Mechanics of Breathing & Gas Exchange

Mechanics of Breathing / Gas Exchange Review

• There are 4 learning objectives on mechanics of breathing / gas exchange

Respiratory System Components

• Includes nose and nasal cavity
• Includes pharynx
• Includes larynx
• Includes trachea
• Includes bronchi and their branches
• Includes lungs, which contain alveoli
• Includes conducting zones and respiratory zone

Pleural Cavity and Pleural Membranes

• The lungs are surrounded by a double layered membrane
• Pleura are serous membranes that line the pleural cavity
• Parietal pleura attaches to the thoracic cavity & diaphragm walls
• Visceral pleura adheres to the surface of the lungs
• Pleural fluid fills and lubricates the space between the pleura in the pleural cavity/sac

Respiratory Pressures

• Described relative to atmospheric pressure (Patm) ~ 760 mmHg
• Negative respiratory pressure is <760 mmHg
• Positive respiratory pressure is >760 mmHg

Intrapulmonary Pressure (Ppul)

• Ppul increases and decreases with breathing phases
• Ppul eventually equalizes with atmospheric pressure
• Ppul determines the direction of air flow

Intrapleural Pressure (Pip)

• Pip is pressure in the plural cavity
• Pip is always ~ 4 mmHg less than Ppul
• Pip maintains the pull on lungs

Transpulmonary Pressure (PTpul)

• PTpul is the difference between Ppul and Pip
• PTpul prevents lungs from collapsing

Volume and Pressure Changes

Boyle’s Law

• States that at a constant temperature, the pressure exerted by a gas varies inversely with the volume
• Volume increase results in decreased pressure
• Volume decrease results in increased pressure
• Pressure changes lead to gas flow, equalising pressure, due to volume changes in the thoracic cavity

Mechanics of Breathing - Inspiration

• Diaphragm and external intercostal muscles (inspiratory muscles) contract causing the rib cage to rise
• The lungs are stretched and the intrapulmonary volume increases
• Intrapulmonary pressure drops below atmospheric pressure
• Air flows into the lungs down the pressure gradient until reaching intrapulmonary pressure = atmospheric pressure

Inspiration Continued

Normal Quiet Breathing (Eupnea)

• Occurs via the relative contribution of the diaphragm and intercostal muscles
• Diaphragm dominates during rest
• Pregnancy causes change to costal breathing

Forced Breathing (Hypernea)

• Involves extra muscle groups

Mechanics of Breathing - Expiration

• Inspiratory muscles relax, rib cage descends due to gravity
• Thoracic cavity volume decreases
• Elastic lungs recoil passively, intrapulmonary volume decreases
• Intrapulmonary pressure rises above atmospheric pressure
• Gases flow out of the lungs down the pressure gradient until reaching intrapulmonary pressure = atmospheric pressure

Factors Affecting Pulmonary Ventilation

Airway Resistance

• Inversely proportional to airway diameter
• Normally insignificant
• Disease increases resistance
  • Mucus narrows airways
  • Irritants
  • Inflammatory chemicals activate parasympathetic reflexes causing bronchoconstriction
• Bronchodilators decrease resistance and increase airflow

Resistance to Airflow

• Greatest in the bronchi near the trachea and in large bronchioles
• Smooth muscle in the bronchiolar wall is very sensitive to neural control and chemicals
• Resistance in smaller bronchioles is important in some disease states
  • Smaller size
  • Muscle contraction
  • Oedema in walls
  • Mucus collection in lumen

Alveolar Surface Tension

• At the water/air interface, the water molecules on the surface have a strong attractive force for one another
• The inside alveoli has the water surfaces attempting to contract and force air out of lungs which results in Alveoli collapse
• Net effect: Elastic contractile force

Surfactant

• Secreted by Type II alveolar epithelia
• Detergent like substance containing phospholipids, proteins and ions
• Reduces surface tension in water
• Increases lung compliance
• A lack of surfactant leads to Infant respiratory distress syndrome
• Premature babies often have little or no surfactant

