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 trachea as the trunk and alveoli as the leaves, where gas exchange occurs by diffusion.

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

• Diffusion: The primary mechanism for gas exchange between alveoli and capillaries.

Structural Plan of Respiratory Organs

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 and Air Purification

Structure and Function of Respiratory Mucosa

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

• Mucus Blanket: Traps inspired irritants such as dust and pollen.

• Cilia: Beat in one direction to move mucus toward the pharynx.

Anatomy of the Upper Respiratory Tract

Nose and Paranasal Sinuses

The nose is divided by the nasal septum and lined with mucous membrane. Paranasal sinuses (frontal, maxillary, sphenoidal, ethmoidal) drain into the nasal cavity and help warm, moisten, and filter air.

• Nasal Polyps: Noncancerous growths associated with chronic inflammation.

• Functions: Air conditioning and olfaction.

Pharynx (Throat)

The pharynx is a muscular tube about 12.5 cm long, divided into nasopharynx, oropharynx, and laryngopharynx. It serves as a passageway for both air and food and contains lymphoid tissue (tonsils) for immune protection.

• Openings: Connects nasal cavities, mouth, esophagus, larynx, and auditory tubes.

• Tonsils: Provide immune defense against inhaled or ingested pathogens.

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.

• Glottis: The space between the vocal cords.

• Epiglottis: Prevents food from entering the larynx during swallowing.

Trachea and Bronchial Tree

Trachea (Windpipe)

The trachea is a tube about 11 cm long, supported by C-shaped cartilage rings that keep it open. It conducts air from the larynx to the bronchi and is lined with mucosa for further air purification.

• Obstruction: Can be life-threatening; abdominal thrusts (Heimlich maneuver) or tracheostomy may be required.

Bronchi, Bronchioles, and Alveoli

The trachea divides into right and left primary bronchi, which branch into smaller bronchi and bronchioles, ending in alveolar sacs. The right bronchus is more vertical, making it more likely for aspirated objects to lodge there.

• Bronchi/Bronchioles: Distribute air to alveoli.

• Alveoli: Microscopic sacs where gas exchange occurs.

Lungs and Pleura

Structure of the Lungs

The lungs are large, spongy organs divided into lobes (three on the right, two on the left). The apex is the narrow upper part, and the base rests on the diaphragm.

• Lobes: Right lung has superior, middle, and inferior lobes; left lung has superior and inferior lobes.

Pleura and Pleural Cavity

The pleura is a double-layered membrane that reduces friction during breathing. Inflammation (pleurisy) or accumulation of air (pneumothorax) or blood (hemothorax) in the pleural space can impair lung function.

• Pleurisy: Inflammation of the pleura, causing pain with breathing.

• Pneumothorax: Air in the pleural space, leading to lung collapse.

• Hemothorax: Blood in the pleural space.

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 cavity returns to resting size; elastic recoil aids passive expiration. Forceful expiration uses internal intercostals and abdominal muscles.

Volumes of Air Exchanged

Respiratory volumes are measured with a spirometer and include:

• 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 of Breathing

Respiratory control centers in the brainstem (medulla and pons) regulate the rate and depth of breathing. Chemoreceptors and stretch receptors provide feedback to adjust ventilation as needed.

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

• Pontine Centers: Modify rhythm based on body needs.

• 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 as dissolved gas, carbaminohemoglobin, or 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 Tract Disorders

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

• Anatomical Conditions: Deviated septum, epistaxis (nosebleed).

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

Respiratory Distress Syndromes

• Infant Respiratory Distress Syndrome (IRDS): Due to lack of surfactant in premature infants.

• Adult Respiratory Distress Syndrome (ARDS): Impaired surfactant function from injury or inhalation of harmful substances.

Breathing Patterns and Terminology

• Eupnea: Normal breathing.

• Hyperventilation: Rapid, deep breathing.

• Hypoventilation: Slow, shallow breathing.

• Dyspnea: Labored breathing.

• Orthopnea: Dyspnea relieved by sitting upright.

• Apnea: Absence of breathing.

• Cheyne-Stokes Respiration: Cycles of apnea and hyperventilation.

• Respiratory Arrest: Failure to resume breathing after apnea.

Summary Table: Key Respiratory Volumes

Key Equations

• Vital Capacity (VC):

• Gas Exchange (Fick's Law):

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).

• What is internal respiration?

• How does the diaphragm operate during inspiration and expiration?

• What nerves stimulate the diaphragm to contract?

• What is the vital capacity? How is it measured?