Breathing is a complex process that keeps us alive. It involves the interplay of muscles, pressure changes, and gas exchange. Understanding how we breathe helps us grasp the body's amazing ability to adapt to different situations.

Our respiratory system is controlled by both automatic and voluntary mechanisms. The brain stem regulates our breathing rhythm, while chemical sensors in our body detect changes in blood gases. This dual control allows us to breathe effortlessly and adjust to various activities.

Mechanics of Breathing

Mechanisms of inhalation and exhalation

• Inhalation (inspiration) occurs when the diaphragm contracts and flattens, while the external intercostal muscles contract to raise the rib cage, increasing the volume of the thoracic cavity and decreasing intrapulmonary pressure, causing air to flow into the lungs
• Exhalation (expiration) occurs when the diaphragm relaxes and rises, while the external intercostal muscles relax, lowering the rib cage and decreasing the volume of the thoracic cavity, increasing intrapulmonary pressure and causing air to flow out of the lungs
• Respiratory muscles (including the diaphragm and intercostal muscles) play a crucial role in the mechanics of breathing by altering the thoracic cavity volume

Pressure-volume-resistance relationships in breathing

• Boyle's Law ($P_1V_1 = P_2V_2$) states that pressure and volume are inversely related, meaning that as the volume of the thoracic cavity increases, the pressure within the lungs decreases, and vice versa
• Airflow resistance is determined by the diameter of the airways, with increased resistance in narrower airways (bronchioles) and decreased resistance in wider airways (trachea)
• Poiseuille's Law ($Flow = \frac{\Delta P \pi r^4}{8 \eta L}$) indicates that airflow is directly proportional to the pressure gradient and the fourth power of the radius, and inversely proportional to the length of the airway and the viscosity of the air
• The partial pressure of gases in the alveoli and blood plays a crucial role in gas exchange and respiratory function

Steps of pulmonary ventilation

1. Inhalation begins with the contraction of the diaphragm and external intercostal muscles, increasing thoracic volume and decreasing intrapulmonary pressure, causing air to flow into the lungs
2. Gas exchange occurs as oxygen diffuses from the alveoli into the bloodstream, and carbon dioxide diffuses from the bloodstream into the alveoli
3. Exhalation involves the relaxation of the diaphragm and external intercostal muscles, decreasing thoracic volume and increasing intrapulmonary pressure, causing air to flow out of the lungs

Respiratory volumes and capacities

• Tidal volume (TV) is the volume of air inhaled or exhaled during a normal breath (approximately 500 mL in adults)
• Inspiratory reserve volume (IRV) is the maximum additional volume of air that can be inhaled after a normal inhalation (approximately 3000 mL in adults)
• Expiratory reserve volume (ERV) is the maximum additional volume of air that can be exhaled after a normal exhalation (approximately 1100 mL in adults)
• Residual volume (RV) is the volume of air remaining in the lungs after a maximal exhalation (approximately 1200 mL in adults)
• Inspiratory capacity (IC) is the maximum volume of air that can be inhaled after a normal exhalation, calculated as TV + IRV
• Functional residual capacity (FRC) is the volume of air remaining in the lungs after a normal exhalation, calculated as ERV + RV
• Vital capacity (VC) is the maximum volume of air that can be exhaled after a maximal inhalation, calculated as TV + IRV + ERV
• Total lung capacity (TLC) is the total volume of air in the lungs after a maximal inhalation, calculated as TV + IRV + ERV + RV

Respiratory rate and influencing factors

• Respiratory rate is the number of breaths per minute, with a normal range of 12-20 breaths per minute in adults
• Factors influencing respiratory rate include age (higher in infants and children), activity level (increases with physical exertion), emotional state (increases with stress or anxiety), body temperature (increases with fever), blood pH (decreases with acidosis, increases with alkalosis), and blood oxygen and carbon dioxide levels

