Body Fluid Compartments

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 Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast in infants
  • 50%–60% in normal adults
  • As low as 45% in older adults
• Weight of total body water accounts for:
  • 60% of lean LEAN Quality Measurement and Improvement weight in men
  • 50% of lean LEAN Quality Measurement and Improvement 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 Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products
    • 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

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 Chloride Inorganic compounds derived from hydrochloric acid that contain the Cl- ion. Electrolytes (Cl^–), 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 relative concentration of solute or water will cause water to shift between compartments.
• 2 processes are responsible for the passive movement of fluid across membranes:
  • Diffusion Diffusion The tendency of a gas or solute to pass from a point of higher pressure or concentration to a point of lower pressure or concentration and to distribute itself throughout the available space. Diffusion, especially facilitated diffusion, is a major mechanism of biological transport. Peritoneal Dialysis and Hemodialysis: 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

Changes in the Fluid Compartment

Hypo-osmotic volume expansion

• Results in an increase in total body water and reduction in osmolality Osmolality Plasma osmolality refers to the combined concentration of all solutes in the blood. Renal Sodium and Water Regulation
• 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 Total protein Liver Function Tests level
  • Hematocrit Hematocrit The volume of packed red blood cells in a blood specimen. The volume is measured by centrifugation in a tube with graduated markings, or with automated blood cell counters. It is an indicator of erythrocyte status in disease. For example, anemia shows a low value; polycythemia, a high value. Neonatal Polycythemia remains the same.
• Endocrine response:
  • Decrease in aldosterone Aldosterone A hormone secreted by the adrenal cortex that regulates electrolyte and water balance by increasing the renal retention of sodium and the excretion of potassium. Hyperkalemia level (suppressed by hypervolemia Hypervolemia Renal Sodium and Water Regulation)
  • Decrease in antidiuretic hormone Antidiuretic hormone Antidiuretic hormones released by the neurohypophysis of all vertebrates (structure varies with species) to regulate water balance and osmolarity. In general, vasopressin is a nonapeptide consisting of a six-amino-acid ring with a cysteine 1 to cysteine 6 disulfide bridge or an octapeptide containing a cystine. All mammals have arginine vasopressin except the pig with a lysine at position 8. Vasopressin, a vasoconstrictor, acts on the kidney collecting ducts to increase water reabsorption, increase blood volume and blood pressure. Hypernatremia level (e.g., diabetes Diabetes Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus insipidus)

Hypo-osmotic volume contraction

• Decrease in total body water along with a decrease in the total osmolality Osmolality Plasma osmolality refers to the combined concentration of all solutes in the blood. Renal Sodium and Water Regulation of solutes
• Usually seen in the case of adrenal insufficiency Adrenal Insufficiency Conditions in which the production of adrenal corticosteroids falls below the requirement of the body. Adrenal insufficiency can be caused by defects in the adrenal glands, the pituitary gland, or the hypothalamus. Adrenal Insufficiency and Addison Disease
• Changes in ECF result in:
  • Reduction in blood pressure due to decrease in volume
  • Decrease in Na⁺ level
  • Increase in total protein Total protein Liver Function Tests level
  • Increase in hematocrit Hematocrit The volume of packed red blood cells in a blood specimen. The volume is measured by centrifugation in a tube with graduated markings, or with automated blood cell counters. It is an indicator of erythrocyte status in disease. For example, anemia shows a low value; polycythemia, a high value. Neonatal Polycythemia
• Endocrine response:
  • Increase in aldosterone Aldosterone A hormone secreted by the adrenal cortex that regulates electrolyte and water balance by increasing the renal retention of sodium and the excretion of potassium. Hyperkalemia (stimulated by hypovolemia Hypovolemia Sepsis in Children 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 Antidiuretic hormone Antidiuretic hormones released by the neurohypophysis of all vertebrates (structure varies with species) to regulate water balance and osmolarity. In general, vasopressin is a nonapeptide consisting of a six-amino-acid ring with a cysteine 1 to cysteine 6 disulfide bridge or an octapeptide containing a cystine. All mammals have arginine vasopressin except the pig with a lysine at position 8. Vasopressin, a vasoconstrictor, acts on the kidney collecting ducts to increase water reabsorption, increase blood volume and blood pressure. Hypernatremia level

