Body Fluid Compartments

The adult human body is made up of 60% water and is divided into extracellular and intracellular fluid compartments. Extracellular fluid is present outside the cells and makes up ⅓ of the total body water. Intracellular fluid is present inside the cells and makes up ⅔ of the total body water. Intracellular and extracellular fluids are separated into compartments by semipermeable membranes, and the transport of fluid and ions is maintained by channels in the cell membrane Cell Membrane A cell membrane (also known as the plasma membrane or plasmalemma) is a biological membrane that separates the cell contents from the outside environment. A cell membrane is composed of a phospholipid bilayer and proteins that function to protect cellular DNA and mediate the exchange of ions and molecules. The Cell: Cell Membrane. Each compartment contains different concentrations of ions and osmolar molecules. The relative charge and osmolarity are maintained rigorously by the transport of water and substances between compartments. Hypernatremia Hypernatremia Hypernatremia is an elevated serum sodium concentration > 145 mmol/L. Serum sodium is the greatest contributor to plasma osmolality, which is very tightly controlled by the hypothalamus via the thirst mechanism and antidiuretic hormone (ADH) release. Hypernatremia occurs either from a lack of access to water or an excessive intake of sodium. Hypernatremia, hyponatremia Hyponatremia Hyponatremia is defined as a decreased serum sodium (sNa+) concentration less than 135 mmol/L. Serum sodium is the greatest contributor to plasma osmolality, which is very tightly controlled via antidiuretic hormone (ADH) release from the hypothalamus and by the thirst mechanism. Hyponatremia, and edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema are the clinical conditions arising from disturbances in the maintenance of osmolarity of the body fluid compartments.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Overview

Water in the adult human body makes up approximately 60% of the total body weight. The fluid is distributed in various organs, organ systems, and tissues. The sum of the water in these tissues is known as total body water.

  • Percentage of water in the body changes with the development of the body:
    • As high as 70% of the body mass in infants
    • 50%–60% in normal adults
    • As low as 45% in older adults
  • Weight of total body water accounts for:
    • 60% of lean weight in men
    • 50% of lean weight in women

Body Fluid Compartments

Overview

The total body water is distributed primarily between 2 compartments, namely, extracellular fluid (ECF) and intracellular fluid (ICF) compartments.

  • ECF is present outside cells:
    • Makes up about ⅓ of total body water
    • ECF includes:
      • Intravascular fluid (¼ of ECF): primary component of plasma
      • Interstitial fluid (¾ of ECF): lies outside blood vessels
  • ICF is present inside cells:
    • Makes up about ⅔ of total body water
    • Principal component of the cytoplasm of cells
Body fluid compartments

Body fluid compartments

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Movement of fluid between compartments

  • Fluid is separated into compartments by semipermeable membranes. The membranes are highly permeable to water but require energy to transport ions.
  • Distribution of fluid between intracellular and extracellular compartments is determined by the concentration of Na+, chloride, and other electrolytes Electrolytes Electrolytes are mineral salts that dissolve in water and dissociate into charged particles called ions, which can be either be positively (cations) or negatively (anions) charged. Electrolytes are distributed in the extracellular and intracellular compartments in different concentrations. Electrolytes are essential for various basic life-sustaining functions. Electrolytes.
  • Water moves between compartments following osmotic gradients.
  • A change in the concentration of solute or water will cause water to shift between compartments.
  • 2 processes are responsible for the movement of fluid across membranes:
    • Diffusion: A substance passes from an area of higher concentration to an area of lower concentration.
    • Osmosis: Water is drawn across a membrane toward a region where there is a higher solute concentration.

