Respiratory Physiology: Alveoli and Gas Exchange

Study Notes

The total gas exchange surface provided by the alveolar walls of a healthy man is approximately 50 to 70 square meters, which is 40 times greater than the surface area of his skin.

Pulmonary ventilation is the process of moving air into and out of the lungs to continuously change and refresh the gases in the air sacs (alveoli) of the lungs.
This process is commonly called breathing.

Gas exchange (oxygen loading and carbon dioxide unloading) occurs between the pulmonary blood and alveoli (exterior).
In external respiration, gas exchanges are made between the blood and the body exterior.

Oxygen and carbon dioxide must be transported to and from the lungs and tissue cells of the body via the bloodstream.
Internal respiration involves gas exchanges between the blood and tissue cells.

Breathing, or pulmonary ventilation, is a completely mechanical process that depends on volume changes occurring in the thoracic cavity.
Volume changes lead to pressure changes, which lead to the flow of gases to equalize the pressure.

The size of the thoracic cavity increases due to the contraction of the inspiratory muscles (diaphragm and external intercostals).

Exhalation in healthy people is largely a passive process that depends on the natural elasticity of the lungs rather than on muscle contraction.
As the inspiratory muscles relax and resume their initial resting length, the ribcage descends and the lungs recoil, decreasing the thoracic and intrapulmonary volumes.
As the intrapulmonary volume decreases, the gases inside the lungs are forced more closely together, and the intrapulmonary pressure rises to a point higher than atmospheric pressure.
This causes the gases to flow out to equalize the pressure inside and outside the lungs.

The internal intercostal muscles are activated to help depress the rib cage.
The abdominal muscles contract to squeeze the abdominal organs upward against the diaphragm.

The normal pressure within the pleural space is always negative.
This negative pressure prevents the collapse of the lungs.
If the intrapleural pressure becomes equal to the atmospheric pressure, the lungs immediately recoil and collapse.

Coughs and sneezes clear the air passages of debris or collected mucus.
Laughing and crying reflect our emotions.
The functional volume of air that actually reaches the respiratory zone and contributes to gas exchange is about 350 ml.

Bronchial sounds are produced by air rushing through the large respiratory passageways (trachea and bronchi).
Vesicular breathing sounds occur as air fills the alveoli, resembling a soft, muffled breeze.

Neural regulation sets the basic rhythm and regulates the activity of the respiratory muscles.
The neural centers that control respiratory rhythm and depth are located in the medulla and pons.
Impulses going back and forth between the pons and medulla centers maintain a rate of 12–15 respirations/minute, referred to as eupnea.