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Chapter 2 · Part 10 of 10
  1. Chapter overview
  2. 1Inside an atom
  3. 2Electron shells and the periodic table
  4. 3Atomic mass, moles, and isotopes
  5. 4Covalent bonds and molecules
  6. 5Polar bonds and weak attractions
  7. 6Ions and chemical reactions
  8. 7Water as a solvent
  9. 8Water's special properties
  10. 9Acids, bases, and the pH scale
  11. 10Buffers and pH balance

Buffers and pH balance

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Healthy human blood stays between pH 7.35 and 7.45, a window only a tenth of a unit wide. The body works hard to hold it there, because H⁺ and OH⁻ bind readily to many kinds of ions and molecules and change how they behave.

Why pH matters, and what shifts it

Because of that binding, the pH of a solution affects:

As a result, many biological processes work well only within a narrow pH range, and even small shifts can do harm. Cells hold their internal pH within about 6.5 to 7.8, and human blood, at 7.35–7.45, is slightly alkaline.

Both surroundings and an organism’s own activity can shift pH. Acid rain and other pollution make the water that reaches a plant’s roots more acidic. Working muscles release lactic acid and carbon dioxide during exercise, and kidney disease and some other illnesses can move blood pH a few tenths of a unit in either direction. Even that much keeps the body’s enzymes from working at their best, which brings on more symptoms.

Organisms fight back in several ways. When a mammal’s blood pH drifts, its kidneys add acidic or alkaline compounds to the blood to correct it. Many kinds of organisms also rely on a chemical defense: pairs of substances that soak up or release H⁺ as needed.

Buffers

Something that keeps the pH of a fluid from changing much, usually a pair of substances made up of an acid and its related base (the form left after the acid gives up an H⁺), is a buffer. A key buffer in the body fluids of animals is the pair carbonic acid (H₂CO₃) and bicarbonate ion (HCO₃⁻):

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

This buffer can push pH in either direction:

If the blood becomes The reaction runs Result
Too alkaline (H⁺ falls, pH rises) Left to right: CO₂ and water form carbonic acid, which releases H⁺ H⁺ rises, and pH falls back
Too acidic (H⁺ rises, pH falls) Right to left: bicarbonate combines with H⁺ to form carbonic acid, which breaks down into CO₂ and water H⁺ falls, and pH rises back; the lungs exhale the extra CO₂

Each buffer works best over a particular pH range, and the buffers an organism relies on are the ones that work best at its own normal pH.

Breathing and blood pH

Your breathing rate is partly controlled by the H⁺ concentration of your blood. During hard exercise, the carbonic acid–bicarbonate buffer and your lungs work together:

  1. Working muscles release extra CO₂ into the blood.
  2. CO₂ combines with water to form carbonic acid.
  3. Carbonic acid breaks apart into H⁺ and bicarbonate.
  4. With more H⁺ in the blood, blood pH falls.
  5. Receptors in the brain detect the drop in pH.
  6. Breathing speeds up, so you exhale more CO₂.
  7. To replace the lost CO₂, carbonic acid breaks down into CO₂ and water.
  8. H⁺ and bicarbonate combine to form more carbonic acid, removing H⁺ from the blood.
  9. Blood pH rises back to normal.
Check yourselfA disease makes a person’s blood too acidic. Which way does the carbonic acid–bicarbonate reaction shift, and what happens to the extra CO₂?

It shifts from right to left. Bicarbonate combines with the extra H⁺ to form carbonic acid, which breaks down into CO₂ and water. Removing H⁺ raises the pH, and the lungs exhale the CO₂.

Practice questions

  1. 1 of 2

    You add the same few drops of acid to two beakers. One holds pure water and the other holds a buffer solution, and both start at pH 7. What happens to their pH?

  2. 2 of 2

    Healthy human blood has a pH of about 7.35 to 7.45. How is blood best described?