The Respiratory System

Overview

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. Understanding its mechanics and pressure relationships is fundamental in Anatomy & Physiology.

Objectives

• Describe the mechanics of breathing

• Explain and compare respiratory volumes and capacities

• Describe various pulmonary function tests

• Understand different pressures involved in breathing

• Define atelectasis

• State and relate Boyle's law, Dalton’s law, and Henry’s law to respiration

Mechanics of Breathing

Pulmonary Ventilation

Pulmonary ventilation consists of two phases that enable the movement of air in and out of the lungs:

• Inspiration: Gases flow into the lungs.

• Expiration: Gases exit the lungs.

Pressure Relationships in the Thoracic Cavity

Atmospheric Pressure (Patm)

Atmospheric pressure is the pressure exerted by the air surrounding the body. At sea level, it is typically 760 mm Hg, which equals 1 atmosphere.

• Negative respiratory pressure: Less than Patm

• Positive respiratory pressure: Greater than Patm

• Zero respiratory pressure: Equal to Patm

Intrapulmonary Pressure (Ppul)

Intrapulmonary (intra-alveolar) pressure is the pressure within the alveoli. It fluctuates with breathing and always eventually equalizes with atmospheric pressure.

Intrapleural Pressure (Pip)

Intrapleural pressure is the pressure within the pleural cavity. It also fluctuates with breathing but is always a negative pressure (less than both Patm and Ppul).

• Fluid level in the pleural cavity must be minimal, maintained by lymphatic drainage.

• If fluid accumulates, positive Pip pressure may develop, leading to lung collapse.

Forces Affecting Intrapleural Pressure

• Two inward forces promote lung collapse:

  • Elastic recoil of lungs decreases lung size.

  • Surface tension of alveolar fluid reduces alveolar size.

• One outward force tends to enlarge lungs:

  • Elasticity of chest wall pulls thorax outward.

Transpulmonary Pressure

Transpulmonary pressure is the difference between intrapulmonary and intrapleural pressures:

• Keeps airways open; greater transpulmonary pressure results in larger lungs.

Pressure Relationships Table

Homeostatic Imbalance: Atelectasis

Atelectasis (Lung Collapse)

Atelectasis refers to the collapse of lung tissue, which can result from:

• Plugged bronchioles leading to collapse of alveoli

• Pneumothorax (air in pleural cavity) due to wound or rupture of pleura

• Treatment involves removing air with chest tubes; healing of pleura allows lung reinflation

Physical Factors Influencing Pulmonary Ventilation

Key Physical Factors

• Airway resistance

• Alveolar surface tension

• Lung compliance

Airway Resistance

Friction in the airways is the major nonelastic source of resistance to gas flow. The relationship is:

• Where is flow, is the pressure gradient, and is resistance.

• Resistance is usually insignificant due to large airway diameters and extensive branching.

• Significant resistance can occur in conditions like asthma, making breathing strenuous.

• Epinephrine dilates bronchioles, reducing resistance.

Alveolar Surface Tension

Surface tension is the attraction of liquid molecules at a gas-liquid interface, which tends to reduce alveolar size and resist expansion.

• Surfactant is a detergent-like substance produced by type II alveolar cells that reduces surface tension and prevents alveolar collapse.

• Insufficient surfactant in premature infants leads to infant respiratory distress syndrome.

Lung Compliance

Lung compliance is a measure of the lung's ability to stretch and expand. It is defined as:

• High compliance means easier expansion; low compliance requires more energy for breathing.

• Compliance is diminished by fibrosis, reduced surfactant, or decreased flexibility of the thoracic cage.

Total Respiratory Compliance

Total compliance includes both lung and thoracic wall compliance. It can be decreased by:

• Thoracic deformities

• Ossification of costal cartilage

• Paralysis of intercostal muscles

Pulmonary Volumes and Capacities

Pulmonary Function Tests

Pulmonary function tests assess respiratory volumes and capacities using a spirometer. They help distinguish between:

• Obstructive pulmonary disease: Increased airway resistance (e.g., bronchitis). TLC, FRC, RV may increase.

• Restrictive disorders: Reduced TLC due to disease or fibrosis. VC, TLC, FRC, RV decline.

Key Pulmonary Function Measurements

• Forced Vital Capacity (FVC): Gas forcibly expelled after a deep breath.

• Forced Expiratory Volume (FEV): Amount of gas expelled during specific intervals of FVC. FEV1 is the amount expelled in the first second; healthy individuals expel about 80% of FVC in the first second.

Relevant Gas Laws in Respiration

Boyle's Law

Boyle's Law describes the inverse relationship between pressure and volume of a gas:

• As volume increases, pressure decreases, and vice versa.

• Essential for understanding how changes in thoracic volume drive air movement during breathing.

Additional info:

• Dalton’s Law: The total pressure of a mixture of gases is the sum of the pressures of each individual gas.

• Henry’s Law: The amount of gas that dissolves in a liquid is proportional to its partial pressure and solubility.