Respiratory system

revised 2025-08-28 1:00:48 PM EDT

The respiratory system is a network of organs and structures that facilitate the process of respiration: bringing oxygen into the body and removing carbon dioxide from the body. These gases are exchanged between the environment, the bloodstream, and the cells of the body, allowing for cellular energy production and waste elimination.

Respiration involves three component processes, which are sometimes called stages or phases despite the fact that they are happening simultaneously:

1. Pulmonary ventilation – the physical movement of air in and out of the lungs (commonly referred to as breathing),
2. External respiration – the exchange of gases between the air within the lungs and the surrounding blood vessels,
3. Internal respiration – the delivery of oxygen to body cells and the removal of carbon dioxide at the tissue level.

Pulmonary ventilation (“breathing”)

Breathing begins when air is drawn in through the respiratory passageways and into the lungs. This movement is made possible by the diaphragm, a dome-shaped skeletal muscle that separates the thoracic and abdominal cavities. When the diaphragm contracts, it flattens and expands the thoracic cavity, reducing pressure and allowing air to flow in. External intercostal muscles between the ribs assist in lifting the ribcage during this process. This action increases the volume of the chest cavity, causing inhalation.

During exhalation, these muscles relax, and the lungs naturally recoil to a smaller size due to their elastic tissue and the tension of the membranes surrounding them. This returns the thoracic cavity to a smaller volume and pushes air out of the lungs.

Conditioning the air

Air typically enters through the nose, which is structured for both protection and conditioning of incoming air. The nasal cavity, divided by the nasal septum, is lined with a mucous membrane rich in blood vessels. This lining warms, humidifies, and filters the air before it travels deeper into the respiratory tract.

Just inside the nostrils are the vestibules, lined with coarse hairs that trap dust and other large particles. Further into the nasal cavity, goblet cells in the epithelium produce mucus that captures smaller particles. Tiny, hair-like cilia help move this mucus toward the throat, where it can be swallowed or expelled.

The nasal conchae, three curled bony projections on the lateral walls of the nasal cavity, create turbulence in the airflow. This allows for better warming and filtration of the air. The superior nasal concha also contains most of the olfactory receptors responsible for our sense of smell.

Tears from the eyes drain into the nasal cavity via the nasolacrimal ducts, helping to keep nasal passages moist. Surrounding the nasal cavity are four pairs of paranasal sinuses: air-filled spaces in the skull that are lined with mucous membranes and connected to the nasal passages. These sinuses assist in resonating the voice and also contribute to the production of mucus that helps trap airborne contaminants.

Pharynx and larynx

The pharynx, or throat, is a muscular tube that extends from the back of the nasal cavity to the esophagus and larynx. It serves as a shared conduit for air (leading to the lungs) and food (leading to the stomach). The pharynx is divided into three regions:

• Nasopharynx – located behind the nasal cavity,
• Oropharynx – behind the mouth,
• Laryngopharynx – leading to the larynx and esophagus.

Air continues its journey through the larynx, commonly known as the voice box. The larynx plays a dual role: it helps direct air into the trachea and food into the esophagus, and it produces sound for speech.

The epiglottis, a flap of cartilage at the top of the larynx, acts as a switch that covers the larynx during swallowing to prevent food from entering the airway. The thyroid cartilage, often externally visible as the “Adam’s apple,” provides structure, while the cricoid cartilage forms the base of the larynx and connects it to the trachea.

Inside the larynx are the vocal folds, or vocal cords: bands of mucous membrane that vibrate when air passes through, producing sound. The tension and length of the folds can be adjusted to change pitch and tone.

The airway network

The trachea, or windpipe, is a flexible tube that carries air from the larynx into the chest. It is lined with ciliated pseudostratified epithelium and goblet cells, which trap and help remove inhaled particles. The cilia beat upward to move debris-laden mucus out of the airway.

To maintain its shape, the trachea is reinforced by C-shaped cartilage rings, which are open at the back to allow space for the adjacent esophagus to expand during swallowing. At its base, the trachea splits into the primary bronchi, with one bronchus entering each lung.

Each primary bronchus branches into secondary (lobar) bronchi, which serve the lobes of the lungs (three on the right and two on the left). These then divide into tertiary (segmental) bronchi, which supply air to the bronchopulmonary segments of each lobe.

As the bronchial tree continues to branch, the airways become smaller and eventually become bronchioles, which lack cartilage and instead rely on smooth muscle for support and regulation of airflow. These bronchioles eventually lead to the terminal bronchioles, and finally to the alveolar ducts and alveoli.

The lungs and their alveoli

The lungs are divided into lobes by deep grooves called fissures. The right lung typically has three lobes (superior, middle, and inferior), while the left has two (superior and inferior) to accommodate space for the heart.

Each lung lobe is subdivided into bronchopulmonary segments, which are further divided into lobules. Each lobule contains clusters of alveoli, the tiny, balloon-like sacs where gas exchange occurs.

The alveoli are composed of thin simple squamous epithelial cells, allowing oxygen and carbon dioxide to diffuse easily across their walls. A dense network of capillaries surrounds each alveolus, facilitating close contact between air and blood. Alveolar macrophages patrol the interior, engulfing and digesting foreign particles and pathogens.

Some alveolar cells produce surfactant, a slippery substance that reduces surface tension and prevents alveolar collapse during exhalation.

Each lung is encased in a double-layered membrane known as the pleura. The parietal pleura is the layer that lines the inner surface of the chest wall, while the visceral pleura is the layer that directly covers the lung tissue. The space between these layers – the pleural cavity – contains a small amount of lubricating fluid that allows smooth movement as the lungs expand and contract with each breath.

Gas exchange with the blood

During external respiration, freshly inhaled air fills the alveoli. Oxygen moves from the alveolar air across the respiratory membrane into the blood in the surrounding capillaries. At the same time, carbon dioxide – a waste product of metabolism all over the body – diffuses from the blood into the alveoli to be exhaled.

Oxygen binds to hemoglobin in red blood cells for transport throughout the body. As this oxygen-rich blood circulates, it reaches capillaries in various tissues. Here, internal respiration takes place: oxygen detaches from hemoglobin and diffuses into nearby cells to support cellular respiration (ATP production inside of cells). In turn, carbon dioxide produced by cells enters the bloodstream and travels back to the lungs to be expelled.