Respiratory Mechanics Overview

Respiratory Mechanics: Inspiration

• Inspiration is initiated by a momentary pause between breaths.
• Thoracic volume increases, causing intra-alveolar pressure (PIAP) to decrease below atmospheric pressure (Patm).
• Air flows into the lungs down the pressure gradient.
• PIAP reaches its lowest value before equalizing with Patm.
• Lung volume is at its maximum.

Respiratory Mechanics: Expiration

• Expiration begins at maximum lung volume with PIAP equal to Patm.
• Thoracic volume decreases, increasing PIAP above Patm.
• Air flows out of the lungs.
• Pressures equalize at the end of the respiratory cycle.
• Pulmonary elasticity facilitates the recoil of the lungs.

Lung Compliance and Elastic Recoil

• Compliance refers to the effort required to stretch the lungs. Low compliance implies difficulty in stretching.
• High compliance means the lungs stretch too easily, characteristic of restrictive pulmonary disease.
• Elastic recoil is the tendency of the lungs to return to their resting volume after being stretched. It's influenced by connective tissue and alveolar surface tension.
• Loss of elastic tissue increases compliance.
• Alveolar surface tension, facilitated by water molecules, aids in elastic recoil but reduces compliance.

Respiratory Muscles and Thoracic Volume Change

• Approximately 60-75% of volume change during inspiration is due to diaphragm contraction.
• The remaining 25-40% is due to rib cage movement by external intercostal muscles.
• During quiet expiration, inspiratory muscle relaxation passively decreases thoracic volume.
• Active expiration (forced) involves accessory muscles contracting to further increase intra-alveolar pressure.

Pneumothorax

• A puncture in the pleural membrane causes air to enter the pleural cavity.
• Intra-alveolar pressure and intrapleural pressure equalize with atmospheric pressure.
• The transmural pressure gradient is lost, leading to lung collapse and outward expansion of the thoracic wall.

Alveolar Stability: Surfactant and Interdependence

• Surfactant lowers surface tension in alveoli, preventing collapse, particularly in smaller alveoli. It's composed of phospholipids and proteins.
• Alveolar interdependence, the interconnectedness of alveoli, contributes to elastic recoil and prevents alveolar collapse. The Law of Laplace describes the relationship between pressure, surface tension, and alveolar radius.

Ventilation-Perfusion Ratio (V/Q)

• V/Q ratio varies across the lung due to gravity. Apex has higher V/Q, while the base has lower V/Q.
• Ventilation exceeds perfusion in the apex.
• Perfusion exceeds ventilation in the base.

Gas Exchange and Transport

• Gas exchange occurs at the alveolar-capillary interface and between blood and tissues.
• Oxygen is transported dissolved in plasma and bound to hemoglobin (Hb).
• Carbon dioxide is transported dissolved, bound to Hb, and as bicarbonate (HCO3-).
• Optimal gas exchange requires adequate oxygen delivery to alveoli, and efficient diffusion across the alveolar-capillary membrane.

Factors Affecting Alveolar Gas Exchange

• Factors influencing alveolar gas exchange include adequate O2 reaching the alveoli (affected by altitude, humidity, ventilation), and effective diffusion across the alveolar-capillary membrane (affected by surface area, diffusion distance, and barrier permeability).

Diffusion

• Diffusion rate is proportional to surface area, concentration gradient, and barrier permeability, and inversely proportional to distance.

Oxygen Transport and Hemoglobin

• Hemoglobin (Hb) is a protein with four subunits, each capable of binding one oxygen molecule.
• Oxygen binding to Hb follows an equilibrium reaction, affected by the partial pressure of oxygen (PO2).
• The Fick equation estimates oxygen consumption based on cardiac output and arterial-venous oxygen difference.

Factors Affecting O2-Hb Binding

• PO2 in plasma is the primary determinant of Hb saturation.
• Other factors influencing O2-Hb binding include pH, temperature, and 2,3-bisphosphoglycerate (2,3-BPG).

Control of Airway Resistance and Perfusion

• Airway resistance is determined by airway diameter, viscosity, and length. Poiseuille's Law describes this relationship.
• Bronchoconstriction increases resistance, while bronchodilation decreases it.
• Changes in airway resistance and perfusion work together to maintain adequate gas exchange. For example, a local decrease in alveolar O2 stimulates vasoconstriction and bronchodilation.

Normal Blood Gas Values

• Arterial PO2: ~95 mmHg
• Venous PO2: ~40 mmHg
• Arterial PCO2: ~40 mmHg
• Venous PCO2: ~46 mmHg
• Arterial pH: ~7.4
• Venous pH: ~7.37

Hypoxia and Hypercapnia

• Hypoxia is insufficient oxygen.
• Hypercapnia is elevated carbon dioxide levels. Different types of hypoxia include hypoxic, anemic, ischemic, and histotoxic.

Explore the fundamentals of respiratory mechanics, including the processes of inspiration and expiration. Understand how thoracic volume changes affect intra-alveolar pressure and learn about lung compliance and elastic recoil in respiratory health. This quiz covers key concepts essential for studying respiratory physiology.