Chapter 30: Acid-Base Regulation

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 30

To read more check out Chapter 11:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-11-normal-electrocardiogram.html

The regulation of hydrogen ion

The regulation of hydrogen ion concentration is vital for keeping the body healthy. Hydrogen ion levels are controlled strictly because almost all enzymes and biochemical reactions in cells depend on a stable pH. Even minor shifts in hydrogen ions can impair normal cellular performance across the whole body. To maintain homeostasis, the body balances hydrogen ion production and intake with its elimination. This regulation uses three primary defenses: chemical buffers, the lungs, and the kidneys.

Chemical buffers act as an immediate defense by binding free hydrogen ions within seconds to prevent large pH changes. The respiratory system responds within minutes by adjusting ventilation to exhale carbon dioxide and control acid levels. Finally, the kidneys act over hours to days to excrete or reabsorb acids and bases, making them the most powerful long-term controller. Together, these systems keep the extracellular fluid pH within a narrow, safe range. The key points from the text are broken down below for medical review.

Importance of Hydrogen Ion Regulation

• Precise hydrogen ion regulation maintains cell homeostasis.
• Enzyme activity depends directly on hydrogen concentration.
• Hydrogen ion changes alter overall cell functions.
• Body fluids maintain low hydrogen ion levels.
• Sodium concentration exceeds hydrogen by millions times.
• Normal hydrogen concentration averages only 0.00004 mEq/L.
• Hydrogen ion variations remain extremely small normally.

Acids, Bases, and Alkalis

• Free hydrogen ions are single released protons.
• Acids are molecules releasing hydrogen ions easily.
• Hydrochloric acid dissociates into hydrogen and chloride.
• Carbonic acid dissociates into hydrogen and bicarbonate.
• Bases accept free hydrogen ions in solutions.
• Bicarbonate combines with hydrogen forming carbonic acid.
• Hydrogen phosphate accepts hydrogen forming dihydrogen phosphate.
• Body proteins act as important internal bases.
• Hemoglobin serves as a major protein base.
• Alkalis combine alkaline metals with basic ions.
• Hydroxyl ions react quickly with hydrogen ions.
• Alkalosis means excess removal of hydrogen ions.
• Acidosis means excess addition of hydrogen ions.

Strong versus Weak Acids and Bases

• Strong acids rapidly release large hydrogen amounts.
• Hydrochloric acid acts as a strong acid.
• Weak acids dissociate less vigor in water.
• Carbonic acid is a weak biological acid.
• Strong bases react rapidly with hydrogen ions.
• Hydroxyl ions remove hydrogen ions very quickly.
• Bicarbonate acts as a typical weak base.
• Extracellular fluid contains mostly weak buffer agents.

Normal pH and Limits of Survival

• Arterial blood hydrogen stays around 40 nEq/L.
• Normal hydrogen concentration varies by 3-5 nEq/L.
• Survival hydrogen limits range between 10-160 nEq/L.
• Logarithmic pH scale expresses low hydrogen numbers.
• Equation defines pH as negative log hydrogen.
• Normal arterial blood pH averages exactly 7.4.
• Venous blood pH averages around 7.35.
• Interstitial fluid contains extra tissue carbon dioxide.
• Acidosis occurs when arterial pH drops below 7.4.
• Alkalosis occurs when arterial pH rises above 7.4.
• Lower lethal survival limit equals pH 6.8.
• Upper lethal survival limit equals pH 8.0.
• Cell metabolism creates lower intracellular fluid pH.
• Intracellular pH ranges between 6.0 and 7.4.
• Tissue hypoxia lowers intracellular pH significantly.
• Urine pH ranges between 4.5 and 8.0.
• Gastric parietal cells secrete highly acidic HCl.
• Parietal cell fluid maintains a low pH 0.8.

Primary Lines of Acid-Base Defense

• Three main systems regulate body fluid pH.
• Chemical buffers react instantly to pH changes.
• Buffers temporarily tie up free hydrogen ions.
• Respiratory center acts within minutes eliminating CO2.
• Kidneys provide slow but most powerful response.
• Kidneys excrete variable rates of acid urine.
• Kidneys readjust extracellular fluid pH over days.