Iso-osmotic volume expansion

• Volume expansion where there is no change in osmolality Osmolality Plasma osmolality refers to the combined concentration of all solutes in the blood. Renal Sodium and Water Regulation
• Seen with IV solution volume expansion
• Changes in ECF result in:
  • Increase in blood pressure
  • No change in Na⁺ level
  • Decrease in total protein Total protein Liver Function Tests level
  • Decrease in hematocrit Hematocrit The volume of packed red blood cells in a blood specimen. The volume is measured by centrifugation in a tube with graduated markings, or with automated blood cell counters. It is an indicator of erythrocyte status in disease. For example, anemia shows a low value; polycythemia, a high value. Neonatal Polycythemia
• Endocrine response to iso-osmolar volume expansion:
  • Decrease in aldosterone Aldosterone A hormone secreted by the adrenal cortex that regulates electrolyte and water balance by increasing the renal retention of sodium and the excretion of potassium. Hyperkalemia (suppressed by volume expansion)
  • No change in antidiuretic hormone Antidiuretic hormone Antidiuretic hormones released by the neurohypophysis of all vertebrates (structure varies with species) to regulate water balance and osmolarity. In general, vasopressin is a nonapeptide consisting of a six-amino-acid ring with a cysteine 1 to cysteine 6 disulfide bridge or an octapeptide containing a cystine. All mammals have arginine vasopressin except the pig with a lysine at position 8. Vasopressin, a vasoconstrictor, acts on the kidney collecting ducts to increase water reabsorption, increase blood volume and blood pressure. Hypernatremia level

Iso-osmotic volume contraction

• Volume contraction occurring without any change in osmolality Osmolality Plasma osmolality refers to the combined concentration of all solutes in the blood. Renal Sodium and Water Regulation
• 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 Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia)
• Changes in ECF result in:
  • Decrease in blood pressure
  • No change in Na⁺ level
  • Increase in total protein Total protein Liver Function Tests level
  • Increase in hematocrit Hematocrit The volume of packed red blood cells in a blood specimen. The volume is measured by centrifugation in a tube with graduated markings, or with automated blood cell counters. It is an indicator of erythrocyte status in disease. For example, anemia shows a low value; polycythemia, a high value. Neonatal Polycythemia
• Endocrine response:
  • Increase in aldosterone Aldosterone A hormone secreted by the adrenal cortex that regulates electrolyte and water balance by increasing the renal retention of sodium and the excretion of potassium. Hyperkalemia levels (stimulated by volume contraction)
  • No change in antidiuretic hormone Antidiuretic hormone Antidiuretic hormones released by the neurohypophysis of all vertebrates (structure varies with species) to regulate water balance and osmolarity. In general, vasopressin is a nonapeptide consisting of a six-amino-acid ring with a cysteine 1 to cysteine 6 disulfide bridge or an octapeptide containing a cystine. All mammals have arginine vasopressin except the pig with a lysine at position 8. Vasopressin, a vasoconstrictor, acts on the kidney collecting ducts to increase water reabsorption, increase blood volume and blood pressure. Hypernatremia level

Hyper-osmotic volume expansion

• Increase in osmolality Osmolality Plasma osmolality refers to the combined concentration of all solutes in the blood. Renal Sodium and Water Regulation alongside the increase in volume
• Seen in cases of:
  • Aldosterone Aldosterone A hormone secreted by the adrenal cortex that regulates electrolyte and water balance by increasing the renal retention of sodium and the excretion of potassium. Hyperkalemia hypersecretion
  • Ingestion of salty foods
  • Individuals with a tumor Tumor Inflammation
• Changes in ECF result in:
  • Increase in blood pressure
  • Increase in Na⁺ level
  • Reduction in total protein Total protein Liver Function Tests level
  • Reduction in hematocrit Hematocrit The volume of packed red blood cells in a blood specimen. The volume is measured by centrifugation in a tube with graduated markings, or with automated blood cell counters. It is an indicator of erythrocyte status in disease. For example, anemia shows a low value; polycythemia, a high value. Neonatal Polycythemia
• Endocrine response:
  • Reduction in aldosterone Aldosterone A hormone secreted by the adrenal cortex that regulates electrolyte and water balance by increasing the renal retention of sodium and the excretion of potassium. Hyperkalemia level
  • Increase in antidiuretic hormone Antidiuretic hormone Antidiuretic hormones released by the neurohypophysis of all vertebrates (structure varies with species) to regulate water balance and osmolarity. In general, vasopressin is a nonapeptide consisting of a six-amino-acid ring with a cysteine 1 to cysteine 6 disulfide bridge or an octapeptide containing a cystine. All mammals have arginine vasopressin except the pig with a lysine at position 8. Vasopressin, a vasoconstrictor, acts on the kidney collecting ducts to increase water reabsorption, increase blood volume and blood pressure. Hypernatremia level