Lung mechanics and protective mechanisms

• The pleura, consisting of visceral and parietal layers, reduces friction during breathing and helps maintain negative intrapleural pressure
• Surfactant, produced by type II alveolar cells, reduces surface tension in the alveoli, preventing alveolar collapse and improving lung compliance
• Lung compliance, which is the ability of the lungs to expand and contract, is influenced by factors such as elastin content, surfactant, and pleural pressure

Respiratory Control

Medulla and pons in breathing control

• The medulla oblongata contains the dorsal respiratory group (DRG), which primarily controls inspiration, and the ventral respiratory group (VRG), which primarily controls expiration, while the rhythmicity center generates the basic breathing rhythm
• The pons contains the pneumotaxic center, which modulates the duration of inspiration, and the apneustic center, which prolongs inspiration

Chemical and physical factors of respiration

• Chemical factors affecting respiratory rate and depth include chemoreceptors that detect changes in blood pH, oxygen, and carbon dioxide levels
  • Central chemoreceptors in the medulla oblongata detect changes in cerebrospinal fluid pH
  • Peripheral chemoreceptors in the carotid and aortic bodies detect changes in arterial blood oxygen and carbon dioxide levels
  • Increased carbon dioxide or decreased oxygen stimulates chemoreceptors, increasing respiratory rate and depth
• Physical factors affecting respiratory rate and depth include:
  • Stretch receptors in the lungs and chest wall that detect changes in lung volume, such as the Hering-Breuer reflex, which inhibits further inhalation and promotes exhalation when the lungs are excessively inflated
  • Irritant receptors in the airways that detect foreign particles or irritants, triggering coughing or bronchoconstriction
  • J receptors in the alveoli that detect pulmonary edema or congestion, causing rapid shallow breathing

Nervous vs chemical breathing control

• Nervous control of breathing involves the medulla oblongata and pons regulating the basic breathing rhythm and pattern, with voluntary control able to override automatic breathing (holding breath, speaking), and stretch and irritant receptors providing feedback to the respiratory centers
• Chemical control of breathing involves chemoreceptors detecting changes in blood pH, oxygen, and carbon dioxide levels, stimulating the respiratory centers to alter the respiratory rate and depth, and is the primary driver of respiratory adjustments during exercise or in response to metabolic changes
• Nervous and chemical control interact, with nervous control setting the basic breathing rhythm and pattern, and chemical control modulating the respiratory rate and depth in response to metabolic needs

Exercise impact on breathing patterns

• Exercise increases metabolic demand, leading to higher oxygen consumption and carbon dioxide production, stimulating chemoreceptors to increase respiratory rate and depth
• Proprioceptors in muscles and joints provide feedback to the respiratory centers, contributing to the exercise-induced increase in ventilation
• Other physiological states impacting breathing patterns include:
  • Sleep, where decreased metabolic rate and reduced chemoreceptor sensitivity lead to slower and more regular breathing, with REM sleep associated with irregular breathing patterns
  • Pregnancy, where increased progesterone levels stimulate the respiratory centers, causing hyperventilation, and the enlarged uterus elevates the diaphragm, reducing functional residual capacity
  • High altitude, where lower atmospheric oxygen pressure leads to hypoxemia, stimulating chemoreceptors to increase respiratory rate and depth, with acclimatization involving increased ventilation and red blood cell production to improve oxygen delivery

Integration of respiratory control mechanisms

• Respiratory control mechanisms work together to maintain blood pH, oxygen, and carbon dioxide levels within normal ranges, with chemoreceptors continuously monitoring blood gases and providing feedback to the respiratory centers
• The medulla oblongata and pons adjust the respiratory rate and depth in response to chemical and physical stimuli, with increased ventilation helping to eliminate excess carbon dioxide and maintain blood pH, and decreased ventilation conserving oxygen and preventing alkalosis
• Nervous and chemical control mechanisms interact to fine-tune the breathing pattern according to the body's metabolic needs, achieving homeostasis through the balance between the production and elimination of carbon dioxide and the supply and consumption of oxygen