Mechanics of Breathing - Study Notes

Pulmonary Ventilation

  • Ventilation is the process of moving air in and out of the lungs.

  • It relies on pressure changes between the lungs and the atmosphere.

  • Two phases:

    • Inhalation – active process (muscle contraction)

    • Exhalation – passive process (normally), active during forceful breathing

Pressure and Gradient Roles in Breathing

  • Air moves from regions of high pressure to low pressure.

  • Key pressures involved:

    • Atmospheric pressure (~760 mmHg)

    • Intrapulmonary (intra-alveolar) pressure

    • Intrapleural pressure

  • Pressure gradients drive airflow during the breathing cycle.

Boyle’s Law (Gas Behavior in the Lungs)

  • Boyle’s law states that the volume of a gas varies inversely with its pressure at constant temperature.

  • In the context of breathing: as lung volume increases, pressure decreases; as volume decreases, pressure increases.

  • Common relation: P1 V1 = P2 V2 (or that pressure is inversely related to volume when temperature is constant)

Mechanics of Inhalation

  • Inhalation is an active process requiring muscle contraction.

  • Key muscles:

    • Diaphragm contracts and moves downward (flattens)

    • External intercostal muscles lift the ribcage

  • Result:

    • Thoracic volume increases

    • Intrapulmonary pressure decreases

    • Air enters the lungs

Mechanics of Exhalation

  • Exhalation is a passive process under resting conditions; active during forceful breathing.

  • Key events:

    • Diaphragm relaxes and moves upward

    • Intercostal muscles relax, ribcage moves down

  • Result:

    • Thoracic volume decreases

    • Intrapulmonary pressure increases

    • Air exits the lungs

Lung Power: Muscles of Breathing

  • Breathing involves respiratory muscles (primary and accessory) that generate the pressure gradients needed for ventilation.

Lung Compliance and Airway Resistance

  • Lung compliance: the ability of the lungs to expand.

    • High compliance = easy expansion

    • Low compliance = stiff lungs, harder expansion

  • Airway resistance: opposition to airflow.

    • Increased by bronchoconstriction

    • Decreased by bronchodilation

The Diaphragm

  • The diaphragm is the major muscle of ventilation.

  • Anatomy:

    • Divided into right and left hemidiaphragms

    • They merge at the midline into the central tendon

Assessing Respiration

  • Diaphragm is the primary muscle of breathing.

  • Inspiration is an active process; expiration is normally passive.

  • Normal adult respiratory rate range: 12 ext{ to } 20 ext{ breaths/min}

Muscles Involved in Breathing

  • Primary muscles:

    • Diaphragm – main muscle for inspiration

    • External intercostal muscles – expand ribcage during inhalation

  • Accessory muscles (active during increased demand):

    • Sternocleidomastoid

    • Scalenus muscles

    • Pectoralis major

    • Trapezius

  • Expiration (active with effort):

    • Internal intercostal muscles

    • Rectus abdominis

    • External oblique and Internal oblique abdominal muscles

Breathing Patterns (Types)

  • Normal to describe pattern:

    • Shoulder elevation, superior rib cage migration, anterior chest expansion, anterior-posterior expansion, anterior abdominal expansion

  • These describe how the chest wall and abdomen move during different breathing patterns

Normal Ventilatory Pattern

  • Composed of:

    • Tidal volume (Vt): about 500 ext{ mL} (7–9 mL/kg)

    • Ventilatory rate: about 15 ext{ breaths/min} (range 12–18)

    • Inspiratory-to-expiratory (I:E) ratio: about 1:2

Eupnea, Bradynea, Tachypnea, and Other Patterns

  • Eupnea: normal range and rhythm; 12 ext{ to } 20 ext{ breaths/min}, moderate depth

  • Bradypnea: regular rhythm with fewer than 12 ext{ breaths/min}

  • Tachypnea: regular rhythm with more than 20 ext{ breaths/min}

  • Apnea: absence of breathing, can lead to respiratory arrest and death

  • Hypoventilation: decreased rate and depth, leading to increased PaCO₂

  • Hyperventilation: increased rate and depth, leading to decreased PaCO₂

  • Cheyne-Stokes: waxing and waning respiration with periods of apnea

  • Kussmaul’s: very increased rate and depth of breathing

  • Biot’s respiration: rapid, deep respirations with abrupt pauses

Abnormal Ventilatory Patterns and Dyspnea

  • Dyspnea: subjective feeling of breathlessness; signs include:

    • Audible labored breathing, tachypnea

    • Retractions of intercostal spaces

    • Use of accessory muscles

    • Distressed facial expression, nostril flaring

    • Paradoxical chest/abdomen movements

    • Gasping

Static Lung Compliance: Calculation and Meaning (Example 1)