Info

1. The diaphragm and external respiratory muscles relax.

2. Elastic tissues of the lungs and thoracic cage, which were stretched during inspiration, suddenly recoil, and surface tension collapses alveolar walls.

1. The diaphragm and external respiratory muscles relax.

2. Elastic tissues of the lungs and thoracic cage, which were stretched during inspiration, suddenly recoil, and surface tension collapses alveolar walls.

3. Tissues recoiling around the lungs increase the intra-alveolar pressure.

4. Air is squeezed out of the lungs.

and the rectus abdominis, can be used to squeeze the abdominal organs inward. Thus, the abdominal wall muscles can increase pressure in the abdominal cavity and force the diaphragm still higher against the lungs, squeezing additional air out of the lungs (fig. 19.25). Table 19.3 summarizes the steps in expiration.

H Describe the events in inspiration.

How does surface tension aid in expanding the lungs during inspiration?

What forces are responsible for normal expiration?

Respiratory Volumes and Capacities

Different degrees of effort in breathing move different volumes of air in or out of the lungs. The measurement of such air volumes is called spirometry, and it describes four distinct respiratory volumes.

One inspiration plus the following expiration is called a respiratory cycle. The amount of air that enters or leaves during a respiratory cycle is termed the tidal volume. About 500 milliliters (mL) of air enter during a normal, resting inspiration. On the average, the same amount leaves during a normal, resting expiration. Thus, the resting tidal volume is about 500 mL (fig. 19.26).

During forced maximal inspiration, a quantity of air in addition to the resting tidal volume enters the lungs. This additional volume is called the inspiratory reserve volume (complemental air), and it equals about 3,000 mL.

During a maximal forced expiration, about 1,100 mL of air in addition to the resting tidal volume can be expelled from the lungs. This quantity is called the expiratory reserve volume (supplemental air). However, even after the most forceful expiration, about 1,200 mL of air remains in the lungs. This is called the residual volume.

Residual air remains in the lungs at all times, and consequently, newly inhaled air always mixes with air already in the lungs. This prevents the oxygen and carbon dioxide concentrations in the lungs from fluctuating greatly with each breath.

Once the respiratory volumes are known, four respiratory capacities can be calculated by combining two or more of the volumes. Thus, if the inspiratory reserve volume (3,000 mL) is combined with the tidal volume (500 mL) and the expiratory reserve volume (1,100 mL), the total is termed the vital capacity (4,600 mL). This capacity is the maximum amount of air a person can exhale after taking the deepest breath possible.

Figure

Respiratory volumes and capacities.

Figure 19.27

A spirometer can be used to measure respiratory air volumes.

The tidal volume (500 mL) plus the inspiratory reserve volume (3,000 mL) gives the inspiratory capacity (3,500 mL), which is the maximum volume of air a person can inhale following a resting expiration. Similarly, the expiratory reserve volume (1,100 mL) plus the residual volume (1,200 mL) equals the functional residual capacity (2,300 mL), which is the volume of air that remains in the lungs following a resting expiration.

The vital capacity plus the residual volume equals the total lung capacity (about 5,800 mL). This total varies with age, sex, and body size.

Some of the air that enters the respiratory tract during breathing fails to reach the alveoli. This volume (about 150 mL) remains in the passageways of the trachea, bronchi, and bronchioles. Since gas exchanges do not occur through the walls of these passages, this air is said to occupy anatomic dead space.

Occasionally, air sacs in some regions of the lungs are nonfunctional due to poor blood flow in the adjacent capillaries. This creates alveolar dead space. The anatomic and alveolar dead space volumes combined equal physiologic dead space. In a normal lung, the anatomic and physiologic dead spaces are essentially the same (about 150 mL).

A spirometer (fig. 19.27) is used to measure respiratory air volumes (except the residual volume). These measurements are used to evaluate the course of respiratory illnesses, such as emphysema, pneumonia, lung cancer, and bronchial asthma. Table 19.4 summarizes the respiratory air volumes and capacities.

D What is tidal volume?

^9 Distinguish between inspiratory and expiratory reserve volumes.

^9 How is vital capacity measured? □ How is the total lung capacity calculated?

Figure 19.27

A spirometer can be used to measure respiratory air volumes.

Alveolar Ventilation

The amount of new atmospheric air that is moved into the respiratory passages each minute is called the minute ventilation. It equals the tidal volume multiplied by the breathing rate. Thus, if the tidal volume is 500 mL and the breathing rate is 12 breaths per minute, the minute ventilation is 500 mL x 12, or 6,000 mL per minute. However, much of the new air remains in the physiologic dead space.

The volume of new air that does reach the alveoli and is available for gas exchange is calculated by subtracting the physiologic dead space (150 mL) from the tidal volume (500 mL). The resulting volume (350 mL) multiplied by the breathing rate (12 breaths per minute) is the alveolar ventilation rate (4,200 mL per minute). The alveolar ventilation rate is a major factor affecting the concentrations of oxygen and carbon dioxide in the alveoli.

Shier-Butler-Lewis: I V. Absorption and I 19. Respiratory System I I © The McGraw-Hill

Human Anatomy and Excretion Companies, 2001

Physiology, Ninth Edition

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

Get My Free Ebook


Post a comment