Anatomy of Respiratory System II: Lower Respiratory Tract

The lower respiratory tract comprises the larynx, trachea, bronchial tree, alveoli, lungs, and pleurae. The larynx houses the epiglottis, vocal cords, and glottis. Air then travels through the trachea, which branches into bronchi that divide into smaller bronchioles, eventually leading to alveoli where gas exchange occurs. The lungs are protected by the pleurae, double-layered serous membranes.

The trachea, or windpipe, is a cylindrical tube about 10-12 centimeters long composed of 16 to 20 C-shaped rings of hyaline cartilage connected by muscles and ligaments. These cartilage rings keep the trachea open for airflow. The tracheal lining comprises ciliated pseudostratified columnar epithelium with goblet cells that produce mucus to trap particles and protect the airway.

The trachea bifurcates into right and left primary bronchi, which branch into lobar bronchi corresponding to lung lobes—three on the right and two on the left. These continue dividing into segmental bronchi and bronchioles. As bronchi branch smaller, cartilage decreases and muscle increases, allowing greater control over airway resistance and airflow distribution through bronchoconstriction and bronchodilation.

Alveoli are tiny sac-like structures at the end of the bronchiolar tree serving as the primary site for gas exchange. Oxygen diffuses from inhaled air into the blood, while carbon dioxide moves from blood into the alveoli for exhalation. This gas exchange is essential for delivering oxygen to body tissues and removing metabolic waste, supporting physiology of respiration i functions of the respiratory system.

The right lung contains three lobes—upper, middle, and lower—while the left lung has only two lobes: upper and lower. This structural difference exists because the heart occupies space on the left side of the chest. Both lungs are elastic organs housed within the thoracic cage and protected by the ribcage and diaphragm.

The pleurae consist of two layers: the visceral pleura directly covering the lung surface and the parietal pleura lining the chest wall and diaphragm. The pleural cavity between these layers contains lubricating fluid that reduces friction during breathing movements, allowing smooth expansion and contraction of the lungs during respiration.

The diaphragm is a dome-shaped muscle positioned between the thoracic and abdominal cavities. During inhalation, it contracts and flattens, expanding the thoracic cavity volume and allowing the lungs to fill with air. This muscular action is fundamental to ventilation, enabling efficient air movement through the lower respiratory tract structures.