Summary

The human body regulates fluid and electrolyte balance by adjusting to internal and external stimuli to maintain homeostasis. When homeostasis is not maintained, the patient is at risk for organ system dysfunction and even death. One of the primary mechanisms by which homeostasis is maintained is through regulation of body fluid within the different fluid compartments. The two fluid compartments in the human body are the extracellular and intracellular fluid compartments. Approximately two-thirds of the total body fluid is found in the intracellular space and one-third is in the extracellular space. For cellular processes to occur normally, the balance of fluid in these two compartments must be maintained and the patient needs to have a net even, or euvolemic, total fluid balance.

Body fluid composition is maintained in a normal physiological range by regulatory mechanisms that control water and electrolyte concentrations in both the intracellular and extracellular spaces. The intracellular and extracellular spaces have different electrolyte concentration levels and there is a narrow window of normal electrolyte ranges. Slight abnormalities in electrolyte levels can have serious consequence; for this reason, it is important for nurses to recognize the signs and symptoms of an electrolyte imbalance and identify appropriate treatments for imbalances. The most common electrolytes in the human body are sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphate.

Keeping electrolytes within a narrow range is necessary for energy production, muscle contractions, stimulation of nerve impulses, and maintenance of a normal fluid balance. Nurses need to recognize how specific changes in electrolyte levels can affect the patient’s overall health. For example, signs of hypernatremia include mental confusion, irritability, and seizures. Patients with hyponatremia may present with confusion, nausea, and a headache. Normal potassium levels are needed for cardiac and muscle cell function. Signs of hyperkalemia included peaked T waves and cardiac dysrhythmias. Patients with low potassium levels may present with muscle weakness, lethargy, a thready pulse, flattened or inverted T waves, U waves, and prolonged ST segments. Normal calcium and magnesium levels are needed for nerve transmission and muscle contraction. Signs of hypercalcemia include nausea, vomiting, and muscle weakness. Signs of hypocalcemia include paresthesia and tetany. Both Chvostek’s sign and Trousseau’s sign are classic physical exam findings in patients with hypocalcemia. Patients with abnormal magnesium levels may present with both cardiac and neuromuscular findings. Common symptoms of hypermagnesemia include bradycardia, muscle weakness, and tremors. Patients with hypomagnesemia may present with vomiting, weakness, and leg cramps. In severe cases, patients may develop dysrhythmias.

Fluid and electrolyte concentrations are maintained in normal ranges through different regulatory mechanisms, passive transport, active transport, and capillary filtration. Passive transport is the movement of water and electrolytes using the principle of concentration gradients. Passive transport is subdivided into two subcategories: osmosis and diffusion. Osmosis is the movement of water of liquid down a concentration gradient whereas diffusion is the movement of solutes, such as electrolytes, down a concentration gradient. Active transport requires energy expenditure and move solutes against their concentration gradients. A prime example of active transport is the sodium–potassium pump, which uses energy to pump sodium to the extracellular space and potassium into the intracellular space. Sodium is the most common extracellular electrolyte and potassium is the most common intracellular electrolyte. Without active transport, the concentration of sodium and potassium would equalize across cell membranes, which would impede the conduction of action potentials and alter fluid balance concentration in both the extracellular and intracellular compartments. Capillary filtration is a regulatory mechanism that delivers oxygen and other nutrients from the arterial blood flow through the capillaries to target tissue. It is counterbalanced by capillary reabsorption, which is the removal of waster products from tissue to the capillaries into the venous blood flow.

Maintaining the human blood in a normal range of acid-base balance is essential for health. Many cellular processes do not function normally if the blood’s acid-base levels are off. The normal pH for the human blood is between 7.35 and 7.45. If the pH is less than 7.35, the patient has an acidosis, and if the pH is greater than 7.45, the patient has an alkalosis. There are multiple processes inside the human body that help maintain the pH within this narrow window.

Understanding acid-base imbalances is a key concept in nursing because lack of acid-base homeostasis can have a profound impact on patients’ health. Acid-base imbalances are broken down into two general categories: metabolic imbalances and respiratory imbalances. Within these two broad categories, acid-base imbalances are further described as either causing an acidosis or an alkalosis, making a total of four subtypes: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis.

The chemical buffer system is a key regulatory mechanism that maintains acid-base levels within a normal range in the intracellular fluid compartment and in the extracellular fluid compartment. The carbonic acid–sodium bicarbonate buffer system is the most widely used buffer system in the human body, accounting for 50 percent of all chemical buffering. However, this buffer system only maintains acid-base balance in the extra cellular fluid compartment.

In the intracellular fluid compartment, there are two different buffering systems: the phosphate buffering system and the protein buffering system. The protein buffering system is more common than the phosphate buffering system. Almost all proteins can act as a chemical buffer because amino acids, which are the building blocks of protein, have both a positive and negative end. By having both electrical charges on one molecule, proteins can neutralize both acids and bases. The phosphate buffering system uses two different molecules: dihydrogen phosphate and hydrogen phosphate. Dihydrogen phosphate neutralizes excess base whereas hydrogen phosphate neutralizes excess acid.

In addition to chemical buffers, there are two systems in the human body that plan an integral role in regulating acid-base homeostasis: the respiratory system, and the renal system. The respiratory system helps maintain acid-base balance by either exhaling or retaining carbon dioxide. Carbonic acid reacts with water to form carbonic acid, which is the most common acid in the bloodstream. The renal system regulates acid-base balance by either reabsorbing or excreting bicarbonate. Bicarbonate is a base. The renal system compensates for the respiratory system if there is an acid-base imbalance that is caused by a respiratory problem. Likewise, the respiratory system compensates for the renal system if there is an alteration in serum bicarbonate levels.

Recognizing cues about fluid, electrolyte, and acid-base imbalances is a cornerstone of providing safe nursing care. By recognizing cues, nurses can intervene early before serious complications occur. The cues that alert nurses to an impending imbalance depend on the alteration that the patient is facing. Nursing interventions to prevent fluid, electrolyte, and acid-base imbalances should target the specific imbalance the patient has or is at risk of developing. Nurses are also responsible for patient education and evaluating patient outcomes.