Overview
This lecture covers the mechanics and regulation of pulmonary ventilation, focusing on how pressure changes drive breathing, lung capacities, and the control of blood pH through respiration.
Mechanics of Breathing
- Pulmonary ventilation moves air in and out of lungs by changing thoracic pressure.
- Breathing has two phases: inspiration (inhalation) and expiration (exhalation).
- Diaphragm is the main muscle for breathing, controlled by the phrenic nerve.
- Inspiration is an active process using muscular contraction; expiration is usually passive (relaxation).
- Boyle’s Law: When lung volume increases, pressure decreases, causing air to flow in; when volume decreases, pressure increases, pushing air out.
- Thoracic volume increases by diaphragm flattening (superior-inferior), rib elevation (lateral), and sternum movement (anterior-posterior).
- Forced inspiration involves additional muscles (external intercostals, sternocleidomastoid, pectorals).
- Forced expiration uses internal intercostals and abdominal muscles.
Surfactant and Lung Function
- Surfactant is a lipid-rich chemical that reduces alveolar surface tension and prevents collapse.
- Newborns, especially preemies, may lack surfactant, leading to respiratory distress syndrome; artificial surfactant is used as treatment.
Neural Control of Breathing
- Phrenic nerve stimulates diaphragm; intercostal nerves activate rib muscles for respiration.
- Control is mostly involuntary, regulated by brainstem centers.
Lung Volumes and Capacities
- Tidal Volume: Air moved in/out during normal breath (~500 mL).
- Inspiratory Reserve Volume: Extra air inhaled after normal inspiration (~3,000 mL).
- Expiratory Reserve Volume: Extra air exhaled after normal expiration (~1,100 mL).
- Residual Volume: Air remaining after maximal exhalation (cannot be measured directly).
- Vital Capacity: Maximum air exhaled after maximal inhalation; key measure of lung health.
- Total Lung Capacity: Vital capacity plus residual volume (~6,000 mL average).
Reflexes and Other Breathing Functions
- Reflexes: Coughing (clears lower tract), Sneezing (clears upper tract).
- Other air movements: Laughing, crying, hiccups (diaphragm spasm), yawning (possibly inflates unused alveoli), speech (air over vocal cords).
- Dead space refers to areas of lungs that do not participate in gas exchange.
Blood pH and Respiratory Control
- Main purpose of respiration is removal of CO₂ to prevent acid buildup.
- CO₂ combines with water to form carbonic acid, increasing blood acidity (lower pH).
- Proper breathing keeps blood pH balanced (7.35–7.45).
- Hypoventilation (not enough CO₂ removal) causes acidosis; hyperventilation (too much CO₂ removal) leads to alkalosis.
Key Terms & Definitions
- Pulmonary Ventilation — Movement of air into and out of the lungs.
- Boyle’s Law — Pressure and volume of a gas are inversely related.
- Diaphragm — Primary muscle for breathing, separates thoracic and abdominal cavities.
- Surfactant — Chemical that prevents alveolar collapse by reducing surface tension.
- Tidal Volume — Air volume in a normal breath.
- Inspiratory/Expiratory Reserve Volume — Additional air inhaled/exhaled beyond normal breath.
- Residual Volume — Air left in lungs after maximal exhalation.
- Vital Capacity — Total air exhaled after maximal inhalation.
- Dead Space — Lung regions where air doesn't participate in gas exchange.
- Phrenic Nerve — Nerve controlling diaphragm contractions.
- Acidosis/Alkalosis — Blood pH imbalances due to improper CO₂ elimination.
Action Items / Next Steps
- Review matching definitions of lung volumes and capacities for exam preparation.
- Understand roles of specific muscles and nerves in breathing.
- Know terms relating to lung function and blood pH regulation.