Overview
This lecture covers the physiology of acid-base balance in the body, including definitions, sources of acids, mechanisms of regulation, and the roles of respiratory and renal systems.
Acid-Base Basics
- Acid: substance that releases hydrogen ions (H+).
- Base: substance that combines with or binds hydrogen ions.
- pH indicates hydrogen ion concentration; higher H+ means lower pH, and vice versa.
- Normal blood pH is 7.35–7.45.
- pH below 7.35 is acidosis; above 7.45 is alkalosis.
Sources of Hydrogen Ions
- Main source: carbonic acid formed from metabolically produced CO2.
- Inorganic acids from protein breakdown (high protein or Western diet increases acid load).
- Organic acids from metabolism, e.g., lactic acid (exercise), fatty acids (fat breakdown).
Acid-Base Regulation Mechanisms
- Three primary mechanisms: buffering systems, respiratory compensation, and renal compensation.
- Buffers are first line of defense; they stabilize pH by neutralizing excess acids or bases.
Buffering Systems
- Bicarbonate/carbonic acid system is the main extracellular buffer.
- Protein buffer system is primary intracellular buffer.
- Hemoglobin buffer system neutralizes H+ during CO2 transport in blood.
- Phosphate buffer system is important in the urine.
Respiratory Compensation
- Changes in breathing rate adjust CO2 and thus pH (quick response).
- Increased H+ shifts reaction left, raising CO2 and increasing breathing rate to expel CO2.
- Decreased H+ reduces breathing rate, retaining CO2.
- Peripheral chemoreceptors sense H+; central chemoreceptors sense CO2.
Renal Compensation
- Kidneys regulate H+, bicarbonate (HCO3-), and ammonia over hours to days (third line of defense).
- Kidneys excrete H+ and reabsorb HCO3-, maintaining blood pH.
- Proximal convoluted tubule (PCT): reabsorbs ~90% of bicarbonate.
- Intercalated cells in distal tubule/collecting duct: Type A cells secrete H+ and reabsorb HCO3- (acidosis); Type B cells do the reverse (alkalosis).
- If urine pH drops below 4.5, phosphate and ammonia buffer excess H+.
Pathophysiology Notes
- Failure of PCT reabsorption causes renal tubular acidosis type 2.
- Failure of distal tubule H+ secretion causes renal tubular acidosis type 1.
Key Terms & Definitions
- Acidosis — condition with blood pH < 7.35.
- Alkalosis — condition with blood pH > 7.45.
- Buffer — substance that resists pH changes.
- Renal tubular acidosis — kidney's inability to properly secrete/reabsorb acids or bases.
- Intercalated cells — kidney cells (Type A: secrete H+, reabsorb HCO3-; Type B: secrete HCO3-, reabsorb H+).
Action Items / Next Steps
- Memorize normal blood pH range and major buffer systems.
- Review causes and types of renal tubular acidosis.
- Prepare for next lecture on acid-base pathology.