Fluid Compartments and Body Fluid Volume

Fluid Compartments

• Intracellular fluid (ICF) constitutes two-thirds of total body fluid in adults.
• Extracellular fluid (ECF) accounts for one-third of total body fluid.
• ICF and ECF have distinct compositions due to cell membrane separation.

Body Fluid Volume States

• Euvolemia indicates normal fluid volume.
• Hypervolemia represents increased body fluid (weight gain).
• Hypovolemia indicates decreased body fluid (weight loss).

Osmosis and Osmotic Pressure

• Osmosis is the movement of water from a low concentration to a high concentration across a semi-permeable membrane.
• Osmotic pressure is the pressure needed to prevent dilution from water movement, influenced by solute concentration.
• Osmotic pressure is a colligative property, correlated with the number of particles, not their chemical identity.

Osmolality vs. Osmolarity

• Osmolality measures solute concentration in osmoles per kilogram of solvent, independent of temperature and pressure (normal: 280-298 mosmol/kg).
• Osmolarity measures solute concentration in osmoles per liter of solvent and is affected by temperature and pressure.
• Calculated serum osmolality formula: 2 x [Na+] + glucose + urea (all in mmol/L).

Serum Osmolal Gap

• The osmolal gap indicates unmeasured solutes in the blood, with a normal range of 5-15 mosmol/kg.
• Significant gaps may arise from substances like alcohols or metabolic conditions (ketoacidosis, lactic acidosis).

Electrolyte Disturbances

• Hyponatremia: Serum sodium < 135 mmol/L; can be mild (< 135) or severe (< 125).
• Hypernatremia: Serum sodium > 150 mmol/L.
• Hypokalemia: Serum potassium < 3.5 mmol/L.
• Hyperkalemia: Serum potassium > 5.0 mmol/L.

Pseudo Hyponatremia

• Characterized by normal osmolality yet decreased sodium due to conditions like hyperglycemia.
• Corrected sodium can be calculated using Corrected Na+ = Na+ + (excess glucose x 0.3).

Anion Gap (AG)

• AG is the difference between cations and anions; normal physiological balance maintains electrostatic neutrality.
• In metabolic alkalosis, the respiratory system compensates by elevating blood CO2 through hypoventilation, but this response is limited.

Renal/Metabolic Compensation

• Kidneys regulate pH through the reabsorption and excretion of acids and bases.
• In respiratory acidosis, kidneys conserve bicarbonate and secrete H+ to restore balance.
• In respiratory alkalosis, kidneys reduce bicarbonate production and reabsorb H+.

Acid-Base Disorders

• Acidosis is represented by pH < 7.4; alkalosis by pH > 7.4.
• Criteria for metabolic acidosis: HCO3- < 24 mmol/L; for respiratory acidosis: PCO2 > 40 mm Hg.
• Mixed acid-base disorders involve multiple primary disorders occurring simultaneously in a patient.

Simple Acid-Base Disorders

• Metabolic acidosis: low pH, low HCO3-, low PCO2.
• Respiratory acidosis: low pH, high HCO3-, high PCO2.
• Metabolic alkalosis: high pH, high HCO3-, high PCO2.
• Respiratory alkalosis: high pH, low HCO3-, low PCO2.

Mixed Acid-Base Disorders

• Mixed disorders involve coexisting primary acid-base derangements, impacting diagnosis and management strategies.

Test your knowledge on fluid compartments, body fluid volume states, osmosis, and the differences between osmolality and osmolarity. Understand the roles of intracellular and extracellular fluids, as well as the significance of fluid volume changes in the body. This quiz will enhance your comprehension of key physiological concepts related to body fluids.