Chapter 23: The Respiratory System – Structure, Function, and Regulation

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The Respiratory System: Overview and Organization

Introduction to the Respiratory System

The respiratory system is essential for gas exchange, supplying oxygen to body tissues and removing carbon dioxide. It supports aerobic metabolism, which is vital for cellular energy production.

Oxygen is obtained from the air via diffusion across lung surfaces.
Carbon dioxide is released from tissues and expelled through the lungs.
Blood transports these gases between the lungs and peripheral tissues.

The Structures of the Respiratory System

Functions of the Respiratory System

Provides a large surface area for gas exchange between air and blood.
Moves air to and from the lungs along respiratory passageways.
Protects respiratory surfaces from dehydration, temperature changes, and pathogens.
Produces sounds for communication.
Detects odors via olfactory receptors in the nasal cavity.

Organization of the Respiratory System

Anatomical divisions:
- Upper respiratory system: Nose, nasal cavity, paranasal sinuses, pharynx
- Lower respiratory system: Larynx, trachea, bronchi, bronchioles, alveoli
Functional divisions:
- Conducting portion: Nasal cavity to larger bronchioles (air passageways)
- Respiratory portion: Smallest bronchioles and alveoli (site of gas exchange)

Histology and Defense of the Respiratory Tract

Respiratory Mucosa

The respiratory mucosa lines the conducting portion and consists of:

Respiratory epithelium: Innermost layer, varies by region (e.g., pseudostratified ciliated columnar in nasal cavity and trachea).
Lamina propria: Areolar tissue beneath the epithelium, contains mucous glands in upper regions and smooth muscle in lower bronchioles.

Diagrammatic view of the respiratory epithelium of the trachea

Respiratory Defense System

Mucus traps particles and pathogens.
Cilia move mucus toward the pharynx (mucociliary escalator).
Alveolar macrophages engulf small particles in the alveoli.

Cilia of the respiratory epithelium

Upper Respiratory System

Nose and Nasal Cavity

Entry point for air; nasal hairs filter large particles.
Olfactory region provides sense of smell.
Mucus moistens air; conchae create turbulence for warming, humidifying, and filtering air.

Nasal cavity and pharynx in sagittal section

Pharynx

Shared by respiratory and digestive systems.
Divided into nasopharynx, oropharynx, and laryngopharynx.
Lined with stratified squamous epithelium for protection against abrasion.

Lower Respiratory System

Larynx

Air passes from pharynx to larynx via the glottis.
Major cartilages: thyroid, cricoid, and epiglottis.
Epiglottis prevents food/liquid from entering the airway during swallowing.

Anterior view of the larynx

Vocal Cords

Vocal folds (true vocal cords) produce sound when air passes through the glottis.
Vestibular ligaments protect the vocal folds.

Glottis in the open position

Trachea and Bronchial Tree

Trachea: tough, flexible tube with C-shaped cartilages to keep airway open.
Branches into right and left main bronchi, which further divide into the bronchial tree.

Anterior view of the trachea and bronchi Cross-sectional view of the trachea and esophagus Bronchial tree and its divisions in the lungs Branching pattern of bronchi in the left lung

Bronchioles and Asthma

Bronchioles: smallest branches, lack cartilage, dominated by smooth muscle.
Bronchodilation (sympathetic) increases airflow; bronchoconstriction (parasympathetic or allergic reaction) decreases airflow.
Asthma: severe bronchoconstriction restricts airflow.

Structure of a single pulmonary lobule

Gas Exchange Structures

Alveoli and Blood-Air Barrier

Respiratory bronchioles connect to alveoli via alveolar ducts and sacs.
Alveoli: surrounded by elastic fibers and capillaries; site of gas exchange.
Pneumocytes type I: simple squamous cells for gas diffusion.
Pneumocytes type II: produce surfactant to reduce surface tension and keep alveoli open.
Alveolar macrophages patrol for debris and pathogens.

Distal end of a single lobule Alveolar structure and capillary network Blood-air barrier

Pneumonia

Inflammation of lung tissue, often due to infection.
Causes swelling, fluid buildup, and impaired gas exchange.

The Lungs

Gross Anatomy of the Lungs

Right lung: three lobes (superior, middle, inferior).
Left lung: two lobes (superior, inferior) and a cardiac notch.
Lungs are located in pleural cavities, separated by the mediastinum.

Lateral views of the lungs Transverse section of lungs and heart

Blood Supply and Pulmonary Embolism

Pulmonary circuit: brings deoxygenated blood to lungs for gas exchange.
Systemic circuit: supplies conducting portions of the lungs.
Pulmonary embolism: blockage of a pulmonary artery by a clot, fat, or air bubble; can cause tissue death or heart failure.

Pleural Cavities and Membranes

Pleura: serous membrane with parietal (outer) and visceral (inner) layers.
Pleural fluid lubricates and reduces friction.
Pleurisy: inflammation of pleura, causing pain due to reduced lubrication.

