How to maintain a healthy water and electrolyte balance

Water and Electrolyte Balance

To maintain water and electrolyte balance, intake and excretion must balance. Water intake is stimulated by thirst, but is not finely regulated this way. Water loss is more closely regulated by the kidneys, which can increase or decrease urinary losses. The kidneys also regulate electrolyte losses in the urine over a wide range of intakes, making electrolyte imbalances unlikely in healthy people.

Thirst

The need to consume water or other fluids is signaled by the sensation of thirst. Thirst is triggered both by sensations in the mouth and signals from the brain. When a person needs water, the mouth becomes dry because less water is available to produce saliva.

When body water levels drop, the thirst center in the brain senses a decrease in the amount of water in blood and an increase in the concentration of dissolved substances in the blood. The feeling of a dry mouth and signals from the brain cause the sensation of thirst and motivate a person to drink.

Thirst is not a perfect regulator of water intake. Thirst is quenched almost as soon as fluid is consumed and long before body water balance has been restored. Also, the sensation of thirst often lags behind the actual need for water.

Athletes exercising in hot weather lose water rapidly, but they do not experience intense thirst until they have lost so much body water that their physical performance is compromised. Also, thirsty people cannot and do not always respond to thirst. To prevent dehydration, the kidneys regulate water loss more closely.

Kidneys regulate water and electrolyte excretion

The kidneys serve as a filtering system that regulates the amount of water and dissolved substances retained in the blood and excreted in urine. As blood flows through the kidneys, water and small molecules are filtered out. Some are reabsorbed and the rest are excreted in the urine. The amount of water and electrolytes that are reabsorbed depends on conditions in the body. Two hormonal systems regulate fluid and electrolyte balance.

One system responds to changes in the concentration of solutes in the blood. When the concentration is high, the pituitary gland secretes antidiuretic hormone (AD H). This hormone signals the kidneys to reabsorb water, reducing the amount lost in the urine. The reabsorbed water is then returned to the blood, preventing the solute concentration from increasing further. When the solute concentration in the blood is low, ADH levels decrease, so less water is reabsorbed and more is excreted in the urine.

The other system is activated by changes in blood pressure and relies on the ability of the kidneys to conserve sodium. Because water follows sodium by osmosis, changes in the amount of sodium retained or excreted result in changes in the amount of body water.

When the concentration of sodium in the blood decreases, water moves out of the blood, decreasing blood volume. This decreases blood pressure. When this happens, the kidneys release an enzyme called renin. renin begins a series of events that lead to the production of angiotensin ii. Angiotensin II increases blood pressure in two ways.

One is by causing blood vessel walls to constrict and the other is by stimulating the release of the hormone aldosterone, which acts on the kidneys to increase the amount of sodium reabsorbed into the blood. Water follows the reabsorbed sodium, and is returned to the blood. As blood pressure returns to normal, the release of renin and aldosterone are inhibited. This keeps blood pressure from continuing to rise.

The kidneys also regulate potassium excretion. The long-term regulation of potassium balance, like that of sodium, depends on aldosterone release, which causes the kidney to excrete potassium and retain sodium.

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