[26.5 - 26.6] Fluid, electrolyte, acid base balance (2026)

Fluid, Electrolyte, and Acid-Base Balance

26.1 Body Fluid Compartments

  • Body Water Content

    • Infants: 73% or more water (low body fat, low bone mass)

    • Adult Males: ~60% water

    • Adult Females: ~50% water (higher fat content, less skeletal muscle mass)

    • Total body water in adults averages ~40 L

    • Water content declines to ~45% in old age.

Body Fluid Distribution

  • Two Main Fluid Compartments:

    • Intracellular fluid (ICF) compartment:

      • Accounts for 2/3 of total body fluid, about 25 L of 40 L total.

    • Extracellular fluid (ECF) compartment:

      • Accounts for one-third of total body fluid:

        • Plasma: 3 L

        • Interstitial fluid (IF): 12 L in spaces between cells.

      • Also considered part of IF: lymph, cerebrospinal fluid (CSF), humors of the eye, synovial fluid, serous fluid, gastrointestinal secretions.

Composition of Body Fluids

  • Water: Universal solvent.

  • Solutes: Substances dissolved in water, classified into:

    • Nonelectrolytes:

      • Most are organic molecules.

      • Do not dissociate in water.

      • Examples: glucose, lipids, creatinine, urea (no charged particles created).

    • Electrolytes:

      • Dissociate into ions in water.

      • Examples: inorganic salts, all acids and bases, some proteins.

      • Ions conduct electrical current, greater osmotic power than nonelectrolytes.

      • Ability to cause fluid shifts due to dissociation into two or more ions. For example:

      • NaCl
        ightarrow Na^+ + Cl^- (electrolyte; 2 particles).

      • MgCl_2
        ightarrow Mg^{2+} + 2Cl^- (electrolyte; 3 particles).

      • Glucose → glucose (nonelectrolyte; 1 particle).

Comparison of Electrolytes in Body Fluids

  • Distinctive patterns of electrolytes in each compartment:

    • Extracellular Fluid (ECF):

      • Similar electrolyte contents:

        • Major cation: Na^+

        • Major anion: Cl^-.

    • Intracellular Fluid (ICF):

      • Contains more soluble proteins than plasma.

      • Low Na^+ and Cl^- content:

        • Major cation: K^+

        • Major anion: HPO_4^{2-} (hydrogen phosphate).

Role of Electrolytes in Body Fluids

  • Electrolytes: most abundant solutes and determine chemical and physical reactions.

  • Bulk of dissolved solutes consists of:

    • Proteins, phospholipids, cholesterol, and triglycerides.

  • Percent distribution: 90% in plasma, 60% in IF, and 97% in ICF.

Fluid Movement among Compartments

  • Regulation Factors:

    • Osmotic and hydrostatic pressures regulate continuous exchange and mixing of fluids.

    • Water moves freely along osmotic gradients.

  • Changes in solute concentration lead to net water flow:

    • Increased ECF osmolality → water leaves the cell.

    • Decreased ECF osmolality → water enters the cell.

Exchanges Between Fluid Compartments

  • Exchanges between plasma and IF occur across capillary walls:

    • Fluid leaks from arteriolar end and is reabsorbed at venule end.

    • Lymphatics pick up remaining fluid and return it to blood.

  • Exchanges between IF and ICF occur across cell membranes:

    • Two-way osmotic flow of water.

    • Nutrients, wastes, gases have unidirectional flow and ions move selectively.

26.2 Water Balance and ECF Osmolality

  • Water Intake and Output:

    • Approximately 2500 ml/day must be balanced.

    • Sources of water intake:

      • Most from ingested foods and beverages; minor from metabolism (metabolic water).

    • Water Output:

      • Urine (60%), insensible water loss (skin and lungs), perspiration, feces.

Osmolality Regulation

  • Maintained around 280–300 mOsm:

    • Rise in osmolality:

      • Stimulates thirst, causes ADH release.

