How Does the Body Excrete Fluid? Treatment of Electrolyte Imbalance

The kidneys are essential for regulating the volume and composition of body fluids. This page outlines the main regulatory systems involving the kidneys that control the volume, sodium and potassium concentrations and pH of body fluids. The most critical concept to understand is how water and sodium regulation are integrated to defend the body against all possible disturbances in the volume and osmolarity of body fluids. Simple examples of such disturbances include dehydration, blood loss, salt intake and plain water intake.

What Is Body Water Balance?

Water balance in the body is achieved by ensuring that the amount of water consumed in food and drink (and produced by metabolism) equals the amount of water excreted. The intake side is regulated by behavioral mechanisms such as thirst and salt cravings. While about one liter of water a day is lost through the skin, lungs and feces, the kidneys are the main site of regulated water excretion. One way the kidneys directly control the volume of body fluids is through the amount of water excreted in the urine. The kidneys can conserve water by producing urine that is concentrated relative to plasma, or eliminate excess water from the body by producing urine that is dilute relative to plasma.

Direct control of water excretion by the kidneys is carried out by vasopressin, or antidiuretic hormone (ADH), a peptide hormone secreted by the hypothalamus. ADH causes water channels to be inserted into the membranes of the cells lining the collecting ducts, allowing water to be reabsorbed. Without ADH, little water is reabsorbed in the collecting ducts and dilute urine is excreted.

ADH secretion is influenced by several factors (note that anything that stimulates ADH secretion also stimulates thirst):

1. By special receptors in the hypothalamus that are sensitive to increases in plasma osmolarity (when the plasma becomes too concentrated). These stimulate ADH secretion.

2. By stretch receptors in the atria of the heart, which are activated when a larger than normal volume of blood returns to the heart from the veins. These inhibit ADH secretion, because the body wants to get rid of the excess fluid volume.

3. By stretch receptors in the aorta and carotid arteries, which are stimulated when blood pressure falls. These stimulate ADH secretion, because the body wants to maintain enough volume to generate the blood pressure needed to deliver blood to the tissues.

What Is Body Sodium Balance?

In addition to total volume, the osmolarity of body fluids (the amount of solute per unit volume) is also tightly regulated. Extreme changes in osmolarity cause cells to shrink or swell, damaging or destroying cellular structures and disrupting normal cellular function.

Osmolarity is regulated by balancing sodium intake and excretion with water. (Sodium is the most important solute in extracellular fluids, so it effectively determines the osmolarity of extracellular fluids.)

An important concept is that the regulation of osmolarity must be integrated with volume regulation, because changes in water volume alone have diluting or concentrating effects on body fluids. For example, when you are dehydrated you lose proportionally more water than solute (sodium), so the osmolarity of your body fluids increases. In this situation, the body must conserve water but not sodium, thereby stopping the rise in osmolarity. However, if you lose a large amount of blood from trauma or surgery, your losses of sodium and water are proportional to the composition of body fluids. In this case, the body must conserve both water and sodium.

As noted above, ADH plays a role in lowering osmolarity (reducing sodium concentration) by increasing water reabsorption in the kidneys, thereby helping to dilute body fluids. To prevent osmolarity from falling below normal, the kidneys also have a regulated mechanism for reabsorbing sodium in the distal nephron. This mechanism is controlled by aldosterone, a steroid hormone produced by the adrenal cortex. Aldosterone secretion is controlled in two ways:

1. The adrenal cortex directly senses plasma osmolarity. When osmolarity rises above normal, aldosterone secretion is inhibited. A lack of aldosterone causes less sodium to be reabsorbed in the distal tubule. Note that in this setting ADH secretion will increase to conserve water, thereby complementing the effect of low aldosterone levels in reducing the osmolarity of body fluids. The net effect on urine excretion is a decrease in the amount of urine excreted and an increase in urine osmolarity.

2. The kidneys sense low blood pressure (which results in lower filtration rates and lower flow through the tubule). This triggers a complex response to raise blood pressure and conserve volume. Specialized cells in the afferent and efferent arterioles (juxtaglomerular cells) produce renin, a peptide hormone that initiates a hormonal cascade that ultimately produces angiotensin II. Angiotensin II stimulates the adrenal cortex to produce aldosterone.

*Note that in this setting, where the body is trying to conserve volume, ADH secretion is also stimulated and water reabsorption increases. Because aldosterone also acts to increase sodium reabsorption, the net effect is the retention of fluid with roughly the same osmolarity as body fluids. The net effect on urine excretion is a decrease in the amount of urine excreted, with a lower osmolarity than in the previous example.

