(1) Perspiration. Water is continuously lost from the body in the form of
perspiration or sweat. With high surrounding temperatures and/or vigorous exercise,
the sweat is obvious. This is called sensible perspiration. Otherwise, the sweat is
usually not obvious, and there is a low level of water loss. This is called insensible
(2) Respiration. The surfaces of the lungs must be moist to ensure the
passage of gases to and from the blood. Air is moistened within the respiratory
passages and the alveoli of the lungs. Thus, moisture passes out of the body along
with the exhaled breath.
(3) Urination. Water is also lost from the body in the form of urine. Urine
carries nitrogenous wastes of protein metabolism, dissolved in the water.
(4) Vomiting and diarrhea. During vomiting and diarrhea, the body loses
large quantities of water and dissolved electrolytes. In infants and the elderly, this loss
of water and electrolytes can be very dangerous. Sometimes, even death may result.
2-8. DISSOLVED SUBSTANCES
As mentioned before, one of the characteristics of water that makes it so
desirable is its capacity to dissolve almost anything ("universal solvent").
a. Gases. Oxygen and carbon dioxide are exchanged between air in the lungs
and the blood. They are also exchanged between the blood and the individual cells of
the body. At least in part, these gases are carried as dissolved substances in the water
of the blood.
b. Nutrients. By nutrients, we mean the end products of digestion, and vitamins
and minerals from the digestive system. By being dissolved in the water of the blood,
these nutrients are distributed to the individual cells of the body.
c. Wastes. Wastes result from the metabolic processes of the body. Wastes
are picked up from the individual cells and delivered dissolved in the water to the
excretory organs of the body, such as the kidneys.
d. Hormones. Hormones are carried from the endocrine glands to specific
target organs while dissolved in the water of the blood.
2-9. TISSUE FLUID CYCLE
That portion of the extracellular fluid found among the cells is called the tissue
fluid, or interstitial fluid. Tissue fluid originates primarily with a fluid portion of the blood
that escapes into the tissues from the capillaries. Part of this escaped fluid enters the