Several Mechanisms in the Body Work Together to Maintain Water Balance
When your body needs water, nerve centers deep within the brain are stimulated, resulting in the sensation of thirst.
The sensation becomes stronger as the body’s need for water increases, motivating a person to drink the needed fluids.
When the body has excess water, thirst is suppressed.
Vasopressin – The Anti-diuretic Hormone
When the body is low in water, the pituitary gland (located at the base of the brain) secretes vasopressin – the anti-diuretic hormone, into the bloodstream, which stimulates the kidneys to conserve water and excrete less urine.
When the body has excess water, the pituitary gland secretes little anti-diuretic hormone, enabling the kidneys to excrete excess water in the urine.
The body can move water from one area to another as needed. When water loss is severe, the amount of water in the bloodstream decreases, so the body moves water from inside the cells to the bloodstream until it can be replaced through increased intake of fluids.
When the body has excess water, the body moves water from the bloodstream into and around the cells. In this way, blood volume and blood pressure can be kept relatively constant.
Electrolytes, or mineral salts, such as sodium and potassium, are dissolved in the water in the body.
Water balance and electrolyte balance are closely linked.
The body works to keep the total amount of water and the levels of electrolytes in the bloodstream constant.
For example, when the sodium level becomes too high, thirst develops. In addition, a hormone secreted by the brain in response to thirst causes the kidneys to excrete less urine. When the sodium level becomes too low, the kidneys excrete more urine, which decreases the amount of water in the bloodstream, again restoring the balance.
Most of the body’s sodium is located in blood and in the fluid around cells.
Sodium helps the body keep fluids in a normal balance. Sodium plays a key role in normal nerve and muscle function.
The body obtains sodium through food and drink and loses it primarily in sweat and urine.
Healthy kidneys maintain a consistent level of sodium in the body by adjusting the amount excreted in the urine.
When sodium consumption and loss are not in balance, the total amount of sodium in the body is affected.
Controlling Blood Volume
The total amount of sodium affects the amount of fluid in blood and around cells.
The body continually monitors blood volume and sodium (and other electrolyte) concentrations.
When either becomes too high or too low, sensors in the heart, blood vessels, and kidneys detect the change and stimulate the kidneys to respond accordingly.
Most of your body’s potassium is located inside the cells.
Potassium is necessary for the normal functioning of cells, nerves, and muscles, but the body must maintain the potassium level in blood within a narrow range.
A potassium level that is too high or too low can have serious consequences, such as an abnormal heart rhythm or even stopping of the heart (cardiac arrest).
Your body can use the potassium stored within cells to help maintain a constant level of potassium in blood, matching the amount of potassium consumed with the amount lost.
You get potassium from food and drinks that contain electrolytes (including potassium).
You lose potassium through urine and sweat, and also through the digestive tract.
Healthy kidneys can adjust potassium excretion to match changes in consumption.