Body Fluid Composition & Osmolarity

Fluid Balance and Osmolarity

• Regulating water balance is fundamental to survival, especially for neurons which are at risk from osmolar stress
• Osmolarity refers to the osmolar concentration of plasma, proportional to the number of particles per litre of solution, expressed in mmol/L.
• Osmolarity is calculated from Na+, K+, urea, and glucose concentrations.
• Osmolarity can be unreliable in conditions like hyperlipidaemia, nephrotic syndrome, and hyperproteinaemia.
• Osmolality refers to the osmolar concentration of plasma, proportional to the number of particles per kilogram of solvent, expressed in mOsmol/kg.
• Osmolality is measured by laboratories using an osmometer.

Osmosis

• Osmosis is the movement of water across a selectively permeable membrane from a region of high water concentration to a region of lower water concentration.

Body Fluid Compartments

• Body fluid compartments are separated into extracellular fluid (ECF) and intracellular fluid (ICF).
• ECF serves as the body's internal environment and transports substances to and from cells
• ICF is the largest fluid compartment, facilitating intracellular chemical reactions essential for life.

ECF and ICF Volume Changes

• Factors that can cause significant changes in ECF and ICF volumes include:
  • Water ingestion
  • Dehydration
  • Intravenous infusion of different solutions
  • Loss of fluids from the gastrointestinal tract
  • Excessive sweating or kidney fluid loss

Electrolytes

• Electrolytes are compounds that dissociate into charged particles (ions) when placed in solution.
• Important cations include sodium (Na+), calcium (Ca++), potassium (K+), and magnesium (Mg++).
• Important anions include chloride (Cl-), bicarbonate (HCO3-), phosphate (HPO4=), and proteins.
• Electrolytes play critical nutrient and regulatory roles in the body.

Shifts of water between body fluid compartments

• Disturbances in body fluid can lead to:
  • Volume contraction: Decrease in ECF volume.
  • Volume expansion: Increase in ECF volume.
• These shifts occur in response to changes in osmolality, which can be categorized as isosmotic, hyperosmotic, and hyposmotic.

Fluid Balance

• Fluid balance is maintained when intake equals output.
• If more water enters the body than leaves, total fluid volume increases.
• If more water leaves the body than enters, total fluid volume decreases.

Body Fluid Compartments

• Extracellular fluid (ECF) is the internal environment of the body.
• ECF provides a constant environment for cells and transports substances to and from them.
• ECF is divided into two compartments: interstitial fluid and plasma.
• Interstitial fluid surrounds cells and is separated from plasma by capillary endothelia.
• Intracellular fluid (ICF) is the largest fluid compartment, containing 25 L of fluid.
• Plasma is the smallest fluid compartment, containing 3 L of fluid.
• Interstitial fluid is the middle ground, containing 12 L of fluid.

Electrolytes and Fluid Compartments

• ECF has a high concentration of sodium (Na+) and a low concentration of potassium (K+).
• ICF has a low concentration of sodium (Na+) and a high concentration of potassium (K+).
• All fluid compartments have the same osmolarity at steady state.

Osmolarity and Osmolality

• Osmolarity is the measurement of the solute concentration of a solution expressed as osmoles per liter (Osm/L).
• Osmolality is the measurement of the solute concentration of a solution expressed as osmoles per kilogram (Osm/kg).
• Osmolality is measured by clinical laboratories using an osmometer.
• The normal osmolality of extracellular fluid is 280-295 mOsmol/kg.

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane from a region of high water concentration to a region of low water concentration.
• Isotonic solutions have the same osmolarity as the cell.
• Hypertonic solutions have a higher osmolarity than the cell.
• Hypotonic solutions have a lower osmolarity than the cell.

Electrolytes

• Electrolytes are compounds that dissociate into charged particles called ions when placed in a solution.
• Important cations include sodium (Na+), calcium (Ca++), potassium (K+), and magnesium (Mg++).
• Important anions include chloride (Cl-), bicarbonate (HCO3-), phosphate (HPO4=), and proteins.

Changes to the ECF and ICF Compartments

• Ingestion of water, dehydration, intravenous infusion, gastrointestinal fluid loss, sweating and kidney function can all cause significant changes in ECF and ICF volume.

Volume Shifts and Homeostasis

• Volume contraction is a decrease in ECF volume.
• Volume expansion is an increase in ECF volume.
• Hyponatremia is a decrease in plasma sodium concentration.
• Hypernatremia is an increase in plasma sodium concentration.

Hyponatremia

• Hyponatremia can result from loss of sodium from the extracellular fluid or the addition of excess water to the extracellular fluid.
• Hyponatremia and dehydration are usually associated with a loss of sodium from the extracellular fluid.
• Conditions that can cause hyponatremia due to sodium loss include diarrhea, vomiting, and overuse of diuretics.

Hypernatremia

• Hypernatremia is an increase in plasma sodium concentration.
• Hypernatremia can result from loss of water from the extracellular fluid or the addition of excess sodium to the extracellular fluid.

Brain Cell Volume Regulation during Hyponatremia

• During acute hyponatremia, water diffuses into cells, causing brain tissue swelling.

Key Concept: Fluid Homeostasis

• Fluid homeostasis is achieved through the process of glomerular filtration of plasma to produce an ultrafiltrate.
• The tubules then process this ultrafiltrate so that the final urine flow rate and solute excretion meet the homeostatic needs of the body.