2.1 - RESPIRATORY PHYSIOLOGY PART 1: MECHANICS

Lecture #11: Respiratory Physiology I: Mechanics

• Lecture presented by Julia M. Hum, Ph.D.
• Schedule: Monday/Wednesday/Friday, 2:00-2:50 PM
• Office Hours: Monday/Wednesday/Friday, 11:00 AM-12:00 PM
• Contact: [email protected]
• Website: marian.edu/medicalschool

Learning Objectives

• List the primary functions of the lung, including non-respiratory functions.
• Compare and contrast the conducting and respiratory zones of the lungs.
• Understand the importance and roles of pneumocytes.
• Illustrate the alveolar surface and alveolar-capillary interface.
• Describe the blood supply of the lung.
• Understand how surface tension is generated in the lungs and the role surfactant plays.
• Recognize the key players in the mechanics of breathing.
• Define dynamic lung mechanics, compliance, and transpulmonary pressure, and predict transpulmonary pressure.
• Draw pressure-volume curves during inflation and exhalation under normal and disease states.

Primary Functions of the Lung

• Pulmonary gas exchange
  • Ventilation (frequency x depth of breathing)
  • Perfusion (cardiac output of right ventricle)
  • Matching of air and blood volume ("ideal")
• Maintenance of partial pressures of gases in tissues (Diffusion)
  • Carries oxygenated blood to tissues
  • Carries deoxygenated blood (CO2) to lungs for elimination

Non-Respiratory Functions of the Lung

• Phonation
• Pulmonary Defense: Protect from pollutants and infection, and removes clots/emboli
• Blood Filter
• Acid-Base Balance:
  • Carbonic anhydrase reaction that converts CO2 + H2O to H2CO3 →H+ + HCO3
• Substrate conversion: Angiotensin I - Angiotensin II (ACE)

Pulmonary Ventilation

• Describes the process of air flow in and out of lungs
• Includes structures like conchae, glottis, pharynx, epiglottis, larynx, vocal cords, trachea, esophagus, alveoli, pulmonary arteries, and pulmonary veins.

Conducting Zone & Respiratory Zone

• Conducting zone: Structures for airflow, including the trachea, bronchi and bronchioles
• Respiratory zone: Structures for gas exchange, including respiratory bronchioles, alveolar ducts, and alveoli
• Diagram shows the progressive decrease in cross-sectional area in the conducting zone and exponential increase in area in the respiratory zone

Alveolar Sacs: Pneumocytes

• Type I pneumocytes: Thin, flat cells forming most of the alveolar surface area (~90%).
• Type II pneumocytes: "Granular" cells present in equal proportion, compact. Contain lamellar bodies that release pulmonary surfactant, and help repair alveolar wall damage.

Alveolar-Capillary Interface

• pulmonary capillaries and alveoli form a blood-gas interface
• alveolar capillary membrane is extremely thin (~ 0.2-0.5 mm).
• gases diffuse across the membrane.

The Lung's Blood Supply

• Two sources:
  • Pulmonary circulations
  • Bronchial circulations

Pulmonary Circulation

• Receives entire output of the heart
• Pulmonary veins carry oxygen-rich blood to the left side of the heart for systemic circulation.
• Pulmonary arteries bring deoxygenated blood from the right ventricle to the blood-gas interface.
• Systemically less smooth muscle than other blood vessels.
• Readily distensible
• Low vascular resistance (2-3 mmHg/L/min)

Bronchial Circulation

• Vascular beds that supply O2 and nutrients to the conducting airways.
• Bronchial arteries arise from the aorta.
• Capillaries anastomose with pulmonary capillaries and drain into pulmonary veins.
• Facilitates bypass of blood-gas interface and supplies the systemic circulation without being oxygenated.

Pulmonary Surface Tension

• Each alveolus is moistened with a thin film of alveolar lining fluid which generates surface tension, crucial for lung performance

Pulmonary Surfactant

• Synthesized and released by Type II pneumocytes.
• Mix of lipids and proteins that serve to counter the effects of surface tension.
• Composition: Dipalmitoyl phosphatidylcholine (DPPC)
• Stored in lamellar bodies and is exocytosed onto the alveolar surface.
• Forms a monolayer that interacts with water molecules, decreasing surface tension

Clinical Connection: Infant Respiratory Distress Syndrome (IRDS)

• Description of clinical presentation and investigation methods to differentiate various lung pathophysiologies
• Emphasis on understanding the mechanisms of lung injury and appropriate therapeutic interventions.

Pulmonary Surfactant: Importance

• Stabilizing alveolar size: LaPlace Law, preventing the smaller alveoli from collapsing.
• Increasing compliance: Decreases the effort required to inflate the lungs. Surfactant reduces surface tension to make it easier to inflate the lungs, decreasing the work of breathing.
• Keeping lungs dry: Surfactant reduces the pressure gradient and helps keep lungs fluid free and prevents a collapsing fluid bubble.

Dynamic Lung Mechanics

• Two opposing forces influencing lung volume at rest: inward and outward forces
• Inward force = elasticity and surface tension
• Outward force = elastic elements in the lungs and muscles/connective tissues of the chest wall
• Net effect: Creates negative pressure (Ppl) within the intrapleural space.

Transpulmonary Pressure

• Transpulmonary Pressure (Ptp) = Palveolar - Pintrapleural
• The difference in pressure across the lung's walls, which drives lung inflation
• Indicates the degree of inflation/deflation
• Positive value = lung expansion, negative = lung collapse

Transpulmonary Pressure & Pneumothorax

• Rupture or puncture in the chest wall leads to air entering the pleural space (pneumothorax).
• Lung retracts to size below residual volume due to loss of negative intrapleural pressure
• Collapse of the lung

Pressure-Volume Curves

• Studying a collapsed lung allows for understanding the effort required to inflate during normal breathing
• Inflation occurs in one or two ways, both modify transpulmonary pressure (PL). PL = Palv - Pleural

Transpulmonary Pressure

• Pressure across alveolar wall - TPP = Palv - Ppl
• If TPP is positive → alveoli are open
• If TPP is 0 → no force across the alveolar wall to prevent collapse. Negative Ppl is needed to keep alveoli open.

Inhalation & Exhalation

• Diaphragm contractions increase volume and decrease intrapulmonary pressure during inspiration, allowing air to enter the lungs
• During expiration, the diaphragm relaxes, decreases intrapulmonary volume, and increases pressure, forcing air out of the lungs

Pressure-Volume Curves: Inflation

• Normal quiet breathing is a gradual and linear function of transpulmonary pressure (PL)
• Collapsed airways require significant transpulmonary pressure to open.

Pulmonary Compliance

• Compliance is the ease with which the lungs can be stretched
• High compliance = easy to inflate; low compliance = difficult to inflate
• Compliance can be affected by diseases such as emphysema and fibrosis.

Clinical Connection...Compliance & Pulmonary Disease

• Emphysema: ↑ TLC, FRC, and compliance; lungs easily distended
• Fibrosis: ↓ TLC, FRC, and compliance; stiffens the lungs.

This quiz explores various aspects of the respiratory system, focusing on alveolar function, gas exchange mechanisms, and the role of surfactant. Test your knowledge on ventilation, perfusion, and the structural components of the lungs.