Measurement of body fluid compartments

Compartment volumes can be measured by determining the volume of distribution of an indicator substance. A known amount of an indicator is added to a compartment, and the indicator concentration in that compartment is measured after allowing sufficient time for uniform distribution throughout the compartment. The compartment volume is calculated as follows:

Volume = amount of the indicator / concentration of the indicator

Table: Body fluid compartments and their associated indicators
Volume Indicators
Total body water 3H2O, 2H2O, antipyrine
Extracellular fluid 22Na, 125I-iothalamate, thiosulfate, inulin
Intracellular fluid Calculated as: (total body water — extracellular fluid volume)
Plasma volume 125I-albumin, Evans blue dye (T-1824)
Blood volume 51Cr-labeled RBCs, or calculated as: (blood volume = plasma volume / (1 − hematocrit))
Interstitial fluid Calculated as: (extracellular fluid volume − plasma volume)

Changes in the Fluid Compartment

Hypo-osmotic volume expansion

  • Results in an increase in total body water and reduction in osmolality
  • Can be seen in cases of:
    • Excessive water intake
    • SIADH SIADH Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder of impaired water excretion due to the inability to suppress the secretion of antidiuretic hormone (ADH). SIADH is characterized by impaired water excretion leading to dilutional hyponatremia, which is mainly asymptomatic but may cause neurologic symptoms. S Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)
  • Changes in ECF constituents:
    • Decrease in Na+ level
    • Decrease in total protein level
    • Hematocrit remains the same.
  • Endocrine response:
    • Decrease in aldosterone level (suppressed by hypervolemia)
    • Decrease in antidiuretic hormone level (e.g., diabetes insipidus Diabetes Insipidus Diabetes insipidus (DI) is a condition in which the kidneys are unable to concentrate urine. There are 2 subforms of DI: central DI (CDI) and nephrogenic DI (NDI). Both conditions result in the kidneys being unable to concentrate urine, leading to polyuria, nocturia, and polydipsia. Diabetes Insipidus)

Hypo-osmotic volume contraction

  • Decrease in total body water along with a decrease in the total osmolality of solutes
  • Usually seen in the case of adrenal insufficiency Adrenal Insufficiency Adrenal insufficiency (AI) is the inadequate production of adrenocortical hormones: glucocorticoids, mineralocorticoids, and adrenal androgens. Primary AI, also called Addison’s disease, is caused by autoimmune disease, infections, and malignancy, among others. Adrenal insufficiency can also occur because of decreased production of adrenocorticotropic hormone (ACTH) from disease in the pituitary gland (secondary) or hypothalamic disorders and prolonged glucocorticoid therapy (tertiary). Adrenal Insufficiency and Addison’s Disease
  • Changes in ECF result in:
    • Reduction in blood pressure due to decrease in volume
    • Decrease in Na+ level
    • Increase in total protein level
    • Increase in hematocrit
  • Endocrine response:
    • Increase in aldosterone (stimulated by hypovolemia and hypokalemia Hypokalemia Hypokalemia is defined as plasma potassium (K+) concentration < 3.5 mEq/L. Homeostatic mechanisms maintain plasma concentration between 3.5-5.2 mEq/L despite marked variation in dietary intake. Hypokalemia can be due to renal losses, GI losses, transcellular shifts, or poor dietary intake. Hypokalemia)
    • No change in antidiuretic hormone level

Iso-osmotic volume expansion

  • Volume expansion where there is no change in osmolality
  • Seen with IV solution volume expansion
  • Changes in ECF result in:
    • Increase in blood pressure
    • No change in Na+ level
    • Decrease in total protein level
    • Decrease in hematocrit
  • Endocrine response to iso-osmolar volume expansion:
    • Decrease in aldosterone (suppressed by volume expansion)
    • No change in antidiuretic hormone level

Iso-osmotic volume contraction

  • Volume contraction occurring without any change in osmolality
  • Seen in cases of iso-osmotic fluid loss (e.g., diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea and vomiting)
  • Changes in ECF result in:
    • Decrease in blood pressure
    • No change in Na+ level
    • Increase in total protein level
    • Increase in hematocrit
  • Endocrine response:
    • Increase in aldosterone levels (stimulated by volume contraction)
    • No change in antidiuretic hormone level

Hyper-osmotic volume expansion

  • Increase in osmolality alongside the increase in volume
  • Seen in cases of:
    • Aldosterone hypersecretion
    • Ingestion of salty foods
    • Individuals with a tumor
  • Changes in ECF result in:
    • Increase in blood pressure
    • Increase in Na+ level
    • Reduction in total protein level
    • Reduction in hematocrit
  • Endocrine response:
    • Reduction in aldosterone level
    • Increase in antidiuretic hormone level

