Blood pH lives in a narrow band — 7.35 to 7.45. Drift much below 7.35 (acidosis) or above 7.45 (alkalosis) and enzymes start to misfold, nerves fire poorly, and the body fails. The system that defends that window has three lines of defense working on three different time scales: chemical buffers in seconds, the respiratory system in minutes, and the kidneys over hours to days. Walk through the three in order and acid-base balance stops looking like a mess of equations.
Why pH Sits in Such a Narrow Window
Every protein in the body has a shape that depends on the local concentration of hydrogen ions (H⁺), because H⁺ binds to the side chains of amino acids and changes their charge. Enzymes are proteins; if pH shifts even a few tenths of a unit, enzyme activity collapses. Hemoglobin's affinity for oxygen also shifts with pH — at low pH, hemoglobin holds less oxygen, exactly when tissues are likely to need more.
The body produces acid all the time. Cellular respiration produces CO₂, which combines with water to form carbonic acid. Protein metabolism produces sulfuric and phosphoric acids. Anaerobic metabolism produces lactic acid. Yet blood pH barely moves, because three buffer mechanisms are working continuously.
First Line: Chemical Buffers (Seconds)
A buffer is a chemical that can either soak up H⁺ or release it, depending on what the blood needs. Buffers act within seconds of any pH change. The body uses three.
The bicarbonate buffer system is the most important in blood and extracellular fluid. The reaction is:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
If H⁺ rises (acidosis), bicarbonate (HCO₃⁻) combines with it to form H₂CO₃, which converts to CO₂ and water, lowering H⁺. If H⁺ falls (alkalosis), the reaction runs the other way. The system is powerful because its two ends — CO₂ and HCO₃⁻ — are managed by two separate organs (the lungs and kidneys), so the body can adjust each independently.
The phosphate buffer system works the same way using H₂PO₄⁻ and HPO₄²⁻. It is most important inside cells and in urine, where phosphate concentration is high.
The protein buffer system uses the side chains of amino acids — especially histidine — to either grab or release H⁺. Hemoglobin is one of the body's most important protein buffers; the rest is contributed by plasma proteins like albumin. Roughly three-quarters of the body's chemical buffering capacity is inside cells, on proteins.
Second Line: The Respiratory System (Minutes)
If buffers were the only defense, they would fill up — bicarbonate would all become carbonic acid, and the system would stop. The lungs fix that by removing CO₂.
Recall the reaction: CO₂ + H₂O ⇌ H⁺ + HCO₃⁻. If blood becomes acidic, chemoreceptors in the medulla and the carotid bodies detect the rise in H⁺ and trigger faster, deeper breathing. Exhaling more CO₂ pulls the reaction to the left and removes H⁺ from the blood. Within minutes, breathing alone can compensate for a moderate acid load.
If blood becomes alkaline (less common), the same chemoreceptors slow breathing down. Holding CO₂ in the blood pushes the reaction to the right, generating more H⁺ and pulling pH back down toward normal.
This respiratory compensation is fast but limited. Slowing breathing too much eventually starves the body of oxygen, so the lungs can only blunt alkalosis, not fully fix it. They cannot remove fixed acids — like the sulfuric acid from protein metabolism — at all, only the volatile acid carried as CO₂.
Third Line: The Kidneys (Hours to Days)
The kidneys handle anything the buffers and lungs cannot, and they are the only system that can permanently fix the problem. They do three things:
- Excrete or retain H⁺. Tubule cells along the nephron secrete H⁺ into the urine when blood is acidic, and reabsorb it when blood is alkaline.
- Reabsorb or excrete bicarbonate. Bicarbonate filtered at the glomerulus is normally reabsorbed almost completely. In alkalosis, the kidneys allow more bicarbonate to escape into urine. In acidosis, they generate new bicarbonate and add it to the blood.
- Make new buffers in the urine. When excess H⁺ has to leave the body, it cannot just be dumped as free hydrogen ions — that would acidify the urine until secretion stopped. Instead, the kidney binds H⁺ to ammonia (forming ammonium, NH₄⁺) and to filtered phosphate (forming H₂PO₄⁻), so H⁺ can be excreted in large amounts.
Kidney compensation is slow, taking 24–72 hours to ramp up, but it can fully correct a disturbance. The respiratory system buys time; the kidneys make the actual repair.
The Four Acid-Base Disorders
Once you can name the three lines of defense, the four named disorders fall out of one table — based on whether the cause is respiratory (the lungs) or metabolic (everything else), and whether pH is too low or too high.
- Respiratory acidosis — hypoventilation traps CO₂; pH falls. Causes include severe asthma, COPD, and opioid overdose. The kidneys compensate by retaining bicarbonate.
- Respiratory alkalosis — hyperventilation blows off too much CO₂; pH rises. Causes include anxiety attacks and high altitude. Kidneys compensate by excreting bicarbonate.
- Metabolic acidosis — bicarbonate is lost (diarrhea) or fixed acid accumulates (diabetic ketoacidosis, lactic acidosis from poor perfusion). The lungs compensate by hyperventilating (Kussmaul breathing) to blow off CO₂.
- Metabolic alkalosis — acid is lost (severe vomiting empties stomach acid) or bicarbonate is gained (antacid overuse). The lungs compensate by hypoventilating slightly to retain CO₂.
The pattern: a respiratory problem changes CO₂ first, then the kidneys try to fix bicarbonate; a metabolic problem changes bicarbonate first, then the lungs change CO₂.
Getting Help
The kidney's role in acid-base balance builds directly on the filtration logic in how nephrons filter blood. For more physiology walkthroughs, see the full set of Anatomy & Physiology study guides.
Conclusion
Acid-base balance is a three-layered defense. Chemical buffers — bicarbonate, phosphate, and protein — react in seconds and absorb sudden H⁺ changes. The lungs respond in minutes by adjusting how much CO₂ leaves the body. The kidneys respond over hours to days by retaining or excreting H⁺ and bicarbonate directly. Knowing which system is responsible for an abnormal pH, and which one is compensating, is the entire framework behind every acid-base question on an exam.
