Respiratory System: Anatomy and Physiology

Introduction

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. Understanding its anatomy and physiology is crucial for comprehending the mechanics of breathing and the process of gas exchange from the atmosphere to the tissues.

Anatomy of the Respiratory System

Lungs

• Lobes: The lungs are divided into five lobes—three on the right and two on the left.

• Bronchopulmonary Segments: Each lobe is further divided into segments called bronchopulmonary segments. Each segment is functionally and anatomically distinct, allowing for surgical removal without compromising the entire lung.

• Clinical Relevance: The segmental structure is important in procedures such as segmentectomy for localized lung disease.

Supporting Body Structures

• Chest Wall: Provides protection and structure for the lungs.

• Diaphragm: The primary muscle of respiration, separating the thoracic and abdominal cavities.

• Pleura: A double-layered serous membrane surrounding each lung.

Pleura

• Visceral Pleura: Covers the external surface of the lungs, including the fissures.

• Parietal Pleura:

  • Covers the thoracic wall

  • Covers the superior face of the diaphragm

  • Forms the lateral walls of the mediastinal enclosure

• Pleural Cavity: The space between the visceral and parietal pleura, containing a thin layer of serous fluid.

• Function: The fluid allows the pleurae to slide smoothly against each other during breathing movements, reducing friction.

Mechanics of Breathing

Overview

Breathing, or pulmonary ventilation, is the mechanical process of moving air into and out of the lungs. It involves coordinated actions of the respiratory muscles and changes in thoracic cavity volume and pressure.

• Inspiration (Inhalation): An active process involving contraction of the diaphragm and external intercostal muscles, increasing thoracic volume and decreasing intrapulmonary pressure, causing air to flow into the lungs.

• Expiration (Exhalation): Normally a passive process where the diaphragm and intercostal muscles relax, decreasing thoracic volume and increasing intrapulmonary pressure, pushing air out of the lungs.

• Forced Breathing: During increased respiratory demand, accessory muscles (e.g., sternocleidomastoid, scalenes, abdominal muscles) assist in inspiration and expiration.

Boyle's Law

Boyle's Law states that at a constant temperature, the pressure of a gas is inversely proportional to its volume:

This principle explains how changes in thoracic volume during breathing alter intrapulmonary pressure, driving airflow.

Gas Exchange

Overview

Gas exchange occurs at two main sites: the alveoli of the lungs (external respiration) and the tissues (internal respiration). It relies on the principles of diffusion and partial pressures of gases.

• Diffusion: Movement of gases from areas of higher concentration (or partial pressure) to lower concentration.

• External Respiration: Exchange of oxygen and carbon dioxide between alveolar air and pulmonary capillary blood.

• Internal Respiration: Exchange of gases between systemic capillaries and tissues.

Dalton's Law of Partial Pressures

Each gas in a mixture exerts its own pressure, called partial pressure. The total pressure is the sum of all partial pressures:

Partial pressures drive the diffusion of gases across respiratory membranes.

Henry's Law

The amount of gas that dissolves in a liquid is proportional to its partial pressure and solubility coefficient:

Where is the concentration of dissolved gas, is the solubility constant, and is the partial pressure.

Summary Table: Lung Lobes and Segments

Additional info: The number of bronchopulmonary segments in the left lung can vary due to fusion of some segments.

Key Terms

• Bronchopulmonary Segment: A distinct region of the lung supplied by its own bronchus and artery.

• Pleura: Double-layered membrane surrounding the lungs.

• Intrapulmonary Pressure: Pressure within the alveoli.

• Intrapleural Pressure: Pressure within the pleural cavity, usually negative relative to atmospheric pressure.

Conclusion

The structure of the respiratory system, including the lungs, pleura, and supporting muscles, is intricately designed to facilitate efficient gas exchange. Understanding the mechanics of breathing and the laws governing gas movement is fundamental for students of anatomy and physiology.