Bronchi & Bronchioles

Primary Bronchi

As the trachea descends toward the diaphragm, it branches into the 2 primary bronchiThe first branches of the trachea that lead into the lungs. Also called lobar bronchi: right and left.  The left bronchus is ever-so-slightly higher than the right one, making room for the heart.  The primary bronchiThe large airways that branch from the trachea into the lungs, dividing into smaller bronchioles. pierce the lung tissue. They branch into secondary bronchiBranches from the primary bronchi that lead to each lung lobe. Also called segmental bronchi. These then further divide into tertiary bronchiBranches from the secondary bronchi that serve bronchopulmonary segments. and so forth.  The trachea and the primary bronchi cause significant resistanceThe opposition to airflow in the respiratory tract, influenced by airway diameter. to air flowThe movement of air into and out of the respiratory tract, driven by pressure differences between th. This resistance occurs in both the conducting system and the respiratory or gas exchange areas. Conditions like bronchitisInflammation of the bronchi, leading to mucus buildup, coughing, and breathing difficulties. and asthmaA chronic condition characterized by airway inflammation, narrowing, and excessive mucus production, affect the trachea and primary bronchi. These conditions substantially increase the resistance to air flow through these tubes. Many people both child and adult who suffer from asthma have lungs that operate perfectly fine. The problem is getting the air through the bronchi to the lungs.

Smooth Muscle

Many of us are familiar with the condition bronchitis.  This can be cause by an infectious agent or by a sympathetic response.  Regardless of the cause, the smooth muscle of the trachea and primary bronchi constricts. This bronchoconstriction considerably decreases air flow to the lungs.  Often accompanying bronchoconstriction is an increased mucus production of the mucus cellsThe basic structural and functional units of life. scattered in the epithelium lining these tubes.  This decreases the diameter of these bronchi and the trachea even more. 

Bronchitis is usually caused by a bacterial infection, causing inflammation of the primary bronchi and the trachea.  Pollutants can also trigger bronchoconstriction Evidence asthma experienced by so many people who are hoarders. Asthma can also be caused by over activity of white blood cells and dry air.  So, I guess the lesion here is to both filter and humidify your air if you have asthma. 

Medications that treat these conditions all focus on causing bronchodilation. Albuterol is the active agent in most inhalers. Epinephrine is an active agent in nebulizer treatments. Both attach to receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter of the sympathetic nervous systemThe organ system that controls body functions using electrical and chemical signals. on bronchi and the trachea. This simulates a fight or flight response from these organs causing bronchodilation.

Secondary & Tertiary Bronchi

Secondary bronchi, branching off of the primary bronchi. I appreciate their alternate name of lobar bronchi. This name indicates how secondary bronchi each lead to one lobe of the lung they serve. There are three secondary bronchi in the right lung. The left lung has only two secondary bronchi. This is due to the additional space the heart occupies in the left lung. Coming off of secondary bronchi are the tertiary bronchi. Naturally, I prefer the term segmental bronchi. It indicates that each tertiary bronchus serves one segment of a lung. As we will see later we divide the lung into sections called bronchopulmonary segmentsFunctional units of the lungs, each served by its own tertiary bronchus and blood supply.. This is why tertiary bronchi are a focus of research.

At the microscopic level, secondary and tertiary bronchi are suspended among the alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood.. Alveoli are the dead end sacs of the lung. The mucosaThe innermost lining of the digestive tract that contains mucus-secreting cells for protection and a is usually WAVY although it does not collapse owing to the dense tissue that surrounds the open bronchus. Running throughout the lung are also capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. and veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge.

