Acid-Base Balance and Homeostasis

Acid-Base Balance

• pH: The negative logarithm of hydrogen ion concentration ([H+]). A measure of acidity or alkalinity
• Normal pH Range: 7.35 - 7.45
• Acidosis: pH less than 7.35, indicating excess hydrogen ions
• Alkalosis: pH greater than 7.45, indicating a lack of hydrogen ions
• Compensation: The body's response to acid-base imbalance (either complete or partial)

Body Fluid Homeostasis

• Water Intake/Output: Average daily intake is 2850 mL, with output also equalling 2850 mL. Intake comprises liquids, food and metabolically produced fluids. Output includes insensible loss (skin/lungs), sweat, faeces and urine.

Ion Balance in Body Fluids

• Plasma, Interstitial Fluid, and Intracellular Fluid: Contain various ions, including sodium (Na+), chloride (Cl-), potassium (K+), bicarbonate (HCO3-), and phosphate (PO43-). These ions are crucial for various physiological processes. The amount of various ions per milliliter of water is different in each of these fluids.

pH Review

• pH Scale: Ranges from 0 to 14, with 7 being neutral. Lower values are increasingly acidic, and higher values are increasingly basic.
• Hydrogen Ions (H+): Crucial in determining pH. Higher concentrations mean lower pH (more acidic)
• Important chemical components in blood pH and acid-base balance are H₂CO₃, HCO3−, and CO2.

Homeostasis in Body Fluids

• Inputs and Outputs: Substances enter and leave the internal pool. Some inputs are through ingestion or inhalation, Absorption through body surface or artificial injection. Outputs include excretion from the kidneys, lungs, digestive tract, sweat, tears or sloughed skin.

Buffers

• Buffer Systems: Substances that resist changes in pH by accepting or releasing hydrogen ions to neutralize external acid or base additions. Common systems include sodium bicarbonate and carbonic acid, and phosphate.
• Action: Buffers take up or release H⁺ as needed, keeping the pH close to its normal range.

Henderson-Hasselbalch Equation

• Equation for pH: pH = pKₐ + log₁₀ ([A⁻]/[HA]). Shows the relationship between pH, pKₐ, and the concentrations of the acid (HA) and its conjugate base (A⁻) in a solution.
• Carbonic Acid-Bicarbonate System: In the body, the major buffer system is based on carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and CO2(). The pKₐ of carbonic acid is 6.1, and the normal ratio of bicarbonate to carbonic acid is approximately 20:1.

Arterial Blood Gases (ABGs)

• Normal pH: 7.4
• PCO2: 40 mmHg
• HCO3-: 24 mEq/L

Chemical Buffers and Their Primary Roles

• H₂CO₃/HCO₃⁻ System: Primary extracellular buffer
• Protein System: Primary intracellular buffer
• Hemoglobin System: Primary buffer for carbonic acid changes
• Phosphate System: Important urinary buffer and ICF buffer

Control of Acids by Buffers

• Buffer Pairs: Buffers are weak acids and their conjugate bases.
• Acid-Base Exchange: Buffers exchange a strong acid or base with a weaker one, reducing the pH change.

Kidney Excretion

• Acid/Base Elimination: Kidneys excrete excess acid or base to maintain pH balance.
• Bicarbonate Regulation: Kidneys regulate bicarbonate levels in the blood
• H+ secretion and HCO3- reabsorption: Control of these are key to kidney's acid-base balance control.

Acid-Base Imbalances

• Acidosis/Alkalosis: Blood pH outside the normal range.
• Compensation: Body attempts to compensate for changes in pH. Compensations may be respiratory or renal.

Renal Control of Acid-Base Balance

• H⁺ Excretion: The kidneys secrete H⁺ ions into the urine.
• HCO₃- Reabsorption: The kidneys reabsorb filtered bicarbonate ions.
• New HCO₃- Production: The kidneys produce new bicarbonate to maintain balance.

Respiratory Mechanisms

• Exhalation of CO₂: Exhaling CO₂ helps regulate pH by altering the H₂CO₃/HCO₃⁻ equilibrium.
• Volatile Acids: Respiratory mechanisms primarily affect volatile acids (like carbonic acid).

PCO₂-Blood pH Relationship

• CO₂ Elevation: Increased CO₂ drives the equation towards more carbonic acid and lower pH.
• CO₂ Decrease: Decreased CO₂ pushes the equilibrium towards more bicarbonate and higher pH.
• Homeostasis: This relationship helps maintain homeostasis in the blood.

Protein Buffers

• Hemoglobin: Essential for carrying O2 in the blood. Exhibits buffering capacities in the blood. Has carboxyl and amino groups that aid in blood buffering.

Respiratory mechanisms

• Exhalation of CO₂: This is a powerful method, but only for volatile acids and doesn't influence fixed acids like lactic acid.

Renal Correction of Acidosis/Alkalosis

• Excess H⁺ removal: The kidneys adjust H⁺ excretion to rectify excess in conditions of acidosis.
• HCO3 replenishment: During acidosis, kidney function regulates HCO3 concentrations to revert to normal levels.

Renal Correction of Alkalosis

• H⁺ Production: The kidneys produce H⁺ to balance the rise in bicarbonate levels (HCO3−) in metabolic alkalosis
• HCO3 excretion: If the HCO3− concentration is elevated in plasma, the kidneys may increase excretion to bring pH back to normal levels.