The Respiratory System: Structure, Function, and Clinical Conditions

Respiratory System Overview

Introduction

The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the body and the environment. It consists of a series of organs and structures that facilitate breathing, protect against pathogens, and contribute to vocalization.

Structural Plan of the Respiratory System

General Organization

• The respiratory system resembles an inverted tree, with the trachea as the trunk and the alveoli as the leaves.

• Gas exchange occurs by diffusion across the thin walls of the alveoli and capillaries.

Respiratory Tracts

Upper and Lower Respiratory Tracts

• Upper respiratory tract: Nose, pharynx, larynx

• Lower respiratory tract: Trachea, bronchial tree, lungs

Head cold affects the upper tract (nose, sinuses, throat), while a chest cold affects the lower tract (trachea, bronchi, alveoli).

Respiratory Mucosa

Structure and Function

• The respiratory mucosa is a mucous membrane lining the airways, producing over 125 mL of mucus daily.

• Mucus traps dust and pathogens, while cilia move the mucus toward the pharynx for removal.

• Respiratory membrane: The thin barrier between alveolar air and capillary blood, essential for gas exchange.

Nose and Paranasal Sinuses

Structure and Functions

• The nasal septum divides the nose into two cavities, lined by mucous membrane.

• Paranasal sinuses (frontal, maxillary, sphenoidal, ethmoidal) drain into the nose and contribute to voice resonance.

• Olfactory receptors for smell are located in the nasal mucosa.

• The nose warms, moistens, and filters inhaled air.

Pharynx (Throat)

Structure and Functions

• The pharynx is about 12.5 cm long and divided into nasopharynx, oropharynx, and laryngopharynx.

• It serves as a passageway for both air (respiratory system) and food (digestive system).

• Tonsils (lymphoid tissue) provide immune protection but can cause breathing difficulty if swollen.

• The eustachian tube connects the pharynx to the middle ear, equalizing pressure.

Tonsillitis

Clinical Condition

• Tonsillitis is inflammation of the tonsils, often causing sore throat and difficulty swallowing.

• Swollen tonsils can obstruct the airway, leading to dyspnea.

Larynx (Voice Box)

Structure and Functions

• The larynx is located below the pharynx and is composed of several cartilages, including the thyroid cartilage (Adam’s apple).

• The epiglottis covers the glottis during swallowing to prevent aspiration.

• Vocal cords produce sound as air passes over them.

Trachea (Windpipe)

Structure and Function

• The trachea is a tube about 11 cm long, supported by C-shaped cartilage rings to keep it open.

• It is lined with ciliated mucosa that moves debris upward toward the pharynx.

Bronchi, Bronchioles, and Alveoli

Structure and Functions

• The trachea divides into right and left primary bronchi, which branch into smaller bronchi and bronchioles.

• Bronchioles end in clusters of alveolar sacs composed of alveoli, where gas exchange occurs.

• Surfactant produced by type II alveolar cells reduces surface tension, preventing alveolar collapse.

Summary Table: Major Structures and Functions of the Respiratory System

Structure	Main Function(s)
Nose	Warms, moistens, and filters air; sense of smell
Pharynx	Passageway for air and food; immune defense (tonsils)
Larynx	Voice production; air passage; prevents aspiration
Trachea	Air passage; removal of debris by cilia
Bronchi/Bronchioles	Air distribution to alveoli
Alveoli	Gas exchange between air and blood

Clinical Conditions of the Respiratory System

Upper Respiratory Infections (URIs)

• Rhinitis: Inflammation of the nasal mucosa (common cold, allergies)

• Pharyngitis: Sore throat due to infection or irritation

• Laryngitis: Inflammation of the larynx, often causing hoarseness

Lower Respiratory Conditions

• Acute bronchitis: Inflammation of the bronchi

• Pneumonia: Infection causing fluid in alveoli

• Tuberculosis: Chronic infection by Mycobacterium tuberculosis

• COPD, asthma, emphysema: Chronic or obstructive disorders affecting airflow

Mechanics of Breathing

Pulmonary Ventilation

• Inspiration: Active process; diaphragm and external intercostals contract, increasing thoracic volume and decreasing pressure, drawing air in.

• Expiration: Usually passive; relaxation of inspiratory muscles and elastic recoil decrease thoracic volume, increasing pressure and expelling air.

Vital capacity (VC) is the maximum amount of air that can be exhaled after a maximal inhalation:

$VC = TV + IRV + ERV$

Where:

• TV = Tidal Volume

• IRV = Inspiratory Reserve Volume

• ERV = Expiratory Reserve Volume

Regulation of Respiration

Control Centers and Receptors

• Medullary rhythmicity area in the brainstem sets the basic rhythm of breathing.

• Pontine centers adjust the rhythm as needed.

• Chemoreceptors in the carotid and aortic bodies respond to changes in CO2, O2, and pH.

• Pulmonary stretch receptors prevent overinflation of the lungs.

Gas Exchange and Transport

Mechanisms of Gas Exchange

• Oxygen diffuses from alveoli into capillary blood, binding to hemoglobin to form oxyhemoglobin.

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

• CO2 is transported in blood as dissolved gas (10%), carbaminohemoglobin (20%), and bicarbonate ions (70%).

$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$

This equation shows the conversion of carbon dioxide to bicarbonate in the blood, catalyzed by carbonic anhydrase.

Summary

• The respiratory system is essential for gas exchange, vocalization, and protection against pathogens.

• Its structure is adapted to maximize surface area for diffusion and to filter, warm, and humidify air.

• Disorders can affect any part of the system, with symptoms and severity depending on the location and cause.