Physiology
Respiratory Physiology
Respiratory physiology for MBBS and NEET-PG: lung volumes, compliance and surfactant, oxygen and carbon dioxide transport, the Bohr effect and control of breathing, mapped to NMC codes PY6.
MedNext Academy | 3 min read
Respiratory Physiology
Respiratory physiology for MBBS and NEET-PG: lung volumes, compliance and surfactant, oxygen and carbon dioxide transport, the Bohr effect and control of breathing, mapped to NMC codes PY6.
Respiratory physiology covers the mechanics of breathing and gas exchange. It deals with lung volumes and capacities, compliance and surfactant, the transport of oxygen and carbon dioxide in the blood, and the chemical and neural control of ventilation.
High-yield: Respiratory Physiology
- Tidal volume is about 500 millilitres and is the volume moved in a normal quiet breath.
- Vital capacity is the maximum volume that can be exhaled after a maximum inspiration.
- Residual volume cannot be measured by spirometry because it remains in the lungs after full expiration.
- The functional residual capacity is the volume left in the lungs at the end of a normal expiration.
- Surfactant from type 2 pneumocytes lowers surface tension and prevents alveolar collapse.
- The oxygen-haemoglobin dissociation curve is sigmoid, and a rightward shift releases more oxygen to the tissues.
- The curve shifts right with a rise in carbon dioxide, temperature, acidity and 2,3-DPG, described by the Bohr effect.
- Most carbon dioxide is carried in the blood as bicarbonate formed by carbonic anhydrase in red cells.
- The chloride shift moves chloride into the red cell as bicarbonate leaves during carbon dioxide transport.
- The central chemoreceptors respond to changes in cerebrospinal fluid pH from carbon dioxide.
- The peripheral chemoreceptors in the carotid and aortic bodies respond mainly to a fall in arterial oxygen.
- Ventilation-perfusion matching is highest at the lung bases in an upright person.
- Compliance is the change in lung volume for a change in pressure and is reduced in fibrosis.
- The respiratory rhythm is generated in the medulla, with the pons refining the pattern.
Oxygen dissociation curve shifts
- **Right shift:** Releases oxygen. Caused by high CO2, acid, heat and 2,3-DPG. Bohr effect.
- **Left shift:** Holds oxygen. Fetal haemoglobin, carbon monoxide, alkalosis and low temperature.
- **CO2 transport:** Mostly as bicarbonate via carbonic anhydrase, with the chloride shift.
- **Surfactant:** From type 2 pneumocytes. Lowers surface tension and prevents collapse.
NMC competencies in this chapter
- **PY6.1:** Functional anatomy of the respiratory tract and mechanics of breathing
- **PY6.2:** Lung volumes, capacities and their measurement
- **PY6.3:** Compliance, surface tension and surfactant
- **PY6.4:** Ventilation, perfusion and their matching
- **PY6.5:** Transport of oxygen and carbon dioxide in blood
- **PY6.6:** Regulation of respiration
- **PY6.7:** Hypoxia and its types
- **PY6.8:** Respiration in altered environments
Frequently Asked Questions
Why can residual volume not be measured by spirometry?
Residual volume is the air that stays in the lungs after a maximal expiration, so it can never be exhaled into a spirometer. It is measured indirectly by helium dilution or body plethysmography.
What is the Bohr effect?
The Bohr effect is the rightward shift of the oxygen-haemoglobin dissociation curve when carbon dioxide and acidity rise, which helps haemoglobin release more oxygen to active tissues.
How is carbon dioxide carried in the blood?
Most carbon dioxide travels as bicarbonate formed by carbonic anhydrase in red cells, with smaller amounts as carbamino compounds on haemoglobin and dissolved in plasma.
What does surfactant do?
Surfactant from type 2 pneumocytes lowers the surface tension inside the alveoli, which reduces the work of breathing and stops the small alveoli from collapsing.
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