Describe the mechanics of breathing. Discuss the muscles involved and the pressure changes occurring during one normal respiratory cycle. Add a note on lung compliance.
Definition
Pulmonary ventilation = movement of air in and out of the lungs, produced by rhythmic changes in thoracic volume.Based on Boyle's law — at constant temperature, P ∝ 1/V.
Muscles of Respiration
A. Quiet inspiration — active
- Diaphragm — chief muscle. Nerve supply: phrenic (C3, C4, C5)
– Contracts → dome flattens → descends 1.5 cm
– ↑ vertical diameter of thorax
– Responsible for 75% of tidal volume
- External intercostals — Nerve supply: intercostal nerves
– Elevate the ribs
– Upper ribs → "pump-handle" movement → ↑ anteroposterior diameter
– Lower ribs → "bucket-handle" movement → ↑ transverse diameter
B. Forced inspiration — accessory muscles
- Sternocleidomastoid → elevates sternum
- Scalenes → elevate 1st and 2nd ribs
- Serratus anterior, pectoralis major and minor
- Alae nasi → nasal flaring
- Diaphragm descends up to 10 cm
C. Quiet expiration — passive
- No muscular contraction
- Due entirely to elastic recoil of lung and chest wall
D. Forced expiration — active
- Anterior abdominal muscles (rectus abdominis, obliques, transversus) → ↑ intra-abdominal pressure → push diaphragm up
- Internal intercostals → pull ribs down and in
Mechanism of Inspiration
Inspiratory muscles contract → ↑ thoracic volume (all 3 diameters) → ↑ negativity of intrapleural pressure → Lung expands → ↓ intra-alveolar pressure (−1 cm H2O) → air enters
Mechanism of Expiration
Muscles relax → Elastic recoil of lung → ↓ thoracic volume → ↑ intra-alveolar pressure (+1 cm H2O) → air leaves
Pressure Changes in One Respiratory Cycle
| Pressure | End expiration | End inspiration | Remarks |
|---|---|---|---|
| Intra-alveolar | 0 (= atmospheric) | −1 cm H2O | Drives airflow |
| Intrapleural | −5 cm H2O | −8 cm H2O | Always negative |
| Transpulmonary(alveolar − pleural) | +5 cm H2O | +7 cm H2O | Distending pressure of lung |
WHY Is Intrapleural Pressure Negative?
- Two opposing elastic forces act across the pleural cavity:
– Lung → recoils inwards (elastic tissue + surface tension)
– Chest wall → springs outwards
- Pleural cavity is a closed space with only a thin fluid film
- Net effect → subatmospheric (negative) pressure
Importance
- Keeps the lung expanded against its own recoil
- Aids venous return — the respiratory pump
- Becomes positive only in forced expiration (cough, Valsalva)
Compliance
Compliance = ΔV / ΔP — the distensibility, or stretchability, of the lung.
- Lungs alone — 200 mL/cm H2O
- Lungs + chest wall — 110 mL/cm H2O
- Determined by: (a) elastic tissue of lung, (b) alveolar surface tension — lowered by surfactant
| ↓ Compliance (stiff lung) | ↑ Compliance (floppy lung) |
|---|---|
| Pulmonary fibrosis | Emphysema |
| Pulmonary oedema | Old age |
| Atelectasis | — |
| RDS of newborn (surfactant deficiency) | — |
Work of Breathing
- Compliance (elastic) work — about 65%
- Tissue resistance work
- Airway resistance work
- Normally only 2–3% of total body energy expenditure
Applied Aspects
- Pneumothorax — air enters pleural cavity → intrapleural pressure = atmospheric → transpulmonary pressure = 0 → lung collapses
- Emphysema — loss of elastic tissue → ↑ compliance → poor recoil → air trapping → barrel chest; expiration becomes active
- Pulmonary fibrosis — ↓ compliance → ↑ work of breathing → rapid shallow breathing
- RDS of newborn — surfactant deficiency → ↑ surface tension → ↓ compliance → atelectasis
CLINICAL PEARL
Note: negative intrapleural pressure is what holds the lung expanded. Any breach of the pleural cavity abolishes it and the lung collapses at once.
All questions in Respiratory System
- Q1Describe the mechanics of breathing. Discuss the muscles involved and the pressure changes occurring during one normal respiratory cycle. Add a note on lung compliance. ★
- Q2Describe the transport of oxygen in the blood. Draw and explain the oxygen–haemoglobin dissociation curve and enumerate the factors that shift it. ★
- Q3Describe the nervous and chemical regulation of respiration. Add a note on the Hering–Breuer reflex. ★
- Q4Describe the transport of carbon dioxide in the blood. Explain the chloride shift and the Haldane effect. ★
- Q5Define the ventilation–perfusion ratio. Describe its regional variation in the upright lung and discuss the consequences of V/Q mismatch. ★
- SN1Write a short note on pulmonary surfactant. ★
- SN2Define lung volumes and capacities. Give their normal values and state which of them cannot be measured by spirometry. ★
- SN3Classify hypoxia. Give the causes and effects of each type. ★
- SN4Write a short note on cyanosis.
- SN5Write a short note on Cheyne–Stokes breathing.
- SN6Define dead space. Describe its types and explain its significance in alveolar ventilation. ★
- SN7Describe the physiological changes that occur during acclimatisation to high altitude. ★