Water is the major constituent of the human body and is maintained by a delicate balance knows as the fluid-electrolyte balance, or commonly as the water-salt balance. Simply this system is maintained by thirst and the intake of water through food and drink or lost through the urine, water vapor from breathing, through the skin as perspiration, as well as in the gut when stool is passed. However in certain medical conditions, under the effect of hormones or due to the action of drugs, this balance can be upset leading to water and salt retention or loss from the body.
Broadly, water exists in two sites within the body, either within the cell (intracellular fluid) or out of the cell (extracellular fluid), in the blood, lymph or interstitial fluid. This balance between the fluid within the cells and out of the cells is maintained by the presence of electrolytes (salts), particularly sodium and potassium. If there is more more electrolytes outside the cell than within the cell, water will travel out of the cell until the pH and electrolyte concentration is balanced. The converse is also applicable until the balance is reached.
This movement of water from an area of low electrolyte concentration to one of high electrolyte concentration across a barrier like the cell membrane is known as osmosis. The difference between the electrolyte concentrations is known as osmotic pressure, the effect or ‘force’ of which determines the amount of water that will move from one site to another. Water is not usually actively transported in or out of a cell but rather depends on the water-salt difference (osmotic pressure) to move from one site to another. Salts on the other hand are actively transported, particularly through the action of hormones on the cell, and this is known as an electrolyte pump.
Types of Water & Salt Deficiency & Excess
Water-salt retention or loss is often a result of one of the following states that may occur for a number of reasons.
General Water Deficiency
This is known as hypo-osmotic fluid loss and is usually caused by :
- dehydration due to a lack of water
- medical conditions like diabetes insipidus
- excessive use of diuretics
This can be treated by administering water orally or through an IV (intravenous) glucose solution.
General Water Retention
This is known as hypo-osmotic fluid retention and may be caused by :
- intravenous fluid administration during hospitalization or surgical procedures
- administering water to patients with a salt deficiency, specifically sodium deficiency
- renal failure
- excess of the hormone, ADH (antidiuretic hormone)
Treatment would involve the administration of a hypertonic solution (fluid high in electrolytes).
Water and Sodium Loss
This state is known as iso-osmotic fluid loss and is caused by :
- dehydration due to persistent vomiting and diarrhea
- excessive perspiration (hyperhidrosis)
- excessive loss through the urine which may be a result of using diuretics, Addison’s disease, nephritis or diabetes mellitus.
This may be treated with an IV saline drip.
Water and Sodium Retention
Also known as iso-osmotic fluid retention and may be caused by :
- excess IV saline administration
- renal disease
Treatment usually involves the use of diuretics.
This is considered to be a lack of potassium within the cells where majority of the body’s potassium is usually held. This may be caused by :
- persistent vomiting and diarrhea as in cholera or viral gastritis
- use of diuretic drugs
- renal disease
Treatment usually involves the administration of potassium but this has to be done cautiously as it can affect the heart muscle.
This refers to an excess of potassium within the blood and interstitial fluid which may be caused by :
- kidney failure
- Addison’s disease
- excess potassium administration
- any cause of extensive destruction of tissue as most potassium is normally held within the cells
This may be treated by administering glucose and insulin which play an important part in increasing the potassium uptake by cells.