Breathing Compliance Overview

Compliance of Breathing

• Compliance is the change in lung volume per unit change in distending (transpulmonary) pressure.
• The formula for compliance is: C = ΔV/ΔP, where C is compliance, ΔV is the change in lung volume, and ΔP is the change in distending pressure.
• The unit for compliance is ml/cm H2O.
• Lung compliance (CL) is 200 ml/cm H2O.
• Pulmonary system compliance (the lungs and thoracic cage together) is about 130 ml/cm H2O.

Causes of Decreased Pulmonary Compliance

• Rigidity of the lungs:
  • Pulmonary fibrosis (increased fibrous tissue in the lung)
  • Pulmonary edema (prevents the inflation of some alveoli)
  • Atelectasis (collapse of some alveoli)
  • Pulmonary congestion (increased venous pressure, lung engorged with blood)
• Rigidity of the thoracic cage:
  • Deformities of the vertebral column (e.g., kyphosis, scoliosis)
  • Poliomyelitis (respiratory muscle disease)
  • Arthritis (joints of the thoracic cage or vertebral column)

Causes of Increased Pulmonary Compliance

• COPD (Chronic Obstructive Pulmonary Disease), specifically emphysema: Loss of alveolar walls makes the lungs loose and floppy, needing less pressure difference to maintain a large volume.
• Old age: Alterations to lung elastic tissue.

Work of Breathing

• Compliance work (elastic resistance): 65%
  • The work needed to stretch the elastic tissue of the lung and chest wall.
  • Increases in conditions associated with decreased lung compliance.
• Tissue resistance work: 7%
  • Work to overcome the viscosity of lung and chest wall structures (non-elastic tissues).
  • Increases in diseases affecting the thoracic cage (e.g., kyphosis) or respiratory muscle diseases (e.g., poliomyelitis).
• Airway resistance work: 28%
  • Work to overcome airway resistance.
  • The major factor is the airway diameter (medium-sized bronchi).
  • Increases in obstructive lung diseases: emphysema, chronic bronchitis, bronchial asthma.

Control of Ventilation

• The respiratory center is composed of groups of neurons in the medulla oblongata and pons of the brain stem.
• Dorsal respiratory group: Located in the dorsal portion of the medulla; mainly responsible for inspiration.
• Ventral respiratory group: Located in the ventrolateral part of the medulla; mainly responsible for expiration.
• Pneumotaxic center: Located in the superior portion of the pons; controls breathing rate and depth.
• Apneustic center: Located in the caudal part of the pons; sends continuous excitatory impulses to the dorsal respiratory group, causing prolonged inspiration and decreasing respiratory rate.

Chemical Control of Respiration

• Central chemoreceptors: Located on the medulla's ventral surface, sensitive to CO2 in the blood. Not affected by H+ ions (blood-brain barrier).
• Peripheral chemoreceptors: Located in the carotid bodies; detect oxygen changes in the blood. Also respond to carbon dioxide and hydrogen ion changes.
• Effect of CO2 and H+: Excess CO2 or hydrogen ions increase the strength of both inspiratory and expiratory motor signals to the respiratory muscles.

Effect of Oxygen (O2)

• Oxygen does not directly affect the respiratory center in controlling respiration.
• It acts on peripheral chemoreceptors in the carotid and aortic bodies, which then send signals to the respiratory center.