Mechanics of Breathing

Mechanics of Gas Movement and Pressure Dynamics

• Initial State (At Rest): When the glottis is open and no air is moving (such as the moment after exhaling and before inhaling), the pressure in the atmosphere ($P_{Atmosphere}$) is equal to the pressure in the alveoli ($P_{Alveoli}$).

• The Inhalation Trigger: To initiate inhalation, a pressure gradient must be established where P_{Atmosphere} > P_{Alveoli}. Air naturally moves from the high-pressure environment of the atmosphere toward the low-pressure environment of the lungs.

• Mechanism of Pressure Decrease: Alveolar pressure falls because the volume of the thoracic cage increases. According to physical gas principles, as the volume of the container (the lungs) increases while the number of gas molecules remains constant, the pressure within that volume drops.

• Pressure Equilibration: As soon as alveolar pressure becomes lower than atmospheric pressure, gas molecules flow into the alveoli. This air movement continues until the expansion of the thoracic cavity stops, at which point the incoming air equilibrates the pressures again until $P_{Atmosphere} = P_{Alveoli}$.

• The Glottis Effect: If the glottis is voluntarily closed during an attempt to inhale, a "suction" sensation is felt in the throat. This is caused by the vacuum created as the lungs expand against an obstruction, preventing air from entering to equalize the pressure.

Muscles of Inspiration and Expiration

• Quiet Inspiration (Resting Breathing):

  • Diaphragm: At rest, the diaphragm is bowed upward into the thoracic cage. Upon contraction, it "straightens out" or flattens, increasing the vertical dimension of the thoracic cavity.

  • External Intercostal Muscles: Their contraction pulls the ribs upward and anteriorly. This causes the ribs to project outward, moving the chest wall away from the spine and increasing the distance between the sternum and the spine by approximately $20\%$.

  • Accessory Muscles: The sternocleidomastoid, anterior serrati, and scaleni muscles assist in elevating the rib cage during inspiration.

• Passive Expiration: In a healthy individual at rest, exhalation is primarily passive. It occurs when the muscles of inspiration relax, allowing the elastic recoil of the lungs and chest wall to decrease thoracic volume and push air out

  • elastic recoil lung goes inward and increase alveolar pressure

• Heavy Breathing or Forced Expiration: During activities like exercise (e.g., walking up 10 steps), exhalation must be faster and is aided by active muscle contraction:

  • Internal Intercostal Muscles: These pull the sternum downward toward its resting position.

  • Abdominal Recti Muscles: They pull downward on the lower ribs.

  • Abdominal Muscles (General): Their contraction forces abdominal organs upward against the diaphragm, further decreasing the thoracic volume and producing a greater pressure difference to expel air rapidly.

Intrapleural Pressure and Measurement Units

• Negative Intrapleural Pressure: The intrapleural cavity maintains a vacuum-like state. At the start of a breath, the pressure is approximately $-5\,\text{cm H}_2\text{O}$. By the end of a full inhalation (roughly 2 seconds into the cycle), as the chest volume is maximized, the pleural pressure reaches its most negative point of $-7.5\,\text{cm H}_2\text{O}$.

• Understanding the Unit (cm H2O): This is a measure of pressure similar to $\text{mm Hg}$. It represents the pressure required to support a column of water of a certain height.

• Respiratory Cycle Timing: In a typical cycle of 12 respirations per minute (5 seconds per cycle), inhalation takes roughly 2 seconds (creating a $-1\,\text{cm H}_2\text{O}$ alveolar pressure difference that moves $500\,\text{ml}$ of air), while exhalation takes approximately 3 seconds.

Surface Tension, Surfactant, and Laplace's Law

• Surface Tension in Alveoli: The inner surface of alveoli is lined with fluid. Water molecules in this fluid pull toward each other, creating a surface tension that exerts an inward force, tending to collapse the alveolus.

• Laplace’s Law: The pressure required to keep an alveolus open (collapse pressure) is described by the formula: $P = \frac{2T}{r}$

  • $P$ = Pressure

  • $T$ = Surface Tension

  • $r$ = Radius

• The Role of Pulmonary Surfactant: Surfactant is a substance that reduces surface tension. It is more effective at reducing surface tension in smaller alveoli than in larger ones. This stabilizes the lungs by ensuring the distending pressure is approximately the same for all alveoli, preventing smaller alveoli (which have a naturally higher collapse pressure due to a smaller radius) from emptying into larger ones.

• Newborn Respiratory Distress Syndrome: Surfactant synthesis and secretion begin late in gestation. Premature infants often lack sufficient surfactant, leading to very high surface tension. Combined with their naturally smaller alveolar radii, these infants must work extremely hard to inflate their lungs, a condition known as newborn respiratory distress syndrome.

Lung Volumes and Capacities

• Tidal Volume ($V_T$): The volume of air inhaled and exhaled during a single normal, resting breath. It is typically around $500\,\text{ml}$.

• Inspiratory Reserve Volume (IRV): The additional volume of air that can be inhaled with maximum effort after a normal tidal inspiration. This is approximately $3{,}000\,\text{ml}$ (3 liters).

• Inspiratory Capacity (IC): The total amount of air that can be inspired starting from the end of a normal expiration. Formula: $IC = IRV + V_T = 3{,}500\,\text{ml}$.

• Expiratory Reserve Volume (ERV): The extra volume of air that can be forcefully exhaled after the end of a normal tidal expiration.

• Residual Volume (RV): The volume of air remaining in the lungs after a maximal forced expiration. This volume cannot be measured by a standard spirometer because it cannot be blown out.

• Functional Residual Capacity (FRC): The volume of air remaining in the lungs at the end of a normal tidal expiration. Formula: $FRC = ERV + RV$.

• Vital Capacity (VC): The maximum amount of air a person can expel from the lungs after first filling the lungs to their maximum. Formula: $VC = IRV + V_T + ERV$.

• Total Lung Capacity (TLC): The total volume of air the lungs can hold. Formula: $TLC = VC + RV$.

• Spirometry: A floating drum spirometer can measure these dynamic volumes by recording air displaced into a collection cylinder, though it cannot directly determine Residual Volume.

Dead Space and Alveolar Ventilation

• Anatomical Dead Space ($V_D$): The volume of air (approximately $150\,\text{ml}$) that fills the conducting airways (nose, mouth, trachea, etc.) where gas exchange does not occur.

• Physiological Dead Space: This includes anatomical dead space plus the volume of air reaching alveoli that are not functional for gas exchange (due to lack of blood flow/perfusion).

  • In a healthy person, this is negligible.

  • In disease states like Emphysema, alveolar walls are destroyed to create larger, fused alveoli. These receive more ventilation but have poor gas exchange capability due to destroyed capillaries, potentially increasing physiological dead space to $1\text{--}2\,\text{liters}$.

• Minute Volumes:

  • Total Minute Volume: The total amount of air moved into the respiratory system per minute.

  • Alveolar Minute Volume ($V_A$): The volume of fresh air that actually reaches the gas exchange surfaces (alveoli) per minute.

  • Calculation: $V_A = \text{Frequency} \times (V_T - V_D)$.

  • Example Case: $(12\,\text{respirations/min}) \times (500\,\text{ml} - 150\,\text{ml}) = 4{,}200\,\text{ml/min}$.

• Importance: Alveolar ventilation is the primary determinant of the partial pressures of oxygen and carbon dioxide in the blood.