The Respiratory System: Structure, Function, and Physiology

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The Respiratory System

Overview and Functional Anatomy

The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the atmosphere and the blood. It is divided into structural and functional components that facilitate ventilation and gas exchange.

Upper respiratory system: Includes the nose, nasal cavity, paranasal sinuses, and pharynx.
Lower respiratory system: Comprises the larynx, trachea, bronchi, bronchioles, and lungs.
Conducting portion: Series of passageways that conduct air into the lungs (nose to terminal bronchioles).
Respiratory portion: Sites of gas exchange (respiratory bronchioles and alveoli).

Labeled overview of the respiratory system, showing upper and lower respiratory structures

Major Functions of the Respiratory System

Provides an extensive surface area for gas exchange between air and circulating blood.
Moves air to and from the exchange surfaces of the lungs along the respiratory passageways.
Protects respiratory surfaces from dehydration, temperature changes, and invasion by pathogens.
Produces sounds for speaking, singing, and other forms of communication.
Detects odors by olfactory receptors in the nasal cavity.

Functions of the respiratory system

Histology and Protection of the Respiratory Tract

Respiratory Mucosa and Epithelium

The respiratory tract is lined by a mucosa that varies in structure along its length, providing protection and facilitating its functions.

Pseudostratified ciliated columnar epithelium: Lines the nasal cavity, upper pharynx, trachea, bronchi, and large bronchioles. Cilia move debris toward the pharynx (mucociliary escalator).
Stratified squamous epithelium: Lines the lower pharynx, providing protection against abrasion.
Cuboidal epithelium: Found in smaller bronchioles.
Simple squamous epithelium: Lines the alveoli, facilitating gas exchange.

Overview of the respiratory mucosa and mucociliary escalator Changes in the respiratory epithelium along the tract

Cystic Fibrosis

Cystic fibrosis is a genetic disorder characterized by the production of thick, sticky mucus that can obstruct airways and lead to recurrent respiratory infections.

Mucus accumulation blocks airflow and increases risk of bacterial infection.
Treatment often involves therapies to thin mucus and clear airways.

Breathing treatment for cystic fibrosis

Anatomy of the Upper Respiratory System

Nose and Nasal Cavity

The nose is the primary entryway for air, which is filtered, warmed, and humidified as it passes through the nasal cavity. The nasal conchae increase surface area and turbulence, trapping particles in mucus.

Paranasal sinuses (maxillary, frontal, ethmoid, sphenoid) open into the nasal cavity and are lined with mucous membrane.
Tears draining through the nasal cavity help clean it.

Anterior view of nasal cartilages and external landmarks Overview of the paranasal sinuses

Pharynx

The pharynx is a muscular tube with three regions:

Nasopharynx: Functions in respiration only.
Oropharynx: Passageway for air and food.
Laryngopharynx: Passageway for air and food, leading to the larynx and esophagus.

Sagittal section view of the upper respiratory system

Larynx and Voice Production

Structure of the Larynx

The larynx (voice box) connects the pharynx to the trachea and is involved in breathing, sound production, and protecting the lower airways.

Thyroid cartilage: Forms the Adam’s apple.
Epiglottis: Prevents food from entering the larynx during swallowing.
Cricoid cartilage: Connects the larynx to the trachea.

Anterior view of the larynx Sagittal section view of the larynx Posterior view of the larynx Cartilages of the larynx

Voice Production

Sound is produced by the vibration of the vocal folds (true vocal cords) in the larynx. The pitch of the sound depends on the tension of the vocal folds, and articulation is achieved by the tongue, teeth, and lips.

Structures associated with the glottis Example of phonation

Lower Respiratory Tract: Trachea, Bronchi, and Bronchioles

Trachea

The trachea (windpipe) extends from the larynx to the primary bronchi. It is supported by C-shaped cartilage rings and lined with pseudostratified ciliated columnar epithelium.

Overview of the trachea The trachea and branching into bronchi Sectional view of the trachea and esophagus

Bronchi and Bronchioles

The trachea divides into right and left primary bronchi at vertebra T5. The right bronchus is wider and more vertical.
The bronchial tree branches into secondary (lobar) and tertiary (segmental) bronchi, then into bronchioles and terminal bronchioles.
Bronchioles have smooth muscle in their walls and are regulated by the autonomic nervous system (bronchodilation and bronchoconstriction).

Air conducting passageways in the lower respiratory tract Overview of bronchi and bronchioles

Lungs and Pulmonary Anatomy

Gross Anatomy of the Lungs

The lungs are paired organs protected by the pleural membranes (visceral and parietal pleura) with a lubricating pleural cavity between them.
The right lung has three lobes (superior, middle, inferior) separated by horizontal and oblique fissures.
The left lung has two lobes (superior, inferior) and a cardiac notch.
Bronchopulmonary segments are supplied by tertiary bronchi and are surgically important subdivisions of the lung.

Features of the lateral surfaces of the lungs Lung lobes and bronchopulmonary segments Features of the medial surfaces of the lungs

Pulmonary Lobules and Alveoli

The smallest functional units of the lung are the pulmonary lobules, which contain alveoli where gas exchange occurs.