How Is Body Water Balance Maintained?

Several mechanisms work together in the body to maintain water balance. These include:

Several mechanisms work together in the body to maintain water balance.

• Thirst
• The interaction of the pituitary gland and the kidneys
• Osmosis

Thirst is one of the most important mechanisms for maintaining water balance. When the body needs water, nerve centers deep in the brain are stimulated and a sensation of thirst arises. The more the body needs water, the stronger the sensation becomes, motivating the person to drink the fluids they need. When there is excess water in the body, thirst is suppressed.

The interaction between the pituitary gland and the kidneys provides another mechanism. When the body is low on water, the pituitary gland releases vasopressin (also known as antidiuretic hormone) into the bloodstream. Vasopressin stimulates the kidneys to conserve water and excrete less urine. When there is excess water in the body, the pituitary gland releases only a small amount of vasopressin, allowing the kidneys to excrete the excess water in the urine.

In osmosis, water, flows passively from one area or compartment of the body to another. This passive flow allows the larger volumes of fluid in the cells and in the spaces around the cells to act as a reservoir that protects the more critical but smaller volume of fluid in the blood vessels from dehydration.

What Is Dehydration?

• Vomiting, diarrhea, excessive sweating, burns, kidney failure and the use of diuretics can cause dehydration.
• People feel thirsty, and as dehydration worsens they may sweat less and pass less urine.
• If dehydration is severe, the person may experience confusion or dizziness.
• Treatment consists of replacing the lost water and the mineral salts dissolved in the blood (electrolytes, such as sodium and potassium), usually by drinking and sometimes with intravenous fluids.

Dehydration occurs when the body loses more water than it takes in. Vomiting, diarrhea, the use of medications that increase urine output (diuretics), excessive sweating (for example, during heat waves, especially with prolonged exertion) and low water intake can lead to dehydration.

Dehydration is especially common among older adults because their thirst centers may not work as well as those of younger people. As a result, some older people may not notice that they are dehydrated. Certain disorders, such as diabetes mellitus, arginine vasopressin deficiency and Addison disease, can increase urine output and thus lead to dehydration.

Dehydration is also common in infants and children, because the amount of fluid lost during diarrhea or vomiting can represent a larger proportion of their body fluids than in older children and adults.

Symptoms of mild to moderate dehydration include:

• Thirst
• Reduced sweating
• Reduced skin elasticity
• Reduced urine output
• Dry mouth

In severe dehydration, the sensation of thirst may decrease and blood pressure may fall, which can cause dizziness or fainting, especially when standing up (a condition called orthostatic hypotension). If dehydration continues, shock and serious damage to internal organs such as the kidneys, liver and brain occur. Brain cells are particularly sensitive to more severe levels of dehydration. Consequently, confusion is one of the best indicators that dehydration has become severe. Very severe dehydration can lead to coma and death.

Overhydration

• Overhydration can develop in people who have a disorder that reduces the body's ability to excrete water or increases the body's tendency to retain water.
• Drinking a lot of water rarely causes overhydration, because normal kidneys easily excrete excess water.
• Most often there are no symptoms, but people with overhydration may experience confusion or seizures.
• Fluid intake is restricted, and diuretics may be given.

Overhydration occurs when the body takes in more water than it loses.

Overhydration can occur when people drink far more water than their body needs. People who drink excessive amounts of water to prevent dehydration, especially athletes, can develop overhydration. People may also drink excessive amounts of water because of a psychiatric disorder called psychogenic polydipsia. The result is too much water and not enough sodium (an electrolyte). Therefore, overhydration usually causes low sodium levels in the blood (hyponatremia), which can be dangerous. However, if the pituitary gland, kidneys, liver and heart are working normally, drinking large amounts of water does not usually cause overhydration. To exceed the body's ability to excrete water, a young adult with normal kidney function would have to regularly drink more than 6 gallons (about 23 liters) of water a day.

Overhydration is much more common in people whose kidneys do not excrete urine normally, for example people with heart, kidney or liver disorders, or premature infants whose kidneys are immature (see Focus on Aging: Water Balance). Certain medications, such as some diuretics and some antidepressants, can also cause overhydration in susceptible people.

Overhydration can also result from the syndrome of inappropriate antidiuretic hormone secretion. In this syndrome, the pituitary gland releases too much vasopressin (also known as antidiuretic hormone), stimulating the kidneys to conserve water when it is not needed.