Acids, Bases & pH — The Chemistry of Blood

Acids and bases are among the most clinically important concepts in chemistry. Every organ in your body operates within a precise pH range, and deviations of even 0.1 units can be life-threatening. The Brønsted-Lowry definition (most useful for biology): - An acid is a substance that donates a proton (H⁺) to another molecule - A base is a substance that accepts a proton (H⁺) When an acid dissolves in water, it releases hydrogen ions (H⁺). These don't float free — they immediately attach to water molecules, forming hydronium ions (H₃O⁺). But for simplicity, we talk about H⁺ concentration. Common acids in biology and medicine: - Hydrochloric acid (HCl) — stomach acid, pH 1–2 - Lactic acid — produced in muscle during intense exercise - Carbonic acid (H₂CO₃) — formed when CO₂ dissolves in blood - Amino acids — their carboxyl groups (–COOH) donate H⁺ - Fatty acids — the acidic end of fat molecules - Nucleic acids (DNA, RNA) — phosphate groups are acidic Common bases: - Sodium bicarbonate (NaHCO₃) — accepts H⁺, neutralises acid - Ammonia (NH₃) — accepts H⁺ in metabolic reactions - The amino group (–NH₂) of amino acids — accepts H⁺

The pH scale measures the concentration of hydrogen ions in a solution. It runs from 0 to 14: - pH 0–6: Acidic (more H⁺ ions). Lower pH = more acidic. - pH 7: Neutral (equal H⁺ and OH⁻, like pure water) - pH 8–14: Basic/alkaline (fewer H⁺ ions, more OH⁻) pH is a logarithmic scale — each unit represents a 10-fold change. pH 5 is 10× more acidic than pH 6, and 100× more acidic than pH 7. This matters clinically: a blood pH of 7.1 vs 7.4 seems like a small difference, but it represents 2.5× more H⁺ ions. pH values you need to know: | Substance | pH | |-----------|-----| | Stomach acid | 1–2 | | Lemon juice | 2–3 | | Cola | ~2.5 | | Black coffee | ~5 | | Urine | 4.5–8 | | Pure water | 7.0 | | Blood plasma | 7.35–7.45 | | Small intestine | 7–8.5 | | Pancreatic juice | 8–8.3 | | Bleach | ~13 | The narrow range of blood pH (7.35–7.45) is maintained with extraordinary precision. Below 7.35 = acidosis; above 7.45 = alkalosis. Either state, if severe, can be fatal.

Respiratory acidosis: Too much CO₂ in blood (hypoventilation). CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ → blood becomes acidic. Causes: severe asthma, pneumonia, opiate overdose (respiratory depression), COPD exacerbation. Treatment: improve ventilation, sometimes mechanical ventilation. Respiratory alkalosis: Too little CO₂ (hyperventilation). Less carbonic acid → fewer H⁺ → blood becomes alkaline. Causes: anxiety/panic attacks, pain, altitude, mechanical ventilation (too fast). Tingling, light-headedness. Treatment: breathing into a bag (re-breathe CO₂) for panic attacks. Metabolic acidosis: Too much acid produced by metabolism, or loss of bicarbonate. Causes: - Diabetic ketoacidosis (DKA): Lack of insulin → fat breakdown → ketone acids produced - Lactic acidosis: Anaerobic respiration in shock or severe exercise - Renal failure: Kidneys can't excrete acid or regenerate bicarbonate - Aspirin overdose: Salicylic acid Metabolic alkalosis: Loss of acid (e.g. severe vomiting → loss of HCl from stomach), or excess bicarbonate intake. Compensation: The body always tries to correct pH. In metabolic acidosis, the lungs compensate by breathing faster (Kussmaul breathing) — blowing off CO₂ to raise pH. In respiratory acidosis, the kidneys compensate by retaining more bicarbonate.

Strong acids completely dissociate in water — all H⁺ is released. HCl → H⁺ + Cl⁻ (no HCl molecules remain). Very low pH. Stomach acid, some industrial acids. Weak acids only partially dissociate — an equilibrium exists between the undissociated acid and its ions. Most biological acids are weak — carbonic acid, lactic acid, acetic acid (vinegar), aspirin. Why this distinction matters in pharmacology: Most drugs are weak acids or weak bases. Their degree of ionisation depends on the pH of the surrounding fluid — and this dramatically affects how they move around the body. Acid trapping: A weak acid drug in an alkaline environment ionises more (loses H⁺) → becomes charged → cannot cross cell membranes → trapped in that compartment. This principle is used in poisoning treatment: - Aspirin overdose: Making urine alkaline (with IV bicarbonate) ionises aspirin in the kidney tubules → it can't be reabsorbed → more is excreted in urine. This is called "urinary alkalinisation." - Cocaine (weak base): More ionised in acidic environments → less absorbed through membranes. Understanding pH and acid-base chemistry isn't abstract — it is directly applied in every intensive care unit, every day.