Demeclocycline

Definition

Electrolytes are positively or negatively charged particles that readily dissolve in water. The predominant positively charged electrolytes in the body are sodium, potassium, calcium, and magnesium; while negatively charged electrolytes include chloride, phosphates, and bicarbonate.

Description

Salts are chemical compounds composed of atoms that carry electrical charges. Dissolved in water, the components in a salt exist as ions. Collectively, these ions are called electrolytes. Electrolytes are dissolved in different compartments of body water including: the serum portion of the blood, inside the cells (intracellular), and outside the cells (extracellular). The concentration of these electrolytes varies considerably from one area to the other. However, there is a narrow concentration limit of these electrolytes that the body must maintain within each of these compartments. The body transfers electrolytes intracellularly and extracellularly as required to maintain electrolyte balance. Electrolyte concentrations of extracellular fluid can be measured in a blood sample.

Function

The kidney filters electrolytes in blood and maintains a balance by excreting the proper amount in the urine. An electrolyte's concentration in a solution of dissolved salts can be measured as the amount in milliequivalents (mEq) per volume of solution (i.e. per liter). Electrolytes have many functions and roles in the body. The concentration of electrolytes must be maintained within a narrow range within the blood, otherwise deleterious physiological effects may occur. Several of the most important electrolytes will be discussed individually in the following sections.

Role in human health

Sodium balance

The largest portion of the body's sodium reserves is in the extracellular fluid, which includes the blood plasma. The kidneys function to control the sodium excreted in the urine; thus the level of sodium in the body is relatively constant on a daily basis. An upset between intake of sodium (through dietary consumption) and output (in urine and sweat) creates an imbalance, affecting the total amount of sodium in the body. Variations in the total amount of sodium are related to the volume of water found in the blood.

A decrease in the overall amount of sodium does not necessarily cause the concentration of blood sodium to fall, but may decrease blood volume. Low blood volume, such as occurs with hemorrhage, signals the kidneys to conserve both water and sodium through stimulation of aldosterone. This helps to return blood volume toward normal, by increasing the amount of extracellular fluid sodium. With an excess of sodium in the body, blood volume may rise. This increase in blood volume initiates an accumulation of extracellular fluid, often in the feet, ankles, and lower legs, resulting in a condition known as edema.

The body maintains extracellular fluid sodium concentration homeostasis through the thirst mechanism and regulation of kidney water excretion by antidiuretic hormone (ADH). When sodium concentration-as opposed to too much total sodium-is too high, thirst prompts water intake and, at the same time, the ADH signals to the kidneys to conserve water, by increasing water absorption by the organs and passing less water into the excretory system.

Common disorders

The electrolytes involved in disorders of salt balance are most often sodium, potassium, calcium, phosphate, and magnesium. The concentration of blood chloride is usually similar to the blood sodium concentration, while bicarbonate is related to acid-base balance.

Sodium balance

Hyponatremia

The most common electrolyte disorder is hyponatremia, it occurs in almost 1% of all patients hospital admissions. Hyponatremia is a condition characterized by low sodium in the blood, below 136 mEq per liter of blood. In hyponatremia, the sodium concentration has been overdiluted by an excess of water or a loss of sodium in the body. Hyponatremia may result from intraveous administration of water to hospitalized patients or can also occur with small amounts of water consumption in those who have impaired kidney function and several other conditions such as liver cirrhosis, heart failure, underactive adrenal glands as with Addison's disease, and various antidiuretic hormone disorders. Over 50% of hospitalized patients with AIDS have been reported to suffer from hyponatremia. Lethargy and confusion are typically the first signs of hyponatremia. Muscle twitching and seizures may occur as hyponatremia progresses with risk of stupor, coma, and death in the most severe cases. Due to the effects on the central nervous system, mortality risk is considerably greater in acute hyponatremia than in chronic hyponatremia. Other factors that reduce survival are the presence of debilitating illnesses such as alcoholism, hepatic cirrhosis, heart failure, or malignancy.

Hypernatremia

Hypernatremia is a condition characterized by a high concentration of sodium in the blood, above 145 mEq per liter of blood. There is too little water compared to the amount of sodium in the blood, often resulting from a low intake of water. Profuse sweating, vomiting, fever, diarrhea, or abnormal kidney function may result in hypernatremia. With age, there is a decreased thirst sensation; therefore, hypernatremia is more common in the elderly. Aging reduces the kidney's ability to concentrate urine; therefore, taking diuretics may further exacerbate hypernatremia. Hypernatremia is very serious, particularly in the elderly. Almost half of individuals hospitalized for this condition will die, although it is often secondary to other illnesses.

Major causes of high sodium levels include:

• limited water access, particularly when combined with any other cause

• excess water loss due to profuse sweating, vomiting, fever, diarrhea

• disorders of other electrolytes

• head trauma or neurosurgery involving the pituitary gland

• use of drugs including lithium, diuretics, demeclocycline

• diabetes insipidus

• sickle cell disease

As with hyponatremia, the major symptoms of hypernatremia result from brain dysfunction. Severe hypernatremia can lead to confusion, muscle twitching, seizures, coma, and death. The effects on central nervous system hyperosmolality and the seriousness of the underlying illness lead to greater mortality in acute hypernatremia compared to chronic hypernatremia.

