Ventilation

Ventilation is the exchange of air between the external atmosphere and the alveoli. It is typically defined as the volume of air entering the alveoli per minute. A complete understanding of ventilation requires the consideration of lung volumes.

Standard lung volumes. The size of the lungs and therefore the lung volumes depend upon an individual's height, weight or body surface area, age, and gender. This discussion will include the typical values for a 70-kg adult. The four standard lung volumes are (see Figure 17.4):

• Residual volume

• Expiratory reserve volume

• Inspiratory reserve volume

The tidal volume (VT) is the volume of air that enters the lungs per breath. During normal, quiet breathing, tidal volume is 500 ml per breath. This volume increases significantly during exercise. The residual volume (RV) is the volume of air remaining in the lungs following a maximal forced expiration. Dynamic compression of the airways causes collapse and trapping

Figure 17.4 Standard lung volumes and lung capacities (typical values for 70-kg adult). Tidal volume (VT) is 500 ml during normal, quiet breathing. Inspiratory reserve volume (IRV) is obtained with a maximal inspiration and is 2.5 l. Tidal volume and IRV together determine the inspiratory capacity (IC), which is 3.0 l. Expiratory reserve volume (ERV) is obtained with maximal expiration and is 1.5 l. The volume of air remaining in the lungs following maximal expiration is the residual volume (RV), which is 1.5 l. The functional residual capacity (FRC) is the volume of air remaining in the lungs following a normal expiration and is 3.0 l. Vital capacity (VC) is obtained with deepest inspiration and most forceful expiration and is 4.5 l. The maximum volume to which the lungs can be expanded is the total lung capacity (TLC) and is approximately 6.0 l in adult males and 5.0 l in adult females.

Figure 17.4 Standard lung volumes and lung capacities (typical values for 70-kg adult). Tidal volume (VT) is 500 ml during normal, quiet breathing. Inspiratory reserve volume (IRV) is obtained with a maximal inspiration and is 2.5 l. Tidal volume and IRV together determine the inspiratory capacity (IC), which is 3.0 l. Expiratory reserve volume (ERV) is obtained with maximal expiration and is 1.5 l. The volume of air remaining in the lungs following maximal expiration is the residual volume (RV), which is 1.5 l. The functional residual capacity (FRC) is the volume of air remaining in the lungs following a normal expiration and is 3.0 l. Vital capacity (VC) is obtained with deepest inspiration and most forceful expiration and is 4.5 l. The maximum volume to which the lungs can be expanded is the total lung capacity (TLC) and is approximately 6.0 l in adult males and 5.0 l in adult females.

of air in the alveoli. Residual volume is normally 1.5 l. It can be much greater in patients with emphysema because of the increased tendency for airway collapse. Expiratory reserve volume (ERV) is the volume of air expelled from the lungs during a maximal forced expiration beginning at the end of a normal expiration. The ERV is normally about 1.5 l. The inspiratory reserve volume (IRV) is the volume of air inhaled into the lungs during a maximal forced inspiration beginning at the end of a normal inspiration. The IRV is normally about 2.5 l and is determined by the strength of contraction of the inspiratory muscles and the inward elastic recoil of the lungs.

The four standard lung capacities consist of two or more lung volumes in combination (see Figure 17.4):

• Functional residual capacity

• Inspiratory capacity

• Total lung capacity

• Vital capacity

The functional residual capacity (FRC) is the volume of air remaining in the lungs at the end of a normal expiration. The FRC consists of the residual volume and the expiratory reserve volume and is equal to 3 l. The inspiratory capacity (IC) is the volume of air that enters the lungs during a maximal forced inspiration beginning at the end of a normal expiration (FRC). The IC consists of the tidal volume and the inspiratory reserve volume and is equal to 3 l. The total lung capacity (TLC) is the volume of air in the lungs following a maximal forced inspiration. In other words, it is the maximum volume to which the lungs can be expanded. It is determined by the strength of contraction of the inspiratory muscles and the inward elastic recoil of the lungs. The TLC consists of all four lung volumes and is equal to about 6 l in a healthy adult male and about 5 l in a healthy adult female. The vital capacity (VC) is the volume of air expelled from the lungs during a maximal forced expiration following a maximal forced inspiration. In others words, it consists of the tidal volume as well as the inspiratory and expiratory reserve volumes. Vital capacity is approximately 4.5 l.

