Lung Structure and Ventilation

Respiratory System Anatomy

The upper respiratory tract includes the nose, pharynx, larynx, and trachea. The lower respiratory tract includes the bronchi, bronchioles, lungs, alveoli, and pleural cavities.

The respiratory system anatomy can be categorized by position and is described as belonging to the upper or lower respiratory tract. The upper respiratory tract consists of all structures involving air passage from the nostrils to the trachea; these include the nose, pharynx, and larynx. The upper respiratory tract is responsible for warming air inspired by the mouth and nose and filtering the air of dust, smoke, and pollen. Filtration is achieved by nasal hairs and mucus which trap foreign particles.

During inspiration, air flows through the nostrils to the nasal vestibule. The nasal septum is a wall of cartilage and bone that separates the right nostril from the left nostril. The nasal conchae are passageways of bone in the nasal cavity that slow the flow of air and make it turbulent. Conchae, also called turbinates, look like shelves made of bone. Air then passes through the nasopharynx and oropharynx and into the larynx, or voice box. Air also passes by the epiglottis. The epiglottis is a flap of cartilage that closes off the trachea (windpipe) when swallowing. This prevents food from entering the trachea. Air then moves past the glottis and into the trachea. The glottis is the portion of the larynx that contains the vocal cords and the opening between them. Cricoid cartilage is a ring of cartilage around the trachea that provides the point of attachment for muscles involved in opening and closing the airway. Thyroid cartilage covers the front of the larynx at the point of the vocal cords.

The Upper Respiratory Tract

The upper respiratory tract includes all the structures through which air passes from the nose and mouth to the trachea. It is responsible for warming and filtering the air inspired in the nose or mouth. Nasal hairs and mucus secreted throughout the respiratory tract trap foreign particles, such as dust, smoke, and pollen. Air passes through the pharynx and then the larynx (voice box) before entering the lower respiratory tract.
Once air moves past the larynx, it enters the trachea and then through to the primary right and left bronchi, or air conduits, that lead to the right and left lungs. Then, much like the branches of a tree, the primary bronchi branch off to the secondary bronchi and then tertiary bronchi, each time becoming smaller and smaller. The tertiary bronchi branch into even smaller bronchioles until they reach the terminal bronchiole. At the end of each bronchiole are grape-like clusters called alveoli. There are approximately 300 million alveoli at the ends of the terminal bronchioles. Gas exchange occurs between the capillaries and the alveoli. Carbon dioxide from the blood diffuses into the alveoli and oxygen from the alveoli moves into the blood. The capillaries flow into the pulmonary vein, which returns oxygen-rich blood to the left atrium of the heart to be circulated throughout the body.

During expiration, air starts at the alveoli, where oxygen has been exchanged for carbon dioxide. It then travels in the opposite direction: through the terminal bronchioles to the tertiary bronchi, secondary bronchi, and primary bronchi. The primary bronchi lead to the trachea, then the larynx and pharynx. Air exits through the nose or mouth and into the environment. The pulmonary artery pumps deoxygenated blood from the right ventricle of the heart to the capillary beds surrounding the alveoli of the lungs.

The lower respiratory tract consists of the trachea, bronchi, bronchioles, lungs, and alveoli. The pleurae are a pair of serous membranes that line the thorax and the surface of the lungs. The space within these two membranes, called the pleural cavity, is filled with serous fluid, called pleural fluid. Pleural fluid is contained within the pleural cavity and acts as a lubricant to enable the lungs to move during breathing. It provides a cushion so that the lungs can easily expand and contract within the thoracic cavity. The visceral pleura is the inner membrane that forms one side of the pleural cavity. It covers the outer surface of the lungs. The parietal pleura is the outer membrane that forms one side of the pleural cavity. It is attached to the inner portion of the thoracic cavity. The pleural cavity is bordered by the inner visceral pleural membrane and the outer parietal pleural membrane.

The Lower Respiratory Tract

Once past the larynx, air moves through the trachea, the main bronchi, and down a smaller series of bronchioles where it reaches the alveoli. The exchange of oxygen and carbon dioxide takes place in the alveolar capillaries. The lungs are lined with visceral pleura and the thoracic cavity is lined with parietal pleura. The space between both pleural membranes is the pleural cavity, which is filled with a fluid that cushions the lungs for easy expansion during respiration.