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

Fluid, Electrolyte, and Acid-Base Homeostasis

Overview of Fluid, Electrolyte, and Acid-Base Homeostasis

This section introduces the concepts of fluid, electrolyte, and acid-base balance, which are essential for maintaining homeostasis in the human body.

• Body fluids include all water-based liquids in the body, such as blood plasma, interstitial fluid, cytosol, cerebrospinal fluid, lymph, and exocrine secretions. Water is the main component.

• Fluid balance refers to maintaining the appropriate volume and concentration of intracellular and extracellular fluids, primarily through water balance.

• Functions of water in the body:

  • Acts as a polar solvent for transporting solutes

  • Distributes body heat

  • Cushions and lubricates organs and tissues

• Water follows the principle of mass balance: what is gained must equal what is lost.

• Factors affecting fluid balance: water intake, physical activity, kidney function, medications, and digestive activities.

• Imbalances in water can disrupt homeostasis and have serious consequences.

Electrolytes and Nonelectrolytes

• Electrolytes are substances that dissociate into ions in water and conduct electricity (e.g., Na+, K+).

• Nonelectrolytes do not dissociate into ions (e.g., glucose).

• Electrolyte balance is also governed by mass balance and is regulated mainly by hormones.

• Ion concentration depends on both the number of ions and the amount of water in a fluid compartment.

Acids, Bases, and pH

• Acids: Chemicals that dissociate in water to release H+ (e.g., HCl in the stomach, H2CO3 in blood).

• Bases: Chemicals that accept H+ (e.g., HCO3- is the main base in the body).

• pH scale measures hydrogen ion concentration:

  • pH < 7: acidic

  • pH = 7: neutral

  • pH > 7: basic

Fluid Homeostasis

Fluid homeostasis involves the regulation of water distribution and movement between body compartments.

• Total body water in a standard 70 kg adult is about 60% of body weight (~42 kg). Varies with age, gender, body mass, and adipose tissue.

Fluid Compartments

• Intracellular fluid (ICF): Fluid within cells; about 60% of body fluids (~26 L).

• Extracellular fluid (ECF): Fluid outside cells; includes plasma and interstitial fluid.

Movement of Water Between Compartments

• Water moves freely between compartments, but solute movement is restricted.

• Movement is influenced by two pressure gradients:

  • Hydrostatic pressure: Pushes water from areas of higher to lower pressure.

  • Osmotic pressure: Pulls water toward higher solute concentration (higher osmolarity).

• At the arteriolar end of capillaries, hydrostatic pressure pushes water out; at the venular end, osmotic pressure pulls water back in.

• Water lost to interstitial fluid is returned to plasma via the lymphatic system.

Tonicity

• Tonicity is the osmotic pressure gradient between two fluid compartments.

• Isotonic: No net water movement.

• Hypotonic ECF: Water enters cells, causing swelling.

• Hypertonic ECF: Water leaves cells, causing shrinkage.

Water Losses and Gains

• Water loss (about 2.5 L/day):

  • Urine (obligatory loss: ~500 mL)

  • Feces (sensible loss: ~100 mL)

  • Skin and lungs (insensible loss: ~900 mL)

• Water gain (about 2.5 L/day):

  • Metabolic water (~250 mL)

  • Ingested liquids (~1500 mL)

  • Food (~750 mL)

• Thirst mechanism is regulated by osmoreceptors in the hypothalamus and the renin-angiotensin-aldosterone system (RAAS).

Hormonal Regulation of Fluid Balance

• ADH (antidiuretic hormone) increases water reabsorption in the kidneys by inserting aquaporins in the distal tubule and collecting ducts.

• Increased ADH: more water reabsorbed, higher ECF volume, lower urine volume.

• Decreased ADH: more water lost in urine, lower ECF volume.

Imbalances of Fluid Homeostasis

• Dehydration: Decreased ECF volume, increased osmolarity. Causes: sweating, diarrhea, vomiting, diuretics. Cells lose water and shrink (crenate).

• Overhydration (hypotonic hydration): Increased ECF volume, decreased osmolarity. Causes: renal impairment, excess ADH, rapid water intake. Cells swell; risk of hyponatremia and cerebral edema.

• Isosmotic imbalances:

  • Hypovolemia: Equal loss of water and solute (e.g., blood loss).

  • Hypervolemia: Excess ECF volume without osmotic change (e.g., edema).

Electrolyte Homeostasis

Electrolyte balance is crucial for nerve and muscle function, fluid balance, and overall cellular activity.

Sodium Ions (Na+)

• Most abundant extracellular cation.

• Maintained by Na+/K+ ATPase pumps and low membrane permeability.

• Critical for depolarization in excitable cells (neurons, muscle).

• Regulated by:

  • Angiotensin-II and aldosterone: increase Na+ retention.

  • ANP (atrial natriuretic peptide): decreases Na+ and water reabsorption.

• Imbalances:

  • Hypernatremia: High Na+, often from dehydration.

  • Hyponatremia: Low Na+, often from overhydration.

Potassium Ions (K+)

• Most abundant intracellular cation.

• Maintained by Na+/K+ ATPase pumps.