Total ventilation. The total ventilation (minute volume) is the volume of air that enters the lungs per minute. It is determined by tidal volume and breathing frequency:

Total ventilation = tidal volume x breathing frequency = 500 ml/breath x 12 breaths/min = 6000 ml/min

With an average tidal volume of 500 ml/breath and breathing frequency of 12 breaths/min, 6000 ml or 6 l of air move in and out of the lungs per minute. These values apply to conditions of normal, quiet breathing; tidal volume and breathing frequency increase substantially during exercise.

Alveolar ventilation. Alveolar ventilation is less than the total ventilation because the last portion of each tidal volume remains in the conducting airways; therefore, that air does not participate in gas exchange. As mentioned at the beginning of the chapter, the volume of the conducting airways is referred to as anatomical dead space. The calculation of alveolar ventilation includes the tidal volume adjusted for anatomical dead space and includes only air that actually reaches the respiratory zone:

Alveolar ventilation

= (tidal volume - anatomical dead space) x breathing frequency = (500 ml/breath - 150 ml dead space) x 12 breaths/min

During exercise, the working muscles need to obtain more oxygen and eliminate more carbon dioxide. Alveolar ventilation is increased accordingly. Interestingly, the increase in tidal volume is greater than the increase in breathing frequency. This is the most efficient mechanism by which to enhance alveolar ventilation. Using the preceding values, a twofold increase in breathing frequency, from 12 breaths/min to 24 breaths/min, results in an alveolar ventilation of 8400 ml/min. In other words, alveolar ventilation also increases by a factor of two. However, a twofold increase in tidal volume, from 500 ml/breath to 1000 ml/breath, results in an alveolar ventilation of 10,200 ml/min. Alveolar ventilation is enhanced more in this case because a greater percentage of the tidal volume reaches the alveoli. At a tidal volume of 500 ml/breath and an anatomical dead space of 150 ml, 30% of the inspired air is wasted because it does not reach the alveoli to participate in gas exchange. However, when the tidal volume is 1000 ml/breath, only 15% of the inspired air remains in the anatomical dead space.

Dead space. Anatomical dead space is equal to the volume of the conducting airways. This is determined by the physical characteristics of the lungs because, by definition, these airways do not contain alveoli to participate in gas exchange. Alveolar dead space is the volume of air that enters unperfused alveoli. In other words, these alveoli receive airflow but no blood flow; with no blood flow to the alveoli, gas exchange cannot take place. Therefore, alveolar dead space is based on functional considerations rather than anatomical factors. Healthy lungs have little or no alveolar dead space. Various pathological conditions, such as low cardiac output, may result in alveolar dead space. The anatomical dead space combined with the alveolar dead space is referred to as physiological dead space:

Physiological dead space = anatomical dead space + alveolar dead space

Physiological dead space is determined by measuring the amount of carbon dioxide in the expired air. Therefore, it is based on the functional characteristics of the lungs because only perfused alveoli can participate in gas exchange and eliminate carbon dioxide.

Pharmacy application: drug-induced hypoventilation

Hypoventilation is defined as a reduction in the rate and depth of breathing. Inadequate ventilation results in hypoxemia, or a decrease in the concentration of oxygen in the arterial blood. Hypoventilation may be induced inadvertently by various pharmacological agents, including opioid analgesics such as morphine. These medications cause hypoventilation by way of their effects on the respiratory centers in the brainstem. Doses of morphine too small to alter a patient's consciousness may cause discernible respiratory depression. This inhibitory effect on the respiratory drive increases progressively as the dose of morphine is increased. In fact, in humans, death due to morphine poisoning is almost always due to respiratory arrest. Although therapeutic doses of morphine decrease tidal volume, the decrease in breathing frequency is the primary cause of decreased minute volume.

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.

Get My Free Ebook


Post a comment