Fluid, Electrolyte, and Acid-Base Balance

Body Fluids

Body fluids are essential for maintaining homeostasis, transporting nutrients, and facilitating biochemical reactions. The distribution and composition of body fluids vary with age, sex, and body composition.

• Body Water Content:

  • Infants: ~73% water

  • Old age: ~45% water

  • Lean men: 57–63% water

  • Lean women: ~50% water

  • Increased body fat decreases water content

• Fluid Compartments:

  • Intracellular Fluid (ICF): ~66% of total body water; fluid within cells

  • Extracellular Fluid (ECF): ~34% of total body water; includes plasma, interstitial fluid (IF), lymph, cerebrospinal fluid, synovial fluid, etc.

• Fluid Composition:

  • Nonelectrolytes: Organic molecules (e.g., glucose, urea, lipids) that do not dissociate in water and carry no charge

  • Electrolytes: Inorganic salts, acids, bases, and some proteins that dissociate into ions in water, affecting osmolarity and water movement

  • Electrolyte concentration is measured in milliequivalents per liter (mEq/L)

• Major Ions in Compartments:

  • ECF: Na+, Cl-, HCO3-

  • ICF: K+, HPO42-, Mg2+, proteins

  • Plasma contains more protein than interstitial fluid

• Fluid Movement:

  • Driven by osmotic and hydrostatic pressures across capillary beds

  • Filtration: Plasma forced out by hydrostatic pressure; proteins pull water back by osmosis

  • Most fluid returns to blood; remainder returns via lymphatic system

  • Solute exchange regulated by size, charge, and active transport

  • Osmolarity changes in ECF cause water to move between compartments

Water Balance

Water balance is maintained by matching intake and output, regulated by thirst and hormonal mechanisms.

• Sources of Water: Ingested fluids and food, metabolic water production

• Water Loss: Lungs, perspiration, gastrointestinal tract, kidneys

• Regulation of Water Intake (Thirst Mechanism):

  • Triggered by decreased plasma volume (5–10%) or increased plasma osmolarity (1–2%)

  • Dry mouth and osmoreceptor activation in hypothalamus stimulate thirst

  • Drinking dampens oral mucosa and activates stomach stretch receptors, inhibiting further thirst

• Regulation of Water Loss (Antidiuretic Hormone, ADH):

  • Osmoreceptors in hypothalamus detect increased ECF osmolality

  • ADH released, causing kidneys to retain water

  • Decreased ECF osmolality inhibits ADH, leading to water excretion

• Disorders:

  • Dehydration: Water loss exceeds intake; causes include burns, hemorrhage, vomiting, diarrhea, excessive sweating, diuretics, endocrine disorders

  • Hypotonic Hydration (Water Intoxication): Excess water dilutes ECF, causing water to enter cells (edema); can lead to brain swelling and death

  • Edema: Accumulation of fluid in ECF; caused by increased capillary hydrostatic pressure, increased permeability, or lymphatic blockage

Electrolyte Balance

Electrolyte balance involves the regulation of salts and ions, crucial for nerve function, muscle contraction, and fluid distribution.

• Salts in the Body:

  • Enter via food and water; small amounts produced metabolically

  • Exit via perspiration, feces, urine

• Role of Sodium (Na+):

  • Most abundant cation in ECF; major determinant of ECF volume and osmolarity

  • Regulation affects blood pressure and volume

  • Hormonal regulation:

    • Aldosterone: Increases Na+ reabsorption in kidneys; water follows

    • Renin-Angiotensin-Aldosterone System (RAAS): Activated by low BP or low filtrate osmolarity; increases aldosterone

    • Atrial Natriuretic Peptide (ANP): Inhibits Na+ and water retention, suppresses ADH, renin, and aldosterone, causes vasodilation

    • Estrogen: Promotes Na+ and water reabsorption

    • Progesterone: Promotes Na+ excretion by blocking aldosterone

    • Glucocorticoids (e.g., cortisol): Enhance Na+ and water reabsorption, but may increase GFR

• Role of Potassium (K+):

  • Most abundant cation in ICF; essential for membrane potential and neuromuscular function

