1.6 Fluid Compartments and Regulation

Fluid Compartments and Regulation

• Body fluids are in constant exchange with the environment.
• Daily water intake averages ~2 liters, while synthesis adds ~0.2 liters.
• Water loss (700ml/day) is through evaporation (~300ml/skin, ~300ml/resp) plus sweat (highly variable).
• Sweat loss ranges from ~0.1 liters/day to ~2 liters/hour depending on activity and environment.
• Urine output varies from ~0.5 liters/day to ~20 liters/day, regulated primarily by the kidneys.

Intracellular and Extracellular Fluid

• Body fluids are mainly distributed among intracellular (28-42 liters) and extracellular (14 liters) compartments.
• Intracellular fluid is inside cells, with similar solute concentrations.
• Extracellular fluid is outside cells, comprising interstitial fluid and plasma.
• Cell membranes are highly permeable to water but impermeable to most electrolytes.
• Intracellular fluid contains little sodium, chloride, and calcium but high amounts of potassium and phosphate.

Osmosis and Fluid Balance

• Osmosis is the movement of water from low to high particle concentration.
• Cell membranes are impermeable to most solutes but permeable to water.
• Aquaporins facilitate rapid water movement.
• Osmotic pressure (~19.3mm Hg) across cell membranes is influenced by impermeable solutes.
• Small changes in solute concentration can cause large changes in cell volume.

Types of Solutions

• Isotonic solutions maintain normal cell volume (equal solute concentration).
• Hypertonic solutions have higher solute concentration, causing cell shrinkage.
• Hypotonic solutions have lower solute concentration, causing cell swelling.
• Water & solute equilibrium is usually reached in seconds to minutes.

Extracellular Fluid Changes

• Adding isotonic solution increases intracellular fluid volume.
• Adding hypertonic solution increases extracellular osmolarity, causing water to leave cells and shrink them.
• Adding hypotonic solution decreases extracellular osmolarity causing water to enter cells, increasing both intracellular and extracellular volume.

Hyponatremia

• Rapid hyponatremia (low sodium) causes brain cell edema and neurological symptoms.
• Slow hyponatremia allows for solute redistribution, mitigating brain edema.
• Hyponatremia can result from water loss or excess sodium in extracellular fluid.

Edema

• Intracellular edema: caused by hyponatremia, impaired metabolism, malnutrition or membrane damage.
• Extracellular edema: caused by excessive fluid leakage from blood vessels (capillaries) into interstitial spaces or lymphatic drainage failure.
• Heart failure can cause both capillary pressure and salt/water retention increasing edema.
• Kidney failure similarly causes edema due to impaired ability to remove excess salt and water.
• Nephrotic syndrome and cirrhosis of the liver can be associated with albumin loss and reduced plasma colloid osmotic pressure which leads to edema.

Preventing Edema

• Low compliance of interstitial tissues helps prevent excess fluid buildup.
• Increased lymph flow removes interstitial fluid effectively.
• High protein concentrations reduce interstitial osmotic pressure.

Additional Body Spaces

• Additional spaces in the body (e.g., pleural, peritoneal, pericardial) have thin layers of fluid for smooth movement.
• Fluid buildup in these spaces can lead to effusions.