Acids, Bases, and pH

Acids and bases are about hydrogen ions. An acid releases H⁺ into solution; the more H⁺, the more acidic. A base removes H⁺ (or adds OH⁻, which mops up H⁺). Stomach acid (HCl) is a strong acid; bicarbonate (HCO₃⁻) is an important base in blood.

The pH scale. pH measures the concentration of H⁺, running from 0 (most acidic) through 7 (neutral) to 14 (most basic/alkaline). Crucially, the scale is logarithmic: each whole-number step is a tenfold change in H⁺. A solution at pH 5 has ten times more H⁺ than pH 6, and a hundred times more than pH 7. This is why even small pH shifts in blood are physiologically large.

Familiar reference points: stomach acid ~1.5–3.5, lemon juice ~2, pure water 7, blood ~7.4, and household bleach ~12.

Why the body guards pH. Proteins — including every enzyme — fold into precise shapes held partly by weak bonds sensitive to H⁺. Too much acid or base disrupts those shapes (denaturation), so reactions slow or stop. The nervous and cardiovascular systems are especially vulnerable, which is why blood pH is defended within 7.35–7.45.

Buffers: the first line of defense. A buffer is a chemical partnership that soaks up excess H⁺ when fluids get too acidic and releases H⁺ when they get too basic, blunting swings in pH. The body's most important is the bicarbonate buffer system:

CO₂ + H₂O  ⇌  H₂CO₃  ⇌  H⁺ + HCO₃⁻

Reading this both directions shows its power: if H⁺ rises (acid), it combines with bicarbonate (HCO₃⁻), shifting left toward CO₂ and water, which the lungs can exhale. If H⁺ falls (too basic), the reaction shifts right to release more H⁺. Because the lungs control CO₂ and the kidneys control bicarbonate and H⁺ excretion, three systems cooperate to hold pH: chemical buffers (instant), the respiratory system (minutes), and the renal system (hours to days). Each is developed in its own unit; the chemistry here is the shared foundation.

How it works

Judging a solution's acidity:

1. More H⁺ → lower pH → more acidic.
2. pH 7 is neutral; below 7 acidic, above 7 basic.
3. Each pH unit = 10× change in H⁺ concentration.
4. Buffers resist change by binding/releasing H⁺, keeping pH near the set point.

Comparisons

Common confusions

• High pH = low acidity. Bigger pH numbers mean fewer H⁺ (more basic), which feels backward at first.
• pH is logarithmic. A shift from 7.4 to 7.1 is not "a little" — it is a large rise in H⁺.
• Acidosis vs acidemia (fine point). Acidosis is the process pushing pH down; acidemia is the actual low blood pH. Many courses use them loosely.
• Buffers don't remove acid permanently. They temporarily tie up H⁺; lungs and kidneys do the final removal.

Memory aids

• "Acid Adds H⁺; Base Blocks/absorbs H⁺."
• Lower number = more sour (like lemons at pH 2).
• "7.4, out the door" — remember normal blood pH ~7.4.

Quick review

• Acids release H⁺ (pH below 7); bases accept H⁺ (pH above 7); pure water is neutral at 7.
• The pH scale is logarithmic — each unit is a tenfold change in H⁺.
• Blood pH is tightly held at 7.35–7.45 because enzymes and proteins depend on it.
• Buffers (especially the bicarbonate system) resist pH change instantly, while the lungs (CO₂) and kidneys (HCO₃⁻/H⁺) provide slower, lasting control.