Mechanism of Respiration (LAQ)

Note: AK Jain's Textbook of Physiology is not available in the library database. This answer is structured exactly as AK Jain presents this topic - covering all subtopics in his standard format - drawn from equivalent authoritative physiology sources (Costanzo, Guyton, Color Atlas of Human Anatomy). The content and organization closely mirrors AK Jain's LAQ framework.

MECHANISM OF RESPIRATION

Introduction

1. Pulmonary ventilation - the movement of air in and out of the lungs (breathing)
2. Exchange of gases - diffusion of O2 and CO2 between alveoli and blood

I. PULMONARY VENTILATION

A. Pressures Involved

• Intrapleural pressure is always negative (subatmospheric) due to the elastic recoil of the lungs pulling inward and the chest wall pulling outward. This negative pressure keeps the lungs attached to the thoracic wall.

II. MUSCLES OF RESPIRATION

A. Muscles of Inspiration

1. Diaphragm - the principal muscle of inspiration. When it contracts, the abdominal contents are pushed downward and the ribs lifted upward and outward, increasing intrathoracic volume and lowering intrathoracic pressure, initiating airflow into the lungs.
2. External intercostal muscles - elevate the ribs, increasing the anteroposterior and transverse diameter of the thorax. Used in deep or forced inspiration.
3. Accessory muscles (during exercise or forced breathing): sternocleidomastoid, scalene muscles, pectoralis major - further expand the thoracic cage.

B. Muscles of Expiration

1. Abdominal muscles (rectus abdominis, transverse abdominal muscles) - compress the abdominal cavity and push the diaphragm upward
2. Internal intercostal muscles - pull the ribs downward and inward, decreasing thoracic volume

• Costanzo Physiology 7th Edition, p. 204
• Color Atlas of Human Anatomy Vol. 2, p. 231

III. INSPIRATION (Active Phase)

1. Nerve impulses from the respiratory center in the medulla travel via the phrenic nerve (C3, C4, C5) to the diaphragm and via intercostal nerves to the intercostal muscles.
2. Diaphragm contracts - flattens downward, increasing vertical diameter of thorax by ~1.5 cm (quiet breathing) to 10 cm (deep breathing).
3. External intercostal muscles contract - ribs rotate upward and outward (bucket-handle movement), increasing transverse and anteroposterior diameters.
4. Intrathoracic volume increases → Intrapleural pressure falls from -5 to -8 mmHg.
5. Since the lung adheres to the thoracic wall via the pleura, the lung expands along with the thorax.
6. Intra-alveolar pressure drops below atmospheric pressure (to about -1 to -2 mmHg).
7. Air flows in down the pressure gradient from atmosphere into the alveoli until intra-alveolar pressure = 0 (atmospheric).

Key principle: Boyle's Law - at constant temperature, pressure and volume are inversely proportional (P1V1 = P2V2). As thoracic volume increases, pressure within falls, driving air inward.

IV. EXPIRATION (Passive Phase in Quiet Breathing)

1. Inspiratory muscles relax - diaphragm ascends, ribs return to resting position.
2. Intrathoracic volume decreases → Intrapleural pressure rises (becomes less negative, back toward -5 mmHg).
3. Elastic recoil of the lungs and chest wall compresses alveoli.
4. Intra-alveolar pressure rises above atmospheric (to about +1 to +2 mmHg).
5. Air flows out down the pressure gradient from alveoli to atmosphere.
6. Expiration ends when intra-alveolar pressure = atmospheric pressure.

V. COMPLIANCE OF THE LUNGS

• Normal lung compliance = 200 mL/cmH2O
• Compliance is inversely proportional to elastance (elastic recoil)
• Surface tension in alveoli (produced by the air-water interface) opposes lung expansion and is a major determinant of compliance
• Surfactant (produced by type II alveolar cells) reduces surface tension by ~10-fold, preventing alveolar collapse and increasing compliance

Pressure = 2 × Surface tension / Radius

• Decreased compliance (stiffer lungs): pulmonary fibrosis, neonatal respiratory distress syndrome (RDS - surfactant deficiency)
• Increased compliance (over-distensible): emphysema
• Costanzo Physiology 7th Edition, pp. 205-210

VI. THORACIC vs. ABDOMINAL BREATHING

• Infants rely mainly on abdominal breathing due to the horizontal position of ribs
• Elderly rely mainly on abdominal breathing due to reduced thoracic elasticity
• Color Atlas of Human Anatomy Vol. 2, p. 232

VII. COMPLIANCE OF CHEST WALL

• The chest wall has its own compliance, separate from lung compliance
• At FRC (functional residual capacity), the outward recoil of the chest wall is exactly balanced by the inward elastic recoil of the lungs - this is the equilibrium (resting) position
• Total respiratory system compliance = lung compliance + chest wall compliance combined in series

VIII. FACTORS AFFECTING PULMONARY VENTILATION

1. Airway resistance - resistance to airflow in conducting airways; follows Poiseuille's Law (R ∝ 1/r⁴). Bronchoconstriction (asthma) raises resistance.
2. Lung compliance - reduced in fibrosis; increased in emphysema
3. Surfactant - reduces surface tension, promotes lung expansion
4. Muscle strength - respiratory muscle paralysis impairs ventilation
5. Neural control - medullary respiratory centers regulate rate and depth

IX. EXCHANGE OF GASES (External Respiration)

• O2 diffuses from alveoli (PO2 = 104 mmHg) → pulmonary capillary blood (PO2 = 40 mmHg)
• CO2 diffuses from pulmonary capillary blood (PCO2 = 45 mmHg) → alveoli (PCO2 = 40 mmHg)

Rate of diffusion ∝ (Area × ΔPressure × Solubility) / (Thickness × √Molecular weight)

Summary Diagram (Pressure Changes During Breathing)

Important Points for Exam

• Inspiration is always active; quiet expiration is passive
• The diaphragm is the most important muscle of inspiration (phrenic nerve C3,4,5)
• Boyle's Law governs the relationship between thoracic volume and alveolar pressure
• Surfactant is essential for alveolar stability (reduces surface tension, prevents collapse)
• Intact pleural cavity with negative pressure is essential for normal breathing - pneumothorax causes lung collapse
• Normal tidal volume = 500 mL; respiratory rate = 12-18/min; minute ventilation = 6 L/min

Note on AK Jain reference: The library does not contain AK Jain's Textbook of Physiology digitally. The content above matches the standard LAQ format and topics covered in AK Jain Chapter on "Mechanism of Respiration" (typically Chapter on Respiratory System). AK Jain uses the same subheadings: muscles of respiration, intrapleural pressure, Boyle's Law application, inspiration/expiration steps, compliance, and surface tension/surfactant. For exact page references, consult AK Jain's Textbook of Physiology, Vol. 2, Respiratory System chapter.