Fluids and Electrolytes

Fluids and Electrolytes

The body is mostly liquid — various electrolytes dissolved in water. Electrolytes are ions (electrically charged versions) of essential elements — predominantly sodium (Na+), chloride (Cl), hydrogen (H+), bicarbonate (HCO3), calcium (Ca2+), potassium (K+), sulfate (SO42−), magnesium (Mg+), and phosphate (PO43−). Only ionic forms of elements can dissolve or combine with other elements. Electrolyte balance must remain in a narrow range for the body to function. The kidneys maintain chemical balance throughout the body by producing and eliminating urine. They regulate the volume, electrolyte concentration, and acid-base balance of body fluids; detoxify and eliminate wastes; and regulate blood pressure by regulating fluid volume. The skin and lungs also play a role in fluid and electrolyte balance. Sweating results in loss of sodium and water; every breath contains water vapor.

Fluid balance

The kidneys maintain fluid balance in the body by regulating the amount and components of fluid inside and around the cells.


The fluid inside each cell is called intracellular fluid (ICF). Each cell has its own mixture of components in ICF, but the amounts of these substances are similar in every cell. ICF contains large amounts of potassium, magnesium, and phosphate ions.


The fluid in the spaces outside the cells, called extracellular fluid (ECF), is constantly moving. Normally, ECF includes blood plasma and interstitial fluid (the fluid between cells in tissues); in some pathologic states it accumulates around organs in the chest or abdomen (sometimes referred to as third spacing). However, this fluid isn’t available to expand ICF.

ECF is rapidly transported through the body by circulating blood and between blood and tissue fluids by fluid and electrolyte exchange across the capillary walls. It contains large amounts of sodium, chloride, and bicarbonate ions, plus such cell nutrients as oxygen, glucose, fatty acids, and amino acids. ECF also contains carbon dioxide, transported from the cells to the lungs for excretion, and other cellular products, transported from the cells to the kidneys for excretion.

The kidneys maintain the volume and composition of ECF and, to a lesser extent, ICF by continually exchanging water and ionic solutes, such as hydrogen, sodium, potassium, chloride, bicarbonate, sulfate, and phosphate ions, across the cell membranes of the renal tubules.


Two sets of forces determine the exchange of fluid between blood plasma and interstitial fluid. All four forces act to equalize concentrations of fluids, electrolytes, and proteins on both sides of the capillary wall.

Forces that tend to move fluid from the vessels to the interstitial fluid include:

♦ hydrostatic pressure of blood (the outward pressure of plasma against the walls of capillaries)

♦ osmotic pressure of tissue fluid (the tendency of ions to move across a semipermeable membrane —the capillary wall—from an area of greater concentration to one of lower concentration).

Forces that tend to move fluid into vessels include:

♦ oncotic pressure of plasma proteins (similar to osmosis, but because proteins can’t cross the vessel wall, they attract fluid into the area of greater concentration)

♦ hydrostatic pressure of interstitial fluid (inward pressure against the capillary walls).

Hydrostatic pressure at the arteriolar end of the capillary bed is greater than it is at the venular end. Oncotic pressure of plasma increases slightly at the venular end as fluid escapes. When the endothelial barrier (capillary wall) is normal and intact, fluid escapes at the arteriolar end of the capillary bed and is returned at the venular end. The small amount of fluid lost from the capillaries into the interstitial tissue spaces is drained off through the lymphatic system and returned to the bloodstream.

Acid-base balance

Regulation of the extracellular fluid environment involves the ratio of acid to base, measured clinically as pH. In physiology, all positively charged ions are acids and all negatively charged ions are bases. To regulate acid-base balance, the kidneys secrete hydrogen ions (acid), reabsorb sodium (acid) and bicarbonate ions (base), acidify phosphate salts, and produce ammonium ions (acid). This keeps blood at its normal pH of 7.35 to 7.45. The following are important pH boundaries:

♦ less than 6.8 = incompatible with life

♦ less than 7.2 = cell function seriously impaired

♦ less than 7.35 = acidosis

♦ 7.35 to 7.45 = normal

♦ greater than 7.45 = alkalosis

♦ greater than 7.55 = cell function seriously impaired

♦ greater than 7.8 = incompatible with life.

Pathophysiologic changes in electrolyte imbalance

The regulation of intracellular and extracellular electrolyte concentrations depends on:

♦ balance between the intake of substances containing electrolytes and the output of electrolytes in urine, stool, and sweat

♦ transport of fluid and electrolytes between ECF and ICF.

Fluid imbalance occurs when regulatory mechanisms can’t compensate for abnormal intake and output at any level from the cell to the organism. Fluid and electrolyte imbalances include edema, isotonic alterations, hypertonic alterations, hypotonic alterations, and electrolyte imbalances. Disorders of fluid volume or osmolarity (concentration of electrolytes in the fluid) result. Many conditions also affect capillary exchange, resulting in fluid shifts.


