The Acid-Base Servo

John Scott Haldane, Lawrence Henderson, and the Medullary Chemoreceptor Loop in Oxford

Arterial blood in a healthy human maintains a free hydrogen ion concentration ($[\text{H}^+]$) of roughly $40\text{ nanomoles per liter}$—one-millionth the concentration of blood sodium ions—corresponding to a pH of 7.40. Yet cellular metabolism pours roughly 15,000 millimoles of volatile carbonic acid and 70 millimoles of non-volatile sulfuric and phosphoric acids into the bloodstream every twenty-four hours. If arterial pH drops below 6.80 or rises above 7.80 for more than a few hours, excess protons alter histidine charge states across proteins. Cardiac ion channels and metabolic enzymes denature, resulting in coma or fatal arrhythmia.

In 1905, at the University of Oxford physiological laboratory, Scottish physiologist John Scott Haldane and his colleague John Gillies Priestley climbed inside an airtight steel respiratory plethysmograph. Their goal was to identify the feedback sensor that prevents this acid flood. For decades, physicians had assumed that the urge to breathe was driven by falling blood oxygen. Haldane and Priestley sampled gas from the deepest part of their own lungs at the end of exhalation. They discovered that alveolar carbon dioxide partial pressure ($P_{\text{CO}_2}$) is held invariant at $40.0\text{ mmHg}$. When they breathed gas mixtures low in oxygen (down to 13 percent $\text{O}_2$), their breathing barely changed. Yet when they increased inspired carbon dioxide so alveolar $P_{\text{CO}_2}$ rose by just $1.5\text{ mmHg}$—from 40.0 to $41.5\text{ mmHg}$—their minute ventilation doubled from 7 liters per minute to 14 liters per minute.

Three years later, in 1908 at Harvard University, chemist Lawrence Joseph Henderson proved why carbon dioxide and blood acidity are mathematically locked together. Dissolved carbon dioxide reacts reversibly with water in the presence of erythrocyte carbonic anhydrase to form carbonic acid, which dissociates into bicarbonate and hydrogen ions. In 1916, Danish physician Karl Hasselbalch expressed Henderson's mass-action equilibrium in logarithmic form as the Henderson-Hasselbalch equation:

In healthy arterial blood, plasma bicarbonate concentration ($[\text{HCO}_3^-]$) sits at $24\text{ mmol/L}$ while arterial $P_{\text{CO}_2}$ is held at $40\text{ mmHg}$, yielding $0.03 \times 40 = 1.2\text{ mmol/L}$ of dissolved $\text{CO}_2$. That locks the ratio inside the logarithm at exactly $20:1$, fixing arterial pH at $6.10 + \log_{10}(20) = 7.40$.

Unlike a sealed chemical buffer in a glass beaker—which is rapidly exhausted as soon as added acid consumes its base—the human bicarbonate system operates as an open, dual-organ proportional-integral servo loop:

• * The Fast Pulmonary Proportional Controller (Denominator $P_{\text{CO}_2}$): Central chemoreceptor neurons located in the retrotrapezoid nucleus on the ventral surface of the medulla oblongata sit behind the blood-brain barrier. While charged $\text{H}^+$ ions in the blood cross the blood-brain barrier poorly, neutral $\text{CO}_2$ gas diffuses across cerebral capillaries instantaneously, acidifying the cerebrospinal fluid and closing pH-sensitive $\text{TASK-2}$ potassium channels on retrotrapezoid neurons. Within seconds, phrenic nerve output to the diaphragm ramps up, blowing off $\text{CO}_2$ to restore the $20:1$ ratio.
• * The Slow Renal Integral Controller (Numerator $[\text{HCO}_3^-]$): Over hours to days, the proximal tubules and collecting ducts of the kidneys adjust the numerator directly by reclaiming filtered bicarbonate and secreting excess protons into the urine via apical $\text{H}^+\text{-ATPase}$ pumps.

During his 1911 Anglo-American expedition to the 14,110-foot summit of Pikes Peak in Colorado, Haldane watched these two controllers interact in real time. Hypobaric hypoxia at altitude triggered peripheral carotid body chemoreceptors to hyperventilate, blowing $P_{\text{CO}_2}$ down to $28\text{ mmHg}$ and driving blood into respiratory alkalosis. That alkaline shift initially braked the medullary center until the kidneys excreted bicarbonate over four days to reset the $20:1$ buffer ratio at the new altitude.