Pulmonary Physiology, Introduction, Dr. D. Penney

Pulmonary Physiology - Introduction (continued...)

Breathing: During inspiration, the volume of the thoracic cavity increases and air is drawn into the lung. The increase in volume is brought about partly by contraction of the diaphragm which causes it to descend, and partly by the action of the intercostal muscles which raise the ribs, thus increasing the x-sectional area of the thorax. Inspired air flows down to about the terminal bronchioles by bulk flow. Beyond that point, the combined x-sectional area of the airways is so enormous that forward movement of the gas becomes very small and diffusion becomes the dominant mechanism.

Elasticity: The lung is elastic and returns passively to its pre-inspiratory volume during resting breathing. It is very easy to distend. For example, a normal breath of about 500 ml requires a distending pressure of less than 3 cm water.

Resistance: The pressure required to move gas through the airways is also very small. During normal inspiration, an air flow rate of 1 liter/sec. requires a pressure drop along the airways of less than 2 cm water.

At Right -> Scanning electron micrograph of a section of lung showing many alveoli and a small bronchiole. The pulmonary capillaries run in the walls of the alveoli. The holes in the alveolar walls are the Pores of Kohn.

Measurements of the newborn and adult human lung.

Parameter Newborn Adult
Body weight (Kg) 3.5 70
Surface area (m2) 0.21 1.90
Lung weight (g) 50 800
Tracheal diameter (mm) 8 18
Bronchiole diameter (mm) 0.1 0.2
Number of Airways (x 106) 1.5 14.0
*Alveolar diameter (microns) 50-100 200-300
Alveolar surface area (m2) 4 80
Number of alveoli (x 106) 24 296
Vital capacity (ml/Kg) 33 52
Functional residual capacity (ml/Kg) 30 34
Dead space (ml/Kg) 2.2 2.2
Tidal volume (ml/Kg) 6 7
Respiratory rate at rest 40 20
Alveolar ventilation (ml/Kg min) 100-150 60
Oxygen consumption at rest (ml/Kg min) 6 3

* Approximate and not taking into account possible variations between apex and base in vivo.

Last Changed 05/01/00

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