Chemical Buffering and Bicarbonate System

• Buffers reversibly bind free hydrogen ions immediately.
• Free hydrogen binds buffer forming weak acid.
• Increased hydrogen forces buffering reaction rightward.
• Decreased hydrogen releases free hydrogen from buffers.
• Daily metabolic processes produce 80 mEq hydrogen.
• Bicarbonate buffer system dominates extracellular fluid space.
• Bicarbonate system combines H2CO3 and NaHCO3 salt.
• Carbonic anhydrase accelerates CO2 and H2O reaction.
• Carbonic anhydrase abounds in lungs and kidneys.
• Carbonic acid ionizes into hydrogen and bicarbonate.
• Sodium bicarbonate ionizes completely into sodium ions.
• Added strong acid combines with bicarbonate ions.
• Buffering strong acid increases carbon dioxide levels.
• Respiration eliminates extra formed carbon dioxide gas.
• Added strong base combines with carbonic acid.
• Strong base generates additional weak bicarbonate ions.
• Decreased carbon dioxide inhibits rapid respiratory rate.
• Kidneys excrete excess bicarbonate during alkalosis states.

Henderson-Hasselbalch Dynamics and Titration

• Acid dissociation constant defines ionized fraction ratio.
• Dissolved CO2 proportional to un-ionized in H2CO3 amount.
• CO2 solubility coefficient equals 0.03 mmol/mm Hg.
• Negative logarithm converts dissociation constant to pK.
• Henderson-Hasselbalch equation calculates fluid pH.
• Bicarbonate pK equals exactly 6.1 units.
• Increased bicarbonate raises pH toward clinical alkalosis.
• Increased CO2 lowers pH toward clinical acidosis.
• Kidneys control extracellular bicarbonate ion concentrations.
• Lungs regulate extracellular partial pressure carbon dioxide.
• Primary bicarbonate changes cause metabolic acid-base disorders.
• Primary CO2 changes cause respiratory acid-base disorders.
• Maximum buffer efficiency occurs near pK value.
• Effective bicarbonate buffer ranges between 5.1-7.1 pH.
• Absolute buffer concentration determines total buffering power.
• High extracellular concentration renders bicarbonate highly effective.
• Renal and respiratory controls create strong buffering.

Phosphate and Protein Buffer Systems

• Phosphate system buffers tubular and intracellular fluid.
• Main phosphate elements include H2PO4 and HPO4.
• Strong acids convert to weak NaH2PO4 salts.
• Strong bases convert to weak Na2HPO4 salts.
• Phosphate buffer pK equals 6.8 units.
• Low extracellular phosphate concentration limits buffering power.
• Tubular fluid concentrates phosphate increasing buffer power.
• Intracellular phosphate concentration exceeds extracellular levels.
• Proteins provide abundant intracellular fluid chemical buffers.
• Carbon dioxide diffuses rapidly through cell membranes.
• Intracellular pH changes mirror extracellular pH shifts.
• Hemoglobin acts as primary red cell buffer.
• Intracellular buffers require hours for maximum effectiveness.
• Protein pKs stay close to intracellular pH.
• Isohydric principle unites all fluid buffer systems.
• All buffers balance single common hydrogen concentration.
• Changing one buffer system alters all others.

Respiratory Regulation of Acid-Base Balance

• Lungs control extracellular carbon dioxide concentration levels.
• Increased alveolar ventilation lowers extracellular hydrogen ions.
• Decreased ventilation elevates extracellular hydrogen ion levels.
• Cell metabolism produces continuous carbon dioxide amounts.
• Dissolved CO2 in extracellular fluid equals 1.2 mol/L.
• Normal PCO2 equals 40 mm Hg value.
• Increased tissue metabolism elevates extracellular PCO2.
• Doubling alveolar ventilation increases arterial pH 0.23.
• Reducing ventilation to one-fourth drops pH 0.45.
• Alveolar ventilation varies up to 15 times.
• Elevated hydrogen ions stimulate alveolar ventilation directly.
• Decreased pH 7.0 increases ventilation fourfold.
• High pH depresses ventilation rate significantly.
• Hypoxia limits respiratory compensation during severe alkalosis.
• Respiratory control operates as negative feedback mechanism.
• Respiratory system efficiency ranges between 50-75 percent.
• Respiratory feedback gain equals 1 to 3.
• Respiratory system buffers more acid than chemicals.
• Lung diseases like emphysema cause respiratory acidosis.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.