Ventilation

Pulmonary Ventilation

relationship among the pleura and pleural fluid

parietal pleura - lines pleural cavities

visceral pleura - covers the lungs

pleural fluid - secreted by pleura and fills the pleural space

the pleural fluid creates a fluid bond (and an associated negative Intrapleural pressure) that pulls the visceral pleura against the parietal pleura

Inspiration

contraction of the diaphragm

• expands the size of the thoracic cavity
• decreases the intrapleural pressure
• pulls the visceral pleura along with the parietal pleura
• decreases the intrapulmonary (alveolar) pressure
• draws air into the alveoli

During forced breathing contraction of the external intercostal muscles, the scalenes, the sternocleidomastoid and the seratus anterior expand the thoracic cavity by elevating the ribs.

Expiration

relaxation of the diaphragm and external intercostal muscles

• permits recoil of the elastic fibers in the walls of the lungs and alveoli
• increases the intrapulmonary (alveolar) pressure
• air is forced out of the alveoli
• pulls the parietal pleura along with the visceral pleura
• decreases the size of the thoracic cavity
• During forced breathing contraction of the internal intercostal muscles, the rectus abdominus and the oblique muscles assist in decreasing the size of the thoracic cavity by depressing the ribs and compressing the abdominal contents.

Gas Exchange

Diffusion between alveoli and pulmonary capillaries

• alveolar O2 diffuses to pulmonary blood
• pulmonary blood CO2 diffuses to alveoli
• Diffusion between systemic capillaries and tissues
• systemic blood O2 diffuses to tissues
• tissue CO2 diffuses to systemic blood

Oxygen Transport

Oxygen combines with hemoglobin in the blood in a reversible reaction

Hb + O₂ ¬ ® Hb-O₂

Carbon Dioxide Transport

Most carbon dioxide combines with water in the blood in a reversible reaction to form carbonic acid, that in turn dissociates in a reversible reaction to form hydrogen ions and bicarbonate ions

CO₂ + H₂O ¬ ® H₂CO₃ ¬ ® H+ + HCO₃^-

© David G. Ward, Ph.D.  Last modified by wardd 23 May, 2006