The Respiratory System: Structure, Function, and Clinical Considerations

The Respiratory System: Overview and Structural Plan

Introduction to the Respiratory System

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. Its structure resembles an inverted tree, with the alveoli acting as the 'leaves' where gas exchange occurs via diffusion.

• Primary Functions: Air distribution, gas exchange, and air purification.

• Gas Exchange: Occurs by diffusion across the alveolar and capillary walls.

Structural Organization of the Respiratory System

Respiratory Tracts

The respiratory system is divided into upper and lower tracts, each with distinct anatomical components and functions.

• Upper Respiratory Tract: Nose, pharynx, larynx

• Lower Respiratory Tract: Trachea, bronchial tree, lungs

Respiratory Mucosa

Structure and Function

The respiratory mucosa lines the airways, producing over 125 mL of mucus daily to trap dust and pathogens. Cilia on mucosal cells move mucus toward the pharynx for removal, serving as a critical air purification mechanism.

• Epithelium: Pseudostratified ciliated columnar epithelium with goblet cells

• Function: Traps and removes particulates and microbes from inhaled air

Anatomy of the Upper Respiratory Tract

Nose and Paranasal Sinuses

The nose is divided by the nasal septum and lined with mucosa. Paranasal sinuses (frontal, maxillary, sphenoidal, ethmoidal) drain into the nasal cavity, aiding in air conditioning and resonance of speech.

• Functions: Warms, moistens, and filters air; houses olfactory receptors

• Clinical Note: Nasal polyps are noncancerous growths associated with chronic inflammation.

Pharynx (Throat)

The pharynx is a muscular tube divided into nasopharynx, oropharynx, and laryngopharynx. It serves as a passageway for both air and food and contains lymphoid tissue (tonsils) for immune defense.

• Divisions: Nasopharynx, oropharynx, laryngopharynx

• Functions: Air and food passage, immune protection via tonsils

Larynx (Voice Box)

The larynx is located below the pharynx and is composed of several cartilages, including the thyroid cartilage (Adam's apple) and the epiglottis. It houses the vocal cords, which are essential for sound production.

• Functions: Air passage, voice production, protection of lower airways

• Clinical Note: Laryngeal cancer is more common in older men and those with alcohol abuse history.

Lower Respiratory Tract

Trachea (Windpipe)

The trachea is a tube supported by C-shaped cartilage rings, preventing collapse and ensuring an open airway. It extends from the larynx into the thoracic cavity, where it divides into the right and left primary bronchi.

• Function: Conducts air to and from the lungs

• Clinical Note: Obstruction can be fatal; abdominal thrusts (Heimlich maneuver) or tracheostomy may be required.

Bronchi, Bronchioles, and Alveoli

The trachea branches into the right and left primary bronchi, which further divide into smaller bronchi and bronchioles, ending in clusters of alveoli. The alveoli are the primary sites for gas exchange.

• Bronchi/Bronchioles: Air distribution

• Alveoli: Gas exchange; Type II cells produce surfactant to reduce surface tension

Lungs and Pleura

Structure of the Lungs

The lungs are divided into lobes (three on the right, two on the left) and are housed in the thoracic cavity. The apex is the narrow upper part, and the base rests on the diaphragm.

• Pleura: Double-layered membrane (parietal and visceral) that reduces friction during breathing

• Clinical Note: Pleurisy is inflammation of the pleura; pneumothorax and hemothorax involve air or blood in the pleural space, respectively.

Respiratory Physiology

Mechanics of Breathing (Pulmonary Ventilation)

Breathing consists of inspiration (active process) and expiration (usually passive). Changes in thoracic volume create pressure gradients that drive airflow.

• Inspiration: Diaphragm and external intercostals contract, increasing thoracic volume and decreasing pressure

• Expiration: Thoracic volume decreases, pressure increases, air is expelled

Volumes of Air Exchanged

Different volumes of air are exchanged during breathing, measurable by spirometry.

• Tidal Volume (TV): Normal breath in or out

• Vital Capacity (VC): Maximum air exhaled after maximum inhalation

• Expiratory Reserve Volume (ERV): Extra air exhaled after normal expiration

• Inspiratory Reserve Volume (IRV): Extra air inhaled after normal inspiration

• Residual Volume (RV): Air remaining after maximal expiration

Regulation of Respiration

Neural Control

Respiratory centers in the brainstem (medulla and pons) regulate the rate and depth of breathing. Chemoreceptors and stretch receptors provide feedback to adjust ventilation according to the body's needs.

• Medullary Rhythmicity Area: Sets basic rhythm (12–18 breaths/min)

• Pontine Centers: Modify rhythm based on physiological demands

• Chemoreceptors: Monitor CO2, O2, and pH levels

• Pulmonary Stretch Receptors: Prevent overinflation of lungs

Gas Exchange and Transport

Pulmonary and Systemic Gas Exchange

Oxygen diffuses from alveoli into capillary blood, binding to hemoglobin to form oxyhemoglobin. Carbon dioxide is transported in three forms: dissolved in plasma, as carbaminohemoglobin, and as bicarbonate ions.

• Oxygen Transport: Mostly as oxyhemoglobin (HbO2)

• Carbon Dioxide Transport: 10% dissolved, 20% as carbaminohemoglobin (HbCO2), 70% as bicarbonate ions (HCO3−)

Clinical Considerations and Disorders

Upper and Lower Respiratory Disorders

• Upper Respiratory Infections (URIs): Rhinitis, pharyngitis, laryngitis, epiglottitis, croup

• Lower Respiratory Disorders: Acute bronchitis, pneumonia, tuberculosis, restrictive and obstructive pulmonary diseases (COPD, chronic bronchitis, emphysema, asthma), lung cancer

• Respiratory Distress Syndromes: IRDS (infants, lack of surfactant), ARDS (adults, surfactant impairment)

Summary Table: Key Respiratory Volumes

Volume	Definition
Tidal Volume (TV)	Amount of air inhaled or exhaled with each breath under resting conditions
Vital Capacity (VC)	Maximum amount of air exhaled after a maximum inhalation
Expiratory Reserve Volume (ERV)	Amount of air that can be forcibly exhaled after normal expiration
Inspiratory Reserve Volume (IRV)	Amount of air that can be forcibly inhaled after normal inspiration
Residual Volume (RV)	Air remaining in lungs after maximal expiration

Key Equations

• Vital Capacity (VC): $VC = TV + IRV + ERV$

• Gas Exchange (Fick's Law): $\text{Rate of diffusion} = \frac{(P_1 - P_2) \times A \times D}{T}$ where $P_1 - P_2$ is the partial pressure difference, $A$ is the surface area, $D$ is the diffusion coefficient, and $T$ is the thickness of the membrane.

Quick Review Questions

• What are the primary functions of the respiratory system?

• Describe the characteristics of the alveoli that enable them to perform their function of gas exchange.

• What distinguishes the upper respiratory tract from the lower respiratory tract?

• What is the role of the respiratory mucosa?

• Name the four paranasal sinuses.

• List the three divisions of the pharynx.

• What keeps the trachea from collapsing?

• What lung structures serve to distribute air, and which serve as gas exchangers?

• Describe the function of surfactant.

• What is the respiratory membrane?

• What causes pleurisy?

• How does a pneumothorax differ from a hemothorax?

• What is the difference between bronchopneumonia and lobar pneumonia?

• Give two examples of chronic obstructive pulmonary disease (COPD).