Fluid, Electrolyte, and Acid-Base Homeostasis: Study Guide

Fluid, Electrolyte, and Acid-Base Homeostasis

Body Fluids & Compartments

Body fluids are distributed in distinct compartments, each with unique solute compositions. Understanding these compartments is essential for grasping fluid and electrolyte balance.

• Factors Affecting Body Water Content: Age, gender, fat, muscle, and bone influence total body water. Muscle contains the most water, bone less, and adipose tissue the least.

• Fluid Compartments:

  • Intracellular Fluid (ICF): Located within cells; comprises about 2/3 of total body fluid. Rich in K+, HPO42-, and proteins.

  • Extracellular Fluid (ECF): Includes plasma and interstitial fluid (IF); contains Na+, Cl-, and proteins, but very low K+.

Osmotic Activity

Osmosis governs water movement between compartments, driven by solute concentration differences.

• Osmosis: Water moves across a semipermeable membrane from areas of higher water concentration (lower solute) to lower water concentration (higher solute).

• Osmolarity: Number of solute particles per liter of solution.

• Osmolality: Number of solute particles per kilogram of solution.

• Electrolytes: Substances that dissociate into ions in water and conduct electricity (e.g., salts, acids, bases, some proteins).

• Non-electrolytes: Solute particles that do not dissociate into ions (e.g., proteins, lipids); contribute less to osmolality.

• Fluid Movement: ICF volume is determined by ECF solute concentration. High ECF solute draws water out of cells; low ECF solute allows water to enter cells.

• Colloidal Osmotic Pressure: Plasma proteins exert inward force; if lower than hydrostatic pressure, fluid leaks into IF.

Water Balance

Water intake and output must be balanced to maintain homeostasis.

• Water Intake: Drinking, solid foods, metabolic water (~2.5 L/day).

• Water Output: Urine (60%), sweat, feces, evaporation from lungs and skin.

• Control Mechanisms:

  • High plasma osmolarity or low blood volume stimulates thirst center in hypothalamus.

  • Urine production (diuresis) removes excess ions and waste; water loss depends on solute levels, diet, fluid intake, and sweating.

  • Na+, ADH, and Aldosterone regulate water levels, blood pressure, and cardiovascular function.

Homeostatic Imbalances

Disruptions in fluid and electrolyte balance can lead to clinical conditions.

• Dehydration: Loss of water and electrolytes increases ECF osmolarity, drawing water out of cells. Causes include hemorrhage, diarrhea, vomiting, sweating, burns, diabetes mellitus, and diabetes insipidus (low ADH).

• Diabetes Mellitus: Glycosuria (excess glucose in urine) draws water into urine by osmosis, causing polyuria.

• Edema: Fluid accumulates in interstitial fluid, lowering blood pressure and impairing circulation.

Maintaining Electrolyte Balance

Electrolytes are vital for physiological functions and are regulated by intake and excretion.

• Sources: Food, drink, metabolism.

• Excretion: Sweat, feces, urine.

• Sodium (Na+): Main solute in ECF; affects osmotic pressure, blood pressure, acid-base balance, muscle and nervous function. Homeostasis is maintained by Na+/K+ pumps.

Regulation of Sodium & Water Balance: Hormonal Control

Hormone	Source	Main Effects
Aldosterone	Adrenal cortex	Increases Na+ reabsorption, K+ secretion; raises BP
ADH (Antidiuretic Hormone)	Posterior pituitary	Promotes water reabsorption in DCT & collecting ducts; raises BP
ANP (Atrial Natriuretic Peptide)	Atria	Suppresses ADH, Renin, Aldosterone; increases Na+ and water loss; lowers BP
Estrogen	Ovaries	Increases Na+ and water retention
Progesterone	Ovaries	Promotes Na+ and water loss; lowers BP
Glucocorticoids	Adrenal cortex	Increase Na+ reabsorption, raise blood glucose and BP

Additional info: Addison's disease (hypoaldosteronism) causes large losses of Na+ and water in urine.

Regulation of Other Ions

• Potassium (K+): Alters membrane potential; imbalances (hyperkalemia/hypokalemia) can cause cardiac issues. Regulated by secretion in collecting ducts and enhanced by aldosterone.

• Calcium (Ca2+): Low levels cause muscle tetany; high levels cause arrhythmias. Regulated by PTH (increases Ca2+ in blood) and calcitonin (decreases Ca2+ in blood).

• Chloride (Cl-): Maintains osmotic pressure; reabsorbed with Na+ in nephron. In acidosis, bicarbonate is reabsorbed as a buffer.

Central Nervous System Regulation

• High BP: Decreased sympathetic stimulation leads to vasodilation, increased GFR, and greater solute/water loss in urine, lowering BP.

• Low BP: Opposite response occurs.

Acid-Base Balance

Maintaining blood pH is critical for physiological function. Acids are proton donors (low pH), bases are proton acceptors (high pH).

• Normal pH: 7.35–7.45

• Acidosis: pH 7.0–7.35

• Alkalosis: pH above 7.45

Types of Acid-Base Imbalances

Type	Cause
Respiratory Acidosis	CO2 retention
Respiratory Alkalosis	CO2 eliminated faster than produced
Metabolic Acidosis	Accumulation of fixed acids (lactic acid, ketone bodies); loss of HCO3-
Metabolic Alkalosis	Excess HCO3-

Sources of Acids

• Protein metabolism yields phosphoric acid.

• Anaerobic respiration yields lactic acid.

• Fat metabolism yields fatty acids and ketone bodies.

• CO2 transport as HCO3- produces H+.

Chemical Buffers in pH Balance

• Buffer Systems: Resist changes in pH.

• Carbonic Acid/Bicarbonate: Buffers acids and bases in ECF. Levels regulated by kidneys.

• Phosphate Buffer: NaH2PO4 and Na2HPO4 buffer in ICF and urine.

• Protein Buffers: Proteins are amphoteric, with both acidic and alkaline ends; potent buffers in ICF.

Respiratory Center in pH Balance

• Acts slower than chemical buffers but is effective.

• Key reaction: $\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-}$

• High blood CO2 (hypercapnia) increases respiratory rate and depth, exhaling CO2 and lowering blood CO2.

• Alkaline blood pH causes shallow breathing, CO2 accumulates, lowering pH.

Kidneys in pH Balance

• Excrete or reabsorb ions as needed.

• Excrete phosphoric, uric, lactic acids, and ketone bodies.

• In acidosis: H+ is excreted.

• In alkalosis: H+ is reabsorbed, HCO3- is excreted.

• Regulate alkali levels and maintain HCO3- in ECF.

• Regenerate chemical buffers, coupled to ion transport.

Infants & Elderly: Homeostatic Imbalances

Infants and elderly are more susceptible to fluid and pH imbalances due to physiological differences.

• Infants: Immature kidneys, large surface area relative to mass, high metabolic rate, high fluid intake/output, low lung residual volume. Respiratory changes affect PCO2 and pH.

• Elderly: Loss of muscle mass reduces ICF water, poor water reserves, prone to dehydration, diminished homeostatic control, age-related issues like congestive heart failure and edema due to hypertension.