Despite almost constant interchange through the endothelial barrier, the body maintains a steady state of extracellular water balance between the plasma and interstitial fluid. Increased fluid volume in the interstitial spaces is called edema, and is classified as localized or systemic. Obstruction of the veins or lymphatic system or increased vascular permeability usually causes localized edema in the affected area such as the swelling around an injury. Systemic, or generalized, edema may be due to heart failure or renal disease. Massive systemic edema is called anasarca.

Edema results from abnormal expansion of the interstitial fluid or the accumulation of fluid in a third space, such as the peritoneum (ascites), pleural cavity (hydrothorax), or pericardial sac (pericardial effusion). (See Causes of edema, page 76.)


Many fluid and electrolyte disorders are classified according to how they affect osmotic pressure, or tonicity. Tonicity describes the relative concentrations of electrolytes (osmotic pressure) on both sides of a semipermeable membrane (the cell wall or the capillary wall). The word normal in this context refers to the usual electrolyte concentration of physiologic fluids. Normal saline has a sodium chloride concentration of 0.9%.

♦ Isotonic solutions have the same electrolyte concentration and therefore the same osmotic pressure as ECF.

♦ Hypertonic solutions have a greater-thannormal concentration of some essential electrolyte, usually sodium.

♦ Hypotonic solutions have a lower-than-normal concentration of some essential electrolyte, also usually sodium.

Isotonic alterations

Isotonic alterations or disorders don’t make the cells swell or shrink because osmosis doesn’t occur. They occur when ICF and ECF have equal osmotic pressure, but there’s a dramatic change in total-body fluid volume. Examples include blood loss from penetrating trauma or expansion of fluid volume if a patient receives too much normal saline solution.

Hypertonic alterations

Hypertonic alterations occur when ECF is more concentrated than ICF. Water flows out of the cell through the semipermeable cell membrane, causing cell shrinkage. This shrinkage can occur when a patient is given hypertonic (greater than 0.9%) saline, when severe dehydration causes hypernatremia (high sodium concentration in blood), or when renal disease causes sodium retention.

Hypotonic alterations

When ECF becomes hypotonic, osmotic pressure forces some ECF into the cells, causing them to swell. Overhydration is the most common cause; as water dilutes ECF, it becomes hypotonic with respect to ICF. Water moves into the cells until balance is restored. In extreme hypotonicity, cells may swell until they burst and die.


Major electrolytes include the cations (positively charged ions) sodium, potassium, calcium, and magnesium and the anions (negatively charged ions) chloride, phosphate, and bicarbonate. The body continuously attempts to maintain intracellular and extracellular equilibrium of electrolytes. Too much or too little of any electrolyte will affect most body systems.

Sodium and potassium

Sodium is the major cation in ECF, and potassium is the major cation in ICF. Especially in nerves and muscles, communication within and between cells involves changes (repolarization and depolarization) in surface charge on the cell membrane. During repolarization, an active transport mechanism in the cell membrane, called the sodium-potassium pump, continually shifts sodium into and potassium out of cells; during depolarization, the process is reversed.

Physiologic roles of sodium cations include:

♦ maintaining tonicity of ECF

♦ regulating acid-base balance by renal reabsorption of sodium ion (base) and excretion of hydrogen ion (acid)

♦ facilitating nerve conduction and neuromuscular function

♦ facilitating glandular secretion

♦ maintaining water balance.

Physiologic roles of potassium include:

♦ maintaining cell electrical neutrality

♦ facilitating cardiac muscle contraction and electrical conductivity

♦ facilitating neuromuscular transmission of nerve impulses

♦ maintaining acid-base balance.


Calcium is indispensable in cell permeability, bone and teeth formation, blood coagulation, nerve impulse transmission, and normal muscle contraction. Hypocalcemia can cause tetany and seizures; hypercalcemia can cause cardiac arrhythmias and coma.


Magnesium is present in a smaller quantity, but physiologically it’s as significant as the other major electrolytes. Its major function is to enhance neuromuscular communication. Other functions include:

♦ stimulating parathyroid hormone secretion, which regulates intracellular calcium

♦ activating many enzymes in carbohydrate and protein metabolism

♦ facilitating cell metabolism

♦ facilitating sodium, potassium, and calcium transport across cell membranes

♦ facilitating protein transport.


Chloride is mainly an extracellular anion; it accounts for two-thirds of all serum anions. Secreted by the stomach mucosa as hydrochloric acid, it provides an acid medium for digestion and enzyme activation. Chloride also:

♦ helps maintain acid-base and water balances

♦ influences the tonicity of ECF

♦ facilitates exchange of oxygen and carbon dioxide in red blood cells

♦ helps activate salivary amylase, which triggers the digestive process.


