Body Fluid Compartments

Fluid Compartments

• TBW typically accounts for 60% of a patient’s weight, with variability depending on patient demographics and composition.^1-4
  • Gender: Increased ICF and ECF in biological males vs biological females.³
  • Body composition: Higher water content in lower BMI individuals.¹
  • Age: TBW declines with age.^2,4
• TBW is divided into ICF and ECF¹ (Table 1).
  • ECF can be further sub-categorized into interstitial fluid and blood plasma.

• Literature describes the “60-40-20 Rule” where 60% of the body’s weight is water, 40% of the TBW is localized within the cell membrane, and 20% of the TBW is localized outside of the cell membrane.¹

ICF

• ICF refers to the fluid within cells, which is surrounded by plasma membranes.^1,2
• The main positively charged molecules located within the ICF are potassium and magnesium.^1,2
• The main negatively charged molecules in the ICF are intracellular proteins, nucleic acids, and phosphate.^1,2

ECF

• ECF refers to the fluid within the body outside the cells. This includes fluid between the cells and within blood plasma.^1,2
• The primary ions that compose the ECF are sodium (Na⁺), chloride (Cl^–), and bicarbonate.^1,2

Regulation of Plasma Osmolarity

• The above fluid distribution and ion concentrations are subject to alteration by the body to maintain homeostasis in response to physiologic disturbances.¹
• Osmolarity is a measure of the number of dissolved osmotically active particles per unit volume of a solution, or simply put, the concentration of ions within a compartment.¹
• Normal serum osmolarity falls within a range of 275-295 mmol/kg.⁵
• The serum (or plasma) osmolarity is determined by the concentration of different solutes in the plasma.¹ Na⁺ salts (mainly Cl^– and bicarbonate), glucose, and urea are the primary circulating solutes.
• Below is the standard calculation for finding serum osmolarity.

• Plasma osmolarity is tightly regulated as it alters the fluid tonicity; tonicity refers to the response of intracellular water to the osmolarity of the surrounding ECF.
  • If the ECF is isotonic, the cells stay the same size
  • If the ECF is hypertonic, the cells shrink due to the extracellular movement of water
  • If the ECF is hypotonic, the cells swell due to the intracellular movement of water by osmosis.
• Changes in the plasma osmolarity are governed by feedback loops that are driven by a sensor, a control center and an effector.⁶
  • Sensor: Osmoreceptors within the central nervous system.⁶
  • Control Center: Central nervous system.⁶
  • Effector: Hormone secretion or stimulation of thirst.⁶
• Feedback loops involving the hypothalamus and the renal system drive regulatory mechanisms that maintain plasma osmolarity homeostasis.⁷ See OA summary on the renal system for more details. Link

Impact of Low Plasma Volume

• A reduction in circulating volume will reduce venous return to the heart causing a drop in preload. Subsequently, this results in a reduction of secreted atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) secretion from the atria.⁶
• The reduction in ANP and BNP are sensed by receptors within the central nervous system which will then upregulate the sensation of thirst and cause secretion of norepinephrine, arginine vasopressin (AVP) and renin. These three hormones will then act on effector organs to retain fluid within the vasculature and increase blood volume.⁶
• This system helps with physiologic maintenance as the human body will typically lose ~1L of fluid to standard diuresis, passage of stool, sweating and respiration.⁶
  • Therefore, a minimum of 1L of fluid intake is typically necessary to balance these physiologic losses and maintain an adequate blood volume.⁶

Impact of High Plasma Volume

• An increase in circulating volume will increase venous return to the heart causing a rise in preload. This results in a greater stretch on mechanoreceptors within the cardiac and pulmonary systems and will induce the release of ANP and BNP.⁶
• The consequence of increased volume along with elevated ANP/BNP will favor greater excretion of Na⁺ into urine from the renal system. This flux of Na⁺ from the vasculature into the renal system will promote diuresis of the excess circulating volume.⁶
• Furthermore, high plasma volume will inhibit the release of renin and aldosterone, thereby increasing volume loss from the renal system.

Hypothalamic Regulation

• AVP, also known as antidiuretic hormone, is produced in the hypothalamus and stored in the posterior pituitary gland. AVP plays a major role in maintaining TBW balance.^2,7
• AVP has two major effects on homeostasis

1. Increase mean arterial pressure via vasoconstriction.²
2. Increase water reabsorption in the renal system by increasing expression of aquaporin channels.^1,2

• AVP is released from the posterior pituitary in response to a sensed increase in plasma osmolarity via hypothalamic osmoreceptors. This mechanism follows negative-feedback physiology.²

Renal Regulation

• The renin-angiotensin-aldosterone system is a major regulator of plasma osmolarity via renal hormone release, with effects primarily exerted on Na⁺, potassium, and water.² See OA summary on renin-angiotensin-aldosterone system for more details. Link
• Aldosterone
  • Aldosterone is an endogenous hormone released from the adrenal gland in response to hyperkalemia or hyponatremia to balance the osmotic concentration of blood plasma.²
  • In a hyperkalemic state, the blood plasma is hypertonic (high osmotic concentration), and removal of potassium will shift the blood plasma back towards homeostatic levels.²
  • In a hyponatremic state, aldosterone will increase reabsorption of Na+ and water to shift the blood plasma back towards homeostatic levels.²

Capillary Fluid Exchange

• Body fluids are regulated at the capillary level via the balance between hydrostatic forces and osmotic (colloidal) pressure gradients.²
• Hydrostatic pressure is generated by the left heart and tonicity of the vascular system, while osmotic pressure is maintained by the composition of solute within the vasculature.²
• Albumin is a major intravascular protein that carries lipid-soluble substances throughout the vasculature and contributes significantly to serum osmolarity and oncotic pressure.⁵ See OA summary on albumin for more details. Link
  • While albumin comprises 5% of the total plasma content, it accounts for 50% of the total protein content in the intravascular compartment.⁵

Clinical Relevance to Anesthesia

• The distribution and composition of body fluid compartments are influenced by osmolarity and tonicity and can be manipulated by administration of IV solutions during operative management.⁸
• Crystalloid solutions are IV fluids with low molecular weight that typically contain salts or glucose, which influence a patient’s osmolarity and tonicity. These solutions are characterized as hypotonic, isotonic, or hypertonic in reference to normal plasma osmolarity.⁸ See OA summary on crystalloid solutions for more details. Link
  • Hypotonic IV solutions are exogenous fluids administered with an osmolarity lower than normal plasma osmolarity.⁸
  • Isotonic IV solutions are exogenous fluids administered with an osmolarity similar to that of normal plasma.⁸
  • Hypertonic IV solutions are exogenous fluids administered that have a higher osmotic concentration than normal plasma osmolarity.⁸
• Colloid solutions are IV solutions with higher molecular weights that do not cross the endothelium as readily as crystalloid solutions and can function as vascular volume expanders when there is plasma loss.⁸ See OA summary on colloid solutions for more details. Link