Respiratory System: Structure, Function, and Ventilation Mechanisms

Respiratory System

Overview

The respiratory system is responsible for the exchange of gases between the body and the environment, supporting cellular respiration and maintaining homeostasis. It consists of anatomical structures such as the nasal cavity, pharynx, larynx, trachea, bronchi, and lungs.

Functions of the Respiratory System

• O2 and CO2 Exchange: Facilitates the transfer of oxygen into the blood and removal of carbon dioxide from the blood.

• Speech: Air movement through the vocal cords enables phonation.

• Smell: Airborne molecules stimulate olfactory receptors in the nasal cavity.

• Acid/Base Regulation: Regulates blood pH by controlling CO2 exhalation.

Ventilation

Definition and Mechanism

Ventilation is the process of moving air into and out of the lungs. Air flows from regions of high pressure to regions of low pressure.

• During inspiration, thoracic volume increases, pressure drops, and air enters the lungs.

• During expiration, thoracic volume decreases, pressure rises, and air exits the lungs.

Boyle's Law

Relationship Between Pressure and Volume

Boyle's Law states that the pressure of a gas varies inversely with its volume:

$P \propto \frac{1}{V}$

• Gas in a large vessel: low pressure

• Gas in a small vessel: high pressure

Thoracic Volumes

Changes During Breathing

• Inspiration: Thorax enlarges, pressure drops below atmospheric, air moves in.

• Expiration: Thorax gets smaller, pressure rises above atmospheric, air moves out.

Muscles of Ventilation

Main Muscles of Inspiration

• Diaphragm: Dome-shaped skeletal muscle; contraction moves it down, increasing thoracic volume.

• External Intercostals: Contraction elevates ribs and sternum, expanding the rib cage.

Accessory Muscles of Inspiration

• Sternocleidomastoids

• Scalenes

• Pectoralis Minor

• These muscles further raise the rib cage during forced inspiration.

Muscles of Expiration

• Normal Expiration: Passive process; inspiratory muscles relax.

• Active Expiration:

  • Internal Intercostals

  • External Oblique (lowers rib cage)

  • Internal Oblique

  • Transversus Abdominis

  • Rectus Abdominis (compresses abdomen)

Respiration Process

Inspiration

• Diaphragm contracts (moves down)

• External intercostals contract (rib cage moves up and out)

• Thorax expands (air rushes in)

Expiration

• Diaphragm relaxes (moves up)

• External intercostals relax (rib cage falls back)

• Thorax contracts (air forced out)

Active Expiration

• Internal intercostals and abdominal muscles contract to force air out during vigorous breathing.

Air Pressures in Respiration

Types of Pressures

• Intrapleural Pressure: Pressure within the pleural cavity.

• Air (Atmospheric) Pressure: Pressure of air outside the body, typically 760 mmHg.

• Intrapulmonary Pressure: Pressure within the alveoli of the lungs.

Intrapleural Pressure

Location and Function

• The intrapleural space is between the parietal and visceral pleura.

• Pressure is always below atmospheric or intrapulmonary pressures, keeping lungs slightly inflated.

• Lungs are "sucked out" due to negative pressure.

Pneumothorax

Pathophysiology

• Presence of air in the pleural cavity due to thoracic wall puncture.

• Loss of negative intrapleural pressure causes lungs to recoil and collapse.

Pressure and Ventilation

Pressure Gradients

• Air moves from high pressure to low pressure.

• Atmospheric pressure: 760 mmHg

• If lung pressure < 760 mmHg, air moves in.

• If lung pressure > 760 mmHg, air moves out.

Pressure at Rest

• Air: 760 mmHg

• Intrapleural: 756 mmHg

• Intrapulmonary: 760 mmHg

• Air doesn’t move when pressures are equal.

Pressure Changes During Breathing

During Inspiration

• Thorax expands, pressure drops.

• Pleura pulled out, intrapleural pressure drops.

• Intrapulmonary pressure drops below atmospheric, air moves in.

• Typical values:

  • Air: 760 mmHg

  • Intrapleural: 754 mmHg

  • Intrapulmonary: 757 mmHg

During Expiration

• Thorax contracts, pressure rises.

• Intrapleural pressure rises.

• Elastic tissues recoil, intrapulmonary pressure rises above atmospheric, air moves out.

• Typical values:

  • Air: 760 mmHg

  • Intrapleural: 756 mmHg

  • Intrapulmonary: 763 mmHg

Compliance

Expandability of Lungs

• Compliance refers to the ability of the lungs to expand.

• Factors affecting compliance:

  • Elastic connective tissue in lungs

  • Mobility of thorax

  • Surfactant

Surfactant

Role in Lung Function

• Mixture of phospholipids and proteins produced by type II alveolar cells.

• Splits bonds between water molecules, reducing surface tension inside alveoli.

• Allows alveoli to expand and prevents collapse.

Surfactant Dynamics

• When lungs are uninflated:

  • Surfactant molecules are concentrated.

  • Reduces surface tension, lungs are easy to inflate.

• When lungs are inflated:

  • Surfactant spreads out, less effect on surface tension, tension rises.

  • Lungs recoil.

Summary Table: Key Pressures in Ventilation

Phase	Air Pressure (mmHg)	Intrapleural Pressure (mmHg)	Intrapulmonary Pressure (mmHg)	Air Movement
At Rest	760	756	760	None
Inspiration	760	754	757	In
Expiration	760	756	763	Out

Additional info: Surfactant is essential for reducing the work of breathing and preventing atelectasis (collapse of alveoli). Pneumothorax is a clinical emergency requiring immediate intervention to restore negative intrapleural pressure.