Body Water Content and Fluid Compartments

Body Water Content and Fluid Compartments

• Body water content varies based on age, body mass, and sex. Infants have ~70% body water, elderly ~45%, men ~60%, and women ~50%.
• Adipose tissue holds ~20% water, while skeletal muscle holds ~75%. More muscle means more body water.
• Total body water is ~40 liters, accounting for ~60% of body weight.
• Intracellular fluid (ICF) accounts for ~25 liters (40% of body weight) and is found inside cells.
• Extracellular fluid (ECF) accounts for ~15 liters (20% of body weight) and is outside cells, with interstitial fluid and plasma as subcompartments.
• Interstitial fluid fills spaces between cells, and plasma is the liquid portion of blood (3 liters, 20% of ECF).
• Non-plasma/non-IF fluids like lymph, CSF (cerebrospinal fluid), and eye or synovial fluids are considered interstitial fluid.

Composition of Body Fluids

• Nonelectrolytes: Primarily composed of covalent bonds and don't dissociate in solution.
• Electrolytes: Dissociate into ions in water, conducting electricity. Examples include inorganic salts, acids, bases, and some proteins. Electrolytes have a stronger osmotic pressure than nonelectrolytes, producing at least 2 ions upon dissociation. Examples: NaCl (two particles), MgCl2 (three particles), as opposed to glucose (one particle).
• Fluid compositions vary between intracellular, interstitial, and plasma. The concentration of particular ions varies dramatically between these compartments.

Comparison of ECF vs ICF

• Sodium and potassium concentrations are opposite in ECF and ICF.
• Characteristic distribution of ions leads to Na+/K+ pumps maintaining resting membrane potentials.

Exchanges Between Fluid Compartments

• Water moves freely between compartments to maintain osmolarity.
• Substances (like nutrients, gases) need to pass through both plasma and interstitial fluid to reach the ICF.
• Plasma acts as a carrier for shuttling materials throughout the body. Exchanges between plasma and interstitial fluid occur across capillary walls. Exchanges between interstitial fluid and intracellular fluid occur across plasma membranes.

Water Intake and Output

• Water intake varies between individuals, influenced by metabolism (production of water), foods, and beverages.
• Water exits the body as urine (60%), sweat (~8%), insensible water loss (skin and lungs, 28%), and feces (~4%).
• Water and sodium are closely linked, with sodium acting as a "water magnet".

Regulation of Water Output

• Obligatory water loss due to urine output (~500 mL/day) is needed to filter out waste products.
• Additional water loss can occur via other mechanisms like fluid intake, diet, and perspiration.

Disturbances in Water Balance

• Hypertonicity: Higher solute concentration outside the cell causes water to move out, causing cell shrinkage.
• Hypotonicity: Higher solute concentration inside the cell causes water to move inside, causing cell swelling or bursting.
• Isotonicity: Balanced solute concentrations inside and outside the cell; no significant net water movement.

Regulation of Sodium

• Despite sodium's importance in water balance, there's no identified sodium receptor.
• Sodium levels are regulated by the kidneys through renin release, angiotensin II stimulation, and aldosterone release, influencing sodium reabsorption in the tubules.

Regulation of Physiological pH

• Optimal pH is crucial for biochemical reactions.
• pH imbalance (acidosis or alkalosis) can affect the body's functions.
• Chemical buffers (like bicarbonate and phosphate systems) can temporarily resist pH changes.
• Proteins in the body can also act as buffers, binding or releasing protons.
• Regulation of pH involves chemical buffers, respiratory mechanisms (controlling CO2), and renal mechanisms (controlling H+ ions, and bicarbonate).

Explore the intricate details of body water content and the various fluid compartments that constitute human physiology. Learn how factors such as age, sex, and body mass influence water distribution in the body, alongside the roles of intracellular and extracellular fluids. This quiz covers the essentials of body fluids and their composition.