Respiratory system Detail for Adults

This photo (courtesy of the Anatomical Institute, Bern) shows a rubber cast of human lungs.


The Human Respiratory System
The Pathway

•Air enters the nostrils
•passes through the nasopharynx,
•the oral pharynx
•through the glottis
•into the trachea
•into the right and left bronchi, which branches and rebranches into
•bronchioles, each of which terminates in a cluster of
•alveoli
Only in the alveoli does actual gas exchange takes place. There are some 300 million alveoli in two adult lungs. These provide a surface area of some 160 m2 (almost equal to the singles area of a tennis court and 80 times the area of our skin!).

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.
◦The diaphragm contracts, drawing 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
◦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.

Link to discussion of gas transport in the blood.
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%






The ease with which oxygen and carbon dioxide can pass between air and blood is clear from this electron micrograph of two alveoli (Air) and an adjacent capillary from the lung of a laboratory mouse. Note the thinness of the epithelial cells (EP) that line the alveoli and capillary (except where the nucleus is located). At the closest point, the surface of the red blood cell is only 0.7 µm away from the air in the alveolus. (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. The concentration of CO2 is monitored by cells in the medulla oblongata. If the level rises, the medulla responds by increasing the activity of the motor nerves that control the intercostal muscles and diaphragm.

However, the carotid body in the carotid arteries does have receptors that respond to a drop in oxygen. Their activation is important in situations (e.g., at high altitude in the unpressurized cabin of an aircraft) where oxygen supply is inadequate but there has been no increase in the production of CO2.

Local Control of Breathing
The smooth muscle in the walls of the bronchioles is very sensitive to the concentration of carbon dioxide. A rising level of CO2 causes the bronchioles to dilate. This lowers the resistance in the airways and thus increases the flow of air in and out.