Chapter 26: Fluids, Electrolytes, and Acid-Base Balance – Structured Study Notes

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Body fluids are distributed in distinct compartments and contain various solutes essential for physiological function.

Body Water Content: Water constitutes 40–75% of body weight, varying with age, sex, and body fat.
Fluid Compartments:
- Intracellular Fluid (ICF): About two-thirds (25L) of body water is inside cells.
- Extracellular Fluid (ECF): The remaining one-third (15L) is outside cells, including plasma and interstitial fluid.
Composition of Body Fluids:
- Electrolytes: Dissolved ions such as sodium, chloride, potassium, bicarbonate, phosphate, and magnesium.
- Nonelectrolytes: Molecules like glucose and urea that do not dissociate into ions.
- Plasma vs. Interstitial Fluid: Plasma contains more proteins; otherwise, ECF compartments are similar.
- ICF: Rich in potassium, phosphate, magnesium, and protein anions.
Fluid Movement: Regulated by osmotic and hydrostatic pressures. Water moves freely by osmosis; solute movement is restricted by size, charge, and transport proteins.

Example: Sodium and chloride are the most abundant ECF electrolytes, while potassium dominates the ICF.

Water balance is maintained by coordinated intake and output mechanisms.

Sources of Water: Ingested foods and fluids, and metabolic water produced during cellular respiration.
Water Loss: Occurs via lungs, skin, gastrointestinal tract, and kidneys.
Regulation of Intake: Increased plasma osmolality activates the thirst mechanism via hypothalamic osmoreceptors.
Regulation of Output:
- Obligatory Water Loss: Unavoidable losses (insensible losses, feces, minimum urine output of ~500 ml/day).
- ADH Influence: Antidiuretic hormone (ADH) increases water reabsorption in renal collecting ducts by inserting aquaporins.
Disorders of Water Balance:
- Dehydration: Water loss exceeds intake; symptoms include thirst, dry skin, decreased urine output, risk of hypovolemic shock.
- Hypotonic Hydration: Excessive water intake dilutes body fluids, causing cell swelling and risk of cerebral edema.
- Edema: Abnormal accumulation of fluid in interstitial space, impairing circulation.

Example: ADH release is triggered by high ECF osmolality or low blood volume/pressure.

Electrolyte levels are tightly regulated to maintain homeostasis.

Sources and Losses: Electrolytes are obtained from diet and lost via sweat, feces, urine, and sometimes vomit.
Role of Kidneys: Primary regulators of electrolyte balance.
Sodium:
- Most abundant ECF solute; controls water volume and distribution.
- Regulation involves baroreceptors, neural and hormonal controls (ADH, aldosterone, renin, atrial natriuretic peptide).
- Aldosterone increases sodium reabsorption; ANP promotes sodium and water excretion.
Potassium:
- 90% reabsorbed in proximal nephron; regulation focuses on excretion.
- Aldosterone and high plasma potassium enhance secretion in distal nephron.
- Principal cells secrete potassium; type A intercalated cells reabsorb during deficit.
Calcium and Phosphate:
- Calcium regulated by parathyroid hormone (PTH), targeting bones, kidneys, intestine.
- PTH decreases renal phosphate reabsorption.
Anion Regulation:
- Chloride is major anion with sodium when pH is normal/high; bicarbonate replaces chloride in acidosis.
- Other anions regulated by transport maximums.

Example: Atrial natriuretic peptide (ANP) is released in response to high blood pressure, promoting sodium and water excretion.

Rapid mechanisms minimize pH changes in body fluids.

Normal pH Range: Arterial blood pH is 7.35–7.45; alkalosis is above, acidosis is below.
Acid Sources: Most acids are generated metabolically (protein breakdown, incomplete oxidation, CO2 transport).
Buffer Systems:
- Chemical Buffers: Weak acids and their salts resist pH shifts by binding/releasing H+.
- Major systems: bicarbonate, phosphate, protein buffers.
Respiratory Regulation:
- CO2 and H2O are in equilibrium with H2CO3 (carbonic acid).
- Acidosis increases respiratory rate/depth, eliminating CO2 and raising pH.
- Alkalosis decreases respiratory activity, retaining CO2 and lowering pH.

Example: The bicarbonate buffer system is crucial for maintaining blood pH.

Key Equation:

The kidneys provide long-term control of acid-base balance by regulating bicarbonate and hydrogen ion concentrations.

Mechanisms:
- Secreted H+ comes from carbonic acid dissociation in tubule cells.
- Filtered bicarbonate is conserved indirectly by absorbing CO2 generated within cells.
- For each bicarbonate reabsorbed, one H+ is secreted into filtrate.
Counteracting Acidosis:
- H+ buffered by bases other than bicarbonate is excreted in urine (phosphate buffer system).
- Ammonium ions (from glutamine catabolism) are excreted in urine.
Counteracting Alkalosis: Bicarbonate is secreted into filtrate; H+ is reabsorbed.

Example: The phosphate buffer system is the major urine buffer.

Acid-base disorders are classified as metabolic or respiratory, with compensatory mechanisms to restore balance.

Respiratory Acidosis: Caused by CO2 retention.
Respiratory Alkalosis: Occurs when CO2 is eliminated faster than produced.
Metabolic Acidosis: Accumulation of nonvolatile acids or loss of bicarbonate.
Metabolic Alkalosis: Excessive bicarbonate levels.
Compensation:
- Respiratory compensation: changes in rate/depth of breathing.
- Renal compensation: modification of blood bicarbonate levels.
pH Extremes: Life is not sustainable below pH 7.0 or above pH 7.8.

Example: In metabolic acidosis, the respiratory system increases ventilation to lower CO2 and raise pH.

Age-related differences affect susceptibility to fluid and acid-base imbalances.

Infants: Higher risk of dehydration and acidosis due to low lung residual volume, high fluid turnover, high metabolic rate, large surface area, and immature kidneys.
Older Adults: Increased risk of dehydration from low body water percentage and reduced thirst sensitivity. Disorders like cardiovascular disease and diabetes increase risk of imbalances.

Example: Elderly individuals may not feel thirsty even when dehydrated, increasing risk during illness or heat exposure.

Compartment	Major Cations	Major Anions
ECF (Plasma & Interstitial Fluid)	Sodium (Na+), Calcium (Ca2+)	Chloride (Cl-), Bicarbonate (HCO3-)
ICF	Potassium (K+), Magnesium (Mg2+)	Phosphate (HPO42-), Protein anions

Additional info: Table summarizes the main electrolytes in each compartment for quick reference.