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Q11 - Acid-Base Balance: Reading a Blood Gas Map

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Homeostasis & Osmoregulation

The pH value of human blood is kept in a strict range, in order to maintain the vital functions of the body. There are three main mechanisms which can help blunting acidic or alkaline shifts:

  • Buffer systems, especially the bicarbonate system.
  • Exhalation of carbon dioxide in the lungs.
  • Excretion of protons and the reabsorption and production of bicarbonate-ions by the kidney.

These three regulatory systems usually operate simultaneously, compensating each other’s malfunctions when needed. Conditions A to F are depicted in the figure below.

Blood-gas map: x-axis arterial blood pH (7.00-7.80), left y-axis plasma [HCO3-] in mmol/L (0-60), right y-axis blood [H+] in nmol/L, with diagonal PCO2 isopleths fanning across the plot. A shaded blob sits centrally labeled “Legend 1 = Normal”. Six lettered regions surround it: A (upper-left, low pH ~7.0-7.2, moderately elevated HCO3-), B (upper-middle, near-normal pH, markedly elevated HCO3- ~36-56), C (upper-right, high pH ~7.5-7.7, markedly elevated HCO3- ~30-56), D (right, high pH ~7.5-7.7, reduced HCO3- ~14-20), E (lower-middle-right, near-normal pH, markedly reduced HCO3- ~4-20), F (lower-left, low pH ~7.0-7.3, reduced HCO3- ~4-16). Fig.1. Axis 1 = Arterial blood pH. Axis 2 = Arterial plasma [HCO3-] (mmol/L). Axis 3 = Arterial blood [H+] (nmol/L). Legend 1 = Normal.

Match the letters from the figure (from A to F) to the corresponding clinical conditions in the list below. Note that acute implies a sudden effect, while chronic conditions last for an extended period and involve compensatory mechanisms. Note that not all the letters are used.

Q11.1. The pH of human blood is kept in a strict range by buffer systems, respiratory CO2 exhalation, and renal H+ excretion/HCO3- handling. Conditions A-F are plotted on a blood-gas map (arterial pH vs. plasma [HCO3-] vs. [H+]), surrounding a central "Normal" zone. Which letter matches: the condition following vomiting?
Q11.2. Which letter matches: acute respiratory acidosis due to ineffective respiration caused by a stroke affecting the brainstem?
Q11.3. Which letter matches: acute respiratory alkalosis caused by hyperventilation?
Q11.4. Which letter matches: chronic respiratory acidosis associated with asthma?
Q11.5. Which letter matches: chronic respiratory alkalosis associated with anaemia?
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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key