15.2A: Human Respiratory System

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The Pathway

alt — Figure 15.2.1.1: Human lungs. 1:Trachea 2:Pulmonary vein 3:Pulmonary artery 4:Alveolar duct 5:Alveoli 6:Cardiac notch 7:Bronchioles 8:Tertiary bronchi 9:Secondary bronchi 10:Primary bronchi 11:Hyoid bone. (CC-BY-SA-4.0; Credit: **Rastrojo**).

Breathing

In mammals, the diaphragm divides the body cavity into the abdominal cavity, which contains the viscera (e.g., stomach and intestines) and the thoracic cavity, which contains the heart and lungs. The inner surface of the thoracic cavity and the outer surface of the lungs are lined with pleural membranes which adhere to each other. If air is introduced between them, the adhesion is broken and the natural elasticity of the lung causes it to collapse. This can occur from trauma. And it is sometimes induced deliberately to allow the lung to rest. In either case, reinflation occurs as the air is gradually absorbed by the tissues. Because of this adhesion, any action that increases the volume of the thoracic cavity causes the lungs to expand, drawing air into them.

During inspiration (inhaling), the external intercostal muscles contract, lifting the ribs up and out. This is accomplished by the contraction of the diaphragm muscle, which draws it down. During expiration (exhaling), these processes are reversed and the natural elasticity of the lungs returns them to their normal volume. At rest, we breath 15–18 times a minute exchanging about 500 ml of air.
In more vigorous expiration, the internal intercostal muscles draw the ribs down and inward and the the wall of the abdomen contracts pushing the stomach and liver upward. Under these conditions, an average adult male can flush his lungs with about 4 liters of air at each breath. This is called the vital capacity. Even with maximum expiration, about 1200 ml of residual air remain.

The table shows what happens to the composition of air when it reaches the alveoli. Some of the oxygen dissolves in the film of moisture covering the epithelium of the alveoli. From here it diffuses into the blood in a nearby capillary. It enters a red blood cell and combines with the hemoglobin therein. At the same time, some of the carbon dioxide in the blood diffuses into the alveoli from which it can be exhaled.

Table 15.2.1.1: Composition of atmospheric air and expired air in a typical subject. Note that only a fraction of the oxygen inhaled is taken up by the lungs.
Component	Atmospheric Air (%)	Expired Air (%)
N₂ (plus inert gases)	78.62	74.9
O₂	20.85	15.3
CO₂	0.03	3.6
H₂O	0.5	6.2
	100.0%	100.0%

alt — Figure 15.2.1.2: Alveoli(Reproduced with permission from Keith R. Porter and Mary A. Bonneville, **An Introduction to the Fine Structure of Cells and Tissues**, 4th. ed., Lea & Febiger, 1973.)

Central Control of Breathing

The rate of cellular respiration (and hence oxygen consumption and carbon dioxide production) varies with level of activity. Vigorous exercise can increase by 20–25 times the demand of the tissues for oxygen. This is met by increasing the rate and depth of breathing. It is a rising concentration of carbon dioxide — not a declining concentration of oxygen — that plays the major role in regulating the ventilation of the lungs. Certain cells in the medulla oblongata are very sensitive to a drop in pH. As the CO₂ content of the blood rises above normal levels, the pH drops

. Those rare people who inherit two defective genes for alpha-1 antitrypsin are particularly susceptible to developing emphysema.