Earth Physiology

If you are a healthy young person weighing 120 pounds, one substance alone, out of the hundreds of compounds present in your body, weighs about 72 pounds, or 60% of your total body weight. This, the body's most abundant compound, is water. Water is tremendously important to the health of the body. In fact, water is the syrup of life and our bodies can go a much longer period of time without food than they can without water.

Inside the body, water is distributed into three physically distinct locations known as fluid compartments. Figure 1 illustrates the relative sizes of these fluid compartments. Note that the largest volume of water by far lies inside the cells and is called, appropriately, intracellular fluid (ICF) Note also that the water that is located outside of the cells - extracellular fluid (ECF) - is located in two compartments: in the microscopic spaces between cells, where it is coiled interstitial fluid (IF); and in the blood vessels, where it is the principal constituent of plasma, the liquid part of blood. In summary, then:

  1. Intracellular Fluid (ICF) = fluid in the cells
  2. Extracellular Fluid (ECF) = fluid outside the cells consisting of:
    1. Interstitial Fluid (IF) = fluid between cells
    2. Plasma = fluid in the blood vessels
A normal body maintains fluid balance. The term "fluid balance" means that the volumes of ICF and ECF, and, therefore, the total volume of water in the body, all remain relatively constant. Under normal conditions, homeostasis (relative uniformity of the body's internal environment) of the total volume of water in the body is maintained or restored primarily by devices that adjust urine output to compensate for greater or less fluid intake (drinking), and secondarily, by mechanisms that adjust fluid intake to respond to the fluid needs of the body. The body's chief mechanism, by far, for maintaining fluid balance is to adjust its fluid output so that, over time, average fluid output equals average fluid intake. Everyone knows this by experience. The more liquid one drinks, the more urine one excretes. Conversely, the less the fluid intake, the less the urine volume output. This balance is maintained mainly through the kidney acting together with certain hormones in the body.

We will discuss the kidney and the formation of urine in a moment; however, let us discuss what electrolytes are. Electrolytes are chemical compounds that dissociate (or break up) in solution into separate positively or negatively charged particles, called ions. For example, table salt (NaCl) is an electrolyte that dissociates in water to form the ions sodium (Na+) and chloride (Cl-). Positively charged particles, such as Na+, are called cations, and negatively charged particles, such as Cl-, are called anions. A variety of anions and cations serve important nutrient or regulatory roles in the body. Important cations include sodium (Na+), calcium (Ca++), potassium (K+), and magnesium (Mg++). Important anions include chloride (Cl-), bicarbonate (HCO3-), phosphate (PO4-3 ), and many proteins. Although blood plasma contains a number of important electrolytes, by far the most abundant one is sodium chloride (ordinary table salt, NaCl).

Figure 1. Names and relative proportions of the main body fluid compartments. Volumes given are for young adults weighing 720 pounds (54.5 Kg); 1 liter is approximately equivalent to 1 quart.

The concentrations of all the electrolytes and ions in the different fluid compartments are controlled very closely by the body, but our discussion will focus primarily on how Na+ levels in the extracellular compartment profoundly affect the amount of urine that is excreted. (Remember, the more salted popcorn you eat at the movies, the less you have to go to the bathroom.) We will also look briefly at how K+ levels affect the production of urine. Let's start with a description of our kidney, the site in our bodies where urine is produced.

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