Respiratory System: Structural Components and Function

Respiratory System: Structural Components

Overview of the Respiratory/Ventilatory System

• Regulates the gaseous state of the body's external environment.
• Provides aeration of body fluids during rest and exercise.
• Pulmonary ventilation: Process by which ambient air is brought into and exchanged.

Lungs and Gas Exchange

• Lungs provide the gas exchange surface.
• Efficient due to:
  • Numerous convolutions.
  • Highly vascularized.
  • Large Surface Area: Splayed out lungs cover about half the size of a tennis court.
• Gas transfer:
  • Oxygen moves from alveolar air into alveolar capillaries.
  • $CO_2$ moves from pulmonary capillaries into the alveolar space, then exhaled.
• Lung volume in adults: 4-6 liters.

Structures of the Respiratory System

Trachea

• Conductive portion of the pulmonary system.
• Functions:
  • Adjusts air to body temperature.
  • Filters air.
  • Humidifies air.

Bronchi

• Large tubes serving as primary conduits into each lung.

Bronchioles

• Numerous divisions that distribute air throughout the lung.
• Anatomical Dead Space: Zones 1-16, not directly involved in respiration; transport gases.
• Bronchiole constriction/expansion: Influenced by conditions like asthma.

Respiratory Bronchioles, Alveolar Ducts, and Alveoli

• Zones 17-23: Terminal end branches.
• Alveoli: Over 600,000,000 in a healthy adult.
  • Provide a large surface area for gas exchange.
  • Pores of Kohn: Allow for gas exchange between alveoli.

Gas Exchange Volumes

• At rest:
  • 250 mL of $O_2$ per minute leave alveoli.
  • 200 mL of $CO_2$ per minute diffuse from blood into alveoli.
• Endurance-trained athletes (maximum intensity):
  • $O_2$ uptake is 25 times greater (6,250 mL per minute).

Mechanics of Ventilation

Boyle's Law

• The volume of a gas is inversely related to its pressure.
• $P<em>1V</em>1 = P<em>2V</em>2$
• Example: Balloon with water; changing the volume affects pressure.
• Expiration: High lung pressure relative to atmospheric pressure.
• Inspiration: Low lung pressure allows air to enter.

Fick's Law

• Gas diffusion rate is proportional to tissue area, diffusion constant, and pressure differential.
• V{gas} = {\text{Area}}{ \text{Thickness}} \cdot D \cdot (P1 - P_2)
  • $V_{gas}$: Volume of gas diffusing per unit time.
  • $A$: Area.
  • $T$: Thickness.
  • $D$: Diffusion constant.
  • $P<em>1$ and $P</em>2$: Partial pressures of gases on either side of the membrane.

Inspiration

• The diaphragm contracts and moves downwards (about 10 cm).
• Increases chest cavity volume, lowers lung pressure below atmospheric pressure.
• Air rushes in until pressures equalize.
• Quiet rest: Primarily diaphragm.
• Exercise: Ribs, sternum muscles assist in expanding the chest cavity.

Expiration

• Rest/light exercise: Diaphragm relaxes, volume reduces, pressure increases, air rushes out.
• Strenuous exercise: Internal intercostals and abdominal muscles contract to reduce chest cavity volume and force air out.

Airflow Velocity

• Decreases due to increased tissue cross-sectional area in terminal bronchioles.
• High-speed airflow in upper respiratory passages slows substantially in respiratory guided zones.

Valsalva Maneuver

• Expiratory muscles used in coughing, sneezing, and torso stabilization during heavy lifting.
• Normal breathing: Lung pressure increases ~3 mmHg during expiration.
• Valsalva: Forced exhalation against a closed glottis increases lung pressure significantly (up to >150 mmHg).

Physiological Consequences

• Acute drop in blood pressure due to decreased venous return.
• Reduced blood flow to the brain causes dizziness, vision changes, or fainting.
• Caution needed when prescribing weight training, especially for patients with cardiovascular disease.

Recommendations

• Breathe in on the lowering phase, breathe out during the concentric phase.
• Useful for increasing intra-abdominal pressure during heavy lifts (>80% of 1RM) but should not last more than three seconds.
• Common in powerlifting.
• Monitor aortic pulse pressure during the maneuver.

Lung Volumes and Capacities

• Tidal Volume (TV): Volume inspired or expired per breath.
• Inspiratory Reserve Volume (IRV): Maximum inspiration at the end of tidal inspiration.
• Expiratory Reserve Volume (ERV): Maximum expiration at the end of tidal expiration.
• Total Lung Capacity (TLC): Volume of lungs after maximum inspiration.
• Residual Lung Volume (RLV): Air remaining in lungs after maximum expiration.
• Forced Vital Capacity (FVC): Maximum volume expired after maximum inspiration.
• Inspiratory Capacity (IC).
• Functional Residual Capacity (FRC).

Explanation of Terms

• Device to measure various lung volumes.
• Residual lung volume allows uninterrupted gas exchange, preventing fluctuations in blood gases.

Examples of Residual Lung Volume

• College-aged female: 0.8-1.2 liters.
• College-aged male: 0.9-1.4 liters.
• Elite football player: 0.96-2.46 liters.

Athlete Comparisons

• Differences in vital capacity, forced vital capacity, FEV1 across various sports.
• Adaptations relate to sporting demands.

Aging and Lung Function

• Residual lung volume increases due to loss of lung elasticity.
• Inspiratory and expiratory reserve volumes decrease.
• Training helps maintain respiratory function.
• Forced vital capacity is higher in trained older adults compared to untrained individuals.
• FEV1 values also show similar trends.

Total Lung Capacity

• TLC = FVC + RLV.
• Indirect measurement of residual lung capacity using spirometer with helium or $O_2$ dilution.
• Temporary increase in TLC after exercise due to open peripheral airways and increased thoracic blood volume.