The Respiratory System: Structure, Function, and Disorders

The Respiratory System

Major Functions of the Respiratory System

• Gas Exchange: Provides a large surface area for the exchange of oxygen and carbon dioxide between air and blood.

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

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

• Sound Production: Produces sounds for communication (speaking, singing, etc.).

• Olfaction: Aids the sense of smell via olfactory receptors in the nasal cavity.

Functional Anatomy of the Respiratory System

Divisions of the Respiratory System

• Upper Respiratory System: Includes the nose, nasal cavity, sinuses, and pharynx. Functions to filter, warm, and humidify incoming air, and protect the lower tract.

• Lower Respiratory System: Includes the larynx, trachea, bronchi, bronchioles, and alveoli. Functions to move air to gas exchange surfaces and facilitate gas exchange with capillaries.

Respiratory Tract

• Conducting Portion: Nasal cavity to bronchioles; conducts air to the exchange area.

• Respiratory Portion: Smallest bronchioles and alveoli; site of gas exchange.

Respiratory Mucosa

• Definition: Lines the conducting portion of the respiratory tract.

• Composition: Consists of epithelium (type varies by location) and underlying areolar tissue (lamina propria).

• Epithelium Types: Pseudostratified ciliated columnar epithelium with mucous cells in the nasal cavity, superior pharynx, trachea, bronchi, and large bronchioles.

• Lamina Propria: Areolar tissue supporting the respiratory epithelium.

• Functions: Warms (via blood vessels), moistens (via mucous glands), and protects (prevents drying of lower surfaces).

Cystic Fibrosis and the Respiratory Mucosa

• Definition: Inherited disease causing thick, sticky mucus that restricts airflow and increases infection risk.

• Treatment: Breathing treatments can temporarily thin mucus.

Upper Respiratory System: From Nose to Pharynx

• Air Pathway: Air enters through the nose (external nares), passes the nasal vestibule (with coarse hairs), through the nasal cavity (with hard and soft palate), and internal nares to the pharynx.

Regions of the Pharynx

Region	Location	Epithelium Type
Nasopharynx	Internal nares to soft palate	Pseudostratified columnar
Oropharynx	Soft palate to base of tongue/hyoid bone	Stratified squamous
Laryngopharynx	Hyoid bone to larynx/esophagus	Stratified squamous

Lower Respiratory System: From Pharynx to Trachea

• Larynx: Beginning of the lower respiratory system; surrounds the glottis (air passage).

• Trachea: Also called the windpipe; branches into right and left primary bronchi.

Trachea and Primary Bronchi

• Trachea: About 2.5 cm in diameter; supported by C-shaped cartilage rings (open posteriorly for esophageal expansion).

• Primary Bronchi: Right bronchus is larger and steeper than the left; each bronchus branches into smaller bronchi and then bronchioles.

Bronchioles

• Structure: Lined with smooth muscle, no cartilage.

• Control: Sympathetic stimulation causes bronchodilation (increased diameter/airflow); parasympathetic causes bronchoconstriction (decreased diameter/airflow).

Asthma

• Definition: Allergic reaction causing extreme bronchoconstriction and inflammation, severely restricting airflow.

Pleura and Pericardium

• Pleural Cavity: Each lung is surrounded by a pleural cavity with parietal pleura (lining chest wall) and visceral pleura (covering lung surface).

• Pleural Fluid: Lubricates and bonds pleural layers.

• Pericardial Cavity: Surrounds the heart.

Gas Exchange at the Alveoli

Alveoli Structure and Function

• Alveoli: Tiny sacs (about 150 million per lung) in clusters, surrounded by capillaries and elastic fibers for expansion and recoil.

• Blood Flow: Pulmonary arteries bring deoxygenated blood to alveoli; pulmonary veins return oxygenated blood to the heart.

Alveolar Epithelium

• Type I Cells: Simple squamous epithelium for gas exchange.

• Type II Cells: Surfactant-secreting cells; surfactant reduces surface tension, preventing alveolar collapse.

• Alveolar Macrophages: Phagocytize debris and particulates.

Respiratory Membrane

• Structure: Very thin (~0.5 µm), composed of alveolar epithelium, fused basement membrane, and capillary endothelium.

• Function: Facilitates rapid diffusion of oxygen and carbon dioxide.

Respiratory Physiology

Types of Respiration

• External Respiration: Exchange of gases between the body's tissues and the external environment.

• Internal Respiration: Absorption of oxygen and release of carbon dioxide by tissue cells.

External Respiration Details

• Pulmonary Ventilation: Physical movement of air into and out of the lungs; maintains alveolar ventilation.

• Gas Diffusion: Movement of O2 and CO2 across the respiratory membrane and capillary walls.

• Impairment: Can lead to hypoxia (low O2) or anoxia (no O2), causing tissue death.

Pressure Changes and Pulmonary Ventilation

• Boyle's Law: In a closed system, increased volume = decreased pressure, and vice versa.

• Mechanics: Changing thoracic cavity shape (via diaphragm and rib movement) changes lung volume and pressure, driving airflow.

Equation:

$P_1 V_1 = P_2 V_2$

Pleura and Lung Expansion

• Pleural Bond: Parietal pleura (chest wall) and visceral pleura (lungs) are bonded by pleural fluid; necessary for lung expansion.

