Physiology of the Lungs and Respiration

$F = P_{alv} - P_{atm}$

Inspiration occurs when the contraction of the diaphragm increases the volume of the thoracic cavity, creating negative pressure relative to the atmosphere, which draws air in.

Airway Resistance and Compliance

The ease with which air flows through the respiratory tract is influenced by airway resistance ($R$). According to the Law of Poiseuille, resistance is highly sensitive to the radius ($r$) of the airway. Because resistance is inversely proportional to the fourth power of the radius ($R \propto \frac{1}{r^4}$), even a small decrease in airway diameter (such as during an asthma attack) leads to a massive increase in the effort required to breathe.

Furthermore, lung compliance describes the "stretchability" of the lungs. It is defined as the change in lung volume ($\Delta V$) per unit change in transpulmonary pressure ($\Delta P$):

$\text{Compliance} = \frac{\Delta V}{\Delta P}$

High compliance means the lungs expand easily, whereas low compliance (stiffness) makes inhalation difficult.

Lung Volumes and Spirometry

To assess respiratory health, clinicians use spirometry to measure various lung volumes and capacities. These measurements help differentiate between different types of lung diseases.

• Tidal Volume (TV): The volume of air moved into or out of the lungs during a single normal, quiet breath.
• Inspiratory Reserve Volume (IRV): The additional volume of air that can be inhaled forcefully after a normal inspiration.
• Expiratory Reserve Volume (ERV): The additional volume of air that can be exhaled forcefully after a normal expiration.
• Residual Volume (RV): The air that remains in the lungs even after the most forceful exhalation; this volume prevents alveolar collapse.
• Vital Capacity (VC): The total amount of exchangeable air, calculated as $VC = IRV + TV + ERV$.
• Total Lung Capacity (TLC): The sum of all lung volumes ($VC + RV$).

It is also important to distinguish between total ventilation and alveolar ventilation ($V_A$). Not all inhaled air reaches the gas-exchange surfaces; some remains in the anatomical dead space (the conducting airways like the trachea and bronchi where no exchange occurs, approximately $150\text{ ml}$). The actual amount of air reaching the alveoli is:

$V_A = \text{respiratory rate} \times (TV - \text{Dead Space})$

Gas Exchange and Diffusion

Gas exchange is driven by the principles of partial pressures and the Ideal Gas Law. Oxygen moves from the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli, following their respective partial pressure gradients. The efficiency of this process is measured as the diffusion capacity, which depends on the surface area of the alveolar-capillary membrane and the thickness of that membrane.

The respiratory quotient (RQ) is a ratio used to describe the metabolic efficiency of gas exchange, representing the ratio of $CO_2$ produced to $O_2$ consumed by the body.

Clinical Pathophysiology

Lung diseases are broadly categorized into two patterns based on how they affect airflow and lung volumes:

1. Obstructive Lung Diseases

These conditions are characterized by an increase in airway resistance, making it difficult to exhale air from the lungs. Common examples include:

• Asthma: Characterized by reversible airway inflammation and bronchoconstriction.
• COPD (Chronic Obstructive Pulmonary Disease): Includes Emphysema (destruction of alveolar walls) and Chronic Bronchitis (inflammation of the airways).
• Cystic Fibrosis: A genetic disorder causing thick, obstructive mucus in the airways.

In obstructive disease, the Tiffeneau Index—the ratio of the Forced Expiratory Volume in one second ($FEV_1$) to the Forced Vital Capacity ($FVC$)—is typically decreased ($\frac{FEV_1}{FVC} < 0.7$) because the patient cannot exhale air quickly.

2. Restrictive Lung Diseases

These conditions are characterized by reduced lung compliance (stiff lungs) or reduced lung tissue, making it difficult to fully expand the lungs.

• Pulmonary Fibrosis: Scarring of the lung tissue increases stiffness and decreases compliance.
• Chest Wall Deformities: Physical restrictions that prevent the lungs from expanding.

In restrictive disease, lung volumes (like $VC$ and $TLC$) are reduced, but because the airways themselves are not obstructed, the Tiffeneau Index often remains normal or even appears high.

Exam Tip: When analyzing spirometry results, always look at the $FEV_1/FVC$ ratio first. A low ratio points toward an obstructive problem (difficulty getting air out), while a normal ratio with low total volumes points toward a restrictive problem (difficulty getting air in).