Air flows from terminal bronchioles to respiratory bronchioles, alveolar ducts, and alveoli.
Alveoli are lined by type I pneumocytes (structural), type II pneumocytes (secrete surfactant), and alveolar macrophages (phagocytic).
Surfactant reduces surface tension, preventing alveolar collapse.

Overview of the respiratory passageways Respiratory bronchioles to alveoli Alveolar structures The respiratory membrane

Respiratory Physiology

External and Internal Respiration

External respiration: Exchange of gases between alveoli and blood.
Internal respiration: Exchange of gases between blood and tissue cells.

Overview of respiration

Physical Principles of Ventilation

Air moves from areas of higher pressure to lower pressure (Boyle’s law: ).
Inhalation occurs when thoracic volume increases (diaphragm and external intercostals contract), lowering intrapulmonary pressure.
Exhalation occurs when thoracic volume decreases (muscles relax or accessory muscles contract), raising intrapulmonary pressure.

Relationship between pressure and volume in a gas Thoracic cavity volume changed by movements of the diaphragm and rib cage Pressures inside and outside the thoracic cavity during inhalation Muscles of respiration Lateral view of inspiratory muscles Overview of pressure changes and air movement during inhalation and exhalation

Pulmonary Volumes and Capacities

Tidal volume (TV): Air moved in and out during normal breathing (~500 mL).
Inspiratory reserve volume (IRV): Additional air inhaled after normal inspiration.
Expiratory reserve volume (ERV): Additional air exhaled after normal expiration.
Residual volume (RV): Air remaining after maximal exhalation.
Vital capacity (VC): IRV + TV + ERV.
Total lung capacity (TLC): VC + RV.

Pulmonary volumes and capacities Graph of tidal volume to calculate respiratory rate

Gas Exchange and Transport

Gas Laws and Diffusion

Dalton’s law: Each gas in a mixture exerts its own partial pressure; total pressure is the sum of all partial pressures.
Henry’s law: The amount of gas dissolved in a liquid is proportional to its partial pressure and solubility.

Partial pressures and normal gas concentrations in air Overview of Henry’s law Overview of gas exchange

Oxygen Transport and Hemoglobin

Most oxygen is transported bound to hemoglobin (Hb) as oxyhemoglobin (HbO2).
Hemoglobin’s affinity for O2 is affected by PO2, pH (Bohr effect), and temperature.
2,3-bisphosphoglycerate (BPG) produced by RBCs decreases Hb’s affinity for O2, promoting release in tissues.

Structure of hemoglobin Factors that affect hemoglobin’s affinity for O2 Oxygen-hemoglobin saturation curve The effects of pH on the oxygen-hemoglobin saturation curve

Carbon Dioxide Transport

CO2 is transported dissolved in plasma, bound to hemoglobin (carbaminohemoglobin), and as bicarbonate ions (HCO3–).
High CO2 in blood promotes O2 release from Hb; high O2 promotes CO2 release from Hb.

Key reaction:

Overview of carbon dioxide transport Transport of carbon dioxide in the lungs and peripheral tissues

Clinical Considerations and Regulation

Pulmonary Disease: Compliance and Resistance

Compliance: Ease with which lungs expand. Decreased in fibrosis, increased in emphysema.
Resistance: Force required to move air. Increased in asthma, bronchitis, and with mucus or inflammation.

Compliance of the lungs Resistance of the lungs Asthma treatment Example of chronic bronchitis Lung x-ray of a patient with emphysema

Neural Control of Respiration

Respiratory centers in the medulla oblongata and pons regulate the rate and depth of breathing.
Higher brain centers (cerebral cortex, hypothalamus, limbic system) can modify respiratory activity.

Multiple levels of respiratory control Events involved in quiet breathing in a person at rest Events involved in forced breathing

Reflex Control: Chemoreceptors and Baroreceptors

Chemoreceptors: Respond to changes in PCO2, PO2, and pH. Increased PCO2 is the strongest stimulus for increased ventilation.
Baroreceptors: Respond to changes in blood pressure, influencing respiratory rate.
Other reflexes include the inflation and deflation reflexes, sneezing, and coughing.

Effect of changes in arterial partial pressure of carbon dioxide on the respiratory rate The inflation reflex The deflation reflex Sneezing as an example of a protective reflex Baroreceptor reflexes

Effects of Aging and Smoking

Aging reduces all aspects of respiratory function.
Cigarette smoke contains carcinogens that can cause progressive changes in the respiratory epithelium, leading to cancer.
Progression: Dysplasia (reversible) → Metaplasia (reversible) → Neoplasia (irreversible) → Anaplasia (malignant, metastatic).

Graph of respiratory performance of nonsmokers to smokers Normal lung versus cancerous lung Progression of events leading to lung cancer

Medical Terminology

Asphyxia: Oxygen starvation due to low atmospheric oxygen or interference with ventilation or respiration.
Aspiration: Inhalation of foreign substances into the bronchial tree.
Dyspnea: Painful or labored breathing.
Epistaxis: Nosebleed.