Calcium balance

The body's calcium reserves are predominately stored in bones, although the blood and cells also contain calcium. Calcium is necessary for proper functioning in many areas of the body including nerve conduction, muscle contraction, and enzyme functions. Like other electrolytes, the body controls calcium levels both in blood and cells. Calcium from the diet is absorbed in the gastrointestinal tract while the excess is excreted in the urine. A minimum of 500-1000 mg of calcium is required daily in order to maintain a normal calcium concentration. Normally, the body transfers calcium to the blood from the bones to maintain calcium homeostasis. If calcium intake falls short of the requirement, too much calcium will be mobilized from the bones, weakening the bones and contributing to osteoporosis.

Parathyroid hormone and calcitonin regulate the amount of calcium in the blood. There are four parathyroid glands located in the neck that increase secretion of parathyroid hormone when the calcium concentration falls too low. Consequently, the gastrointestinal tract is stimulated to absorb more calcium from the blood, release a greater amount of calcium from the bones, and to excrete less in the urine. At the same time, parathyroid hormone induces the kidneys to activate vitamin D which increases uptake of calcium from the gastrointestinal tract. Calcitonin is a hormone produced by the parathyroid, thyroid, and thymus glands. It acts to lower the calcium concentration in blood by enhancing uptake of calcium into the bones.

Common disorders

Calcium balance

Hypocalcemia

A low calcium blood level is referred to as hypocalcemia. Calcium is measured in extracellular fluid in two forms: total calcium concentration and ionized calcium concentration. About 50% of the total calcium concentration in the plasma exists in ionized form, which is the form that has biological activity at cell membranes. The remainder is either bound to the plasma proteins (about 40%) or complexed in the non-ionized form (about 10%) with anions such as phosphate. In hypocalcemia the total calcium concentration falls below 2.4 mEq/l in the extracellular fluid. Hypocalcemia can result from a number of problems. The most common reason is an inability to mobilize calcium from the bones or a chronic loss of calcium in the urine.

Other causes of hypocalcemia include:

• low blood albumin concentration

• hypoparathyroidism

• vitamin D deficiency

• renal failure

• magnesium depletion

• acute pancreatitis

• hypoproteinemia

• septic shock

• hyperphosphatemia

• drugs such as those used to treat hypercalcemia; anticonvulsants

• excessive secretion of calcitonin

An abnormally low blood calcium concentration may not produce any symptoms. However, over time the lack of calcium in the blood can affect brain function causing neurologic symptoms such as memory loss, depression, confusion, delirium, and hallucinations. Once calcium levels return to normal, these symptoms are reversible. Very severe cases of hypocalcemia can lead to seizures, tetany, and muscle spasms in the throat, affecting breathing. The condition is usually first discovered during routine blood tests because often there are no symptoms evident.

Hypercalcemia

A high calcium blood level is referred to as hypercalcemia. The blood calcium concentration rises above 10.5 mg per deciliter of blood. Increased gastrointestinal tract absorption or increased intake of calcium may lead to hypercalcemia. Individuals who consume large amounts of calcium or who take calcium containing antacids can develop hypercalcemia. Absorption of calcium can be increased in the gastrointestinal tract with an overdose of vitamin D. The condition is usually first discovered during routine blood tests because hypercalcemia often doesn't have any symptoms at all.

If symptoms occur, typically the earliest are:

• constipation

• loss of appetite

• nausea and vomiting

• abdominal pain

Large amounts of urine may be produced by the kidneys. Due to excess urine production, fluid levels in the body decrease and may lead to dehydration. Severe hypercalcemia may induce brain dysfunction symptoms such as weakness, confusion, emotional disturbances, delirium, hallucinations, and coma. Additionally, abnormal heart rhythms and death may follow. In chronic conditions, kidney stones or calcium-containing crystals that can cause permanent damage may form.

Potassium balance

Potassium plays a major part in cell metabolism and in nerve and muscle cell function. Most of the body's potassium is located intracellularly, not extracellularly or in the blood. Too high or low concentrations of blood potassium can have serious effects such as an abnormal heart rhythm or cardiac arrest. The potassium concentration in the blood is maintained with the assistance of intracellular potassium. Like other electrolytes, potassium balance is regulated through gastrointestinal tract absorption of potassium in food, and by excretion of potassium by the kidneys. Some potassium is lost in the gastrointestinal tract, but most is lost through urine. Some conditions and drugs influence potassium balance intracellularly, also affecting blood concentrations.

High sources of dietary potassium are:

• bananas

• melons

• tomatoes

• oranges

• potatoes and sweet potatoes

• green leafy vegetables such as spinach, turnip greens, collard greens, kale etc.

