Physiology, Body Fluids

Cellular Level

The distribution of fluid throughout the body can be broken down into 2 general categories: intracellular fluid and extracellular fluid. Intracellular fluid is approximately 40% of the total body weight. It is the total space within cells primarily defined as the cytoplasm of cells. In general, intracellular fluids are stable and do not readily adjust to rapid changes. This space is where many chemical reactions occur. As such, it is important to maintain an appropriate osmolality. The extracellular fluid comprises approximately 20% of total body weight and is further subcategorized as plasma at approximately 5% of body weight and interstitial space, which is approximately 12% of body weight. Additional fluid spaces are possible in pathological scenarios and are categorized as transudate or exudate based on location and etiology.

The exact chemical composition of body fluid is highly variable. This is dependent on which portion of the body, as well as which organ of the body, contains the fluid. Extracellular fluid and interstitial fluid are similar in composition. Extracellular spaces contain high concentrations of sodium, chloride, bicarbonate, and proteins but are relatively lower in potassium, magnesium, and phosphate. Interstitial fluids physiologically tend to have a low concentration of proteins. Intracellular fluids tend to be inversed with high levels of phosphate, magnesium, potassium, and proteins but lower sodium, chloride, and bicarbonate.[2][3][4]

Mechanism

Fluid moves throughout cellular environments in the body by passively crossing semipermeable membranes. Osmolarity is defined as the number of particles per liter of fluid. Physiologic blood plasma osmolarity is approximately 286 mOsmoles/L. Less than this is hypoosmotic, and greater is hyperosmotic. Cellular osmotic concentration gradients are maintained largely through the active pumping of transmembrane ionic transport proteins. However, rapid changes in fluid volume without changes in ionic components cause dilation or concentration of those components. Blood plasma osmotic gradients are maintained through the absorption of solutes from the gastrointestinal tract or secretion into the gastrointestinal tract or urine. In addition to ionic components, osmolarity is partially composed of proteins such as albumin in the serum. Another important osmotically active component to consider is glucose. Fluid moves towards hyperosmotic compartments and away from hypoosmotic compartments. All body fluids should have an ionic net electrical charge close to zero, indicating a balance of cations and anions. Ionic components diffuse through fluids selectively depending on the presence of permeable membranes. If a membrane is non-permeable to an ion, this creates a gradient of relatively higher concentration osmolarity. Solute gradients can be physiologically created by membrane-pumping proteins, which expend energy in the form of ATP to move components from areas of low concentration into higher concentrations against their diffusion gradient. These processes create a cellular environment to osmotically “pull” water into fluid compartments. In addition to the osmotic pull of fluids, fluid movement within the body relies on created and maintained hydrostatic pressures. This is best utilized in the movement of fluid from plasma in the extracellular blood space into the interstitial spaces of tissue across the capillary membrane. Hydrostatic pressure is the “push” factor on fluid movement where increased pressures force fluid out of space. The combined “push” of hydrostatic forces and the “pull” of osmotic forces create a net fluid movement. This is mathematically explained using the Starling equation:

• Jv = Kfc ([Pc - Pi] - n [Op-Oi])

Where Jv is the net rate of capillary fluid movement, Kfc is a capillary filtration fluid coefficient, Pc is capillary hydrostatic pressure, Pi is interstitial hydrostatic pressure, n is the osmotic reflection coefficient, Op is plasma oncotic pressure, and Oi is interstitial oncotic pressure.[4]

Clinical Significance

A variety of pathological conditions induce abnormalities in fluid balance. Fluid balance abnormalities are either an overload of fluid or a decrease in effective fluid. Fluid overload is clinically known as edema. Edema occurs most commonly in soft tissues of the extremities but can occur in any tissue. Decreases in fluid load are commonly referred to as dehydration. Edema manifests as swelling in the soft tissues of the limbs and face, followed by an increase in size and tightness of the skin. Peripheral edema is reducible by increasing the pressure in the interstitial space and is measured by pressing a finger into the tissue, temporarily creating a dimple in the edematous skin. Likewise, wearing compression stockings can reduce peripheral edema by increasing interstitial hydrostatic pressure, forcing fluid back into the capillaries.

