
How much of our body is water?
Water makes up about 60 percent of our total body weight, typically a little more for men and a little less for women. For instance, a 175-pound man might attribute more than 100 pounds of his weight to water. Roughly two-thirds of our body water is found within our cells as intracellular fluid, while the remaining one-third is extracellular fluid found bathing our cells. As mentioned earlier, extracellular fluid includes both the fluid between our cells and also the plasma portion of our blood.
When looking at certain body tissue, skeletal muscle is a little more than 70 percent water (by weight), while fat tissue is less than 10 percent water (see Figure 7.1). By and large, it is the ratio of skeletal muscle to fat tissue that has the greatest impact on the amount of water in the body. Because men tend to have a higher percentage of muscle and a lower percentage of fat compared to women, they tend to have a higher percentage of body water. However, regardless of gender, a lean muscular person will have a higher percentage of body water while a nonmuscular, overweight person will have a lower percentage of body water.
Figure 7.1. Difference in composition between skeletal muscle and adipose (fat) tissue. Skeletal muscle is largely water and then protein while adipose tissue is mostly fat and very little water, protein and other material
Why do we have so much water in our body?
Water is the most abundant substance in the body because it provides the medium or environment for the body. That means that all other substances within the body are either dissolved, suspended, and/or bathed within water. In general, substances such as carbohydrates, protein, and electrolytes dissolved well into body water. Meanwhile, lipids do not and the transport of lipid materials in our blood requires water soluble transporters such as proteins or lipoproteins. For instance, fat-soluble vitamin D hitches a ride upon a vitamin D binding protein (DBP), while sex hormones (i.e., estrogen, testosterone) can latch onto sex hormone binding protein (SHBP). In the meantime, fats and cholesterol are transported in lipoproteins, which are in essence submarines carrying lipid cargo.
How does water help us regulate our body temperature?
Water has the capability to absorb heat to keep us from overheating (hyperthermia) and release it at times overcooling (hypothermia). In comparison to other materials, water can absorb a lot of heat before its own temperature changes. This allows body water to absorb the heat generated during normal metabolism and during times of extra heat production such as exercise. Water then facilitates the removal of extra heat from our body by sweating (discussed below). On the other hand water can give up heat to help keep tissue warm when we are in cooler environments.
What other roles does water play?
Water also provides the basis for the lubricating substances found in our joints. This helps cushion the joint and reduce the physical stress and friction between the bones in the joint. Water is the basis of amniotic fluid that cushions and protects a fetus during pregnancy. In addition, of our urine, bile, saliva, mucus, lacrimal fluid (tears), and digestive secretions all are water based.
