Learn more about the respiratory system anatomy. How does air cycle through the respiratory system and where does gas exchange occur?
About the Respiratory System
The human respiratory system is the primary means through which gas exchange occurs in the body. Gas exchange is necessary to provide oxygen for the aerobic metabolic processes within the cells that make up the tissues throughout the body. These cellular metabolic processes result in waste products, such as carbon dioxide. The gas exchange function of the respiratory system expels these wastes away from the body.
The respiratory system anatomy is divided into upper and lower tracts. The upper tract primarily transports or conducts air into the lower tract, upon inspiration, and then provides a pathway for waste gases from the body upon expiration. The lower tract shares conduction and respiration responsibilities. The respiratory system is also responsible for the production of sound and aids in balancing the pH balance of the blood.
Most breathing in healthy humans occurs through the nose. As air enters the nasal cavity, it is filtered of large debris by hairs lining the inside of the nose, and then further filtered through the nasal conchae, an area of folded, moist tissues; this also warms the air to near body temperature when it is cold. The conchae are lined with special cells and will be discussed further in the next section. Other receptors in the conchae are responsible for smell. The nasal septum divides the nose into a right and left portion.
The nasal conchae, sinuses, the pharynx, the larynx and the trachea are lined with a layer of epithelial cells that have cilia and secrete mucus. These specialized cells perform two vital functions. The mucus traps contaminants in the inspired air, while the cilia acts as an “elevator", moving the contaminants up towards the mouth for expulsion.
There are four paranasal sinuses in the skull. These hollow sections of the skull are lined with the same mucosa as other parts of the respiratory system. They warm and moisten air much like the conchae.
The pharynx is subdivided into three portions: the nasopharynx at the top, the oropharynx in the middle and the laryngopharynx at the bottom. The nasopharynx usually only conducts air. When swallowing food, the uvula raises to block food from entering the nasopharynx. The sides of the nasopharynx connect to the eustachian tubes, which lead to the middle ear. The oropharynx is a shared space for food that is swallowed and air on its way to the lungs. The laryngopharynx is the section that leads directly to the larynx. The laryngopharynx terminates at the top of the larynx, where the epiglottis—a small flap of cartilage that keeps food out of the airway—is located.
The larynx is supported by nine pieces of cartilage and produces sound with expired air. Loudness of sound is based upon the force with which air is expelled. Tone and enunciation are controlled by muscles of the face, tongue and lips.
Below the larynx and in front of the esophagus, the trachea is a rigid tube conducting air to the lungs. Rings of cartilage provide support and structure to ensure the trachea stays open at all times. Damage to the trachea is a medical emergency. At the bottom, the trachea separates into the right and left primary bronchi.
Humans have two lungs: a right and a left. These are divided into lobes. The right lung has three lobes, while the left only has two. Within the lungs, gas exchange occurs.
Bronchi are tubes that conduct air to the part of the lungs where gas exchange occurs. They are differentiated because gas exchange does not occur across their membrane. The left and right primary bronchi divide into secondary bronchi, supplying each lobe. The secondary bronchi divide into tertiary or segmental bronchi which conducts air to portions of the lungs called bronchopulmonary segments. Bronchopulmonary segments are made up of bronchioles which finally branch into terminal bronchioles.
Terminal bronchioles branch into respiratory bronchioles which give rise to alveolar ducts. At the end of the alveolar ducts, alveoli are closely associated with the pulmonary capillaries where gases cross the the membranes of the alveoli and capillaries, a distance of approximately 0.5 μ. Gases move to the area that has the highest concentration. For example, during expiration, there is a higher concentration of carbon dioxide in the bloodstream, so it crosses into the alveoli and is then exhaled.
The human body demands a vast amount of constant oxygen supply. Each lung contains 300 to 400 million alveoli. If the alveoli were opened and laid end to end, the area would be roughly the size of a tennis court.
Not an actual part of the respiratory system, the diaphragm is a muscle that forms the floor of the thoracic cavity. When it contracts, it pulls down and opens the lungs allowing them to fill with air during the inspiration phase of breathing. When it relaxes, it moves upwards and expiration occurs. A few other smaller muscles also aid in the breathing process.