Pulmonary edema is when excess fluid swells into the interstitial tissues of the lung. Symptoms include shortness of breath and chest pain. Orthopnea, or impaired respiration while lying flat, may also be present as the excess fluid is distributed across the entire lung. Pulmonary edema is life-threatening as it compromises gas exchange in the lungs, and conditions can quickly decompensate. Pulmonary edema is associated with cardiac failure and renal failure. Classically, cardiac failure causes pulmonary edema through decreased pumping efficiency and capacity of the left atrium and ventricle. This creates back pressure in the pulmonary veins, increasing pressure in the vessels. Subsequently, hydrostatic pressures in the pulmonary capillaries are increased, “pushing” fluid into the interstitial lung space following the Starling equation. Renal failure causes edema through a failure to remove fluids and osmotic components from the body. The net result is increased osmotic pull into tissues and increased hydrostatic push out of capillaries.[5]

Liver disease is also capable of inducing edema. This is due to a failure to produce osmotically active proteins—specifically, a failure to produce albumin. Albumin is found physiologically primarily in the plasma of the extracellular blood. It is typically not found in the interstitial space. As such, a decrease in body albumen directly decreases the “pull” of osmotic pressure into the capillaries. According to Starling forces, this results in the fluid moving into the interstitial spaces.[6] Additionally, fluid overload can be iatrogenically induced by excessive fluid replacement via intravenous access.

Edema is treated for symptomatic relief using a variety of medications, including diuretics, to remove fluid from the body via the renal system. Diuretics are closely associated with inducing contraction metabolic alkalosis. Albumin may be supplemented in cases of low plasma albumin. Lifestyle changes can include reducing sodium intake, restricting fluid intake, and wearing compression stockings. However, targeting the underlying pathology to improve cardiac, hepatic, or renal function offers better results than symptomatic treatment by removing fluid, replacing osmotic components, or other lifestyle changes.

Dehydration is largely due to inadequate water intake to meet the body’s metabolic needs. The average adult has an obligatory intake requirement of 1600 mL per day. This value increases depending on activity and metabolism. Primary sources of normal fluid loss include urine, sweat, respiration, and stool. Pathological causes include diarrhea, vomiting, infection, and increased urination secondary to diabetes mellitus or diabetes insipidus. Dehydration manifests clinically as decreased urine output, dizziness, fatigue, tachycardia, increased skin turgidity, and fatigue or confusion in severe cases. Whenever possible, oral fluid replacement should be attempted. In more urgent situations, intravenous fluid replenishment should be based on bolus supplementation of the deficit of fluids and a maintenance replenishment of obligatory intake requirements. The fluid deficit can be calculated when the pre-dehydration weight and post-dehydration weight are known. The equation in males is:

• Deficit = 0.6 X weight in kilograms X [1-(140/measured Na)]

In females, the equation is:

• Deficit = 0.5 X weight in kilograms X [1-(140/measured Na)]

This equation is highly useful in determining the initial fluid deficit. However, it has limitations in accuracy and can underestimate total fluid loss by more than 40%. While the above equation can be useful in initial fluid resuscitation, a more accurate approach uses plasma osmolarity instead of sodium, using 290 mmol/kg as the standard value. In pediatric patients, the fluid deficit is directly correlated to body weight loss from pre-illness compared to post-illness. One liter of free water weighs 1 kg. Therefore, a 10-kg pre-illness child that weighs 9 kg in illness has a fluid deficit of 1 L. In emergency scenarios, a bolus volume of 30 mL/kg is used to replace the loss. In obese patients, however, this leads to over-repletion of free water. Therefore, it is recommended to base bolus fluid resuscitation on adjusted ideal body weight (AIBW) in obese patients. This is derived from the ideal body weight (IBW) and the actual body weight (ABW).