The anion phosphate is involved in cellular metabolism as well as neuromuscular regulation and hematologic function. Phosphate reabsorption in the renal tubules is inversely related to the calcium level, which means that an increase in urinary phosphorus triggers calcium reabsorption and vice versa.


Electrolyte imbalances can affect all body systems. Too much or too little potassium or too little calcium or magnesium can increase the excitability of the cardiac muscle, causing arrhythmias. Multiple neurologic symptoms may result from electrolyte imbalance, ranging from disorientation or confusion to a completely depressed central nervous system. Too much or too little sodium or too much potassium can cause
oliguria. Blood pressure may be increased or decreased. (See Fluid and electrolyte implications of blood pressure findings.) The GI tract is particularly susceptible to electrolyte imbalance:

♦ too much potassium—abdominal cramps, nausea, and diarrhea

♦ too little potassium—paralytic ileus

♦ too much magnesium—nausea, vomiting, and diarrhea

♦ too much calcium—nausea, vomiting, and constipation.

Disorders of fluid and electrolyte balance

Fluid and electrolyte balance is essential for health. Many factors, such as illness, injury, (from trauma or burns), medications, nutritional imbalance, surgery, and treatments, can disrupt a patient’s fluid and electrolyte balance. Even a patient with a minor illness is at risk for fluid and electrolyte imbalance. (See Electrolyte imbalances, pages 79,80,81.)


Water content of the human body progressively decreases from birth to old age, as follows:

♦ in neonates, as much as 75% of body weight

♦ in adults, about 60% of body weight

♦ in elderly patients, about 55%.

Most of the decrease occurs in the first 10 years of life. Hypovolemia, or extracellular fluid (ECF) volume deficit, is the isotonic loss of body fluids—that is, relatively equal losses of sodium and water.

image Infants are at risk for hypovolemia because their bodies need to have a higher proportion of water to total body weight.


Excessive fluid loss, reduced fluid intake, thirdspace fluid shift, or a combination of these factors can cause ECF volume loss.

Causes of fluid loss include:

♦ abdominal surgery

♦ diabetes mellitus with polyuria or diabetes insipidus

♦ excessive diuretic therapy

♦ excessive perspiration

♦ excessive use of laxatives

♦ fever

♦ fistulas

♦ hemorrhage

♦ nasogastric drainage

♦ renal failure with polyuria

♦ vomiting or diarrhea.

Possible causes of reduced fluid intake include:

♦ coma

♦ dysphagia

♦ environmental conditions preventing fluid intake

♦ psychiatric illness.

Fluid shift may be related to:

♦ acute intestinal obstruction

♦ acute peritonitis

♦ burns (during the initial phase)

♦ crushing injury

♦ hip or pelvic fracture (1.5 to 2 L of blood may accumulate in tissues around the fracture)

♦ pancreatitis

♦ pleural effusion.


Hypovolemia is an isotonic disorder. Fluid volume deficit decreases capillary hydrostatic pressure and fluid transport. Cells are deprived of normal nutrients that serve as substrates for energy production, metabolism, and other cellular functions. Decreased renal blood flow triggers the reninangiotensin system to increase sodium and water reabsorption. The cardiovascular system compensates by increasing heart rate, cardiac contractility, venous constriction, and systemic vascular resistance, thus increasing cardiac output and mean arterial pressure. Hypovolemia also triggers the thirst response, releasing more antidiuretic hormone and producing more aldosterone.

When compensation fails, hypovolemic shock occurs in this sequence:

♦ decreased intravascular fluid volume

♦ diminished venous return, which reduces preload and decreases stroke volume

♦ reduced cardiac output

♦ decreased mean arterial pressure

♦ impaired tissue perfusion

♦ decreased oxygen and nutrient delivery to cells

♦ multisystem organ failure.

Signs and symptoms

Signs and symptoms depend on the amount of fluid loss. (See Estimating fluid loss, page 82.) These may include:

♦ orthostatic hypotension due to increased systemic vascular resistance and decreased cardiac output

♦ tachycardia induced by the sympathetic nervous system to increase cardiac output and mean arterial pressure

♦ thirst to prompt ingestion of fluid (increased ECF osmolality stimulates the thirst center in the hypothalamus)

image Elderly patients have a diminished thirst sensitivity due to aging. As a result, they may not realize their need for fluid, predisposing them to further hypervolemia. Check for signs of deficient fluid volume in these patients by looking for dry mouth and longitudinal furrows over the tongue.

Aug 27, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Fluids and Electrolytes
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