Respiration: External and Internal

Processes of Respiration

External respiration: Exchange of gases between interstitial fluid and external environment (lungs).
Internal respiration: Exchange of gases between blood and tissue cells.

Overview of key steps in respiration

Steps in External Respiration

Pulmonary ventilation (breathing)
Gas diffusion across blood-air barrier and systemic capillaries
Transport of gases in blood

Hypoxia: Low tissue oxygen levels; Anoxia: Complete lack of oxygen, leading to cell death.

Pulmonary Ventilation

Physical Principles of Ventilation

Air moves from high to low pressure (down a pressure gradient).
Boyle’s Law: Pressure and volume of a gas are inversely related.

Relationship between gas pressure and volume

Volume Changes and Pressure Gradients

At rest: pressures inside and outside lungs are equal; no air movement.
Inhalation: chest volume increases, pressure decreases, air flows in.
Exhalation: chest volume decreases, pressure increases, air flows out.

Pulmonary ventilation at rest Pulmonary ventilation during inhalation Pulmonary ventilation during exhalation

Muscles of Respiration

Inhalation: diaphragm contracts (flattens), external intercostals elevate ribs.
Exhalation: passive at rest (muscle relaxation); active exhalation uses internal intercostals and abdominal muscles.

Primary and accessory respiratory muscles

Pressure Changes During Breathing

Normal atmospheric pressure: 1 atm = 760 mm Hg.
Intrapulmonary pressure: varies with inhalation/exhalation (−1 to +1 mm Hg at rest).
Intrapleural pressure: always lower than atmospheric, assists venous return to heart.

Pressure and volume changes during inhalation and exhalation

Pulmonary Volumes and Capacities

Tidal Volume (TV): Air moved per breath (average 500 mL).
Inspiratory Reserve Volume (IRV): Extra air inhaled after normal inspiration.
Expiratory Reserve Volume (ERV): Extra air exhaled after normal expiration.
Residual Volume: Air remaining after maximal exhalation.
Vital Capacity: Maximum air moved in/out in a single breath.
Total Lung Capacity: Total volume of the lungs.

Pulmonary volumes and capacities

Gas Exchange: Physical Principles

Partial Pressures and Gas Laws

Dalton’s Law: Each gas in a mixture exerts its own partial pressure.
Henry’s Law: Amount of gas dissolved in a liquid is proportional to its partial pressure.

Henry's Law and solubility

Factors Affecting Gas Exchange

Partial pressure gradients
Distance for diffusion
Lipid solubility of gases
Surface area for exchange
Ventilation-perfusion ratio

External and Internal Respiration

External respiration: Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli.
Internal respiration: Oxygen diffuses from blood to tissues; carbon dioxide diffuses from tissues to blood.

External respiration partial pressures Internal respiration partial pressures

Gas Transport in Blood

Oxygen Transport

1.5% dissolved in plasma; 98.5% bound to hemoglobin (Hb).
Each Hb binds up to four O2 molecules (forms oxyhemoglobin).
Hemoglobin saturation depends on partial pressure of O2, pH, temperature, and metabolic activity.

Oxygen-hemoglobin saturation curve Effect of pH on hemoglobin saturation Effect of temperature on hemoglobin saturation

Carbon Dioxide Transport

70% as bicarbonate ions (HCO3−) in plasma
23% bound to hemoglobin (carbaminohemoglobin)
7% dissolved in plasma

Carbon dioxide transport in blood Summary of primary gas transport mechanisms

Control of Respiration

Local and Neural Regulation

Local factors: CO2 and O2 levels regulate blood flow and airflow in the lungs.
Neural control: Respiratory centers in the medulla oblongata and pons regulate rate and depth of breathing.
Voluntary control: Cerebral cortex can override involuntary centers for activities like speaking or holding breath.

Control of respiration in the brain Respiratory centers and reflex control

Respiratory Reflexes

Chemoreceptors: Detect changes in CO2, O2, and pH; stimulate changes in respiratory rate.
Baroreceptors: Respond to blood pressure changes; affect respiratory rate.
Stretch receptors: Prevent overexpansion or excessive deflation of the lungs (Hering–Breuer reflexes).
Other stimuli: Pain, temperature, and visceral sensations can influence breathing.

Chemoreceptor response to increased CO2 Chemoreceptor response to decreased CO2

Integration with Other Body Systems

Respiratory and Cardiovascular System Interactions

Respiratory and cardiovascular systems coordinate to maintain oxygen and carbon dioxide homeostasis.
Integration improves gas exchange efficiency, regulates blood pressure, and adjusts cardiac output as needed.

Integration of the respiratory system with other body systems

Additional info: This guide covers the structure, function, and regulation of the respiratory system, including gas exchange, transport, and integration with other organ systems, as outlined in a standard Anatomy & Physiology college course.