Acid–Base Balance: Buffers, Lungs and Kidneys

A tiny range with big consequences

The pH of your blood normally stays between 7.35 and 7.45. That might look like a wide-enough window, but pH is a logarithmic scale: each whole unit is a tenfold change in H⁺ concentration. At pH 7.35 the blood has about 26% more H⁺ than at pH 7.45. Push pH much beyond this range and enzymes change shape, nerve and muscle cells misfire, and hemoglobin's grip on oxygen changes.

Yet your cells are constantly making acid. Cellular respiration produces CO₂ (which forms carbonic acid), hard exercise produces lactic acid, and breaking down proteins and fats produces other acids. So how does the blood stay so steady? Three lines of defence: buffers (instant), the lungs (minutes) and the kidneys (hours to days).

Line of defence 1: the bicarbonate buffer

A buffer is a mixture of a weak acid and its conjugate base. It soaks up added H⁺ or OH⁻ so that pH changes only a little. The most important buffer in blood is the carbonic acid–bicarbonate system:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

The enzyme carbonic anhydrase (in red blood cells) makes the first step very fast.

• If acid is added (for example lactic acid during a sprint), the extra H⁺ combines with HCO₃⁻ to form H₂CO₃, which breaks down into CO₂ and water. The equilibrium shifts left. The H⁺ is "used up", so pH drops only slightly, and the extra CO₂ can be breathed out.
• If base is added (or H⁺ is lost), H₂CO₃ breaks apart to release more H⁺. The equilibrium shifts right, again limiting the change.

Proteins (including hemoglobin) and phosphate ions also act as buffers, especially inside cells.

Buffers act instantly, but they do not remove acid from the body. They only hold it. For that you need the lungs and kidneys.

Line of defence 2: the lungs (minutes)

Because CO₂ is part of the buffer equation, changing breathing changes pH. Chemoreceptors in the medulla and in the carotid and aortic bodies (in the walls of large arteries) detect rising CO₂ and H⁺. The medulla then increases the rate and depth of breathing. Breathing out more CO₂ pulls the equilibrium to the left, removing H⁺ from the blood, so pH rises back toward normal. If pH rises too high, breathing slows, CO₂ builds up and pH falls. This is a negative feedback loop that works within minutes.

Line of defence 3: the kidneys (hours to days)

The kidneys are the only organs that can actually remove non-volatile acids (acids that can't be breathed out) from the body. Tubule cells:

• secrete H⁺ into the filtrate (as you saw in the nephron lesson);
• reabsorb almost all filtered HCO₃⁻ and make new HCO₃⁻ to return to the blood;
• use buffers in the urine (phosphate and ammonia, which picks up H⁺ to form ammonium, NH₄⁺) so that lots of H⁺ can be excreted without making urine dangerously acidic.

This is why urine pH varies a lot (from about 4.5 to 8), while blood pH barely moves.

When the balance fails: acidosis and alkalosis

Doctors classify acid–base problems by direction (acidosis or alkalosis) and by cause (respiratory or metabolic):

Treatments and everyday connections

Treatment always targets the cause: for example, insulin and fluids for diabetic ketoacidosis, help with breathing for respiratory acidosis, or slow breathing techniques for anxiety-related hyperventilation. (The old trick of breathing into a paper bag is no longer recommended, because it can be unsafe if the cause is actually a heart or lung problem.) People on long-term dialysis need their bicarbonate carefully managed because their kidneys can no longer do this job, which you will see in the next lesson.

You may also notice a familiar word: bicarbonate. Baking soda is sodium bicarbonate, and many antacid tablets contain bicarbonate or carbonate, which neutralize stomach acid.