The Lab / Core medicine / Lesson 11
Your body makes acid all day, yet the blood barely notices. Build up how a chemical buffer, your lungs and your kidneys share the job, then watch two kinds of diuretic, sodium bicarbonate and an aspirin overdose tip the balance.
Educational, not for clinical use
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pH is a scale of how acidic or alkaline a liquid is. Lemon juice is acidic; soap is alkaline, the opposite of acidic. Water sits in the middle. Blood sits just on the alkaline side, and the body holds it inside a narrow band.
Build the idea
Working cells burn fuel and give off carbon dioxide, a waste gas, and other acids, such as lactic acid from hard-working muscle. An acid releases hydrogen ions: tiny charged particles that push pH toward acidic. Something has to deal with them.
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A buffer is a pair of chemicals that grabs extra acid, or extra base (anything that mops up hydrogen ions), so pH barely moves. Drop acid into plain water and pH swings a long way; drop it into blood and it hardly shifts.
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Bicarbonate is a base dissolved in blood. When it catches a hydrogen ion it becomes carbonic acid, a weak acid that holds on to that ion instead of letting it loose. Blood keeps far more bicarbonate than carbonic acid, ready for incoming acid.
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Carbonic acid easily splits into carbon dioxide and water, and the two can join back. Carbonic anhydrase, an enzyme (a protein that speeds up a chemical reaction), makes this swap fast. So carbon dioxide in blood acts as acid, and breathing it out removes acid.
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Picture a seesaw. On one side, carbon dioxide, the acid side; on the other, bicarbonate, the base side. pH depends on the balance between them, not on the amount of either. In the right proportion the beam sits level and pH stays in its band.
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Breathing faster or deeper blows off more carbon dioxide, lightening the acid side so pH rises. Breathing slower lets carbon dioxide build up, so pH falls. Sensors in the arteries and brain adjust breathing within minutes (more on the lungs in Lesson 10).
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The kidneys control bicarbonate. Cells lining their tubes push hydrogen ions out into the urine and return bicarbonate to the blood, using carbonic anhydrase. They can keep more or let more go, but this takes hours to days (Lesson 2 covers the kidney’s tubes).
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Buffers act in seconds, catching acid the moment it appears. The lungs respond in minutes by changing breathing. The kidneys are slowest, taking hours to days, but they clear acids that breathing cannot, and rebuild bicarbonate.
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Blood that is too acidic is in acidosis; too alkaline, alkalosis. If a breathing problem changed the carbon dioxide, it is respiratory. If bicarbonate changed first, it is metabolic. Two directions, two causes: four disorders.
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When one side goes wrong, the other side moves the same way to restore the balance. This is compensation. Here bicarbonate has fallen, so the lungs breathe off extra carbon dioxide. pH comes back toward normal, but compensation never pushes it past normal.
Build the idea
Buffers catch acid at once, the lungs set carbon dioxide, the kidneys set bicarbonate. A blood test reads three key measures: pH says acidosis or alkalosis; carbon dioxide and bicarbonate say whether the cause is respiratory or metabolic, and whether compensation has begun.
What changes it
Carbonic anhydrase inhibitors are diuretics, drugs that make the kidneys pass more urine. They block carbonic anhydrase in the kidney, so less bicarbonate returns to the blood and more is lost in urine. The base side lightens and blood tips toward acidosis.
What changes it
Some diuretics used for high blood pressure drain potassium, a mineral that competes with hydrogen ions to be swapped for sodium in the kidney (Lesson 2). With less potassium, more hydrogen ions go out and more bicarbonate is kept: blood tips toward alkalosis.
What changes it
Sodium bicarbonate, also found in some antacids for heartburn, is bicarbonate itself. Taken in, it adds straight to the base side, so blood tips toward alkalosis. Breathing slows a little to hold on to carbon dioxide, but the lungs compensate less well for alkalosis.
What changes it
Salicylate toxicity means too much aspirin in the body. Its breakdown product, salicylic acid, adds acid that uses up bicarbonate. Breathing also speeds up strongly and blows off carbon dioxide. Both sides lighten, so pH can swing first acidic, then alkaline.
Start with Next, or use the arrow keys. Pictures are illustrative, not measurements.
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Normal teaching baseline.
The blue line on each bar is an illustrative teaching model, not a patient measurement. Where the source gives a range, the bar shades it; where it gives a single value, the bar shows it as a black tick and the model is called close when it is within a tenth of it. Arrows come from the cited source for the selected state. The reference ranges and typical values come from the OpenStax passages quoted under each row; the blue model markers and their printed readings, and the end points of each bar, are illustrative teaching-model choices, not reference values.
An illustrative teaching model of the bicarbonate buffer. Change bicarbonate (a metabolic change) or carbon dioxide (a respiratory change), then let the other side compensate. The faint dashed shapes are the normal state; on the map below, the hollow ring is the state before compensation.
The buffer reactions, which the lesson draws as pictures: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3–. Carbon dioxide and water form carbonic acid (carbonic anhydrase speeds this up), and carbonic acid dissociates into a hydrogen ion and bicarbonate; every step runs both ways.
The model uses the Henderson–Hasselbalch equation for the bicarbonate buffer: pH = 6.1 + log10(HCO3 ÷ (0.03 × PCO2)), with bicarbonate in mM and carbon dioxide in mm Hg. The two constants, 6.1 and 0.03, belong to the equation; they are not from the cited excerpts. Its baseline carbon dioxide sits at the centre of OpenStax’s male and female ranges taken together, and its baseline bicarbonate is set so the bicarbonate to carbonic acid ratio is OpenStax’s 20 to 1, which gives a pH inside OpenStax’s normal range.
The two top sliders set the first change as a multiple of the baseline. Compensation then moves the other side in the same direction as the first change, a fraction of the way toward the value that would restore the normal ratio: up to three quarters of the way when the lungs answer an acidosis, half when they answer an alkalosis (OpenStax notes they are less efficient at that), and seven tenths when the kidneys answer a respiratory change. Because it never goes all the way, compensation can bring pH back toward normal but never past it. If both sides have been pushed toward the same disorder there is nothing left to compensate with, and if they already pull in opposite directions the model adds nothing more.
Left out: the protein and phosphate buffers, time (the compensation slider stands in for minutes in the lungs and hours to days in the kidneys), potassium and chloride, and limits on how far real breathing or kidneys can go. The sizes of the drug and disease changes in the lesson are illustrative; only their directions are meant to teach.