5.3: Ventilation

Ventilation involves two phases: inhaling (inspiration) and exhaling (expiration). Inspiration moves air into the nostrils and mouth, and down the airways to the deepest parts of the lungs, where the O₂ it contains can diffuse into the blood. Expiration moves air containing CO₂ from the innermost parts of the lungs up and out of the body.

To understand how ventilation occurs, one must realize that air around the body is under atmospheric pressure. Materials normally move from areas of higher concentration or pressure to areas of lower concentration or pressure. For example, air moves into a balloon and inflates it when more air pressure is applied to its opening than is already in the balloon. Conversely, releasing the opening of an inflated balloon results in air rushing out because the pressure within the balloon is higher than atmospheric pressure.

Inspiration

Inspiration occurs for the same reason that a balloon becomes inflated. Air moves into the body when the air pressure outside the body is greater than that inside the respiratory system. A person creates this difference in pressure by contracting muscles to move the floor or walls of the thoracic cavity.

The floor of the thoracic cavity consists of the dome‑shaped diaphragm, a thick sheet whose edge is muscle and whose center is fibrous material. The muscular edge slants downward sharply and is attached around its perimeter to the body wall. When the muscle contracts, the rounded central region is pulled downward within the body wall, moving like a piston downward in a cylinder. When the central region is pulled downward and the diaphragm flattens somewhat, the pressure within the thoracic cavity decreases. Because moisture on the outer surface of the lungs causes them, in effect, to adhere to the diaphragm, a similar decrease in pressure occurs within the lungs. Since the pressure in the lungs is then lower than atmospheric pressure, air flows through the airways and into the lungs, resulting in inspiration (Figure 5.2).

Figure 5.2 Inspiration (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

The walls of the thoracic cavity contain many bones, including the ribs, the sternum, and some vertebrae. The cartilage and joints that connect these bones allow them to move somewhat when the muscles attached to the bones contract. When muscles move the ribs and sternum upward and outward, pressure in the thoracic cavity decreases. The lungs, whose surfaces are stuck to the thoracic walls by fluid, also have a decrease in internal pressure, and inspiration occurs.

Inspiration usually involves simultaneous movement of both the diaphragm and the bones of the thorax. Some individuals rely mostly on movement of the diaphragm (diaphragmatic breathing), while in others movement of the ribs (costal breathing) makes the major contribution. Inspiration ends when parts of the body stop moving and enough air has come in to raise the pressure in the lungs to atmospheric pressure. If the muscles are held in this position, no further air movement occurs and the lungs remain inflated. People in this state are truly holding their breath.

Inspiration is called an active process because it requires the use of energy. Obtaining this energy uses some of the oxygen that inspiration helps obtain. The energy expended and the oxygen used are called the work of breathing. Usually not more than 5 percent of the oxygen brought in by inspiration is consumed in this process; the rest is available for use by other body cells. Since diaphragmatic breathing is more efficient and requires less energy than does costal breathing, it consumes less oxygen. These differences leave more of the oxygen obtained from inspiration for use by other body cells.

Expiration

Expiration for a person who is resting and breathing quietly, normally requires no muscle contraction because the movements of inspiration set up conditions that allow it to occur automatically. For example, when the diaphragm moves downward, it pushes on the organs below it in the abdominal cavity, and this increases the pressure in the abdominal cavity. Also, the movements of the ribs and sternum stretch and bend elastic and springy structures in the thoracic wall such as ligaments, cartilage, and the ribs themselves. Finally, the lungs, which are elastic, are stretched outward.

As a result, as soon as the muscles of inspiration relax, the abdominal organs, structures in the thoracic wall, and the lungs start to spring back to their original positions. This elastic recoil increases the pressure in the lungs. The pressure quickly rises above atmospheric pressure, and expiration occurs. The process is similar to what happens when the opening of an inflated balloon is released. Each expiration is followed shortly by the next inspiration (Figure 5.3).

Figure 5.3 Passive expiration (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Since normal quiet expiration requires no muscle contraction or energy, it is called a passive process. However, when a person becomes very active, passive expiration occurs too slowly to meet the needs of the body. Then respiratory muscles and energy can be used to perform active forced expiration. For example, abdominal muscles can squeeze on the abdominal organs, causing them to push upward on the diaphragm more forcefully, and chest muscles can pull the ribs downward. The resulting increase in pressure in the lungs pushes air out of the respiratory system quickly (Figure 5.4).

Figure 5.4 Forced expiration (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Rate of Ventilation (Minute Volume)

Ventilation usually occurs continuously to provide ongoing replacement of the O₂ being consumed and elimination of the CO₂ being produced. The rate of ventilation must be high enough to maintain homeostatic levels of these gases in the body. The rate of ventilation is called the respiratory minute volume, the volume of air inspired per breath times the number of breaths per minute. The number of breaths per minute is called the respiratory rate. Minute volume can be expressed mathematically:

Minute volume = volume per breath x breaths per minute

Lung Volumes

The volume of air inspired equals the amount of air expired. When a person is at rest and breathing quietly, this volume is called the tidal volume (TV).

When a person is active and has to exchange gases more quickly, inspiratory and expiratory volumes can be increased considerably by increasing the distance the respiratory muscles contract. The extra amount a person can inspire is called the inspiratory reserve volume (IRV); the extra amount a person can expire is called the expiratory reserve volume (ERV). The combination of tidal volume, inspiratory reserve volume, and expiratory reserve volume is called the vital capacity (VC). Vital capacity is the most air a person can expire after taking the deepest possible inspiration. This can be expressed mathematically:

TV + IRV + ERV = VC

Besides increasing the volume of air respired with each breath, a person can increase the speed at which the air flows. This is accomplished by increasing the speed and force of respiratory muscle contractions, which can magnify pressure changes in the lungs more than 25‑fold.

A person who expires as much as possible still has some air left in the lungs. This volume is called the residual volume (RV). Thus, the total amount of air the lungs can hold equals TV + IRV + ERV + RV and is called total lung capacity (TLC). A small amount of the TLC does not reach the alveoli but remains in the lower airways. This volume of air – the dead space – cannot be used for gas exchange because only the alveoli are thin enough for this process to occur.

Respiratory Rate

A normal person may have a respiratory rate of 15 to 20 breaths per minute, but this rate can change as needed. If the volume per breath remains high, an increase in the rate of respiration increases the minute volume and therefore the rate of gas exchange. Decreases in the respiratory rate have the opposite effect on the minute volume and the rate of gas exchange. Such adjustments in the respiratory rate occur as changes in bodily activity alter the need for gas exchange.

When the rate of respiration increases, there is less time for each inspiration and expiration. If a person does not increase the rate of airflow, breathing becomes rapid but shallow. Such breathing delivers little fresh air to the lungs for gas exchange.