Stages of respiration and properties of alveolar air
The physiological process of respiration consists of five consecutive stages. It all starts with external respiration (pulmonary ventilation) and gas exchange between the alveoli and pulmonary capillaries. This is followed by the transport of gases by the blood, tissue exchange, and finally, internal (tissue) respiration—oxidative processes in the mitochondria.
The foundation of pulmonary gas exchange is purely physical laws, specifically the difference in partial pressures of gases. At the same time, the volume and gas composition of alveolar air normally remain strictly constant, differing from atmospheric air. The main regulator of this constancy is the $CO_2$ concentration.
The exhaled portion of air always contains less carbon dioxide than the alveoli themselves. This is because, upon expiration, the saturated alveolar gas is mixed with air from the dead space, which is practically devoid of $CO_2$.
Primary respiratory volumes
To assess ventilation, the method of spirometry is most commonly used. There are four basic (primary) volumes:
- Tidal volume (TV): the portion entering during a single quiet breath (normally 500 mL).
- Inspiratory reserve volume (IRV): the maximum amount that can be forcibly inhaled on top of a normal breath (1500 mL).
- Expiratory reserve volume (ERV): the air that can be forcefully exhaled after a standard expiration (1500 mL).
- Residual volume (RV): the portion that inevitably remains in the lungs and airways even after the deepest expiration (about 1000 mL).
Please note: residual volume cannot be measured with a conventional spirometer. The dilution method is used for this purpose. The subject breathes a gas mixture of a known volume (3 L) containing 1 L of inert argon for 5–10 minutes. The target parameter is calculated after exhaling the mixture based on the degree of argon dilution in it.
Pulmonary capacities and ventilation parameters
Capacities represent clinically significant sums of several primary volumes:
- Vital capacity (VC): the maximum expiration volume after a maximal inspiration (sum of TV, IRV, and ERV).
- Functional residual capacity (FRC): the amount of air in the lungs after a quiet expiration (sum of ERV and RV). This reserve is constantly present in the alveoli.
- Total lung capacity (TLC): the total capacity at the peak of inspiration (VC + RV).
- Inspiratory capacity: the sum of TV and IRV.
In addition to static parameters, dynamic ones are evaluated. The volume passing through the system per minute is called the minute ventilation (MV). If a person breathes at maximum frequency and depth, the maximal voluntary ventilation (MVV) is recorded, reaching 150–200 L/min.
Airway biomechanics and dead space
The airways begin with the trachea (diameter 15–30 mm), which divides into two main bronchi at the level of the 5th thoracic vertebra. Each branching of the bronchial tree forms a new generation.
The first 16 generations form the conducting zone. Transport here occurs by convection, and gas exchange does not take place. This zone (oral cavity, trachea, large bronchi) forms the anatomical dead space with a volume of about 150–180 ml (~170 ml).
This is followed by the transitional zone (3 generations after terminal bronchioles) and the respiratory zone (the last 4 generations: 20–23). Starting from the 20th generation, the main transport mechanism becomes diffusion.
Physiologists distinguish two types of dead space:
- Anatomical: only the airways without gas exchange.
- Physiological (functional): anatomical space plus the volume of ventilated but non-perfused alveoli.
Real alveolar ventilation is the portion of fresh air that reaches the alveoli with each breath. During quiet breathing, it is: $500 \text{ mL} - 170 \text{ mL (dead space)} = 340 \text{ mL}$.
Briefly: answers to topical questions
What stages does internal (tissue) respiration consist of?
Internal respiration is a complex of processes ensuring gas exchange between the blood and tissues, as well as cellular oxidation.
It consists of the following stages:
- Pulmonary diffusion — exchange of gases between the alveoli and blood.
- Gas transport — transfer of gases by blood to organs and tissues.
- Tissue diffusion — exchange of gases between systemic capillary blood and tissue cells.
- Cellular respiration — oxidative processes occurring directly in the mitochondria of cells.
What methods, other than the argon dilution method, are used to measure residual lung volume?
To assess residual lung volume, body plethysmography is used in addition to the argon dilution method.
Standard spirometry is not used to determine this parameter because residual air remains in the lungs and airways and cannot be exhaled.
To easily remember the composition of VC, use the rule of "three voluntary actions": we can consciously take a normal breath (TV), inhale even deeper (Inspiratory reserve volume), and forcibly exhale (Expiratory reserve volume). Everything that is not subject to will is the Residual volume.
Class and exam questions
Why cannot residual volume be determined by spirometry?
A spirometer measures only the air that passes through the device (is inhaled or exhaled). Residual volume is by definition impossible to exhale, which is why an inert gas dilution method (using argon) is employed to calculate it.
How does anatomical dead space differ from physiological dead space?
Anatomical dead space includes only airways structurally unsuited for gas exchange (about 170 mL). Physiological space is broader: it additionally includes those alveoli that receive air but are temporarily unwashed by blood (lack perfusion).
Why is the carbon dioxide concentration in exhaled air lower than in alveolar air?
During expiration, carbon dioxide-saturated air from the alveoli mixes with a portion of air from the anatomical dead space. Since there is practically no $CO_2$ in the airways, the final concentration in the exhaled mixture decreases.
What else to study on this topic
- Mechanisms of gas diffusion across the blood-air barrier
- Transport of oxygen and carbon dioxide by blood (the role of hemoglobin)
- Gas exchange in tissues and internal (tissue) respiration
- Regulation of respiration and the role of blood gas partial pressures
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