The Respiratory System: Structure and Function

The Respiratory System

Introduction

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It consists of a series of organs and structures that work together to ensure efficient ventilation and respiration.

Learning Outcomes

• Identify the main structures and functions of the respiratory system.

• Describe the mechanism of breathing (respiratory) with reference to pressure and volume changes.

• Outline the implications to health and disease.

Major Functions of the Respiratory System

• Gas Exchange: Facilitates the exchange of oxygen and carbon dioxide between the air and blood.

• Protection: Protects respiratory surfaces from dehydration, temperature changes, and pathogens.

• Sound Production: Produces sounds for speaking and singing via the larynx.

• Olfaction: Houses olfactory receptors for the sense of smell.

• Movement of Air: Moves air to and from the exchange surfaces of the lungs.

Major Organs and Structures of the Respiratory System

The respiratory system is divided into the upper and lower respiratory tracts, each with specialized structures.

Structure	Function
Nasal Cavity	Warms, moistens, and filters air as it is inhaled; contains olfactory receptors for smell.
Pharynx	Passageway for air and food; leads to the trachea and esophagus.
Larynx (Voice Box)	The voice box, where vocal cords are located; produces sound and protects the airway during swallowing.
Trachea (Windpipe)	Keeps the windpipe "open" with C-shaped hyaline cartilage; lined with cilia to move debris out of the airway.
Bronchi	Two main branches at the end of the trachea, each leading to a lung.
Bronchioles	Smaller branches of the bronchi that lead into lung tissue and ultimately to the alveoli.
Alveoli	The functional respiratory units where gas exchange occurs between air and blood.
Lungs	Large organs that occupy the thoracic cavity; contain the bronchi, bronchioles, and alveoli.
Diaphragm	Major muscle of respiration; contracts and flattens to allow inhalation.

Example:

When you inhale, air enters through the nasal cavity, passes through the pharynx and larynx, travels down the trachea, and moves into the bronchi, bronchioles, and finally the alveoli, where gas exchange occurs.

Respiratory Membrane and Gas Exchange

The respiratory membrane is a thin barrier between the alveolar air and the blood in pulmonary capillaries. Gas exchange occurs by simple diffusion across this membrane.

• Oxygen diffuses from the alveoli into the capillary blood.

• Carbon dioxide diffuses from the blood into the alveoli to be exhaled.

Key Features of the Respiratory Membrane

• Composed of a single layer of squamous epithelial cells (alveolar cells) and capillary endothelium.

• Extremely thin to facilitate rapid gas exchange.

Mechanism of Breathing (Ventilation)

Breathing involves the movement of air into and out of the lungs, driven by pressure and volume changes in the thoracic cavity.

Boyle's Law

• States that pressure and volume are inversely proportional in a closed system.

• Mathematically: $P_1V_1 = P_2V_2$

Inspiration (Active Process)

• Diaphragm contracts and flattens, increasing thoracic volume.

• External intercostal muscles contract, raising the rib cage.

• Intrapulmonary pressure drops below atmospheric pressure, causing air to flow into the lungs.

Expiration (Passive Process)

• Diaphragm and intercostal muscles relax, decreasing thoracic volume.

• Intrapulmonary pressure rises above atmospheric pressure, pushing air out of the lungs.

Key Pressures in Breathing

• Atmospheric Pressure: The pressure of air outside the body (typically 760 mmHg at sea level).

• Intrapulmonary Pressure: The pressure within the alveoli.

• Intrapleural Pressure: The pressure within the pleural cavity, usually slightly less than atmospheric pressure to keep the lungs inflated.

Common Respiratory Disorders

• Sinusitis: Inflammation of the sinuses, often due to infection or allergy, causing blockage and pressure.

• Tonsillitis: Inflammation and swelling of the tonsils, usually due to infection.

• Laryngitis: Inflammation of the larynx, often resulting in hoarseness or loss of voice.

Summary Table: Major Respiratory Structures and Functions

Structure	Function
Nasal Cavity	Warms, moistens, and filters incoming air; houses olfactory receptors.
Pharynx	Passageway for air and food; aids in sound resonance.
Larynx	Produces sound; protects lower airway during swallowing.
Trachea	Conducts air to bronchi; lined with cilia to remove debris.
Bronchi/Bronchioles	Distribute air to each lung and further into lung tissue.
Alveoli	Site of gas exchange between air and blood.
Lungs	House the bronchi, bronchioles, and alveoli; main organs of respiration.
Diaphragm	Primary muscle of inspiration.

Additional info:

• The respiratory system is divided into the conducting zone (nasal cavity to terminal bronchioles) and the respiratory zone (respiratory bronchioles, alveolar ducts, and alveoli).

• Accessory muscles (e.g., pectoralis minor, sternocleidomastoid) assist in forced inspiration.

• Respiratory mucosa and cilia play a crucial role in trapping and removing pathogens and debris from the airway.