• Atelectasis: Collapse of lung (or part) if air enters pleural cavity and bond is lost.

Air Pressure Terms

• Atmospheric Pressure: Air pressure outside the lungs.

• Intrapulmonary Pressure: Air pressure inside the lungs.

• Alveolar Pressure: Air pressure within the alveoli.

Respiratory Muscles

Muscles of Inhalation

• Primary Inspiratory Muscles: Diaphragm and external intercostals (active during quiet breathing).

• Accessory Inspiratory Muscles: Sternocleidomastoid, scalenes, pectoralis minor (active during deep or forceful breathing).

Muscles of Exhalation

• Primary Expiratory Muscles: None; quiet exhalation is passive.

• Accessory Expiratory Muscles: Internal intercostals, transversus thoracis, external and internal obliques, rectus abdominis (active during forceful exhalation).

Pulmonary Volumes and Capacities

Volume/Capacity	Definition	Average Value
Tidal Volume (VT)	Air moved in/out during one cycle	500 mL
Dead Space	Air in conducting zone (not for gas exchange)	150 mL
Inspiratory Reserve Volume (IRV)	Extra air inhaled beyond tidal volume	Varies
Expiratory Reserve Volume (ERV)	Extra air exhaled beyond normal exhalation	Varies
Vital Capacity	Max air moved in/out in one cycle	Varies
Residual Volume	Air left after max exhalation	Varies
Total Lung Capacity	Vital capacity + residual volume	4200 mL (females), 6000 mL (males)

Gas Diffusion and Transport

Gas Diffusion Principles

• Oxygen: Moves from high to low concentration (alveoli to blood in lungs; blood to tissues in body).

• Carbon Dioxide: Moves from high to low concentration (blood to alveoli in lungs; tissues to blood in body).

Gas Transport in Blood

• Oxygen: Carried mainly bound to hemoglobin in red blood cells.

Equations:

$\mathrm{Hb} + \mathrm{O}_2 \rightarrow \mathrm{HbO}_2$

$\mathrm{HbO}_2 \rightarrow \mathrm{Hb} + \mathrm{O}_2$

• Carbon Dioxide: Transported in three ways:

  1. Dissolved in plasma

  2. Bound to hemoglobin ($\mathrm{CO}_2 + \mathrm{Hb} \rightarrow \mathrm{HbCO}_2$)

  3. As bicarbonate ion (via carbonic acid): $\mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{CO}_3$ $\mathrm{H}_2\mathrm{CO}_3 \rightarrow \mathrm{H}^+ + \mathrm{HCO}_3^-$

• Chloride Shift: Bicarbonate exchanged for chloride ions in plasma.

Respiratory Control

Neural Control Centers

• Medulla Oblongata: Contains dorsal respiratory group (DRG, inspiratory center for every cycle) and ventral respiratory group (VRG, active during increased demand).

• Pons: Respiratory centers adjust the rate set by the medulla.

Homeostatic Regulation

• CO2 Levels: Main trigger for breathing rate adjustments.

• Hypercapnia: High CO2 (from hypoventilation) stimulates increased breathing rate.

• Hypocapnia: Low CO2 (from hyperventilation) causes decreased breathing rate.

Shallow Water Blackout

• Hyperventilation before swimming lowers CO2 levels, delaying the urge to breathe and risking fainting before CO2 rises enough to trigger breathing.

Respiratory Disorders

Chronic Obstructive Pulmonary Disease (COPD)

• Definition: General term for disorders restricting airflow, including asthma, chronic bronchitis, and emphysema.

Asthma

• Features: Extreme airway sensitivity, bronchoconstriction, increased mucus, inflammation.

• Triggers: Allergies, toxins, exercise, cold, stress.

Chronic Bronchitis

• Features: Long-term inflammation, excess mucus, airway clogging, frequent infections.

• Causes: Mainly cigarette smoking, also other irritants.

• Symptoms: Swelling, bluish skin ("blue bloaters").

Emphysema

• Features: Alveoli expand and merge, loss of elastic tissue, reduced surface area, shortness of breath, overexpanded lungs.

• Symptoms: Heavy breathing, pink skin ("pink puffers").

Lung Cancer

• Prevalence: Over 12% of new cancer cases; high mortality rate.

• Risk Factors: 85–90% due to smoking (including secondhand smoke); living with a smoker increases risk by 20–30%.

Effects of Aging on the Respiratory System

• Changes: Less elastic/flexible connective tissue, some emphysema after age 50, overall decline in respiratory performance (worse with smoking).

Summary Table: Respiratory Volumes and Disorders

Disorder	Main Features	Causes/Risk Factors
Asthma	Bronchoconstriction, inflammation, mucus	Allergies, toxins, exercise, cold, stress
Chronic Bronchitis	Inflammation, excess mucus, airway clogging	Smoking, irritants
Emphysema	Alveolar destruction, loss of elasticity	Smoking, aging
Lung Cancer	Uncontrolled cell growth, high mortality	Smoking, secondhand smoke

Additional info: The notes above expand on the original content by providing definitions, explanations, and context for key terms and processes, as well as summarizing the main disorders and their features for exam preparation.