Unit
- Primary function: The respiratory system facilitates gas exchange, supplying oxygen to the blood and removing carbon dioxide. This process occurs through a series of structures including the nasal cavity, pharynx, larynx, trachea, bronchi, and lungs.
- Upper respiratory tract: The nose and pharynx filter, warm, and humidify incoming air. The nasal mucosa traps particles, and the pharynx directs air to the larynx while also serving the digestive tract.
- Lower respiratory tract: The larynx, trachea, bronchi, and bronchioles form the conducting airways. The larynx protects the airway during swallowing, and the trachea branches into two main bronchi that enter the lungs.
- Lungs and pleura: Each lung is enclosed in a pleural sac with visceral and parietal layers. The pleural cavity contains fluid that reduces friction during breathing and maintains lung expansion.
- Alveoli: The functional units of gas exchange are thin-walled sacs surrounded by capillaries. Their enormous surface area and thin epithelium allow rapid diffusion of oxygen and carbon dioxide.
- Blood supply: The pulmonary circulation delivers deoxygenated blood to the lungs for oxygenation, while bronchial arteries provide oxygenated blood to lung tissue itself.
- Lung volumes and capacities: Tidal volume, inspiratory reserve, expiratory reserve, residual volume, and total lung capacity are measured via spirometry and help assess lung function.
- Control of breathing: The medulla oblongata and pons regulate respiratory rate and depth through chemoreceptors sensitive to pH, carbon dioxide, and oxygen levels.
- Ventilation mechanics: Inspiration occurs when the diaphragm and external intercostals contract, increasing thoracic volume and decreasing intrapleural pressure. Expiration is normally passive, relying on elastic recoil of the lungs and chest wall. In forced expiration, abdominal and internal intercostal muscles actively compress the chest.
- Partial pressures and gas diffusion: Oxygen and carbon dioxide move down their partial pressure gradients across the alveolar capillary membrane. Normal arterial partial pressure of oxygen is 80 to 100 mmHg, while carbon dioxide is 35 to 45 mmHg. Diffusion efficiency depends on membrane thickness, surface area, and ventilation perfusion matching.
- Pulmonary surfactant: Produced by type II alveolar cells, surfactant reduces surface tension within the alveoli, preventing collapse at low lung volumes. It also helps maintain alveolar stability and decreases the work of breathing. Deficiency in premature infants can lead to respiratory distress syndrome.
- Common respiratory conditions: Asthma involves reversible airway obstruction due to inflammation and bronchoconstriction. Chronic obstructive pulmonary disease (COPD) includes emphysema and chronic bronchitis, leading to airflow limitation. Pneumonia is an alveolar infection with fluid accumulation, impairing gas exchange.
Terminology
- Tidal volume (Vt)
- The volume of air inhaled or exhaled during normal, quiet breathing.
- Vital capacity (VC)
- The maximum volume of air exhaled after a maximal inhalation.
- Residual volume
- The volume of air remaining in the lungs after a maximal exhalation.
Quick Revision
- The respiratory system consists of upper and lower tracts with conducting and respiratory zones.
- Gas exchange occurs in the alveoli across a thin membrane.
- Lung volumes are measured through spirometry and are key to diagnosis.
- Breathing is controlled by the brainstem in response to chemical stimuli.