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), oxygen (O2), hydrogen (H+), bicarbonate (HCO3), calcium (Ca2+), potassium (K+), sulfate (SO42-), 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 vital role in fluid and electrolyte balance. Sweating results in loss of sodium and water, and every breath contains water vapor.


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

Intracellular Fluid

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

Extracellular Fluid

The fluid in the spaces outside the cells, called extracellular fluid, is constantly moving. Normally, extracellular fluid includes blood plasma and interstitial fluid. In some pathologic states, it accumulates in a so-called third space, the space around organs in the chest or abdomen.

Extracellular fluid 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. It also contains carbon dioxide (CO2), 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 extracellular fluid and, to a lesser extent, intracellular fluid 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.

Fluid Exchange

Four forces act to equalize concentrations of fluids, electrolytes, and proteins on both sides of the capillary wall by moving fluid between the vessels and the interstitial fluid. Forces that move fluid out of blood vessels are:

  • hydrostatic pressure of blood

  • osmotic pressure of interstitial fluid.

Forces that move fluid into blood vessels are:

  • oncotic pressure of plasma proteins

  • hydrostatic pressure of interstitial fluid.

Hydrostatic pressure is higher at the arteriolar end of the capillary bed than at the venular end. Oncotic pressure of plasma increases slightly at the venular end as fluid is drawn into the blood vessel. 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.

Alterations in Tonicity





  • Intracellular fluids and extracellular fluids have equal osmotic pressure, but there’s a dramatic change in total body fluid volume.

  • No cellular swelling or shrinkage exists because osmosis doesn’t occur.

  • Blood loss from penetrating trauma

  • Expansion of fluid volume if a patient receives too much normal saline


  • Extracellular fluid is more concentrated than is intracellular fluid.

  • Water flows out of the cell through the semipermeable cell membrane, causing cell shrinkage.

  • Administration of hypertonic (>0.9%) saline

  • Hypernatremia from severe dehydration

  • Sodium retention from renal disease


  • Decreased osmotic pressure forces some extracellular fluid into the cells, causing them to swell.

  • In extreme hypotonicity, cells may swell until they burst and die.

  • Overhydration

Normal Electrolyte Values



135-145 mEq/L

8.5-10.5 mg/dL



3.5-5 mEq/L

1.8-2.5 mEq/L



96-106 mEq/L

2.5-4.5 mg/dL

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 the blood at its normal pH of 7.35 to 7.45. Important pH boundaries include:

  • <6.8 incompatible with life

  • <7.2 cell function seriously impaired

  • <7.35 acidosis

  • 7.35 to 7.45 normal

  • >7.45 alkalosis

  • >7.55 cell function seriously impaired

  • >7.8 incompatible with life.


The regulation of intracellular and extracellular electrolyte concentrations depends on these factors:

  • balance between intake of substances containing electrolytes and output of electrolytes in urine, feces, and sweat

  • transport of fluid and electrolytes between extracellular and intracellular fluid.

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 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. It’s 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).

Major Electrolytes




  • Major extracellular fluid cation

  • Maintains tonicity of extracellular fluid

  • Regulates acid-base balance by renal reabsorption of sodium ion (base) and excretion of hydrogen ion (acid)

  • Facilitates nerve conduction and neuromuscular function

  • Facilitates glandular secretion

  • Maintains water balance


  • Major intracellular fluid cation

  • Maintains cell electrical neutrality

  • Facilitates cardiac muscle contraction and electrical conductivity

  • Facilitates neuromuscular transmission of nerve impulses

  • Maintains acid-base balance


  • Mainly an extracellular fluid anion

  • Accounts for two-thirds of all serum anions

  • Secreted by the stomach mucosa as hydrochloric acid, providing an acid medium for digestion and enzyme activation

  • Helps maintain acid-base and water balances

  • Influences tonicity of extracellular fluid

  • Facilitates exchange of oxygen and carbon dioxide in red blood cells

  • Helps activate salivary amylase, which triggers the digestive process


  • Indispensable to cell permeability, bone and teeth formation, blood coagulation, nerve impulse transmission, and normal muscle contraction

  • Plays a vital role in cardiac action potential and is essential for cardiac pacemaker automaticity


  • Present in small quantities, but physiologically as significant as the other major electrolytes

  • Enhances neuromuscular communication

  • Stimulates parathyroid hormone secretion, which regulates intracellular calcium

  • Activates many enzymes in carbohydrate and protein metabolism

  • Facilitates cell metabolism

  • Facilitates sodium, potassium, and calcium transport across cell membranes

  • Facilitates protein transport


  • Involved in cellular metabolism as well as neuromuscular regulation and hematologic function

  • Phosphate reabsorption in the renal tubules inversely related to calcium levels (an increase in urinary phosphorous triggers calcium reabsorption and vice versa)

Disorders of Fluid Balance: Hypovolemia






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. Hypovolemia results from these causes:

Fluid loss

  • Hemorrhage

  • Excessive perspiration

  • Renal failure with polyuria

  • Surgery

  • Vomiting or diarrhea

  • Nasogastric drainage

  • Diabetes mellitus with polyuria or diabetes insipidus

  • Fistulas

  • Excessive use of laxatives; diuretic therapy

  • Fever

Reduced fluid intake

  • Dysphagia

  • Coma

  • Environmental conditions preventing fluid intake

  • Psychiatric illness

Fluid shift from extracellular fluid

  • Burns (during the initial phase)

  • Acute intestinal obstruction

  • Acute peritonitis

  • Pancreatitis

  • Crushing injury

  • Pleural effusion

  • Hip fracture

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 (MAP). 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 MAP

  • impaired tissue perfusion

  • decreased oxygen and nutrient delivery to cells

  • multiple organ dysfunction syndrome.

  • Orthostatic hypotension (in major blood or fluid loss)

  • Tachycardia

  • Thirst

  • Flattened jugular veins

  • Sunken eyeballs

  • Dry mucous membranes

  • Diminished skin turgor

  • Rapid weight loss

  • Decreased urine output

  • Prolonged capillary refill time

  • Increased blood urea nitrogen

  • Elevated serum creatinine level

  • Increased serum protein, hemoglobin, and hematocrit (unless caused by hemorrhage, when loss of blood elements causes subnormal values)

  • Rising blood glucose

  • Elevated serum osmolality (except in hyponatremia, where serum osmolality is low)

  • Serum electrolyte and arterial blood gas analysis may reflect associated clinical problems resulting from the underlying cause of hypovolemia or the treatment regimen

  • Urine specific gravity > 1.030

  • Increased urine osmolality

  • Urine sodium level < 50 mEq/L

  • Oral fluids

  • Parenteral fluids

  • Fluid resuscitation by rapid I.V. administration

  • Blood or blood products (with hemorrhage)

  • Antidiarrheals as needed

  • Antiemetics as needed

  • I.V. dopamine (lntropin) or norepinephrine (Levophed) to increase cardiac contractility and renal perfusion (if patient remains symptomatic after fluid replacement)

  • Autotransfusion (for some patients with hypovolemia caused by trauma)

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Sep 22, 2018 | Posted by in ANATOMY | Comments Off on Fluids and Electrolytes
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