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 have ~45%
• Young men have ~60% body water; young women have ~50%
• Adipose tissue holds less water (up to 20%) compared to other tissues
• Muscle tissue holds ~75% water
• Proportionately more muscle mass correlates with more body water

Solutes in Body Fluids

• Water is a universal solvent
• Two solute classifications: electrolytes and nonelectrolytes
• Electrolytes dissociate into ions, conducting electricity; examples include salts, acids, and bases. Dissociation can produce more than one ion.
• Nonelectrolytes do not dissociate and usually have covalent bonds

Fluid Compartments

• Water occupies two main compartments: Intracellular Fluid (ICF) and Extracellular Fluid (ECF)
• ICF makes up 2/3 of body water; found inside cells
• ICF includes plasma (3L, 20% of ECF). Interstitial fluid (IF) (fluid between cells approximately 12L with remaining 15L) makes up 1/3 of body water.
• Non-plasma/non-IF fluids e.g. lymph, CSF, eye humors are considered part of IF

Comparison of ECF and ICF

• Sodium and potassium concentrations are opposite in ECF and ICF
• Na+/K+ pumps maintain resting membrane potential

Fluid Exchanges

• Water moves freely between compartments, maintaining overall osmolality
• Substances cross both plasma and interstitial fluid to reach intracellular fluid
• Plasma acts as a transport system for various materials
• Exchange between plasma and interstitial fluid occurs across capillary walls.
• Exchange between interstitial fluid and intracellular fluid occurs across plasma membranes

Water Intake and Output

• Water intake varies considerably between people
• Water input sources include metabolism, food, and beverages
• Water output involves urination, feces, sweat, insensible water loss

Regulation of Water Output

• Obligatory water losses are roughly 500 mL of urine/day
• Kidneys flush about 600 mmols of solutes per day; this necessitates water loss as well.
• Beyond obligatory loss, factors like fluid intake, diet, and perspiration affect water output

Disturbances in Water Balance

• If the environment outside the cell has a higher solute concentration (hypertonicity), water moves out of the cell leading to shrinking
• If the environment outside the cell has a lower solute concentration (hypotonicity), water moves into the cell, potentially causing swelling or bursting.

Regulation of Sodium

• Although sodium is crucial for water balance, no specific sodium receptor has been identified.
• Increased plasma potassium concentration leads to renin release leading to angiotensin II production
• Angiotensin II stimulates the adrenal cortex to release aldosterone
• Aldosterone targets kidney tubules, increasing sodium reabsorption and potassium secretion to restore homeostasis of plasma sodium and potassium.

Regulation of Physiological pH

• Optimal pH varies in different body parts, especially arterial blood (~pH 7.4)
• Deviations (acidosis, alkalosis) affect biochemical reactions
• Chemical buffers (like bicarbonate and phosphate) temporarily resist pH changes
• Breathing and kidneys regulate pH long term; removing potential imbalances.

This quiz explores the concepts of body water content in relation to age, sex, and tissue types. Learn about the classifications of solutes found in body fluids and the major compartments of water within the body. Test your understanding of how these factors influence overall health.