• AIBW = IBW + 0.4 (ABW - IBW)

Where ideal body weight is calculated as:

• Males: IBW = 50 kg + 2.3 kg for each inch over 5 feetFemales: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet

Maintenance fluid is also determined using a formula based on weight. Fluid should be replaced at a rate of:

• 4 mL/kg/hr for kg 1-10 + 2 mL/kg/hr for kg 10-20 + 1 mL/kg/hr above 20 kg

In other words, a patient who weighs 55 kilograms would require:

• 40 mL/hr + 20 mL/hr + 35 mL = 95 mL/hr of free water

Intravenous fluid replacement options include normal saline (0.9% NaCl), one-half normal saline (0.45% NaCl), Dextrose 5% in either normal saline or one-half normal saline, and lactated Ringer solution. The choice of replacement fluids is patient scenario-specific and dependent on the electrolyte status of laboratory evaluation.[7]

Burn patients require specialized increases in fluid replacement secondary to the immense loss of free water through their wounds. The needed fluid resuscitation in adults is calculated using the Parkland and Brooke formulas. The modified Brooke formula is:

• 2 mL/kg/% body surface area burned

The modified Parkland formula is:

• 4 mL/kg/% body surface area burned

Both formulas estimate the first 24-hour fluid requirements from the time of the burn, with half the amount to be given in the first 8 hours. While both formulas give widely different values, they give equivalent outcomes. Final fluid needs should be based on the urine output rate.[8] 

Diabetic ketoacidosis is a complication of diabetes mellitus that results when the body fails to utilize glucose for energy production. Glucose is an osmotically active substance that is excreted in the urine at high concentrations. This leads to extreme fluid loss through the urine and dehydration, necessitating large-volume resuscitation of 6 to 9 L of normal saline on average. Hyperosmolar hyperglycemic non-ketotic acidosis is similar to diabetic ketoacidosis, except it lacks ketone production. It requires a similar fluid resuscitation. Hypernatremic patients who undergo fluid replacement with rapid subsequent correction of hypernatremia are at an increased risk for developing cerebral edema. This develops due to increased intracellular and extracellular fluid loads and increased pressure within the brain space. This leads to neurological deficits and, ultimately, death. This condition can be avoided by slowly infusing fluids such that sodium levels are reduced at an initial rate of 2 to 3 mEq/L per hour for a maximum total change of 12 mEq/L per day until sodium is in a normal range.

Conversely, rapid correction of hyponatremia may lead to central pontine myelinolysis syndrome. Brain cells adapt to chronic states of hyponatremia by shifting organic osmoles, such as amino acids, from the intracellular compartment to the extracellular compartment. This allows the cells to maintain their original volume. When hyponatremia is rapidly corrected, brain cells shrink, and the tight junctions of the blood-brain barrier are disrupted, leading to cell damage and demyelination of neurons.[9] This can lead to what is known as “locked-in syndrome,” which is characterized by paralysis, dysphagia, and dysarthria. The serum sodium should be increased by approximately 1 to 2 mEq/L per hour until the neurologic symptoms of hyponatremia subside or until plasma sodium concentration is over 120 mEq/L.

Crystalloid fluid resuscitation offers complications as it alters the ionic load of the serum. Specifically, normal saline replacement may lead to non-gap hyperchloremic metabolic acidosis. One-half normal saline, if not monitored closely, may dilute ionic components, leading to hyponatremia or, less often, hypokalemia. Abdominal compartment syndrome in septic shock patients is possibly secondary to fluid overload with the subsequent leak of fluid from capillaries into extravascular spaces. Colloid fluid resuscitation has its risks as well. The 2 major colloids used are albumen and hydroxyethyl starch. In the SAFE trial, which compared 4% albumin fluid with 0.9% normal saline, it was determined that outcomes were equivalent. However, in specific cases involving neurological injury, 4% albumin has an increased mortality rate compared to normal saline. As such, albumin should be avoided in this situation. Hydroxyethyl starch was studied in comparison and found to carry an increased risk of death or end-stage renal failure when compared to lactated Ringer's solution when used in sepsis patients.[10][11][12]