Short context: Normal arterial blood pH is 7.35–7.45 (slightly alkaline). A fall below 7.35 is called acidemia caused by acidosis (process). Persistent or severe acidemia impairs cellular enzymes, heart and brain function, and can be life-threatening.
Quick Answer
If blood pH decreases below normal (acidosis), the blood becomes too acidic. This disrupts enzyme activity, weakens the nervous system, reduces oxygen transport, and may cause confusion, irregular heartbeat, and even coma in severe cases. Maintaining pH balance is vital for proper cell and organ function.
Detailed Answer
The physiology (why pH matters)
- The body’s chief buffer is bicarbonate (HCO₃⁻):
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (Henderson–Hasselbalch).- Lungs regulate CO₂ (acid component) via ventilation.
- Kidneys regulate HCO₃⁻ (base component) via reabsorption and acid excretion (H⁺, NH₄⁺).
- When pH drops, proteins (including enzymes and hemoglobin) change conformation → reduced enzyme efficiency and disturbed metabolism.
Types, core signs, and classic causes
| Type | Primary change | Typical ABG clue | Common causes | Key compensatory response |
|---|---|---|---|---|
| Metabolic acidosis | ↓ HCO₃⁻ | ↓ pH, ↓ HCO₃⁻; PaCO₂ falls secondarily | DKA, lactic acidosis (sepsis/shock), renal failure, diarrhea (HCO₃⁻ loss), renal tubular acidosis, toxins (methanol, ethylene glycol, salicylate) | Hyperventilation (Kussmaul breathing) to blow off CO₂ |
| Respiratory acidosis | ↑ PaCO₂ (hypoventilation) | ↓ pH, ↑ PaCO₂; HCO₃⁻ rises (if chronic) | COPD/asthma exacerbation, CNS depression (sedatives, head injury), neuromuscular disorders, airway obstruction | Renal retention of HCO₃⁻ (hours–days) |
System-wise effects of a pH decrease (acidemia)
1) Brain & nerves
- Headache, confusion, lethargy → stupor/coma if severe (pH ≤ 7.1 often dangerous).
2) Lungs
- Rapid, deep breathing (Kussmaul) in metabolic acidosis as compensation.
- In respiratory acidosis, compensation is renal (breathing remains inadequate).
3) Heart & circulation
- ↓ Myocardial contractility, vasodilation, and arrhythmias (risk of ventricular instability).
- Right shift of the oxyhemoglobin curve (Bohr effect): more O₂ unloading to tissues; useful initially, but severe acidosis + reduced cardiac output → tissue hypoxia.
4) Kidneys & electrolytes
- Hyperkalemia (H⁺ shifts into cells, K⁺ shifts out) → muscle weakness, arrhythmias.
- Chronic acidosis: bone buffering → bone resorption, possible osteopenia.
5) Metabolic/enzymatic
- Enzyme activity falls in acidic milieu → impaired energy production, lactic acidosis can worsen.
How it’s identified (exam-style essentials)
- Arterial Blood Gas (ABG):
- Acidemia: pH < 7.35
- Respiratory acidosis: PaCO₂ > 45 mmHg (primary)
- Metabolic acidosis: HCO₃⁻ < 22 mEq/L (primary)
- Anion gap (AG):AG = Na⁺ – (Cl⁻ + HCO₃⁻); normal ≈ 8–12 mEq/L (lab-dependent).
- High-AG MA → added acids (lactate, ketones, toxins).
- Normal-AG (hyperchloremic) MA → HCO₃⁻ loss (diarrhea, RTA).
Compensation quick checks (for viva/MCQs)
- Metabolic acidosis (Winter’s formula) expected PaCO₂ ≈ 1.5 × HCO₃⁻ + 8 (±2).
- Respiratory acidosis compensation:
- Acute: HCO₃⁻ ↑ ~1 mEq/L per 10 mmHg ↑PaCO₂
- Chronic: HCO₃⁻ ↑ ~3–4 mEq/L per 10 mmHg ↑PaCO₂
Clinical red flags (know these for one-liners)
- pH < 7.2: diminished contractility, hypotension risk.
- Kussmaul breathing: hallmark of DKA/severe metabolic acidosis.
- Peaked T waves on ECG: think hyperkalemia from acidemia.
Management (conceptual for exams)
- Treat the cause (insulin + fluids in DKA; fluids/oxygen/antibiotics for sepsis; antidotes for methanol/ethylene glycol; dialysis for uremia/toxins).
- Ventilatory support for respiratory acidosis (improve alveolar ventilation).
- Sodium bicarbonate: reserved for severe acidemia (typically pH ≤ 7.1) or specific poisonings—because indiscriminate use can worsen CO₂ load, sodium overload, and paradoxical CNS acidosis.
Exam tip: They love asking when to use bicarbonate—answer: selectively in severe acidemia or certain toxicities; primary therapy is correcting the underlying cause.
Exam Relevance / Extra Facts
- Blood pH balance is a frequently tested concept in Biology & General Science sections of exams.
- UPSC & SSC Previous Year Example:
- “What is the normal range of human blood pH?” (SSC CGL 2012)
- “Which condition occurs when blood pH falls below 7.35?” (Railways 2018)
Quick Recap / Key Points Box
- ✅ Normal blood pH = 7.35–7.45
- ✅ pH < 7.35 → Acidosis (too acidic)
- ✅ Metabolic acidosis: ↓HCO₃⁻; respiratory acidosis: ↑PaCO₂.
- ✅ Causes: Respiratory (CO₂ buildup) & Metabolic (acid accumulation)
- ✅ Effects: Enzyme failure, weak nerves, irregular heartbeat, possible coma
- ✅ Important for UPSC, SSC, Railways, Defence Exams
FAQs
Q1. What is the normal pH of blood?
👉 The normal blood pH is 7.35–7.45 (slightly alkaline).
Q2. What happens if blood becomes too acidic?
👉 It leads to acidosis, causing fatigue, confusion, breathing issues, and severe health risks.
Q3. Which organs help regulate blood pH?
👉 The lungs and kidneys maintain pH balance by removing CO₂ and acids.
Q4. Can blood pH be higher than normal?
👉 Yes, if it rises above 7.45, it is called alkalosis (opposite of acidosis).
👉 Did this answer help you? Save it for quick revision and share with friends preparing for exams!
Keep practicing daily – small concepts like these often fetch easy marks in competitive exams.
