Pulmonary Volumes and Capacities

Stages of Respiration and Properties of Alveolar Air

The physiological process of respiration consists of five consecutive stages. It begins 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, exchange in the tissues, and finally, internal (tissue) respiration—oxidative processes in the mitochondria.

The foundation of pulmonary gas exchange relies entirely on physical laws, specifically the difference in partial pressures of gases. Meanwhile, 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 alveolar air itself. This is because, during expiration, the saturated alveolar gas is mixed with air from the dead space, which is virtually devoid of $CO_2$.

Primary Respiratory Volumes

To assess ventilation, spirometry is most commonly used. There are four basic (primary) volumes:

1. Tidal Volume (TV): The volume inspired or expired during a single quiet breath (normally 500 mL).
2. Inspiratory Reserve Volume (IRV): The maximum volume that can be forcibly inhaled above a normal tidal inspiration (1500 mL).
3. Expiratory Reserve Volume (ERV): The maximum volume that can be forcibly exhaled after a normal tidal expiration (1500 mL).
4. Residual Volume (RV): The volume of air remaining in the lungs and airways even after a maximal forced expiration (about 1000 mL).

Note: Residual volume cannot be measured with a standard spirometer. Instead, the helium dilution method or nitrogen washout (gas dilution method) is applied. For instance, the subject breathes a gas mixture of a known volume containing an inert gas like argon; the target parameter is calculated based on the degree of gas dilution after exhalation.

Pulmonary Capacities and Ventilation Parameters

Capacities represent clinically significant combinations of two or more primary volumes:

• Vital Capacity (VC): The maximum volume of air exhaled after a maximal inspiration (sum of TV, IRV, and ERV).
• Functional Residual Capacity (FRC): The volume of air remaining in the lungs at the end of a normal tidal expiration (sum of ERV and RV). This reserve constantly resides in the alveoli.
• Total Lung Capacity (TLC): The maximum volume of air the lungs can contain at the peak of a maximal inspiration (VC + RV).
• Inspiratory Capacity (IC): The maximum volume of air that can be inspired following a normal tidal expiration (sum of TV and IRV).

In addition to static parameters, dynamic parameters are evaluated. The volume passing through the system per minute is called the minute ventilation (MV). When a person breathes with maximum frequency and depth, the maximal voluntary ventilation (MVV) is registered, reaching 150–200 L/min.

Biromechanics of Airways and Dead Space

The respiratory tract begins with the trachea (diameter 15–30 mm), which bifurcates into the 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 via 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 past terminal bronchioles) and the respiratory zone (the last 4 generations: 20–23). Starting from the 20th generation, diffusion becomes the primary transport mechanism.

Physiologists distinguish two types of dead space:

1. Anatomical dead space: The conducting airways alone, devoid of gas exchange.
2. Physiological (functional) dead space: Anatomical dead space plus the volume of ventilated alveoli that are not perfused with blood.

Real alveolar ventilation is the volume of fresh air that reaches the alveoli with each breath. During quiet breathing, it is calculated as: $500 \text{ mL} - 170 \text{ mL (dead space)} = 340 \text{ mL}$.