• most peas and beans

• potassium supplements

• salt substitutes (potassium chloride)

Common disorders

Potassium balance

Hypokalemia

A low potassium blood level is referred to as hypokalemia. It occurs when the blood potassium concentration falls below 3.8 mEq per liter of blood. Hypokalemia is common in the elderly. Common causes include decreased intake of potassium during acute illness, nausea and vomiting, and treatment with thiazide or loop diuretics. About 20% of patients receiving thiazide diuretics develop hypokalemia, which is dose-dependent but usually mild. Since several foods contain potassium, hypokalemia is not typically due to a low intake. It is usually due to malfunction of the kidneys or abnormal loss through the gastrointestinal tract. People with heart disease have to be especially cautious regarding hypokalemia (particularly when taking digoxin), because they are prone to developing abnormal heart rhythms.

Potassium usually can be replaced relatively easily by eating foods rich in potassium or by taking potassium salts (potassium chloride) orally.

Hyperkalemia

A high level of potassium in the blood is referred to as hyperkalemia. It occurs when the blood potassium concentration rises above 5.0 mEq per liter of blood. Hyperkalemia typically results when the kidneys excrete too little potassium.

Some common causes are due to:

• drugs which block potassium excretion (angiotensin-converting enzyme [ACE] inhibitors, triamterene, and spironolactone)

• Addison's disease

• kidney failure

• a sudden release of potassium from the cell reservoir in such cases as when a large amount of muscle tissue is destroyed (crush injury) or severe burn injuries, or an overdose on crack cocaine

The kidney's ability to excrete potassium is overwhelmed due to a rapid influx into the blood, resulting in life-threatening hyperkalemia. Generally, hyperkalemia is more dangerous than hypokalemia. A blood potassium concentration above 5.5 mEq/liter starts to affect the electrical conducting system in the heart. If the concentration continues to increase, the heart rhythm becomes irregular which may cause the heart to eventually stop.

Mild hyperkalemia often may not produce any symptoms. Symptoms may include an irregular heartbeat that could be experienced as palpitations. Hyperkalemia is typically first diagnosed during a routine blood test or by examining changes in an electrocardiogram. Severe deficiencies may lead to muscular weakness, twitches, and paralysis.

Magnesium balance

Magnesium influences the function of many enzymes. Dietary intake is essential to maintain normal levels. The body's magnesium stores are predominately found in bone with little appearing in the blood. Excess is excreted in the urine or stool.

Common disorders

Magnesium balance

Hypomagnesemia

A low level of magnesium in the blood is known as hypomagnesemia. The level of magnesium in the blood decreases below 1.6 mEq per liter of blood. Metabolic and nutritional disorders are usually the culprit of hypomagnesemia, most often when intake of magnesium is decreased during starvation or intestinal malabsorption compounded with greater kidney excretion.

Symptoms of hypomagnesemia may include:

• loss of appetite

• nausea and vomiting

• sleepiness

• weakness

• personality changes

• muscle spasms

• tremors

When hypomagnesemia occurs along with hypocalcemia, the magnesium must be replaced before successful treatment of the calcium disorder.

Hypermagnesemia

A high level of magnesium in the blood is referred to as hypermagnesemia. The blood magnesium concentration rises above 2.1 mEq per liter of blood. Hypermagnesemia is quite rare unless people with kidney failure are given magnesium salts or consume magnesium-containing drugs such as antacids. Weakness, low blood pressure, and impaired breathing can result and the heart may stop if the concentration increases above 12 to 15 mEq per liter.

Phosphate regulation

Phosphorus occurs in the body almost solely in the form of phosphate, which is composed of one phosphorus and four oxygen atoms. Phosphate is found mostly in bones, although a significant amount is found intracellularly. It plays a role in energy metabolism and acid-base regulation, and it is used as a building block for DNA. Phosphate is excreted in the urine and stool.

Sources of phosphate include:

• spinach, turnip greens, collard greens, kale, and other green leafy vegetables

• milk and dairy products

• nuts

• chocolate

• many peas and beans

• dark-colored soft drinks

Key Terms

Antidiuretic hormone (ADH)

A hormone that encourages the kidney to retain water when body stores are low.

Bicarbonate

A salt of carbonic acid produced by neutralizing a hydrogen ion.

Dehydration

A deficit of body water that results when the output of water exceeds intake.

Diuretic

An agent or drug that eliminates excessive water in the body by increasing the flow of urine.

Edema

An increase in blood volume instigates an accumulation of extracellular fluid resulting in swelling of the feet, ankles, and lower legs.

Electrolyte

A substance such as an acid, bases, or salt. An electrolyte's water solution will conduct an electric current and ionizes. Acids, bases, and salts are electrolytes.

Homeostasis

An organism's regulation of body processes to maintain internal equilibrium in temperature and fluid content.

Hypoparathyroidism

A condition resulting from an absence or deficiency in parathyroid hormone. It is characterized by hypocalcemia and hyperphosphatemia.

Tetany

A general stiffening and spasms of the muscles that can occur in severe cases of hypocalcemia.