    • Decrease in osmolality:

      • Causes inhibition of ADH.

Regulation of Water Intake

  • Thirst Mechanism:

    • Governed by hypothalamic thirst center:

      • Activated by increased plasma osmolality (1–2% increase), dry mouth, decreased blood volume/pressure (signaled by Angiotensin II or baroreceptor input).

  • Feedback Mechanisms:

    • Drinking water inhibits thirst center:

      • Includes relief of dry mouth, activation of stomach and intestinal stretch receptors.

Regulation of Water Output

  • Obligatory water losses:

    • Include insensible water loss (lungs/sweat) and sensible water loss (urine, sweat, feces).

  • Fluid intake influences urine volume and concentration:

    • Urine volume and solute concentration are dependent on fluid intake, diet, and other water loss avenues.

Influence of Antidiuretic Hormone (ADH)

  • ADH Effect on Water Reabsorption:

    • Proportional to ADH release:

      • Increased ADH leads to concentrated urine and increased body fluid volume.

      • Decreased ADH leads to dilute urine and decrease in body fluid volume.

  • Mechanism:

    • Triggers aquaporin expression in collecting ducts; regulated by ECF solute concentration.

Disorders of Water Balance

  • Dehydration:

    • ECF water loss from hemorrhage, burns, vomiting, sweating, deprivation, or diuretic abuse.

    • Symptoms: sticky oral mucosa, thirst, dry skin, oliguria, weight loss, confusion, hypovolemic shock.

  • Hypotonic Hydration (Water Intoxication):

    • Cellular overhydration due to renal insufficiency or excess rapid water intake.

    • Results in hyponatremia and cellular swelling.

    • Symptoms: metabolic disturbances, nausea, vomiting, muscle cramps, cerebral edema, death is possible.

    • Treatment involves hypertonic saline.

  • Edema:

    • Abnormal accumulation of interstitial fluid causes tissue swelling.

    • Can impair tissue function.

26.3 Electrolyte Balance

  • Electrolyte balance primarily concerns salts:

    • Salt control fluid movement, provide minerals for excitability and membrane permeability.

    • Salts enter through ingestion and metabolism, lost through perspiration, feces, urine, vomiting.

Regulation of Sodium Balance

  • Major cation in ECF:

    • Sodium salts contribute to 280 mOsm of total ECF solute concentration.

    • Sodium affects plasma volume, blood pressure, ECF, and IF volumes.

    • No known receptors for monitoring Na⁺; however, the macula densa monitors filtrate.

Role of Hormones in Sodium Regulation

  • Aldosterone:

    • Major role in sodium regulation via kidneys.

    • Renin-angiotensin-aldosterone mechanism is the main trigger for aldosterone release:

      • In response to sympathetic stimulation, decreased NaCl concentration, or decreased blood pressure.

    • High concentrations lead to increased Na⁺ reabsorption and greater fluid volume.

    • Low concentrations result in urinary sodium loss and decreased fluid reabsorption.

  • Atrial Natriuretic Peptide (ANP):

    • Released by atrial cells due to increased blood pressure:

      • Decreases blood volume and pressure, inhibiting ADH and renin/aldosterone production.

      • Increases sodium and water excretion.

Regulation of Other Electrolytes

  • Potassium Balance:

    • Affects the resting membrane potential of neurons and muscles:

      • Hyperkalemia decreases RMP and causes depolarization.

      • Hypokalemia leads to hyperpolarization.

    • Regulatory site: DCT and collecting duct, where secretion is controlled.

  • Calcium and Phosphate Regulation:

    • 99% calcium in bones; controlled by parathyroid hormone (PTH).

    • PTH increases calcium levels by acting on bones, kidneys, and small intestine.

    • Phosphate reabsorption in kidneys is influenced by hormones like insulin and glucagon.

  • Anion Regulation:

    • Chloride (Cl⁻) follows sodium in ECF to maintain osmotic pressure and is actively reabsorbed alongside Na⁺.