  • Regulated by renal excretion; increased ICF K+ leads to secretion into filtrate

  • Aldosterone increases K+ secretion in exchange for Na+

  • H+ concentration (pH) affects K+ secretion: acidosis decreases K+ secretion

• Role of Calcium (Ca2+):

  • Regulated primarily by Parathyroid Hormone (PTH): increases Ca2+ release from bone, absorption in intestine, and reabsorption in kidneys

  • Calcitonin (thyroid hormone) may decrease plasma Ca2+ at therapeutic levels

• Role of Magnesium (Mg2+):

  • Stored in bone, muscle, and liver

  • Aldosterone may stimulate Mg2+ secretion

• Role of Chloride (Cl-):

  • Major anion in ECF; reabsorbed in kidneys with Na+

  • Less reabsorbed when blood pH is acidic

Acid-Base Balance in Body Fluids

Maintaining a stable pH is vital for cellular function. The body uses buffer systems, respiratory, and renal mechanisms to regulate acid-base balance.

• Normal pH Values:

  • Arterial blood: 7.4

  • Venous blood & ECF: 7.35

  • ICF: 7.0

  • Alkalosis: pH > 7.45

  • Acidosis: pH < 7.35

• Sources of Acids:

  • Protein catabolism (phosphoric acid)

  • Anaerobic respiration (lactic acid)

  • Fat catabolism (fatty acids, ketone bodies)

  • Stomach acid, CO2 loading in blood

Chemical Buffer Systems

• Buffer: A system that resists changes in pH by binding or releasing H+ ions

• Three main buffer systems:

  • Bicarbonate Buffer System (ECF):

    • Components: H2CO3 (carbonic acid) and NaHCO3 (sodium bicarbonate)

    • In acid environment:

    • In basic environment:

    • Bicarbonate regulated by kidneys; carbonic acid by respiratory system

  • Phosphate Buffer System (ICF and urine):

    • Components: Na2HPO4 (sodium hydrogen phosphate) and NaH2PO4 (sodium dihydrogen phosphate)

    • In acid environment:

    • In basic environment:

  • Protein Buffer System:

    • Proteins act as amphoteric molecules (can act as acid or base)

    • Carboxyl groups (–COOH) release H+; amino groups (–NH2) accept H+

    • Hemoglobin in RBCs buffers H+ in blood

Respiratory System Regulation of [H+]

• CO2 + H2O H2CO3 $\leftrightarrow$ H+ + HCO3-

• Increased CO2 or H+ stimulates medullary chemoreceptors, increasing respiratory rate and depth

• More CO2 is exhaled, shifting equilibrium to the left, raising pH

• Alkalosis depresses respiratory center, retaining CO2 and lowering pH

Renal Mechanisms of Acid-Base Balance

• Kidneys excrete metabolic acids and regulate blood levels of alkaline substances

• Excrete H+ and conserve/replenish bicarbonate ions

• Hydrogen Ion Secretion:

  • CO2 + H2O H2CO3 $\rightarrow$ HCO3- + H+

  • H+ secreted into filtrate; Na+ reabsorbed

  • Rate of H+ secretion tied to CO2 levels

• Conservation of Filtered Bicarbonate:

  • Bicarbonate generated in tubule cells is shunted into capillary blood for each H+ secreted

• Buffering of Excreted H+:

  • Phosphate and ammonia buffer systems in urine buffer secreted H+

Abnormalities of Acid-Base Balance

• Effects:

  • Acidosis: CNS depression, coma, death if pH < 7.0

  • Alkalosis: CNS overactivity, tetany, convulsions, death if respiratory muscles affected

• Compensation:

  • Respiratory system compensates for metabolic imbalances by altering ventilation

  • Renal system compensates for respiratory imbalances by adjusting HCO3- retention or excretion

Example: Bicarbonate Buffer System in Action

• When a strong acid (HCl) is added to blood, bicarbonate ions buffer the excess H+:

• Carbonic acid (H2CO3) then dissociates to CO2 and H2O, which are exhaled or excreted

Additional info: For a comprehensive understanding, review related chapters on renal physiology, respiratory system, and hormonal regulation of fluid balance.