The blood contains numerous chemical substances, many of which have clinical significance. Abnormalities in blood chemistry may be indicative of the presence of a disease process. Electrolytes, the salts in the body, are a specific group of chemicals that must remain in balance for the body to function normally. Electrolytes analysis may be ordered by a physician as a general screening for disease or to evaluate the health of specific organs in the body. Electrolytes exist as charged particles, called ions, and are responsible for the electrical communication required for many bodily functions including brain, muscle and nerve activity. Those commonly measured include sodium, potassium, chloride, calcium, phosphorus, magnesium and bicarbonate.
Sodium is the most abundant electrolyte in the body and exists in the fluid surrounding the cells. It is a positively charged ion, a cation, that maintains the distribution of water in the body's various fluid compartments, as well as the movement of water into and out of the cells. The kidneys and adrenal glands regulate sodium concentration. Any excess sodium is excreted by the kidneys. It is a threshold substance, meaning that when levels in the body are too low, the kidneys will stop eliminating it. The normal values for serum sodium range from 135 to 145 millimoles per litre (mmol/L). Abnormal values result when there is either too much or too little sodium in relation to the amount of water present.
In contrast to sodium, which is the major electrolyte outside the cell, potassium is the primary intracellular cation. Potassium plays key roles in muscle contraction, respiration and heart function. Unlike sodium, there is no threshold level for potassium. As a result, even if blood electrolyte levels are low, the kidneys will continue to excrete it. A regular dietary intake of potassium is necessary to maintain adequate potassium levels. The normal serum potassium level ranges from 3.5 to 5.0 mmol/L.
Chloride is the primary negatively charged blood electrolyte (anion) found in the fluid surrounding the cells. Chloride anions balance sodium cations. Like sodium, this electrolyte is important in maintaining proper water distribution in the fluid compartments outside the cells. The normal range for serum chloride is 98 to 106 mmol/L.
Bicarbonate is the second most abundant anion in the blood where it serves as part of the body's buffering system. This system, in conjunction with the respiratory system and kidney function, ensure that the pH (acidity) of the blood remains within a very narrow range. Bicarbonate also acts as a means of transportation for carbon dioxide from the tissues to the lungs where it is expired as a waste product. A range of 22 to 30 mmol/L constitutes normal values. Results outside the normal may be seen in metabolic conditions, respiratory disease and kidney disease.
Calcium and Phosphorus
A close interrelationship exists between the electrolytes calcium and phosphorus. The bones contain more than 99 per cent of the body's calcium and 80 per cent of the phosphorus. Nearly all the calcium in the blood is found in the serum. In contrast, most of the phosphorus is found in the cells, with only a small amount present in the serum. A balance between these two electrolytes exists. As a result, if the level of one goes up, the level of the other goes down. The parathyroid gland regulates blood calcium and phosphorus via the release of parathyroid hormone (PTH). When calcium levels are low, the release of additional PTH causes the resorption of bone. Calcium from the bones enters the bloodstream and levels rise. Normal calcium levels are 8.5 to 10.4 milligrams per decilitre (mg/dl).
Phosphorus is necessary for the utilisation of carbohydrates and fats, and the synthesis of proteins. It is also essential in energy transport. The normal range for phosphorus is 3.0 to 4.5 mg/dl.
Magnesium is second to potassium in intracellular cations. More than 50 per cent of it is found in the bones. Most of the remainder exists in the soft tissues. The body utilises this electrolyte to activate numerous enzymatic reactions including those involving ATP (adenosine triphosphate), an important factor in energy metabolism. It is the breakdown of ATP that supplies the energy for numerous important cellular reactions. The body possesses a great ability to preserve magnesium. Levels are considered normal if they range between 1.7 to 3.0 mg/dl.
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