Acid-Base Regulation and Buffer Systems

Acid-Base Regulation

• Precise regulation of hydrogen ion (H+) concentration is crucial for nearly all enzyme functions in the body
• Extracellular fluid H⁺ concentration is normally kept low
• Normal variation is minimal compared to other ions like sodium
• Acids release hydrogen ions; bases accept hydrogen ions
• Buffering systems (e.g., bicarbonate, phosphate, proteins) resist changes in H⁺ concentration in the body fluids.

Bicarbonate Buffer System

• The most significant extracellular buffer is the bicarbonate system
• It involves carbonic acid (H₂CO₃), and bicarbonate (HCO₃⁻)
• CO₂ combines with water to form carbonic acid
• Carbonic acid dissociates to form H⁺ and HCO₃⁻
• The buffer system resists pH changes by absorbing excess H⁺ or releasing H⁺, depending on the situation.
• The reverse reaction occurs in the lungs and kidneys to regulate the removal and addition of bicarbonate (HCO₃⁻).

Phosphate Buffer System

• Phosphate buffers play a key role in renal tubular fluid and intracellular fluids but less significant in extracellular fluid.
• It involves H₂PO₄⁻ and HPO₄²⁻ ions
• These ions can bind or release H⁺ depending on the direction of change in fluid pH.
• The concentration of phosphate is higher in intracellular fluid compared to extracellular fluid, making it a more effective buffer within cells.
• High phosphate concentration in renal tubules allows it to be a valuable buffer in these regions.

Protein Buffer Systems

• Proteins are crucial intracellular buffers due to their abundant presence.
• Proteins can bind or release H⁺ depending on body fluid pH.
• Their buffering capacity is affected in response to changes in H⁺ concentrations.

Respiratory Regulation of Acid-Base Balance

• Ventilation rate impacts the concentration of carbon dioxide (CO₂) in the body.
• Increased ventilation removes CO₂ which reduces H⁺ concentration and raises blood pH.
• Decreasing ventilation increases CO₂ which increases H⁺ concentration and lowers blood pH.

Renal Control of Acid-Base Balance

• Kidneys regulate acid-base balance by excreting acidic or basic urine.
• Acid excretion lowers the concentration of H⁺ in the blood
• Base excretion removes HCO₃⁻ , thus increasing H⁺ concentration
• Maintaining acid/base balance requires an interplay of all three buffering systems (pulmonary, renal, and chemical): for instance, high CO₂ leads to high H⁺. pH decreases in response, thereby increasing respiration rate.
• Ammonia buffer is a major component of renal acid-base regulation.
• Glutamine metabolism is a primary source of ammonia production. Ammonia combines with hydrogen to form ammonium, which is excreted and results in formation of new bicarbonate for the blood.