What is the relationship between fluids and electrolytes?

The human body contains fluid both inside its cells, called intracellular fluid (ICF), and outside its cells, called extracellular fluid (ECF). The two types of ECF are blood plasma and interstitial fluid, which is found in the microscopic spaces between cells. All body fluids contain electrolytes, which are atoms that have a positive or negative charge and are essential for the function of the nervous system and muscles, including the heart, and for maintaining blood pressure and acid-base balance in the body, or potent hydrogen (pH). The lungs, kidneys, and hypothalamus play important roles in regulating fluid and electrolyte levels in the body, as well as osmosis. An imbalance between the body's fluid and electrolytes can lead to serious illness or death.

There are five main electrolytes in the human body. They are sodium (Na+), which helps nerve cells signal each other and helps maintain electrolyte balance; potassium (K+), which helps nerves and muscles function and helps maintain the pH of body fluids; calcium (CA2+), which plays a role in blood clotting and nerve and muscle function; chloride (Cl-), which serves as a balance against positive ions; and bicarbonate (HCO3-), which helps maintain the proper pH in body fluids. Sodium, potassium, and calcium are cations, or positively charged ions, and chloride and bicarbonate are anions, or negatively charged ions. Ions have a positive or negative charge, so they can affect the pH of body fluids. The body can tolerate only very little alteration of the pH of its fluids and still function properly, so balanced levels of electrolytes are necessary for survival.

Severe dehydration causes the kidneys to stop excreting fluid in an effort to prevent further fluid loss. This causes an imbalance in electrolytes that leads to metabolic acidosis, a condition in which the pH of body fluids is too low. Acidosis causes rapid breathing, lethargy, and confusion, and can lead to shock and death. An extreme loss of Cl- due to prolonged vomiting leads to a metabolic alkalosis, in which the pH of body fluids is too high. Signs and symptoms of alkalosis include confusion, muscle spasms or spasms, feeling light-headed, nausea, and tingling in the hands or face, and coma.

The kidneys help maintain electrolyte balance by controlling the amount of fluid and electrolytes released into the urine, and the lungs remove carbon dioxide from the blood, making the blood less acidic. This is why someone with acidosis breathes rapidly to correct the condition, and why hyperventilation can lead to respiratory alkalosis. Excessive use of sedative medications can slow down the breathing process enough to cause respiratory acidosis.

In addition to maintaining fluid levels and pH, the body must also maintain a healthy concentration of fluids and electrolytes between ICF and ECF. Water moves across cell membranes through a passive process called osmosis, which works to maintain the same concentration of fluids and electrolytes, especially Na+, inside and outside the cell membrane. If there is a higher Na+ concentration outside the cell, water will move from ICF to ECF to balance the concentration. Too much of this water movement causes cells to become dehydrated, blood pressure to rise, and the brain's hypothalamus to trigger a sensation of thirst. This is called osmotic thirst, and this is why a person feels thirsty after eating salty foods.

When the same person drinks water, the concentration of Na + in the blood falls, and the water returns to the cells, restoring fluid and electrolyte balance. As a person loses body fluid through sweating, urinating, vomiting, diarrhea, or bleeding, another type of cell within the hypothalamus triggers thirst to replace fluid volume. This is called hypovolemic thirst.

The passive process of osmosis and the functions of the lungs and kidneys work together to maintain the correct levels of fluids and electrolytes within the body. This ensures that each electrolyte can do its job of keeping the heart beating and the nervous system functioning. Too much or too little electrolyte can cause serious problems. For example, too little K+ leads to acidosis, and too much K+ can stop the heart, which is the cause of death for many kidney disease patients. The lungs and kidneys also help maintain a balance of cations and anions to maintain the proper pH within body fluids so that the organs can function.

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