Jethalal S. Zaveri
Regulation of Total Body Fluids
A human body passes through an evolution as it develops. An embryo is 97% water, a newborn infant is 77% and an adult man only about 60 to 65%. Because of more water-free adipose tissues in a woman, water-content is 54%. The composition of the fluid media remains relatively constant. The dynamic equilibrium is maintained by the complex interaction of respiratory, circulatory, digestive, excretory, endocrine and nervous systems. It is one of the most important aspects of homeostasis i.e. optimum internal conditions of health.
There is a high degree of internal organisation—structural differentiation and compartmentalization—of the body fluids. About two-thirds of the total body fluid is intracellular (found within the cells). Interstitial fluid which bathes the cells, fluid in the blood and lymph vessels, the cerebrospinal fluid and some other special fluids are extracellular.
Water enters the body through the digestive tract in the form of liquid intake and food consituents. It leaves the body through the lungs, kidneys, the bowel and the skin. To maintain balance, intake must precisely balance the output. If the lost fluid is not replaced promptly, the person would rapidly become dehydrated, placing an increasing strain on the body systems. On the other hand, if fluid intake exceeded output, edema—an excess fluid in the interstitial space—would result, placing a strain on the cardio-vascular system. A dynamic equilibrium is maintained by continual adjustments both of intake—through the thirst mechanism and of output—mainly by variation of urine formation.
Input
The mechanism of thirst is apparently simple, but actually rather complex and not entirely understood. A drying of mucous membrane of the mouth and a cessation of saliva flow play an important role in the immediate sensation of thirst and serve as a stimulus to fluid intake. But these sensations are controlled by the thirst centre in the hypothalamus. When the extracellular fluids are too concentrated, impulses from the control centre somehow give rise to the sensation of thirst. The feedback system is tuned to conditions of excess loss of fluid. For example, hot weather or participation in sports or other strenuous exercises force us to drink frequently, because fluid pours out by the sweat glands as the body works to dissipate the excess heat. The moistening of the mouth and the distension of the stomach, resulting from the act of drinking, initiate feedback impulses which turn off the stimulation of the thirst centre. When large quantities of fluids are taken in, water is rapidly absorbed from the intestinal tract into the plasma compartment. Both the blood volume and cardiac output may be temporarily increased. But soon the fluid passes into the interstitial compartment and the kidneys step up their elimination of water. Balance is rapidly restored. Thus within the body there is a continual interchange of water and other materials among the compartments. Interstitial fluid plays an important role in the body's fluid dynamics.
Output
We have an efficient system of sweat glands to lose heat through evaporation of water. Even on a cool day, we continually lose water through the skin. This mechanism is an important part of the body's temperature-control-system but maybe a threat to the body's fluid-balance, if intake is limited. Under normal condition the fluid-loss through skin is about 500 ml. per day. Exhaled air is also laden with moisture. Water-loss through the lungs varies the humidity and temperature of the air. On the average, it comes to about 300 ml. per day. Under extreme conditions, loss through the skin and exhaled air may reach as much as 20 litres per day.
Dietary fibre makes bowel function more rapid and effective. The feces are softer and move more rapidly through the bowel. A lack of fibre and an insufficient fluid intake may result in the feces becoming hard and difficult to pass, setting up a vicious cycle of constipation. Normally 200 ml. of water is lost in the feces each day.
Kidneys are the main spillway for the body's excess water. It provides wide latitude for regualtion of fluid output. The average daily output in the urine is about 1500 ml., but this amount may vary greatly, depending on the fluid-intake and the amount of water lost through other channels of excretion. If intake is less, or if more water is lost in sweating or diarrhea, the kidneys put out small amounts of concentrated urine. If intake is excessive, it is excreted by producing large quantities of dilute urine. Regardless of fluid-intake, a certain minimum amount of urine must be produced, containing sufficient water to keep dissolved the excreted wastes".
Body Water-reserves
One type of stress that the body is sometimes called on to bear is a loss of water. Prolonged vomiting or diarrhea can place strenuous demands on the body's water reserves. Under such conditions, people with the large percentage of body-water stand the best chance for survival. Obesity is one factor that reduces the body's fluid reserves, since adipose tissue is essentially water-free. The progressive tissue-dehydration that occurs in old age produces great depletion of water-reserve.
Practical Considerations
A pregnant woman is usually advised to cut down on salt in her diet to avoid troublesome swelling of feet and ankles. Fluid retention may also be a problem during menstrual periods. An excess of fluid in the interstitial space is called 'edema'.
Obstruction of the venous system or a local dilation of arterioles may raise the capillary blood-pressure causing fluid to leak out. Lymphatic drainage may be blocked by a tumor or inflammation, causing excess fluid to build up in the tissue spaces. When the kidneys fail to put out enough urine, edema results. An imbalance of electrolytes[1] in the body produces a disturbance of the fluid-balance and vice versa.
Profuse sweating, diarrhea etc. can result in dehydration (decrease in the fluid-volume). Consequently, circulation is impaired, the blood-flow to the kidneys is reduced, and waste-products accumulate. Fluids must be replaced promptly either by mouth or by direct infusion into the blood-stream in order to prevent permanent damage.
Burns over large areas present special problem, since they exude large quantities of plasma. When patients cannot take in sufficient liquids and food by mouth, intravenous infusion of carbohydrate solutions provides a ready source of energy and effective replacement of fluid.
Repeated enemas of water may wash out ions and produce electrolyte imbalance. It is preferable to use an isotomic solution of common salt (sodium chloride 0.85%) for such treatment.
