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Air passageway

Air passageway

Atmospheric pressure of 760 mm Hg on the outside

Figure 19.21

When the lungs are at rest, the pressure on the inside of the lungs is equal to the pressure on the outside of the thorax.

Atmospheric pressure of 760 mm Hg on the outside

Figure 19.21

When the lungs are at rest, the pressure on the inside of the lungs is equal to the pressure on the outside of the thorax.

Figure 19.22

Figure 19.22

Moving the plunger of a syringe causes air to move in (a) or out (b) of the syringe. Air movements in and out of the lungs occur in much the same way.

Air pressure is exerted on all surfaces in contact with the air, and because people breathe air, the inside surfaces of their lungs are also subjected to pressure. In other words, when the respiratory muscles are at rest, the pressures on the inside of the lungs and alveoli and on the outside of the thoracic wall are about the same (fig. 19.21).

Pressure and volume are related in an opposite, or inverse, way. For example, pulling back on the plunger of a syringe increases the volume inside the barrel, lowering the air pressure inside. Atmospheric pressure then pushes outside air into the syringe (fig. 19.22a). In contrast, pushing on the plunger of a syringe reduces the volume inside the syringe, but the pressure inside increases, forcing air out into the atmosphere (fig. 19.22b). The movement of air into and out of the lungs occurs in much the same way.

If the pressure inside the lungs and alveoli (intra-alveolar pressure) decreases, outside air will then be pushed into the airways by atmospheric pressure. This is what happens during normal inspiration, and it involves the action of muscle fibers within the dome-shaped diaphragm.

The diaphragm is located just inferior to the lungs. It consists of an anterior group of skeletal muscle fibers (costal fibers), which originate from the ribs and sternum, and a posterior group (crural fibers), which originate from the vertebrae. Both groups of muscle fibers are inserted on a tendinous central portion of the diaphragm (reference plate 71).

The muscle fibers of the diaphragm are stimulated to contract by impulses carried by the phrenic nerves, which are associated with the cervical plexuses. When this occurs, the diaphragm moves downward, the thoracic cavity enlarges, and the intra-alveolar pressure falls about 2 mm Hg below that of atmospheric pressure. In response to this decreased pressure, air is forced into the airways by atmospheric pressure (fig. 19.23).

While the diaphragm is contracting and moving downward, the external (inspiratory) intercostal muscles and certain thoracic muscles may be stimulated to contract. This action raises the ribs and elevates the sternum, increasing the size of the thoracic cavity even more. As a result, the intra-alveolar pressure falls farther, and atmospheric pressure forces more air into the airways.

Lung expansion in response to movements of the diaphragm and chest wall depends on movements of the pleural membranes. Any separation of the pleural membranes decreases pressure in the intrapleural space, holding these membranes together. In addition, only a thin film of serous fluid separates the parietal pleura on the inner wall of the thoracic cavity from the visceral pleura attached to the surface of the lungs. The water molecules in this fluid greatly attract the pleural membranes and each other, helping to hold the moist surfaces of the pleural membranes tightly together, much as a wet coverslip

Atmospheric pressure (760 mm Hg)

Intra-alveolar pressure (760 mm Hg)

Intra-alveolar pressure (760 mm Hg)

Figure 19.23

Atmospheric pressure (760 mm Hg)

Intra-alveolar pressure (758 mm Hg)

Figure 19.23

(a) Prior to inspiration, the intra-alveolar pressure is 760 mm Hg. (b) The intra-alveolar pressure decreases to about 758 mm Hg as the thoracic cavity enlarges, and atmospheric pressure forces air into the airways.

1. Nerve impulses travel on phrenic nerves to muscle fibers in the diaphragm, contracting them.

1. Nerve impulses travel on phrenic nerves to muscle fibers in the diaphragm, contracting them.

2. As the dome-shaped diaphragm moves downward, the thoracic cavity expands.

3. At the same time, the external intercostal muscles may contract, raising the ribs and expanding the thoracic cavity still more.

4. The intra-alveolar pressure decreases.

5. Atmospheric pressure, which is greater on the outside, forces air into the respiratory tract through the air passages.

6. The lungs fill with air.

sticks to a microscope slide. As a result of these factors, when the intercostal muscles move the thoracic wall upward and outward, the parietal pleura moves too, and the visceral pleura follows it. This helps expand the lung in all directions.

Although the moist pleural membranes play a role in expansion of the lungs, the moist inner surfaces of the alveoli have the opposite effect. Here the attraction of water molecules to each other creates a force called surface tension that makes it difficult to inflate the alveoli and may actually cause them to collapse. Certain alveolar cells, however, synthesize a mixture of lipoproteins called surfactant, which is secreted continuously into alveolar air spaces. Surfactant reduces the alveoli's ten-

dency to collapse, especially when lung volumes are low, and makes it easier for inspiratory efforts to inflate the alveoli. Table 19.2 summarizes the steps of inspiration.

Born two months early, Benjamin McClatchey weighed only 2 pounds, 13 ounces. Like many of the 380,000 "preemies" born each year in the United States, Benjamin had respiratory distress syndrome (RDS). His lungs were too immature to produce sufficient surfactant, and as a result, they could not overcome the force of surface tension enough to inflate.

A decade ago, Benjamin might not have survived RDS. But with the help of a synthetic surfactant sprayed or dripped into his lungs through an endotra-cheal tube and a ventilator machine designed to assist breathing in premature infants, he survived. Unlike conventional ventilators, which force air into the lungs at pressures that could damage delicate newborn lungs, the high-frequency ventilator used on preemies delivers the lifesaving oxygen in tiny, gentle puffs.

If a person needs to take a deeper than normal breath, the diaphragm and external intercostal muscles may contract to an even greater extent. Additional muscles, such as the pectoralis minors and sternocleidomas-toids, can also be used to pull the thoracic cage further upward and outward, enlarging the thoracic cavity, and decreasing alveolar pressure even more (fig. 19.24).

Intercostal Muscle Pull

External intercostal muscles pull ribs up and out

Diaphragm contracts

Sternum moves up and out

External intercostal muscles pull ribs up and out

Diaphragm contracts

Sternocleidomastoid elevates sternum

Pectoralis minor elevates ribs

Diaphragm contracts more

Sternocleidomastoid elevates sternum

Pectoralis minor elevates ribs

Diaphragm contracts more

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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