• Essential for resting membrane potential in excitable cells.

• Regulated by insulin, aldosterone, and epinephrine (stimulate K+ uptake by cells).

• Excess K+ is excreted in urine.

• Imbalances:

  • Hyperkalemia: High K+, dangerous—depolarizes cells, can cause cardiac arrest.

  • Hypokalemia: Low K+, often from diuretics—hyperpolarizes cells, reduces excitability.

Calcium and Phosphate Ions (Ca2+ and PO43-)

• Bound together in bone as hydroxyapatite; also critical for muscle contraction, cardiac action potentials, signaling, blood clotting, and synaptic transmission.

• Regulated by:

  • Parathyroid hormone (PTH) and Vitamin D3 (calcitriol)

  • Bone, kidneys, and small intestine adjust Ca2+ levels.

• Imbalances:

  • Hypercalcemia: Excess Ca2+, reduces neuron excitability.

  • Hypocalcemia: Low Ca2+, increases neuron excitability.

Other Important Ions

• Chloride (Cl-): Major ECF anion, forms HCl in the stomach, involved in bicarbonate formation.

• Magnesium (Mg2+): Enzyme activator, important in bone tissue.

Acid-Base Homeostasis

Maintaining a stable pH (7.35–7.45) is vital for enzyme function and cellular metabolism.

Buffer Systems

• Chemical buffer systems (immediate response):

  • Carbonic acid-bicarbonate buffer (main in blood): $\mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \leftrightarrow \mathrm{H}_2\mathrm{CO}_3 \leftrightarrow \mathrm{HCO}_3^- + \mathrm{H}^+$

  • Phosphate buffer (important in cytosol and kidneys): $\mathrm{H}_2\mathrm{PO}_4^- \leftrightarrow \mathrm{HPO}_4^{2-} + \mathrm{H}^+$

  • Protein buffer (carboxyl groups of amino acids): $\mathrm{COOH} \leftrightarrow \mathrm{COO}^- + \mathrm{H}^+$

• Physiological buffer systems (slower, but powerful):

  • Respiratory system: Regulates CO2 (volatile acid) via ventilation.

  • Urinary system: Excretes fixed acids and regulates HCO3- reabsorption and production.

Renal Regulation of Acid-Base Balance

• Kidneys secrete H+ and reabsorb or generate HCO3- as needed.

• Proximal tubule: H+ secretion, HCO3- reabsorption.

• Distal tubule and collecting duct: Major site of urine acidification via intercalated cells.

• New HCO3- can be generated from glutamine metabolism.

Acid-Base Imbalances

• Acidosis: pH < 7.35 (acidemia = low blood pH)

  • Causes: Excess H+ or loss of HCO3-

  • Effects: Neuronal depression, CNS symptoms

• Respiratory acidosis: Due to hypoventilation (CO2 retention)

  • Compensation: Increased ventilation, renal H+ secretion, HCO3- reabsorption

• Metabolic acidosis: Addition of metabolic acids or loss of HCO3-

  • Compensation: Hyperventilation, renal H+ secretion, HCO3- generation

• Alkalosis: pH > 7.45 (alkalemia = high blood pH)

  • Causes: Excess base or loss of H+

  • Effects: Neuronal hyperexcitability, muscle symptoms, seizures

• Respiratory alkalosis: Due to hyperventilation (CO2 loss)

  • Compensation: Renal excretion of HCO3-, retention of H+

• Metabolic alkalosis: Loss of H+ (vomiting, diuretics) or excess HCO3- (antacids)

  • Compensation: Hypoventilation, renal retention of H+, excretion of HCO3-

Arterial Blood Gases (ABGs)

ABGs are used clinically to assess acid-base status. They measure pH, PCO2, and HCO3- in arterial blood.

Disorder	pH	PCO2	HCO3-	Compensation
Respiratory Acidosis	Low	High	High (if compensated)	Renal: ↑ HCO3-
Metabolic Acidosis	Low	Low (if compensated)	Low	Respiratory: ↓ PCO2
Respiratory Alkalosis	High	Low	Low (if compensated)	Renal: ↓ HCO3-
Metabolic Alkalosis	High	High (if compensated)	High	Respiratory: ↑ PCO2

Example: Homeostatic Response to Dehydration

• Decreased total body water leads to:

  • Decreased blood volume and pressure

  • Increased ECF electrolyte concentration (especially Na+)

  • Increased ECF osmolarity (water leaves cells)

  • Potential metabolic acidosis (due to higher acid concentration)

• Juxtaglomerular cells release renin → activates RAAS → forms angiotensin-II.

• Angiotensin-II effects:

  • Vasoconstriction (raises blood pressure)

  • Increases Na+ and water reabsorption

  • Stimulates thirst, ADH, and aldosterone secretion

  • Aldosterone increases K+ and H+ secretion (helps restore pH)

• Fluid, electrolyte, and acid-base homeostasis is restored.

Additional info: This summary expands on the original outline by providing definitions, mechanisms, and clinical relevance for each topic, as would be expected in a mini-textbook study guide for college-level Anatomy & Physiology.