Hyper-osmotic volume contraction

• Reduction of total body water but osmolality Osmolality Plasma osmolality refers to the combined concentration of all solutes in the blood. Renal Sodium and Water Regulation increases:
  • Seen in cases of dehydration Dehydration The condition that results from excessive loss of water from a living organism. Volume Depletion and 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 Total protein Liver Function Tests level
  • No change in hematocrit Hematocrit The volume of packed red blood cells in a blood specimen. The volume is measured by centrifugation in a tube with graduated markings, or with automated blood cell counters. It is an indicator of erythrocyte status in disease. For example, anemia shows a low value; polycythemia, a high value. Neonatal Polycythemia
• Endocrine response:
  • Increase in aldosterone Aldosterone A hormone secreted by the adrenal cortex that regulates electrolyte and water balance by increasing the renal retention of sodium and the excretion of potassium. Hyperkalemia level
  • Increase in antidiuretic hormone Antidiuretic hormone Antidiuretic hormones released by the neurohypophysis of all vertebrates (structure varies with species) to regulate water balance and osmolarity. In general, vasopressin is a nonapeptide consisting of a six-amino-acid ring with a cysteine 1 to cysteine 6 disulfide bridge or an octapeptide containing a cystine. All mammals have arginine vasopressin except the pig with a lysine at position 8. Vasopressin, a vasoconstrictor, acts on the kidney collecting ducts to increase water reabsorption, increase blood volume and blood pressure. Hypernatremia level

Body Fluid Composition

• Body fluid compartments contain water, ions and osmolar molecules at different concentrations.
• Relative concentration of the components depends on the exchange between cells and tissues in the body which is determined by the activity of transporters on cell membranes.
• Composition of each fluid compartment is rigorously maintained by the continuous movement of water and other substances between compartments.

Clinical Relevance

• 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: a decrease in Na⁺ levels in the body. 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 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 Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia, excessive diuretic use, and Addison disease. 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 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 Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification damage, which presents with neurological symptoms of headache Headache The symptom of pain in the cranial region. It may be an isolated benign occurrence or manifestation of a wide variety of headache disorders. Brain Abscess, nausea Nausea An unpleasant sensation in the stomach usually accompanied by the urge to vomit. Common causes are early pregnancy, sea and motion sickness, emotional stress, intense pain, food poisoning, and various enteroviruses. Antiemetics, and lethargy Lethargy A general state of sluggishness, listless, or uninterested, with being tired, and having difficulty concentrating and doing simple tasks. It may be related to depression or drug addiction. Hyponatremia. Treatment is with gradual Na+ repletion, as a rapid replacement can lead to osmotic demyelination Demyelination Multiple Sclerosis 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 Severe Dehydration Fluid Replacement Therapy in Children. 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 Dextrose Intravenous Fluids solution.
• 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: the presence of excess fluid in body tissues. Both ECF and ICF compartments may be involved. 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 is often clinically apparent in the lower extremities and occurs when the osmotic gradient between the intravascular space and interstitial space promotes the flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of water from the vasculature into the tissues. 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 can be treated through diuresis or the addition of osmolar molecules (e.g., albumin Albumin Serum albumin from humans. It is an essential carrier of both endogenous substances, such as fatty acids and bilirubin, and of xenobiotics in the blood. Liver Function Tests) to the vascular space.