Ch 25 Fluid, Electrolyte, and Acid-Base Balance Notes

Water constitutes a significant portion of the human body, ranging from 45% to 75%. This percentage varies based on factors such as age, adipose tissue, and muscle tissue. Below is a detailed breakdown of average water content by age group:

Fluid Compartments

Total body water (TBW) is approximately 40 liters. This total body water is divided into:

• Intracellular Fluid (ICF): Makes up \frac{2}{3} of the total body water, approximately 25 liters, and is found inside cells.

• Extracellular Fluid (ECF): Makes up \frac{1}{3} of the total body water, approximately 15 liters, and is found outside cells.

The ECF compartment can be further detailed as follows:

Electrolyte Content in Fluid Compartments

Intracellular Fluid (ICF) contains:

• Major cations: Potassium (K^+), Magnesium (Mg^{2+})

• Major anions: Phosphate (PO_4^{3-}), negatively charged proteins

Extracellular Fluids (ECFs) contain:

• Major cations: Sodium (Na^+), Calcium (Ca^{2+})

• Major anions: Chloride (Cl^-), Bicarbonate (HCO_3^-)

Plasma has a higher protein content than interstitial fluid (IF). In both compartments, ions play critical roles in maintaining electrical gradients and osmotic balance.

Fluid Movement Between Compartments

Fluids and solutes constantly move between compartments due to osmotic and hydrostatic pressures. Water moves along the osmotic gradient from a hypotonic solution to a hypertonic solution until concentrations are equal. Examples of fluid movement are detailed below:

Drinking Water:

• Water enters the blood from the digestive system, decreasing plasma osmolarity.

• Water then moves to the interstitial fluid and into cells (plasma >> IF >> cells).

Dehydration:

• Blood plasma osmolarity increases, becoming hypertonic to both interstitial fluid and cells.

• Water moves from cells to interstitial fluid, then into the plasma (cells >> IF >> plasma).

Water Intake and Output

Maintaining proper hydration requires that water intake equals water output. Below is the average daily requirement:

Obligatory and Facultative Water Losses

Disorders of Water Balance: Dehydration

Water loss is greater than fluid intake, leading to increased ECF osmolality. This causes net osmosis of water out of tissue cells, causing shrinkage. Common causes include:

• Insufficient water intake

• Profuse sweating

• Diabetes mellitus

• Hyposecretion of ADH

• Alcohol intake

• Overexposure to cold weather

Disorders of Water Balance: Hypotonic Hydration

Also known as water intoxication, this condition is caused by rapid, excess water ingestion or ADH hypersecretion, leading to hypotonic blood plasma. This can cause:

• Cerebral edema

• Atypical accumulation of interstitial fluid

Regulation of Water Intake and Output

The thirst mechanism drives water intake through:

• Hypothalamic thirst center: Detects ECF osmolarity.

• Inhibition occurs when:

  • Drinking water relieves dry mouth,

  • Stomach and intestinal stretch receptors are activated,

  • Decreased blood osmolarity or increased blood pressure.

Fluid output is mainly regulated through kidneys by controlling urine output, influenced by:

• Hormones:

  • Angiotensin II, ADH, and aldosterone decrease urine output and increase blood volume and pressure.

  • Atrial natriuretic peptide (ANP) increases urine output and decreases blood volume and pressure.

Electrolyte Balance

Electrolyte balance, particularly sodium (Na^+) balance, is essential for fluid balance. Sodium is affected by: