Respiratory System: Mechanics, Muscles, Membranes, and Volumes

Respiratory System

Mechanics of Breathing

The process of breathing involves changes in the volume and pressure within the thoracic cavity, which drive air movement into and out of the lungs. The rib cage and diaphragm play crucial roles in these volume changes.

• Superior movement of the rib cage increases the depth and width of the thoracic cavity, increasing its volume and decreasing pressure within it.

• Diaphragm contraction causes it to tense and move inferiorly, further increasing thoracic cavity volume and decreasing pressure.

• Air flows from high to low pressure: As the thoracic cavity expands (inhalation), air pressure drops (~2 mm Hg), and air flows in. As the cavity contracts (exhalation), air pressure increases, and air is forced out.

• Air movement is based on the difference between atmospheric pressure and intrapulmonary pressure. Atmospheric pressure at sea level is approximately 760 mm Hg.

Example: During inhalation, the diaphragm contracts and moves downward, the rib cage lifts, and the lungs expand, causing air to flow in due to decreased pressure.

Respiratory Muscles

Muscles involved in breathing are classified as primary or accessory, depending on their role and activity level.

• Primary muscles: Diaphragm and external intercostals. These are active during quiet breathing at rest.

• Accessory inspiratory muscles: Used during increased respiratory demand (e.g., exercise). Includes scalene muscles, pectoralis minor, and serratus anterior.

• Accessory expiratory muscles: Active during forced exhalation. Includes internal intercostals, transversus thoracis, rectus abdominis, and internal oblique.

Example: During vigorous exercise, accessory muscles increase the speed and amount of rib movement to accommodate greater oxygen demand.

Pleural Membrane

Lung expansion depends on the behavior of the pleural membranes, which 'tack' the lung to the chest wall via serous fluid and pressure.

• Attractions between the two layers of the pleural membrane (visceral and parietal) are due to water in serous fluid and pressure within the pleural cavity.

• If this is disrupted (e.g., thoracic wall punctured), pneumothorax occurs, causing lung collapse and possible bulging of the chest wall.

Example: A stab wound to the chest can introduce air into the pleural cavity, breaking the seal and causing the lung to collapse.

Respiratory Volumes

Types of Respiratory Volumes

Respiratory volumes measure the amount of air moved during different phases of the breathing cycle. These volumes are important for assessing lung function.

• Tidal Volume (TV): Amount of air inhaled or exhaled during one cycle of quiet breathing (~500 mL).

• Inspiratory Reserve Volume (IRV): Amount of air that can be forcibly inhaled after normal inhalation (~3000 mL).

• Expiratory Reserve Volume (ERV): Amount of air that can be forcibly exhaled after normal exhalation (~1100 mL).

• Residual Volume (RV): Amount of air remaining in the lungs after maximal exhalation; necessary to keep lungs from collapsing (~1200 mL).

• Minimal Volume: Air that remains in lungs even after collapse; cannot be measured in healthy individuals.

• Dead Space: Air that remains in the trachea and airways, not used in gas exchange (~150 mL).

Capacities Derived from Volumes

• Inspiratory Capacity (IC): Total amount that can be physically inhaled. $IC = TV + IRV$

• Vital Capacity (VC): Total amount of air that can be exhaled after maximal inhalation. $VC = TV + IRV + ERV$

• Total Lung Capacity (TLC): Maximum amount of air that can be contained in the lungs. $TLC = TV + IRV + ERV + RV$

• Functional Residual Capacity (FRC): Air in lungs after normal exhale. $FRC = ERV + RV$

Regulation of Breathing

Both air volume and breath rate can be increased to meet metabolic demand. Regulation involves neural and chemical mechanisms.

• Respiratory rate: Number of breaths per minute (12-18 for adults at rest; 18-20 for children).

• Minute ventilation: Volume of air moved each minute. $MV = ext{Respiratory Rate} imes ext{Tidal Volume}$ Example: $12 ext{ breaths/min} imes 500 ext{ mL} = 6000 ext{ mL/min} = 6 ext{ L/min}$

• Alveolar ventilation: Accounts for dead space; not all air reaches alveoli for gas exchange.

Respiratory Control Areas

Groups of neurons in the brainstem control breathing rate and rhythm.

• Medullary respiratory center and pontine respiratory area coordinate breathing.

• Regulated by chemoreceptors (monitor CO2, H+, and pH), lung stretch receptors, and emotional/physical activity.

• Inflation reflex (Hering-Breuer reflex): Prevents overinflation of the lungs.

Gas Diffusion and Transport

Gas Laws and Partial Pressures

Gas exchange in the lungs is governed by physical laws describing the behavior of gases.

• Atmospheric pressure at sea level: 760 mm Hg.

• Dalton's Law: The total pressure of a mixture of gases is the sum of the partial pressures of each individual gas.

• Henry's Law: At a given temperature, the amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with the liquid.

Example: Oxygen makes up ~21% of atmospheric air, so its partial pressure is $0.21 imes 760 ext{ mm Hg} ightarrow 160 ext{ mm Hg}$.

Oxygen Transport

Oxygen is primarily transported in the blood bound to hemoglobin within red blood cells.

• Oxygen saturation: Percentage of heme units containing bound O2 at any moment; dependent on partial pressures.

• Usually does not drop below 75%, even in venous blood.

• Carbon monoxide poisoning: CO irreversibly binds to heme, preventing oxygen transport.

Carbon Dioxide Transport

CO2 is transported in the blood in three forms:

• Dissolved in plasma: ~7% of total CO2.

• As bicarbonate ion (HCO3-): ~70% of total CO2. Formed by the reaction: $CO_2 + H_2O ightarrow H_2CO_3 ightarrow H^+ + HCO_3^-$

• Bound to hemoglobin (carbaminohemoglobin): ~23% of total CO2.

Example: Most CO2 produced by tissues is converted to bicarbonate for transport in plasma.

Summary Table: Respiratory Volumes and Capacities

Volume/Capacity	Definition	Approximate Value (mL)
Tidal Volume (TV)	Air inhaled/exhaled in one cycle	500
Inspiratory Reserve Volume (IRV)	Max air inhaled after normal inhale	3000
Expiratory Reserve Volume (ERV)	Max air exhaled after normal exhale	1100
Residual Volume (RV)	Air remaining after maximal exhale	1200
Inspiratory Capacity (IC)	Total air that can be inhaled	3500
Vital Capacity (VC)	Total air exhaled after maximal inhale	4600
Total Lung Capacity (TLC)	Max air in lungs	5800
Functional Residual Capacity (FRC)	Air in lungs after normal exhale	2300

Additional info: Some values and definitions have been inferred and expanded for completeness and clarity based on standard anatomy and physiology textbooks.