Makindo Medical Notes

Related Subjects: Asthma |Acute Severe Asthma |Respiratory Failure |Cardiac Physiology |Renal Physiology |Liver Physiology |Pulmonary Physiology |Pancreas Physiology |Spleen Physiology |Gastrointestinal tract Physiology |Brainstem Physiology |Bone Physiology

Ventilation-perfusion mismatch increases A–a gradient (normally <2 kPa).

🔎 Lung Anatomy

• Trachea: cricoid → carina (T4/5).
• Right main bronchus = shorter, wider, more vertical → common site of aspiration.
• Pleura: visceral + parietal with thin fluid layer; pain via intercostal & phrenic nerves (referred to chest wall/shoulder).
• Lobes: Right = 3 lobes (upper, middle, lower); Left = 2 lobes (+ lingula).
• Bronchopulmonary segments: surgically resectable units with their own artery/bronchus/vein.

📊 Airway Levels

🔬 Alveolus–Capillary Interface

• Surface area ≈ 90 m²; ~300 million alveoli (diameter <0.2 mm).
• Gas barrier ≈ 0.4 µm (epithelium + BM + endothelium).
• Type I pneumocytes: thin, for gas exchange.
• Type II pneumocytes: secrete surfactant, can differentiate into Type I.
• Surfactant: phospholipid (lecithin, sphingomyelin); lowers surface tension, prevents collapse. Deficiency = neonatal RDS.

🧬 Oxygen–Haemoglobin Curve

• Sigmoid shape due to cooperative binding.
• Left shift: O₂ held tighter (low CO₂, alkalosis, ↓ temp, COHb, fetal Hb).
• Right shift: O₂ released easier (high CO₂, acidosis, ↑ temp, ↑ 2,3-DPG).
• Anaemia (Hb <7 g/dl) → severely limits O₂ delivery.

💨 Oxygen Therapy

• Room air: 21% O₂.
• Nasal cannula: 24–40% FiO₂ (flow-dependent).
• Venturi mask: accurate FiO₂ → best for COPD.
• Non-rebreather: very high FiO₂ (not for CO₂ retainers).

🧠 Respiratory Control

• Central chemoreceptors (medulla): respond to CO₂ via CSF H⁺.
• Peripheral chemoreceptors: carotid body (CN IX), aortic body (CN X) → sense low PaO₂ (<8 kPa), high CO₂, acidosis.
• Sensation of dyspnoea: muscle receptors, J-receptors, chemoreceptors.
• Normal driver: CO₂. In COPD → hypoxic drive can predominate.

🌬️ Causes of Hypoxia

• Normal V/Q ratio ≈ 0.8 (4 L/min ventilation ÷ 5 L/min perfusion).
• PE: V normal, Q ↓ → dead space.
• Pneumonia/asthma: V ↓, Q normal → shunt.

📐 Physics of Respiration

• Dalton’s law: total pressure = sum of partial pressures.
• Atmospheric pressure at sea level = 760 mmHg (101 kPa).
• pO₂ at sea level = 0.21 × 760 ≈ 159 mmHg (21.2 kPa).
• On Everest (252 mmHg): 0.21 × 252 ≈ 52 mmHg (6.9 kPa) → severe hypoxia, needs supplemental O₂.