Respiratory System Structures

Structures of the Respiratory System

• Nasopharynx: Located behind the nasal cavity, extends to the soft palate, lined with pseudostratified ciliated columnar epithelium, facilitates equalization of air pressure between the middle ear and atmosphere via auditory tubes.

• Oropharynx: Serves both respiratory and digestive purposes, acts as a passage for air, food, and fluid, lined with nonkeratinized stratified squamous epithelium for abrasion protection.

• Laryngopharynx: Begins at the hyoid bone, opens to the esophagus and larynx, lined with nonkeratinized stratified squamous epithelium, facilitates air and food passage, acts as a resonating chamber for speech, houses tonsils for immune response.

• Larynx (Voice Box): Connects the laryngopharynx to the trachea, located anterior to the esophagus, has varying epithelial linings: nonkeratinized stratified squamous epithelium above vocal folds and pseudostratified ciliated columnar epithelium below.

• Trachea (Windpipe): Extends inferiorly from the larynx and divides the lungs into lobes, also features a right horizontal fissure that creates a middle lobe.

• Lung Parenchyma: Portion responsible for gas exchange, contains alveoli that open into alveolar ducts, crucial for respiratory function.

• Lobule of the Lung: Small compartments within bronchopulmonary segments, wrapped in elastic connective tissue, includes lymphatic vessels, arterioles, venules, and branches from terminal bronchioles.

Alveoli

• Alveolus: Cup-shaped sac at the bronchiole's end, main site for gas exchange, lined with two types of alveolar epithelial cells.

• Type I Alveolar Cells: Predominant in number, squamous cells forming a continuous lining for gas exchange.

• Type II Alveolar Cells: Fewer in number, secrete alveolar fluid and surfactant to maintain alveolar stability by lowering surface tension.

Gas Exchange Principles

• Partial Pressure: Concentration of gas in a mixture is proportional to its pressure, with gases diffusing from areas of higher to lower concentration.

• Concentration of Oxygen at Sea Level: The partial pressure of oxygen (21% of 760 mmHg) equals 160 mmHg.

• Henry’s Law: The relationship between partial pressure and solubility of gases, critical for understanding gas behavior in solutions.

Factors Affecting Gas Diffusion

• Thickness of Respiratory Membrane: Increased thickness reduces diffusion efficiency, commonly caused by ailments like edema or pulmonary fibrosis.

• Surface Area of Respiratory Membrane: Reduced surface area decreases diffusion, for example, due to removal of lung tissue or conditions like emphysema.

• Diffusion Coefficient: Indicator of diffusion rate, influenced by gas solubility and molecular weight.

Diffusion Capacity

• Diffusion Capacity: Volume of gas diffusing through the membrane per minute for a partial pressure difference of 1 mmHg. Normal oxygen diffusion capacity at rest is 21 ml/min/mmHg.

Hemoglobin and Oxygen Transport

• Oxygen Transportation: 97% of oxygen binds to hemoglobin (Hb) for transport; remaining 3% is dissolved in plasma.

• Hemoglobin Structure: Comprises 4 polypeptide chains (2 alpha, 2 beta), each with a heme group capable of binding oxygen.

• Oxygen-Carrying Capacity: A typical 100 ml blood contains 15 g of hemoglobin, allowing for an oxygen capacity of 20.1 ml, with approximately 5 ml released to tissues under normal conditions.

Oxygen-Hemoglobin Dissociation Curve

• Dissociation Curve: Reflects hemoglobin's oxygen saturation levels rising with increased blood oxygen partial pressure.

• Bohr Effect: An increase in carbon dioxide and hydrogen ions shifts the dissociation curve to the right, facilitating oxygen release to tissues.

• Carbon Monoxide Binding: CO competes with oxygen for hemoglobin binding sites with a much higher affinity, critically affecting oxygen transport efficiency.