Body Fluids and Their Functions

Body Fluids

• Body fluids are crucial for maintaining homeostasis
• Total body water (TBW) is approximately 60% of body weight in adults
• Intracellular fluid (ICF) is about 2/3 of TBW
• Extracellular fluid (ECF) is approximately 1/3 of TBW
• Interstitial fluid (ISF) comprises about 3/4 of ECF
• Vascular compartment contains blood volume, including plasma and red blood cells; about 1/4 of ECF
• Cell membranes and capillary membranes act as barriers regulating fluid and solute exchange

Functions of Body Water

• Medium for chemical and enzymatic reactions
• Essential for diffusion and filtration
• Involved in regulating pH and osmolality
• Regulates body temperature through heat absorption, distribution, and evaporation
• Lubricant in joints and potential spaces (e.g., pleura)
• Refractive medium in the eye
• Cerebrospinal fluid acts as a mechanical buffer protecting the brain
• Medium for O2 and CO2 exchange in lungs and tissues

Composition of Extracellular Fluid (ECF)

• Composition is similar throughout ECF, except for protein concentration
• Primarily composed of Na+ (about 142 mEq/L) as the main cation and Cl- (about 106 mEq/L) as the main anion
• Other cations include K+ (about 4.2 mEq/L), Ca2+, and Mg2+
• Other anions include HCO3-(about 24 mEq/L), proteins, and small amounts of HPO4-2 and SO4-2
• Also contains non-electrolytes like glucose, cholesterol, urea, uric acid, creatinine, bile pigments, and phospholipids
• ECF pH is approximately 7.4
• ECF osmolality is about 300 mOsm/L

Composition of Intracellular Fluid (ICF)

• Main cations are K+ (about 140 mEq/L) and Mg2+ (about 20 mEq/L) along with small amounts of Na+ and Ca2+
• Main anions include HPO4-2 and proteins, along with small amounts of Cl-,HCO3-, and SO4-2
• ICF pH is less than that of ECF (around 7)
• ICF osmolality is similar to ECF osmolality

Fluid Exchange Between ICF and ECF

• Cell membranes are highly permeable to water, leading to continuous fluid exchange and equal osmolality
• Factors such as hypertonic or hypotonic NaCl injections alter osmolality, causing water to move between ICF and ECF until equilibrium is reached

Osmosis

• Osmosis is the diffusion of water across a semipermeable or selectively permeable membrane
• Movement of water from a region of higher-water concentration to a region of lower-water concentration
• Water concentration in a solution is determined by the solute concentration
• The number of particles, not the type of particle is crucial to osmolality

Osmotic Pressure

• Osmotic pressure is the amount of pressure needed to stop osmosis in a sodium chloride solution
• Differences in osmotic pressure create a driving force for water movement across a membrane

Water Balance

• Normal water balance involves equal daily water gain and loss (approximately 2300 mL/day)
• Water intake is primarily from exogenous sources such as ingested fluids and food
• Water loss occurs through various mechanisms like urinary, fecal, respiratory, and evaporative losses

Water Gain sources

• Drinking fluids: approximately 1400 ml daily
• Foodwater: approximately 600 ml daily
• Endogenous synthesis: approximately 300 ml daily

Water Loss sources

• Urine: approximately 1400 ml daily
• Feces: approximately 100 ml daily
• Respiratory tract: approximately 350 ml daily
• Skin: approximately 450 ml daily, often called insensible water loss

Summary

• Important role of body fluids in maintaining homeostasis
• Composition and functions of intracellular and extracellular fluids
• Mechanisms of fluid exchange, particularly osmosis, for maintaining equilibrium
• Water balance as an aspect of overall body homeostasis