Acid-Base Balance and Homeostasis

Acid-Base Balance

• Acid-base balance refers to precise regulation of free hydrogen ion (H+) concentration in body fluids.
• Total body water = 40 Liters, 60% of body weight.
• Major fluid compartments: Intracellular fluid (ICF) - 25 L (40% body weight), Interstitial fluid (IF) - 12 L (80% of ECF), Extracellular fluid (ECF) - 15 L (20% body weight)
• Plasma (3 L, 20% ECF), comprises primarily Na+, Cl-, and HCO3-.
• ICF (25 L) contains high amounts of K+ and PO43-.
• Body fluids consist of water and solutes in three main compartments.

Homeostasis

• Homeostasis of the body is regulated by water intake and output, Na+, K+, and Ca2+ regulation, and pH (acid-base balance) regulation.

Importance of Acid-Base Balance

• Changes in nerve & muscle cell excitability are influenced by acid-base balance.
• Enzyme activity is influenced by acid-base balance.
• Acid-base balance influences K+ levels in the body; acidosis decreases K+ secretion, while alkalosis increases it.

Sources of H+ Ions in the Body

• Carbonic acid formation during metabolic processes.
• Inorganic acids produced during breakdown of nutrients.
• Organic acids from intermediary metabolism.

Acid Dissociation Constant

• Acid dissociation constant (Ka) shows acid strength.
• HA ⇌ H⁺ + A⁻
• pKa = -log10Ka

Henderson-Hasselbalch Equation

• pH of a solution containing an acid or base can be calculated using the Henderson-Hasselbalch equation.
• This equation is derived from the behavior of weak acids (and bases) in solution, and uses the kinetics of reversible reactions.
• pH = pKa + log ([A⁻]/[HA])

Buffer

• A buffer is a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid.
• Key buffer systems in the body include bicarbonate, phosphate, and protein.
• pH = pKa + log [conjugate base]/[acid]

Buffering Capacity

• Effectiveness of a buffer depends on concentration and pKa value.
• Effective buffering occurs when the solution pH is within ±1 pH unit of the buffer pKa.
• Buffers only work ±1 unit on either side of their pKa values.

Kₐ and pKₐ Values for Weak Acids

• A table of various weak acids and their Kₐ and pKₐ values.

pH Values of Some Common Substances

• A table of various substances and their corresponding pH values.

Ideal Buffer Characteristics for Biological Purposes

• Impermeability to biological membranes.
• Biological stability and lack of interference with metabolic and biological processes.
• Minimal absorption of ultraviolet or visible light.
• Minimal formation of insoluble complexes with cations.
• Minimal effect of ionic composition or salt concentration.
• Limited pH change in response to temperature.

pH Regulation by Respiration

• A rising plasma H+ concentration excites the respiratory center, stimulating deeper and faster respiration, removing CO2, and decreasing H+ concentration.
• When blood pH rises, the respiratory center is depressed, respiratory rate drops, CO2 accumulates, and H⁺ is increased.

Respiratory Adjustments to Acidosis and Alkalosis

• Provides a table of expected ventilation, CO2 removal rates, etc., in various physiological states.

Renal Mechanism of Acid-Base Balance

• Key functions include conservation and reabsorption of HCO3⁻, generation of new HCO3⁻, and excretion of HCO3⁻.

Altered Blood H⁺ Concentration

• Renal mechanisms adjust to changes in blood H⁺ concentration.
• Renal excretion of H⁺ depends on CO2 levels in the peritubular capillary blood.

Bicarbonate Reabsorption & H⁺ Ion Secretion in Distal Tubules

• Crucial process for maintaining acid-base balance. A detailed explanation of the mechanisms.
• Shows how different types of intercalated cells operate.

Renal Function to Regulate Acid-Base Balance

• Conservation of bicarbonate ions (HCO⁻₃)
• Generation of new bicarbonate ions (HCO⁻₃) through excretion of buffered H⁺ and NH₄⁺ excretion
• Excretion of HCO⁻₃
• Importance of peritubular capillary blood CO₂ concentration.

Buffer pairs

• Bicarbonate (e.g., NaHCO₃, KHCO₃), Plasma proteins, Hemoglobin, and Phosphate (e.g., Na₂HPO₄, NaH₂PO₄) are important buffer pairs in the body.

Regulation of pH by Buffers

• Bicarbonate Buffer System: This system involves the interaction of carbonic acid (H₂CO₃) and its salt, sodium bicarbonate (NaHCO₃) in the same solution. HCl (strong acid) reaction is demonstrated, and reactions with a strong base NaOH.
• Phosphate Buffer System: Operation and comparison to the bicarbonate system. Acids and bases' reactions in the buffer system are explained.
• Protein Buffer System: R-COOH ↔ R-COO⁻ + H⁺ and R-NH₂ + H⁺ ↔ R-NH₃⁺. Reactions in acid and alkaline mediums, buffering capacity is explained.

Intracellular pH Regulation

• Intracellular buffering plays a crucial role.
• Example: Buffering actions of proteins
• Erythrocyte buffering in acidosis and alkalosis is explicitly stated.

Response to Metabolic Acidosis and Alkalosis

• The response of the body's systems, particularly cells and the kidneys, to metabolic acidosis and alkalosis for pH control.

Causes of Acid-Base Disorders

• Explains the causes of Metabolic Acidosis and Metabolic Alkalosis, with various examples. Details are provided, such as overproduction of fixed H⁺ and loss of HCO₃.

Summary of Renal Responses to Acidosis and Alkalosis

• Comprehensive summary table of the renal responses.

Factors Influencing Increased H+ Secretion

• Key factors that increase H⁺ secretion are highlighted.
• Importance of aldosterone, K⁺ levels, and CO2 on secretion.

Control of Tubular H+ Secretion and HCO3⁻ Reabsorption

• Explanation of the body's mechanisms for regulating the secretion of H+ and reabsorption of HCO₃⁻ to keep up with cellular processes.

Respiratory and Metabolic Compensation

• Explanation of physiological compensation responses to maintain blood pH homeostasis.

Summary of Acid-Base Disorders

• Shows a summary regarding the imbalances of CO₂, H₂O, H⁺ and HCO₃⁻, explaining roles of respiratory and renal compensation.

Causes of Metabolic Acidosis and Alkalosis

• This section details causes and examples of metabolic acidosis and alkalosis.

Respiratory and Renal Compensation

• This section details how the respiratory system and kidney adjust to maintain balance.

Intracellular Acid-Base Balance

• Mechanisms for maintaining intracellular pH.