Hyper-osmotic volume contraction

  • Reduction of total body water but osmolality increases:
    • Seen in cases of dehydration
    • Sweat contains more water than solute; thus, excessive sweating results in hyperosmotic volume contraction.
  • Changes in ECF result in:
    • Decrease in blood pressure
    • Increase in Na+ level
    • Increase in total protein level
    • No change in hematocrit
  • Endocrine response:
    • Increase in aldosterone level
    • Increase in antidiuretic hormone level

Body Fluid Composition

  • Body fluid compartments contain ions and osmolar molecules at different concentrations.
  • Relative concentration of the components depends on the exchange between cells and tissues in the body.
  • Composition of each fluid compartment is rigorously maintained by the continuous movement of water and other substances between compartments.
Table: Normal levels of ions and osmolar molecules in the plasma, interstitial fluid, and intracellular fluid
Plasma Interstitial fluid Intracellular fluid
(mOsm/L) (mOsm/L) (mOsm/L)
Na+ 142 139 14
K+ 4.2 4.0 140
Ca2+ 1.3 1.2 0
Mg2+ 0.8 0.7 20
Chloride (Cl) 106 108 4
HCO3 24 28.3 10
HPO42–, H2PO4 2 2 11
SO42– 0.5 0.5 1
Phosphocreatine 45
Carnosine 14
Amino acids 2 2 8
Creatinine 0.2 0.2 9
Lactate 1.2 1.2 1.5
ATP 5
Hexose monophosphate 3.7
Glucose 5.6 5.6
Proteins 1.2 0.2 4
Urea 4 1 4
Others 4.8 3.9 10
Total mOsm/L 299.8 300.8 301.2
Corrected osmolar activity 282.0 281.0 281.0

Clinical Relevance

  • Hyponatremia: a decrease in Na+ levels in the body. Hyponatremia most often occurs in the setting of severe diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea and vomiting, excessive diuretic use, and Addison disease. Hyponatremia can also occur less commonly as a result of SIADH SIADH Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder of impaired water excretion due to the inability to suppress the secretion of antidiuretic hormone (ADH). SIADH is characterized by impaired water excretion leading to dilutional hyponatremia, which is mainly asymptomatic but may cause neurologic symptoms. S Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) and progress to cell edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema that can ultimately lead to brain damage, which presents with neurological symptoms of headache, nausea, and lethargy. Treatment is with gradual Na+ repletion, as a rapid replacement can lead to osmotic demyelination syndrome.
  • Hypernatremia Hypernatremia Hypernatremia is an elevated serum sodium concentration > 145 mmol/L. Serum sodium is the greatest contributor to plasma osmolality, which is very tightly controlled by the hypothalamus via the thirst mechanism and antidiuretic hormone (ADH) release. Hypernatremia occurs either from a lack of access to water or an excessive intake of sodium. Hypernatremia: an increase in Na+ levels in the body. Hypernatremia Hypernatremia Hypernatremia is an elevated serum sodium concentration > 145 mmol/L. Serum sodium is the greatest contributor to plasma osmolality, which is very tightly controlled by the hypothalamus via the thirst mechanism and antidiuretic hormone (ADH) release. Hypernatremia occurs either from a lack of access to water or an excessive intake of sodium. Hypernatremia most often occurs as a result of severe dehydration. Affected individuals are often asymptomatic, as symptoms typically occur when the Na+ level is approximately 160 mmol/L. Hypernatremia Hypernatremia Hypernatremia is an elevated serum sodium concentration > 145 mmol/L. Serum sodium is the greatest contributor to plasma osmolality, which is very tightly controlled by the hypothalamus via the thirst mechanism and antidiuretic hormone (ADH) release. Hypernatremia occurs either from a lack of access to water or an excessive intake of sodium. Hypernatremia can be corrected by administering hypo-osmotic NaCl or dextrose solution.
  • Edema: the presence of excess fluid in body tissues. Both ECF and ICF compartments may be involved. Edema is often clinically apparent in the lower extremities and occurs when the osmotic gradient between the intravascular space and interstitial space promotes the flow of water from the vasculature into the tissues. Edema can be treated through diuresis or the addition of osmolar molecules (e.g., albumin) to the vascular space.

References

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