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Fluid Balance Within the Body

By Matthew Fox, MD ; Updated July 18, 2017

The human body is mostly composed of water. By weight, about 60 percent of the body is water, and it is separated into different compartments. Electrolytes, which are charged minerals, play a vital role in determining total fluid in the body and the individual compartments. Many diseases have a component of fluid imbalance, and methods are available to increase and decrease fluid levels. The diagnosis and treatment of disease should be undertaken only by a healthcare professional.

Body Compartments

The total amount of water in the human body is divided into intracellular fluid, which is the fluid found inside of cells, and extracellular fluid, which is found outside of cells. Intracellular fluids make up two-thirds of the total body water, and extracellular fluids make up one-third, so the majority of fluid is found inside cells. The extracellular fluid is also divided into different compartments. About three-fourths of the extracellular fluid is known as interstitial fluid. This is the fluid that surrounds cells and tissues but does not circulate. About one-fourth of the extracellular fluid is found circulating in blood. The small amount of remaining fluid in the body is found outside these compartments, such as around the brain and spinal cord and in the digestive tract, according to Linda S. Costanzo in her book, "Physiology."

Determinants of Body Fluid

In health, the body has a set point of fluid balance that it tightly regulates. Electrolytes such as sodium carry an electrical charge that attracts water. As such, electrolyte regulation is an integral part of fluid balance. Proteins also carry an electrical charge, and their levels in the blood and interstitial fluid also determine fluid balance. Hormones released by the nervous system, the adrenal gland, kidneys and other organs signal the body to conserve or expel fluids and electrolytes. For example, if blood pressure is low, the kidneys release a hormone called renin. Renin leads to the production of angiotensin I in the blood. Angiotensin I is converted to angiotensin II in the lungs. This hormone raises the blood pressure by narrowing the blood vessels. It also stimulates the release of the hormone aldosterone from the kidneys. Both aldosterone and angiotensin II stimulate the kidneys to conserve sodium and fluid, raising the blood pressure, explains Costanzo.

Diseases of Body Fluid

Diseases of the kidneys are often accompanied by fluid imbalances. If the kidneys shut down altogether, they will not be able to rid the body of fluid through the urine, and it will build up. If the blood vessels in the kidneys are damaged, they may leak protein into the urine. This can cause fluid to leak from the blood vessels into the tissues. Similarly, in liver disease, the liver may not make blood proteins, and this causes fluid to leak into the tissues. Some types of heart diseases are also associated with fluid imbalances. Brain and hormonal disorders can also affect fluid levels, as can trauma with blood loss.

Decreasing and Increasing Body Fluid

Many medical options are available to decrease and increase body fluid. Decreasing body fluid often involves limiting the fluid intake in the diet and taking medicines called diuretics. These medicines act on the kidneys to increase urine production. Dialysis involves the use of a machine to filter the blood and modify its composition and fluid content, like an artificial kidney. Increasing body fluid may involve administering fluids or blood products intravenously. While these types of methods can improve fluid balance, the most active approach is to alleviate the underlying cause as much as possible.

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