Introduction
The respiratory system is divided into upper and lower respiratory zones. The tracheobronchial tree is divided into 23 generations according to Webel's model. From the trachea to the sixteenth terminal, the bronchioles are called the conducting zone or the anatomical dead space where no gaseous exchange occurs. From the seventeenth terminal, bronchioles, to the twenty-third terminal, alveoli are known as the respiratory zone where gaseous exchange occurs.
There are approximately 300 million alveoli present in both lungs. The alveolar cells line the alveoli. Alveolar cells are of two types- type 1 and type 2. Type 1 alveolar cells or pneumocytes occupy 96-98% of the surface area of alveoli, whereas type 2 alveolar cells or pneumocytes occupy only two to four percent of the surface area of alveoli. The major function of these two types of alveolar cells is the gaseous exchange and secreting surfactant, respectively. Terminal bronchioles are the seat of resistance in respiration.
The major function of the respiratory system is the gaseous exchange between the environment and the body. It specifically provides oxygen and removes carbon dioxide from the body. The other non-respiratory functions of the respiratory system are acid-base balance, secreted surfactant, pulmonary circulation, defense mechanism, articulation and phonation, and serotonin metabolism.
What Is Alveolar Ventilation?
Alveolar ventilation is the exchange of gases between the alveoli and the external environment. It is a process in which oxygen is brought into the lungs and carbon dioxide is carried into the lungs in the mixed venous blood is released from the body. It can be termed as the volume of fresh air entering the alveoli per minute.
What Is the Mechanism Behind Alveolar Ventilation?
The volume of air entering and exiting the nose or mouth per minute, the minute volume, is not equal to the volume of air entering and leaving the alveoli per minute. Therefore, alveolar ventilation is only part of the respiratory minute volume that reaches alveolar surfaces.
It is less than the respiratory minute volume because the last part of each inspiration remains in the conducting airways and does not reach the alveoli. Therefore, it cannot be measured directly but can be obtained from the tidal volume, breathing frequency, and dead space ventilation.
Similarly, the last part of each expiration remains in the conducting airways and is not expelled from the body. No gas exchange occurs in the conducting airways for anatomic reasons. The walls of the conducting airways are too thick for much diffusion; mixed venous blood does not come into contact with the air. The conducting airways are therefore referred to as the anatomic dead space.
The volume of anatomical dead space is 150 ml (milliliter). The anatomic dead space can be determined by using Fowler’s method. This method uses a nitrogen meter to analyze the expired nitrogen concentration after a single inspiration of 100 percent oxygen. The expired gas volume is measured simultaneously.
Tidal volume is the volume of air inhaled or exhaled in one breath. The normal tidal volume in a healthy individual is 500 ml. Since the anatomical dead space is 150 ml, a total of 350 ml goes to the lungs for gaseous exchange.
What Is the Formula for Alveolar Ventilation?
Alveolar ventilation = (tidal volume - dead space ) x respiratory rate = (500-150) x 12 = 4200 ml/minute (milliliter per minute).
Determined by respiratory rate and tidal volume,
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For a given respiratory rate: Increasing tidal volume increases alveolar ventilation rate.
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For a given tidal volume: Increasing respiratory rate also increases alveolar ventilation.
What Are the Effects of Breathing Patterns on Alveolar Ventilation?
The respiratory rate or breathing patterns can be defined as the frequency of breaths over time. Breathing patterns comprise tidal volume and respiratory rate in a person. The average breathing pattern is 12 breaths per minute and 500 ml volume per breath. Normal breathing at rest is known as eupnea.
Altered or abnormal breathing patterns typically indicate either too fast and rapid breathing, too slow and shallow breathing, or increased or decreased tidal volume. It is referred to as a change in the respiratory rate or the amount of air exchanged during breathing.
The autonomic nervous system mainly controls the respiratory rate. The respiratory centers of the medulla oblongata and pons control the overall respiratory rate based on various chemical stimuli secreted from the body. The hypothalamus also influences the respiratory rate during stress responses and emotional breakouts.
Various types of altered breathing rates are symptoms of many diseases.
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Rapid, shallow breathing is not good as tidal volume decreases, most of the air is lost in dead space, and little or no air goes to the alveoli. As a result, the respiratory rate is increased to 18. As a result, the alveolar ventilation becomes 2.7 liters per minute. The person may get unconscious within a few minutes. This will result in the rapid removal of carbon dioxide to restore the pH, a condition typically known as hyperventilation.
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Slow, deep breathing increases the tidal volume and decreases the respiratory rate to 10. Therefore, the alveolar ventilation is increased to 5.5 liters per minute. This will result in the accumulation of carbon dioxide in the blood, and the pH of the blood becomes alkaline, a condition typically known as hypoventilation.
What Is the Relation Between Alveolar Ventilation and Partial Pressure of Oxygen and Carbon Dioxide?
As the alveolar ventilation increases, the alveolar partial pressure of oxygen will also increase. However, doubling the alveolar ventilation doesn't double the partial pressure of oxygen in a person whose alveolar partial pressure is already 104 mm Hg because the highest partial pressure of oxygen one could achieve the inspired partial pressure of oxygen at sea levels of 149 mm Hg.
As the alveolar ventilation increases, the partial pressure of carbon dioxide decreases. For example, as the alveolar ventilation is doubled, the alveolar and arterial partial pressure of carbon dioxide is reduced to one-half. And, if alveolar ventilation is halved, then the alveolar and arterial partial pressure of carbon dioxide is doubled.
Conclusion
Alveolar ventilation is the amount of air entering the alveoli per minute. It is always less than pulmonary ventilation or respiratory minute ventilation. Rapid and shallow breathing can cause decreased alveolar ventilation, whereas deep and slow breathing can cause increased alveolar ventilation.