Lung Compliance

• The extent to which the lung volume expands given an increase in transpulmonary pressure
• Involves elastic forces of the lung tissue and alveolar surface tension
• Healthy lungs have high compliance
• Decreased compliance results in increased force required to fill and empty the lungs
• Decreased elasticity of lungs, fibrosis, leads to decreased compliance
• Decreased surfactant production leads to decreased compliance
• Decreased thoracic mobility, arthritis, paralysis, leads to decreased compliance
• Alveolar damage, emphysema, leads to increased compliance where air moves in and out of the lung more easily, but loss of respiratory surfaces restricts gas exchange

Pneumothorax

• Occurs when air enters the pleural cavity which it shouldn't normally
• Can occur when the chest wall is punctured
  • Air flows down the pressure gradient from the atmosphere into the pleural space
  • Lung collapses
• Can occur with a hole in the lung wall, which is a disease state

General Principles of Gas Exchange

• Involves the diffusion of gases and gas laws

Gas Exchange: Diffusion of Gases and Gas Laws

Dalton’s Law of Partial Pressures

• The total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each gas
• Atmospheric air consists of PN2 (78.6%)+ PO2 (20.9%) + PCO2 (0.04%) + PH2O (0.46%) and has a pressure of 760 mm Hg (100%)

Henry’s Law

• States that when a mixture of gases is in contact with a liquid, each gas will dissolve in proportion to its partial pressure
• How much gas dissolves is dependent on its solubility
• CO2 is twenty times more soluble in water than O2

Composition of Alveolar Air

• Atmospheric air contains mostly O2 and N2
• Alveolar air is mostly CO2 and water vapour
• Differences in the gases are due to gas exchange in the lungs, humidification, and mixing of alveolar gas

Pulmonary Gas Exchange

• Factors that influence the movement of O2 and CO2 across the respiratory membrane include,
  • Partial pressure gradients and gas solubilities, matching of alveolar ventilation and pulmonary blood perfusion, and structural features of the respiratory membranes

Partial Pressure Gradients & Gas Solubilities

• Alveolar partial pressure compared to Venous partial pressure

• PO2 of venous blood is 40mmHg, PO2 in the alveoli is 104mmHg

• O2 diffuses rapidly down the pressure gradient from the alveoli into the capillary blood

  • Equilibrium in 0.25s
  • RBCs remain in capillaries 0.75s

• PCO2 is 45mmHg in capillaries and 40mmHg in alveoli

• Although there is a steeper gradient for O2, equal amounts of the gases are exchanged

Ventilation-Perfusion Coupling

• Requires close coupling between alveolar ventilation and blood flow in pulmonary capillaries, perfusion, for efficient gas exchange

Features of Respiratory Membranes

• Healthy lungs have a respiratory membrane thickness of 0.5 - 1.0 μm thick which makes gas exchange very efficient

• Accumulation of fluid in lungs causes exchange membranes to thicken resulting in inadequate gas exchange

• Healthy lungs have a surface area of ~90m2 in adult males

• Functional surface area is reduced in emphysema, from tumours, and from inflammatory disease

Oxygen Transport

• 98.5% of is carried bound to hemoglobin within RBCs

• 1.5% is dissolved in plasma

• The rate at which Hb binds or releases O2 is regulated by: PO2, temperature, blood pH, PCO2, and 2,3-DPG

• These factors ensure adequate delivery of O2 to tissue cells

Carbon Dioxide Transport

• Transported from tissue cells to the lungs
• Dissolved in plasma, 7-10%
• Chemically bound to hemoglobin, ~20%, as carboxyhemoglobin and globin
• As bicarbonate ions in plasma, ~70%
  • Converted to bicarbonate mainly in RBCs and then released into the plasma
  • Some conversion in plasma