Sodium is one of the body's electrolytes Overview of Electrolytes Well over half of the body's weight is made up of water. Doctors think about the body's water as being restricted to various spaces, called fluid compartments. The three main compartments are... read more , which are minerals Overview of Minerals Minerals are necessary for the normal functioning of the body’s cells. The body needs relatively large quantities of Calcium Chloride Magnesium Phosphate read more that the body needs in relatively large amounts. Electrolytes carry an electric charge when dissolved in body fluids such as blood. (See also Overview of Electrolytes Overview of Electrolytes Well over half of the body's weight is made up of water. Doctors think about the body's water as being restricted to various spaces, called fluid compartments. The three main compartments are... read more .)
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 (see About Body Water About Body Water Water accounts for about one half to two thirds of an average person’s weight. Fat tissue has a lower percentage of water than lean tissue and women tend to have more fat, so the percentage... read more ). 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. The amount (concentration) of sodium in the blood may be
Controlling blood volume
The total amount of sodium in the body affects the amount of fluid in blood (blood volume) and around cells. The body continually monitors blood volume and sodium concentration. When either becomes too high, sensors in the heart, blood vessels, and kidneys detect the increases and stimulate the kidneys to increase sodium excretion, thus returning blood volume to normal.
When blood volume or sodium concentration becomes too low, the sensors trigger mechanisms to increase blood volume. These mechanisms include the following:
The kidneys stimulate the adrenal glands Overview of the Adrenal Glands The body has two adrenal glands, one near the top of each kidney. They are endocrine glands, which secrete hormones into the bloodstream. Each adrenal gland has two parts. Medulla: The inner... read more to secrete the hormone aldosterone. Aldosterone causes the kidneys to retain sodium and to excrete potassium. When sodium is retained, less urine is produced, eventually causing blood volume to increase.
The pituitary gland secretes vasopressin (sometimes called antidiuretic hormone). Vasopressin causes the kidneys to conserve water.
Maintaining fluid and sodium balance in older people
As people age, the body is less able to maintain fluid and sodium balance for several reasons:
Decreased thirst: As people age, they sense thirst less quickly or less intensely and thus may not drink fluids when needed.
Changes in the kidneys: Aging kidneys may become less able to reclaim water and electrolytes from the urine (concentrate urine), and, as a result, more water may be excreted in urine.
Less fluid in the body: In older people, the body contains less fluid. Only 45% of body weight is fluid in older people, compared with 60% in younger people. This change means that a slight loss of fluid and sodium, as can result from a fever or from not eating and drinking enough (sometimes for only a day or two), can have more serious consequences in older people.
Inability to obtain water: Some older people have physical problems that prevent them from getting something to drink when they are thirsty. Others may have dementia Dementia Dementia is a slow, progressive decline in mental function including memory, thinking, judgment, and the ability to learn. Typically, symptoms include memory loss, problems using language and... read more , which may prevent them from realizing they are thirsty or from saying so. These people may have to depend on other people to provide them with water.
Drugs: Many older people take drugs for high blood pressure High Blood Pressure High blood pressure (hypertension) is persistently high pressure in the arteries. Often no cause for high blood pressure can be identified, but sometimes it occurs as a result of an underlying... read more
, diabetes mellitus Diabetes Mellitus (DM) Diabetes mellitus is a disorder in which the body does not produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high. Urination and thirst are... read more , or heart disorders that can make the body excrete excess fluid or magnify the ill effects of fluid loss.
The above situations can result in losing fluid or not consuming enough fluid and thus can cause a high sodium level in blood (hypernatremia Hypernatremia (High Level of Sodium in the Blood) In hypernatremia, the level of sodium in blood is too high. Hypernatremia involves dehydration, which can have many causes, including not drinking enough fluids, diarrhea, kidney dysfunction... read more ) and/or dehydration Dehydration Dehydration is a deficiency of water in the body. Vomiting, diarrhea, excessive sweating, burns, kidney failure, and use of diuretics may cause dehydration. People feel thirsty, and as dehydration... read more . Because these situations are more common among older people, hypernatremia is also more common among them. Hypernatremia is poorly tolerated by older people and can result in confusion, coma, and death if severe.
Excess fluid and sodium also occur more commonly in older people because disorders that usually result in excess fluid (fluid overload Overhydration Overhydration is an excess of water in the body. People can develop overhydration if they have a disorder that decreases the body’s ability to excrete water or increases the body's tendency... read more )— heart failure Heart Failure (HF) Heart failure is a disorder in which the heart is unable to keep up with the demands of the body, leading to reduced blood flow, back-up (congestion) of blood in the veins and lungs, and/or... read more 
, liver disorders, and kidney disease—are also more common in older people.
A low sodium level in blood (hyponatremia Hyponatremia (Low Level of Sodium in the Blood) In hyponatremia, the level of sodium in blood is too low. A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics... read more ) is more common among older people. Hyponatremia usually results when the body retains too much fluid, as occurs in heart failure or liver disease. Hyponatremia also occurs in older people who take certain types of diuretics (thiazide diuretics such as hydrochlorothiazide), particularly if the kidneys are not functioning normally. Diuretics, which are sometimes called water pills, are drugs that help the body eliminate excess fluid. Using liquid nutritional supplements or receiving intravenous fluids that are low in sodium while in the hospital also may cause hyponatremia in older people.
