The Respiratory System: Structure, Function, and Physiology

The Respiratory System

Overview and Importance

The respiratory system is essential for providing oxygen to body cells and removing carbon dioxide, a waste product of metabolism. This system works closely with the cardiovascular system to ensure efficient gas exchange and transport throughout the body.

• ATP Production: Body cells require oxygen for ATP synthesis and produce carbon dioxide as a byproduct.

• Gas Exchange: Oxygen is taken in and carbon dioxide is expelled via diffusion across lung surfaces.

• Transport: Gases are transported to and from tissues by the cardiovascular system.

Major Organs and Divisions

The respiratory system is divided into upper and lower regions, each with specialized structures and functions.

• Upper Respiratory System: Nose, nasal cavity, paranasal sinuses, and pharynx. Functions include filtering, warming, and humidifying air, and olfaction.

• Lower Respiratory System: Larynx, trachea, bronchi, bronchioles, and alveoli. Functions include sound production, air conduction, and gas exchange.

Functional Organization

Conducting vs. Respiratory Portions

The respiratory tract is divided into:

• Conducting Portion: Nasal cavity to terminal bronchioles; conducts air only.

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

Basic Functions

• Pulmonary Ventilation: Movement of air in and out of the lungs.

• Pulmonary Gas Exchange: Exchange of gases between lungs and blood.

• Gas Transport: Movement of gases in the blood.

• Tissue Gas Exchange: Exchange of gases between blood and tissues.

• Other Functions: Sound production, olfaction, protection, and acid-base balance.

Histology of the Respiratory System

Respiratory Mucosa

The respiratory mucosa lines the conducting portion and consists of an epithelial layer and a deep areolar layer (lamina propria). It contains mucous glands and, in some areas, smooth muscle.

Types of Epithelium

• Pseudostratified ciliated columnar epithelium: Nasal cavity, nasopharynx, and superior lower respiratory tract.

• Stratified squamous epithelium: Inferior pharynx and oropharynx, for protection against abrasion.

• Cuboidal epithelium: Smaller bronchioles.

• Simple squamous epithelium: Alveoli, for efficient gas exchange.

Respiratory Defense System

• Filtration by nasal hairs: Removes large particles.

• Mucous/goblet cells: Trap debris and pathogens.

• Cilia: Sweep mucus toward the pharynx.

• Alveolar macrophages: Engulf small particles in the lungs.

Anatomy of the Upper Respiratory Tract

Nose and Nasal Cavity

• Functions: Warm, humidify, and filter air; house olfactory receptors.

• Nasal Conchae and Meatuses: Create turbulence, increasing contact with mucosa.

Pharynx

• Nasopharynx: Lined with pseudostratified ciliated columnar epithelium; contains pharyngeal tonsil.

• Oropharynx: Lined with stratified squamous epithelium; passage for food and air.

• Laryngopharynx: Lined with stratified squamous epithelium; leads to larynx and esophagus.

Larynx

• Glottis: Opening through which air passes.

• Cartilages: Thyroid, cricoid, and epiglottis support and protect the airway.

• Epiglottis: Prevents food from entering the respiratory tract during swallowing.

• Sound Production: Air passing through the glottis vibrates vocal folds.

Lower Respiratory Tract

Trachea

• Structure: Supported by C-shaped rings of hyaline cartilage to maintain patency.

• Carina: Contains sensory receptors that trigger coughing if foreign material is detected.

Bronchial Tree

• Primary Bronchi: Right and left branches from the trachea.

• Secondary (Lobar) Bronchi: Branch from primary bronchi.

• Tertiary (Segmental) Bronchi: Branch from secondary bronchi.

• Bronchioles: Smallest airways, lack cartilage, lined by simple cuboidal epithelium.

• Terminal Bronchioles: Final part of conducting airways.

Alveoli and Respiratory Membrane

• Alveoli: Air-filled sacs where gas exchange occurs; lined by simple squamous epithelium (type I alveolar cells).

• Type II alveolar cells: Produce surfactant to reduce surface tension and prevent alveolar collapse.

• Alveolar macrophages: Phagocytize debris and pathogens.

• Respiratory Membrane: Composed of alveolar epithelium, fused basement membrane, and capillary endothelium.

Lungs and Pleurae

Lung Structure

• Pulmonary Lobules: Smallest compartments of the lung, each supplied by a terminal bronchiole.

• Trabeculae: Fibrous partitions dividing the lung into lobules.

• Alveolar Ducts and Sacs: Terminal structures for gas exchange.

Pleural Cavities and Membranes

• Pleural Cavities: Each lung is enclosed in a pleural cavity lined by parietal and visceral pleura.

• Pleural Fluid: Lubricates and reduces friction, holds membranes together via surface tension.

Respiratory Physiology

External and Internal Respiration

• External Respiration: Exchange of O2 and CO2 between lungs and blood.

• Internal Respiration: Exchange of O2 and CO2 between blood and tissues.

Pressure-Volume Relationships (Boyle's Law)

Boyle’s Law states that the pressure of a gas is inversely proportional to its volume, provided the number of gas molecules remains constant:

• Decreasing container size increases pressure.

• Increasing container size decreases pressure.

Equation:

$P_1 V_1 = P_2 V_2$

Pulmonary Ventilation

• Inhalation: Diaphragm contracts, thoracic cavity volume increases, pressure decreases, air flows in.

• Exhalation: Diaphragm relaxes, thoracic cavity volume decreases, pressure increases, air flows out.

• Tidal Volume: Amount of air moved in and out during a single respiratory cycle.

Mechanics of Breathing

• Inspiration: Active process involving diaphragm and external intercostals.

• Expiration: Passive at rest (elastic recoil); active during forceful breathing (accessory muscles).

Physical Factors Influencing Ventilation

• Airway Resistance: Anything that impedes airflow.

• Alveolar Surface Tension: Reduced by surfactant to prevent alveolar collapse.

• Pulmonary Compliance: Ability of lungs and chest wall to stretch.

Gas Exchange and Transport

Pulmonary and Tissue Gas Exchange

• Pulmonary Gas Exchange: O2 diffuses from alveoli to blood; CO2 diffuses from blood to alveoli.

• Tissue Gas Exchange: O2 diffuses from blood to tissues; CO2 diffuses from tissues to blood.

Oxygen Transport

• Most O2 is transported bound to hemoglobin in erythrocytes.

• Oxygen loading and unloading depend on partial pressures and affinity of hemoglobin.

Carbon Dioxide Transport

• Dissolved in plasma: 7–10% of total CO2.

• Bound to hemoglobin: 20% as carbaminohemoglobin.

• As bicarbonate ions: 70% converted in erythrocytes.

Control of Respiration

Neural Control

• Medullary Respiratory Centers: Dorsal (DRG) and Ventral (VRG) groups regulate rhythm and depth.

• Pontine Respiratory Group: Modifies output from medullary centers.

• Phrenic Nerve: Innervates the diaphragm.

Chemoreceptor Regulation

• Central and Peripheral Chemoreceptors: Monitor blood and CSF for O2, CO2, and pH changes.

• Stretch Receptors: Respond to changes in lung volume.

• Irritant Receptors: Trigger protective reflexes.

Noninfectious Respiratory Diseases

Restrictive Lung Diseases

• Decreased pulmonary compliance, reduced inspiration, and lower lung volumes.

Obstructive Lung Diseases

• Increased airway resistance, decreased efficiency of expiration.

• Examples: Chronic Obstructive Pulmonary Disease (COPD), emphysema, asthma.