Mucosa

As air travels from the nasal cavityThe internal space behind the nose that filters, warms, and humidifies incoming air. through the pharynxThe muscular passageway connecting the mouth to the esophagus and larynx. and trachea, it moves through the primary and secondary bronchi. The respiratory mucosa transitions from pseudostratified columnar cells into simple columnar, then into simple cuboidal cells. As you travel through the conducting system, there are also fewer mucus cells scattered among the epithelium. At some point that’s not well defined bronchi transition into bronchiolesSmall airways branching from the bronchi that lead to alveoli; lack cartilage and control airflow wi. I’m kidding there’s a certain point we just don’t need to know the measurement at which that happens. Bronchioles are much smaller. However, there are so many bronchioles that they cumulatively do not produce any resistance to air flow. This is unlike blood vessels in which arteriolesSmall arteries that regulate blood flow into capillaries through vasoconstriction and vasodilation were very small. Arterioles were not very numerous, so they provided resistance to blood flow.

Bronchioles

As air moves further into the lungs, it passes through branches of bronchi and bronchioles. Gases reach an area of very little resistance with much less airflow. Here is the opportunity to exchange gases. A terminal bronchiole is not the last bronchiole. Air flows through other bronchioles before reaching alveolar clusters. These clusters are the dead end cul de sacs of the lung. A terminal Bronchiole is more like a highway and the respiratory bronchiolesSmallest bronchioles leading to alveolar ducts and alveoli. are the exits. this picture has numerous respiratory bronchioles leading to alveolar clusters. I guess I should mention now that your lungs have close to 3,000,000 alveolar clusters. This one picture here does not show the magnitude of these structures. Terminal bronchiole I have no mucus cells and a simple cuboidal lining. The smooth muscle becomes less and less prevalent around the bronchioles as we headRounded proximal end that fits into the acetabulum of the hip bone. toward the alveolar clusters. There are some elastic fibers but not many that are supporting the shape of the terminal and respiratory bronchioles. The respiratory bronchioles lead directly to an alveolar cluster. There is no smooth muscle, very few elastic fibers, and simple squamous cells through which gas exchange can happen. This gas exchange ability is the dividing line between the conducting system and the respiratory systemThe organ system responsible for gas exchange (oxygen and carbon dioxide).. This is why respiratory bronchioles get their name respiratory and not terminal. They are the 1st place where oxygen can diffuse into blood vessels and carbon dioxide can diffuse out of blood vessels

As air moves inferiorly…

Imagine yourself as a molecule of oxygen. You are being breathed in the nasalTwo small rectangular bones forming the bridge of the nose. cavity, funneled down the pharynx and trachea. Then you split off into bronchi and bronchioles and finally dead end in an alveolar cluster. This oxygen molecule has traveled the length of the respiratory mucosa. As this oxygen molecule has traveled, some trends have emerged in the epithelium. Changes are also evident in the submucosa and other tissues surrounding the air conduits. Closer to the opening of the nasal cavity and mouthThe opening of the digestive tract where food enters and mastication begins. are more mucus cells to trap particulates. Mucus cells toward the respiratory areas of the lung would inhibit gas exchange. A lot of cartilageA flexible connective tissue found in joints, the ear, nose, and rib cage. Cartilage can be of three and smooth muscle is found in the trachea and primary bronchi. This contrasts with the bronchioles and alveolar clusters. Those areas of a larger diameter need to be held open to achieve their diameter. Bronchioles and alveolar clusters open and close with pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. differences in your thoracic cavityThe body cavity housing the heart and lungs.. Elastin fibers are not needed in the areas supported open by cartilage. However, they are important in the respiratory bronchioles and alveoli. These structures open and close because of pressure differences in the thoracic cavity. Elastin fibres make this movementA fundamental property of life involving motion of the body or its parts. possible. Once an oxygen molecule gets to the dead end cul-de-sacs of the alveoli, two types of cells are present. These simple squamous cells can be called type 1 pneumocytes or something else that I can’t remember right now. These are the cells that are responsible for gas exchange and run closely to a capillary to achieve that exchange. Type 2 pneumocytes, also called alveolar great cells, act as resident macrophages. They are there to clean up the last of any particulate matter that may have slipped through the mucociliary escalatorThe mechanism by which cilia move mucus and trapped particles up toward the throat for removal..