Pulmonary Ventilation

Respiration - exchange of gases between atmosphere and body tissues

1. Pulmonary ventilation - inflow and outflow of air between the lungs & atmosphere
2. Diffusion of gases from lungs to blood to tissues
3. Regulation of ventilation

Pulmonary ventilation

Gases flow from areas of higher pressure to areas of lower pressure. For inspiration, atmospheric pressure > alveolar pressure, the reverse for expiration.

Airtight thoracic cavity divided into 2 pleural cavities containing lungs

• Pleural cavity is lined with parietal pleura & lungs are covered with visceral pleura, intrapleural pressure < atmospheric pressure helps them to adhere together
• Pleural fluid secreted by the pleural membranes fills the small space between the pleura and helps the pleura slide over each other but not pull apart
• Lung tissue is elastic
• Movement of the thoracic cavity walls causes movement of lung walls, changing the lung volume. Compliance measures ease of lung expansion and contraction

Boyle's Law - pressure of a given quantity of gas is inversely proportional to its volume (constant temperature)

Diaphragm

Dome-shaped skeletal muscle that forms floor of thoracic cavity.

Inspiration

Contraction of diaphragm causes it to flatten & lengthen the pleural cavities, thus increasing intrathoracic volume. Lungs expand, decreasing intrapulmonary (alveolar) pressure within lungs & air flows in (tidal volume). Active process

Expiration

Diaphragm relaxes & shortens pleural cavities, thus decreasing intrathoracic volume. Lungs elastically recoil, chest wall & abdominal organs compress lungs, increasing intra pulmonary pressure & air flows out. Passive process

Contraction of abdominal muscles during forced breathing causes rapid expiration. Active process

Ribs

Form walls of thoracic cavity.

Inspiration

Contraction of external intercostal muscles pull ribs & sternum upward and outward, increases anterior-posterior thoracic diameter by 20%. Active process

Contraction of sternocleidomastoids, scalenes & pectoralis minor further elevate upper ribs during forced inspiration.

Expiration

External intercostals relax, ribs & sternum move downward and inward, decreases anterior-posterior diameter. Passive process

Internal intercostals and abdominal muscle layers contract during forced expiration. Active process