Breathing

Breathing Learning Objectives

• By the end of this section, you will be able to:

  • Describe how the structures of the lungs and thoracic cavity control the mechanics of breathing.

  • Explain the importance of compliance and resistance in the lungs.

  • Discuss problems that may arise due to a ventilation/perfusion (V/Q) mismatch.

Anatomy and Physiology of the Lungs

• Location and Protection:

  • Mammalian lungs are situated in the thoracic cavity, surrounded by:

  • Rib cage

  • Intercostal muscles

  • Chest wall

  • Bottom of the lungs is bounded by the diaphragm, a skeletal muscle crucial for breathing.

• Mechanistic Coordination: Breathing involves coordination between:

  • Lungs

  • Chest wall

  • Diaphragm

Types of Breathing

• Amphibians:

  • Young amphibians (tadpoles) use gills for respiration and live in water.

  • Adult amphibians:

  • May lack or possess a reduced diaphragm, causing forced lung breathing.

  • Utilize skin diffusion (must keep skin moist) as an alternative respiration method.

• Birds:

  • Face unique challenges due to flying, which requires high oxygen consumption.

  • Have evolved:

  • Lungs specializing in gas exchange.

  • Air sacs for unidirectional airflow, enhancing efficiency of gas exchange.

  • Air flows:

  • From posterior air sacs to lungs, out through anterior air sacs.

  • Opposite of blood flow, allowing for optimal utilization of oxygen even at high altitudes.

  • Requires two full cycles of intake and exhalation to clear air from lungs.

Evolutionary Connections

• Birds evolved from theropod dinosaurs, which had a similar flow-through respiratory system involving lungs and air sacs.

• Fossil evidence supports the connection between ancient flying dinosaurs and modern birds, indicating long-term evolutionary changes in respiratory systems.

Human Breathing Mechanics

• Boyle’s Law:

  • In a closed system, pressure and volume are inversely related. As volume decreases, pressure increases, and vice versa.

  • This law explains the mechanics of breathing:

  • Inhalation:

    • Lung volume increases due to diaphragm contraction, lowering thoracic pressure.

    • Air flows into the respiratory passages due to pressure differential.

    • Chest wall expands assisted by intercostal muscle contraction.

  • Exhalation:

    • Lungs recoil passively, expelling air as the diaphragm relaxes and thoracic pressure increases.

Respiratory Structures

• Lungs are surrounded by:

  • Visceral pleura: Covers the lung tissue.

  • Parietal pleura: Lines thoracic cavity interior.

• Intrapleural Space: Contains fluid to reduce friction and protect lung tissue.

• Pleurisy: A painful condition results from inflamed pleura, increasing thoracic pressure and reducing lung volume.

The Work of Breathing

• Respiratory Rate:

  • Average is 12–15 breaths/minute under resting conditions.

  • Influences alveolar ventilation, essential for preventing CO2 buildup.

• Ventilation Regulation:

  • Constant ventilation can be obtained by adjusting:

  • Increasing respiratory rate while decreasing tidal volume (shallow breathing).

  • Decreasing respiratory rate while increasing tidal volume.

• Types of Work during Breathing:

  • Flow-resistive Work: Related to the work of alveoli and lung tissues.

  • Elastic Work: Pertains to intercostal muscles, chest wall, and diaphragm.

  • Changes in respiration rate affect types of breathing work: higher rate increases flow-resistive work, lowering elastic work.

Surfactant

• Surfactant comprises phospholipids and lipoproteins that reduce surface tension in alveoli.

• Functions include:

  • Preventing alveolar collapse by lowering surface tension, facilitating easier lung inflation.

  • Analogy: Surfactant's function is akin to detergent used in inflating balloons - it reduces the effort required for inflation.

• Premature infants may lack sufficient surfactant, risking respiratory distress syndrome due to increased inflation effort.

• Importance: Surfactant helps prevent small alveoli from collapsing relative to larger ones.

Lung Resistance and Compliance

• Pulmonary Diseases: Affect gas exchange through changes in lung compliance and resistance.

  • Compliance: The elasticity of the lungs.

  • Resistance: The degree of obstruction in airways.

• Restrictive Diseases: Examples include respiratory distress syndrome and pulmonary fibrosis:

  • Characterized by decreased compliance, making lung tissue stiff.

  • Result:

    • Positive intrapleural pressure causes airway collapse during exhalation.

    • Lower forced vital capacity (FVC) and prolonged exhalation times.

• Obstructive Diseases: Include emphysema, asthma, and pulmonary edema:

  • Emphysema:

  • Causes destruction of alveolar walls, lowering surface area for gas exchange.

  • Increased compliance due to loss of elastic fibers, trapping air during exhalation.

  • Asthma:

  • Triggered by inflammation, causing airway obstruction.

  • Results from factors such as edema, bronchoconstrictions, and increased mucus.

  • Result:

  • Increased volume of trapped air post-exhalation, leading to compensatory high lung volumes.

• FEV1/FVC Ratio:

  • Used to diagnose restrictive vs. obstructive diseases:

  • Restrictive:

    • Lower FVC but reasonable exhalation speed.

  • Obstructive:

    • Slow exhalation and reduced FVC.

    • FEV1/FVC ratio in obstructive disease < 69% vs. > 88% in restrictive.

Dead Space and Ventilation/Perfusion (V/Q) Mismatch

• V/Q Mismatch: Refers to discrepancies between ventilation (V) and perfusion (Q), causing reduced gas exchange efficiency.

• Types of Dead Space:

  • Anatomical Dead Space: Originates from structural failures in airway function (e.g., gravity's effect on lung position).

  • Ineffective ventilation in specific lung regions leads to reduced gas exchange.

  • Physiological Dead Space: Results from functional impairment (such as infection or edema) obstructing ventilation without impacting perfusion.

• Compensatory Mechanisms: The lung can adapt to V/Q mismatches:

  • If ventilation exceeds perfusion:

  • Arterioles dilate, and bronchioles constrict to enhance perfusion.

  • If perfusion exceeds ventilation:

  • Arterioles constrict, and bronchioles dilate to balance gas exchange.