About the Respiratory System

The respiratory system is a vital system for life. Its job is to bring oxygen into the body, remove carbon dioxide from the body and to do this safely with minimal risk of infection, toxin absorption or blockage. We often divide the respiratory system into two sections – the upper respiratory tract and the lower respiratory tract. The upper respiratory tract ends and the lower respiratory tract begins where the throat divides into the trachea and the oesophagus.

Upper Respiratory System

The upper respiratory tract refers to all of the structures involved in respiration above the level of the beginning of the trachea. These structures include the nasal cavity, the oral cavity and the pharynx which is made up of three parts – the nasopharynx, the oropharynx and the laryngopharynx. Air is drawn into the body through the nasal cavity and the oral cavity.

Nasal Cavity

The nasal cavity is the air-filled space behind the nose. There are two sites separated by the nasal septum and each site corresponds to one nostril. The nasal cavity has several functions. One of its functions is to warm or cool air to body temperature. Another is to humidify incoming air. It also functions to trap dust, pathogens and other particles as well as providing our sense of smell.

Oral Cavity

The other entry point for air is the oral cavity. The oral cavity is the air-filled space within the mouth. The oral cavity has a few respiratory functions. Similarly to the nasal cavity, it acts to warm or cool air to body temperature as well as functioning to humidify incoming air. It also provides our sense of taste. Special senses such as taste and smell are important in identifying the presence of potentially harmful air.

Inhaled air is brought from the nasal cavity and oral cavity into the pharynx – the area you might know as the throat. There are two pipes leaving the pharynx – the trachea and the esophagus. The pharynx guides air into the trachea and food into the oesophagus. It also has a role in speech for producing vowel sounds. The pharynx has three parts – a part from the nasal cavity which is the nasopharynx, a part from the oral cavity which is the oropharynx, and a common part where these two meet which is the laryngopharynx. The laryngopharynx ends at the beginning of the trachea marking the end of the upper respiratory tract. Now we'll follow inhaled air down into the lower respiratory tract.

Lower Respiratory Tract

The lower respiratory tract is made up of several structures. The components in order are the trachea, the bronchi, the bronchioles, the alveoli, and more generally the lungs.

Trachea

The beginning of the trachea marks the top of the lower respiratory tract. It travels down the neck into the thorax where it splits into two. Known more commonly as the windpipe, the trachea is responsible for bringing air down the neck and into the chest. It also produces mucus to trap pathogens and particles that managed to get past the defenses of the upper respiratory tract. The trachea is located towards the front of the neck in the center. It's surrounded by cartilage arranged in C-shaped bands making it firm but expandable to allow air in.

Bronchi

Next on our way down are the bronchi – a branching network of airways arranged in an inverted tree formation. Like the trachea, they have cartilaginous rings around them to hold them open. The amount of cartilage decreases as they go deeper into the lungs as more pliability of the airway is required. There are three levels of bronchi. First, we have the primary bronchi also known as the principle bronchi, then we have the secondary or lobar bronchi, and finally we have the tertiary or segmental bronchi.

The trachea splits into two primary bronchi. This split happens deep to the Angle of Louis which is the bump near the top of your breast bone. Each primary bronchus corresponds to one lung. Once they enter the respective lung, each primary bronchus splits into several smaller secondary bronchi which then split into many even smaller tertiary bronchi. In this way, they direct air to and from the bronchioles.

The Bronchioles

The bronchioles are the smallest airways in the lung even smaller than the tertiary bronchi. They are distributed throughout the lungs and do not have cartilaginous rings. Their primary function is to carry air in and out of the alveoli, however, they can also secrete mucus to trap any particles or pathogens which may be inhaled that far into the respiratory tract.

Continuous with the bronchioles are the alveoli which are tiny air sacs and the terminal points of the airways. The alveoli exist as clumps at the end of each bronchiole so are also distributed throughout the lungs. Oxygen from inhaled air is absorbed through the walls of the alveoli into the blood stream. Similarly, carbon dioxide and certain other waste products diffuse through the wall from the blood and are subsequently exhaled. This process is called gas exchange and is their role in respiration.

The Lungs

The lungs are the organs responsible for gas exchange. The secondary and tertiary bronchi – the bronchioles and the alveoli – all branch within the lungs. The lungs also consist of the pulmonary blood vessels and the tissues lining and supporting the airways. They are located within the thoracic cavity and inflate and deflate with the chest wall movement. Each lung is divided into lobes. There are three lobes on the right and two lobes on the left. There are only two lobes on the left-hand side to allow space for the heart.

Blood Supply

The bronchi, bronchioles and lungs benefit from a dual blood supply and drainage. The first supply is through the bronchial arteries which fuel the cells of the tract. They bring oxygenated blood from the thoracic aorta and drain into the bronchial veins and the pulmonary veins. The second supply comes from the pulmonary arteries which bring deoxygenated blood from the right ventricle of the heart to the alveoli of the lungs for gas exchange. They then drain into the pulmonary veins to take the now oxygenated blood to the left atrium of the heart, ready to be pumped into the systemic circulation to supply the tissues of the body. Through this pulmonary circulation, the lungs enact their function of removing waste from the blood and supplying the body with oxygen.