6.4 Acid-Base Regulation in Biological Systems

The respiratory and renal systems work together to maintain acid-base balance in the body. They regulate pH through different mechanisms, with the lungs controlling CO2 levels and the kidneys managing bicarbonate and hydrogen ion concentrations.

Understanding how these systems interact is crucial for grasping acid-base homeostasis. Imbalances can lead to acidosis or alkalosis, affecting various organ systems and cellular functions. The body's compensatory mechanisms help restore pH to normal levels.

Respiratory and Renal Regulation of Acid-Base Balance

Respiratory system in pH regulation

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• Carbon dioxide (CO2) and pH relationship drives acid-base balance as CO2 forms carbonic acid in blood causing increased CO2 to decrease pH
• Respiratory control of CO2 occurs through breathing rate affecting CO2 elimination while chemoreceptors detect changes in blood pH and CO2 levels
• Alveolar gas exchange facilitates CO2 diffusion from blood to alveoli allowing exhaled air to remove CO2 from the body
• Bicarbonate buffer system maintains pH balance through reversible reaction $CO_2 + H_2O ⇌ H_2CO_3 ⇌ H^+ + HCO_3^-$
• Respiratory compensation adjusts pH through hyperventilation to decrease CO2 and increase pH or hypoventilation to increase CO2 and decrease pH

Renal system in acid-base balance

• Renal tubule functions include blood filtration in glomerulus and reabsorption and secretion in tubules (proximal, distal, collecting)
• Hydrogen ion secretion occurs actively in proximal tubule and collecting duct with intercalated cells specializing in H+ secretion
• Bicarbonate reabsorption happens primarily in proximal tubule reabsorbing 80-90% of filtered HCO3- catalyzed by carbonic anhydrase enzyme
• Ammonia buffer system allows for H+ excretion as ammonium ion through reaction $NH_3 + H^+ → NH_4^+$
• Titratable acid excretion involves phosphate and other buffers binding H+ for excretion
• New bicarbonate generation occurs in α-intercalated cells producing HCO3- during acid secretion

Physiological Consequences and System Interplay

Consequences of acid-base imbalances

• Acidosis (pH < 7.35) manifests as metabolic (decreased HCO3-) or respiratory (increased CO2) with effects on organ systems:
  • Cardiovascular: decreased contractility, arrhythmias
  • Nervous: confusion, coma
  • Respiratory: hyperventilation (compensatory)
• Alkalosis (pH > 7.45) presents as metabolic (increased HCO3-) or respiratory (decreased CO2) impacting organ systems:
  • Neuromuscular: tetany, seizures
  • Cardiovascular: vasoconstriction
  • Respiratory: hypoventilation (compensatory)
• Cellular effects of pH changes include protein structure and function alterations, enzyme activity disruption, and electrolyte imbalances (K+, Ca2+)

Respiratory vs renal acid-base homeostasis

• Compensatory mechanisms involve respiratory compensation for metabolic disturbances and renal compensation for respiratory disturbances
• Time course of compensation differs with respiratory system responding rapidly (minutes to hours) and renal system responding slower (hours to days)
• Integrated control systems include central chemoreceptors in brainstem and peripheral chemoreceptors in carotid and aortic bodies
• Feedback loops maintain pH within normal range through negative feedback while positive feedback can occur in severe disturbances
• Clinical assessment utilizes arterial blood gas analysis and anion gap calculation
• Acid-base disorders classified as simple (one primary disturbance with compensation) or mixed (multiple primary disturbances)