Breathing Mechanics & Gas Exchange

Mechanics of Breathing and Gas Exchange

• The session will cover mechanics of breathing, gas exchange, and the respiratory membrane.
• After the session, students should be able to:
  • Describe pressure changes in the thoracic cavity.
  • Outline the mechanics of breathing.
  • Describe factors affecting pulmonary ventilation.
  • Explain gas exchange principles, including oxygen and carbon dioxide transport.
• Recommended reading includes Chapter 22 of "Human Anatomy and Physiology" by Marieb and Hoehn and Chapters 38, 40, and 41 of "Textbook of Medical Physiology" by Guyton and Hall.

Respiratory System Components

• Key anatomical structures include:
  • Nose, nasal cavity, pharynx, larynx, and trachea.
  • Followed by the bronchi and their branches then lungs.

Pleural Cavity and Membranes

• The lungs are surrounded by a double-layered membrane.
• Pleura is a serous membrane that lines the pleural cavity.
• Parietal lining that attaches to the thoracic cavity & diaphragm walls.
• Visceral lining adheres to the lung surface.
• Pleural fluid lubricates the space between the pleura to reduce friction in the pleural cavity/sac

Respiratory Pressures

• Respiratory pressures are discussed relative to atmospheric pressure, which is approximately 760 mmHg.
• Negative respiratory pressure is less than 760 mmHg
• Positive respiratory pressure is more than 760 mmHg
• Intrapulmonary pressure (Ppul) changes with breathing phases and equalizes with atmospheric pressure.
• Intrapulmonary pressure determines airflow direction.
• Intrapleural pressure (Pip) remains about 4 mmHg less than intrapulmonary pressure, maintaining lung inflation.
• The transpulmonary pressure (PTpul) is the difference between intrapulmonary and intrapleural pressures, which prevents lung collapse.

Volume and Pressure Relationship

• Boyle's Law states at constant temperature, pressure and volume are inversely related.
• increased volume results in decreased pressure
• decreased volume results in increased pressure.
• Thoracic cavity volume changes drive pressure changes, which lead to gas flow to equalize pressure.

Inspiration

• Diaphragm and external intercostal muscles contraction elevates rib cage.
• Increased lung volume causes intrapulmonary volume increases.
• Intrapulmonary pressure drops below atmospheric pressure.
• Air flows into the lungs down the pressure gradient until intrapulmonary pressure equals atmospheric pressure.

Inspiration Types

• Normal quiet breathing (Eupnea), diaphragm and intercostal muscles relatively contribute
  • Rest: Diaphragm dominates
  • Pregnancy: Chest cavity breathing alters
• Forced breathing (Hypernea): Involves extra muscle groups

Expiration

• Inspiratory muscles relax and rib cage descends due to gravity.
• Thoracic cavity volume decreases.
• Lungs recoil passively and intrapulmonary volume decreases.
• Intrapulmonary pressure rises above atmospheric pressure.
• Gases flow out of the lungs down the pressure gradient until intrapulmonary pressure equals atmospheric pressure.

Pulmonary Ventilation Factors- Airway Resistance

• Airway resistance is inversely proportional to diameter.
• Normally airway resistance is insignificant.
• Diseases such as mucus or inflammatory chemicals narrow airways increasing resistance.
• Bronchodilators reduce resistance and increase airflow.

Resistance to airflow

• Resistance is more significant in bronchi near trachea and in large bronchioles
• Smooth muscle in bronchiolar wall is sensitive to neural control and chemicals.
• Resistance is important in smaller bronchioles in some disease states as their size is smaller.
• Mucus collection, muscle contraction or edema in walls

Pulmonary Ventilation Factors-Alveolar Surface Tension

• Water molecules at the air/water interface have strong attractive forces.
• Water surfaces in alveoli attempting to contract and force air out of them.
• Alveoli collapse without surfactant.
• Results in Elastic contractile force

Pulmonary Ventilation Factors-Surfactant

• Surfactant is secreted by Type II alveolar epithelia.
• Reduces surface tension with phospholipids, proteins and ions like detergent
• ↑ lung compliance
• ↓surface tension with less water
• Infant respiratory distress syndrome is caused due to premature babies having little or no surfactant.

Pulmonary Ventilation Factors-Lung Compliance

• Lung compliance measures the extent to which the lung volume expands for a given rise in transpulmonary pressure
  • Due to elastic forces of the lung tissue
  • Related to alveolar surface tension
• Healthy lungs have high compliance
• Decreased compliance requires more force to fill and empty the lungs.
• ↓ elasticity of lungs (fibrosis) ↓ compliance
• ↓ surfactant production leads to ↓ compliance
• Decrease in thoracic mobility (arthritis, paralysis) decreases in lung compliance.
• ↑ compliance occurs in Alveolar damage (emphysema) where air moves in and out of the lung faster reducing gas exchange function

Pneumothorax

• Air typically does not enter the pleural cavity
• Punctured chest wall (stab wound or broken rib) causes air flows into the pleural space causing lung collapse.
• Lung wall hole(disease state)

Gas exchange

• Gases diffuse and obey gas laws.
• Dalton’s Law of Partial Pressures states total pressure equals exerted pressures mixture.
• Atmospheric air is composed of:
  • PN2 (78.6%)+ P02 (20.9%) + PC02 (0.04%) + PH2O (0.46%)
  • Total pressure equals = 760 mm Hg (100%).
• Henry's Law says mixture of gases in contact with a dissolves per partial pressure
• Soluble of gas dependent
• Solubility of carbon is twenty times larger than oxygen.

Alveolar Air Composition

• Atmospheric air is composed more of oxygen and nitrogen.
• Alveolar air is more of carbon dioxide and water vapor.
• Due to Gas exchange, humidification, Mixing of alveolar gas

Pulmonary Gas Exchange: Factors Influencing Gas Movement

• Partial pressure gradients.
• Gas solubility matching between alveolar ventilation and lung blood perfusion.
• Structural features of Membranes

Partial Pressure & Gas Solubilities

• in venous blood PO2 is 40mmHg but in the alveoli PO2 is 104mmHg
• Oxygen diffuses through partial pressure in alveoli to capillary blood in 0.25s to equilibrium
• Blood cells reach equilibrium in capillaries in 0.75s
• In capillaries PC02 is 45mmHg
• Alveoli PC02 is 40mmHg
• Although steeper gradients exist for oxygen exchange amounts

Ventilation-Perfusion Coupling

• Gas is coupled in alveolar ventilation
• and blood flow is pulmonary

Respiratory Membranes

• Membrane thickness should be 0.5 - 1.0 to enable gas exchange
• fluid in membranes thickens area
• Surface area Large (~90m2 in adult male) for gas exchange, if functional will gas exchange is optimal
• reduced surface area due to emphysema
• or inflammation can reduce surface area

Oxygen Transport

• Most oxygen (98.5%) is bound to hemoglobin in red blood cells
• Small amounts of oxygen(1.5%) dissolve in blood plasma
• The rate at which hemoglobin binds or releases oxygen depends on:
  • Partial pressure of oxygen, and carbon dioxide.
  • Temperature.
  • Blood pH.
  • levels of 2,3-DPG.
• These factors ensure adequate O2 delivery to tissues

Carbon Dioxide Transport

• Transport carbon from lungs.
• Carried by dissolution in plasma (7-10%)
• Carbon is chemically bound to hemoglobin (~20%):- Carboxyhemoglobin and Globin
• As bicarbonate ions in plasma (~70%) are, Converted to bicarbonate mainly in red blood cell then released some in plasma