The respiratory system
C. Simon Herrington in Muir's Textbook of Pathology, 2020
The trachea divides into the left and right main bronchi. The right continues in the general direction of the trachea; the left diverges at a greater angle. Thus, aspiration is more common in the right lung, especially the middle and lower lobes. Bronchial cartilage progressively decreases with increasing distance from the trachea. At the terminal bronchiole (sixteenth division), it has disappeared. The airways continue dividing into respiratory bronchioles, alveolar ducts, and finally alveoli (Figure 8.1). The terminal bronchiole is the smallest airway that is lined by bronchial epithelial cells. The terminal bronchiolar walls consist almost entirely of smooth muscle. The small bronchioles, i.e. terminal and those just proximal, play a larger role in determining airflow than the bronchi. This is because they have much more smooth muscle, which constricts easily. As they are small, they are easily occluded, as in bronchial asthma and chronic obstructive pulmonary disease. The respiratory bronchiole bears alveoli.
The Respiratory System and Its Disorders
Walter F. Stanaszek, Mary J. Stanaszek, Robert J. Holt, Steven Strauss in Understanding Medical Terms, 2020
Within the lungs, the left and right bronchi branch into smaller and smaller passageways. As these tubes become small enough to remain barely visible without a microscope, they still contain the cartilage rings found in the trachea and left and right bronchi, so they are still referred to as bronchi. As the size continues to decrease, however, only muscle tissue remains. These smallest branches are called bronchioles, meaning "little bronchi." Bronchioles terminate in small air sacs called alveoli from which oxygen passes into the bloodstream for transport throughout the body (Figure 9.1).
Physiology of the Airways
Anthony J. Hickey, Sandro R.P. da Rocha in Pharmaceutical Inhalation Aerosol Technology, 2019
The airways are often described as the pulmonary tree in that their overall form resembles a tree. The tree trunk is analogous to the trachea of the airways that bifurcates to form main bronchi. These divide to form smaller bronchi that lead to individual lung lobes: three lobes on the right side and two on the left side. Inside each lobe, the bronchi undergo further divisions to form new generations of smaller caliber airways: the bronchioles. This process continues through the terminal bronchioles (the smallest airway not involved with an alveolus), the respiratory bronchioles (which exhibit alveoli protruding from their walls), alveolar ducts, and terminates with the alveolar sacs. In the classic model of the airways, as described by Weibel (1963), each airway divides to form two smaller “daughter” airways (Figure 2.1), and, as a result, the number of airways at each generation is double that of the previous generation. The model proposes the existence of 24 airway generations in total, with the trachea being generation 0 and the alveolar sacs being generation 23.
Nanocrystals based pulmonary inhalation delivery system: advance and challenge
Published in Drug Delivery, 2022
Pengfei Yue, Weicheng Zhou, Guiting Huang, Fangfang Lei, Yingchong Chen, Zhilin Ma, Liru Chen, Ming Yang
The lungs are the organs that contact with the exchange of air between the organism and the outside world. They are divided into two main regions: the conducting airway region and the respiratory region. The airway is a continuous branch from the bronchi to the lungs and consists mainly of bronchi, bronchioles, and terminal bronchioles. As the bronchi continue to branch, the diameter of the tubes becomes smaller, the tube wall becomes thinner, and the structure of the tube wall changes gradually. The annular smooth muscles of the bronchi contract or relax under splanchnic nerves innervation and it is responsible for the regulation of airflow passage into the alveoli. This is the site where the lung tissue completes gas exchange consisting of respiratory bronchioles, alveolar ducts, lung sacs, and alveoli. The respiratory bronchiole is the transitional pipes between the pulmonary airway and the respiratory site. Each respiratory bronchiole branch is divided into 2–3 alveolar ducts. The alveolar sacs are the common opening of several alveoli and are connected to the alveolar ducts. The gut is the main site for the digestion and absorption of nutrients.
Simulation of respiratory tract lining fluid for in vitro dissolution study
Published in Expert Opinion on Drug Delivery, 2021
Rakesh Bastola, Paul M. Young, Shyamal C. Das
The respiratory system consists of mouth and nose, pharynx, larynx, trachea, bronchi, bronchioles and alveoli [9]. The pharynx is approximately 12–15 cm in length and it is divided into nasopharynx, oropharynx and laryngopharynx [10]. The larynx works as a sphincter and transmits air from oropharynx and nasopharynx to the trachea [11]. The trachea is divided into the left and right primary bronchi. Each bronchus is divided into secondary bronchi, which are further divided into many tertiary bronchi. Branching of tertiary bronchi gives many tiny bronchioles which finally, lead to terminal and respiratory bronchioles. Respiratory bronchioles are further divided into alveolar ducts which end at the alveoli (alveolar sacs) [11]. There are more than 300 million alveoli in the lungs. Each alveolus is lined with pulmonary capillaries, which forms a massive network that includes more than 280 billion capillaries with a surface area of around 70 m2 [12].
Endogenous lung stem cells for lung regeneration
Published in Expert Opinion on Biological Therapy, 2019
Jonathan L. McQualter
The luminal surface of the entire airway tract is lined by a continuous epithelial layer, which, in addition to facilitating airflow, functions as a physical barrier to the external environment and serves vital functions in mucosal immunity, microbial defense and mucociliary clearance [36]. Based on structure and function, mammalian airways can be conceptually divided into upper and lower regions, which vary in relative distribution, abundance, and type of epithelial cells. The upper cartilaginous airways (i.e. trachea and primary bronchi) are the first line of defence against everything we inhale; hence, the epithelium is mostly composed of mucus-secreting goblet cells and ciliated cells, which are central to respiratory host defence. Invagination of upper airway epithelium also forms submucosal glands, which harbor ciliated cells, goblet cells, and serous cells, which secrete electrolytes, antioxidants and antimicrobial peptides [37]. The lower conducting airways (i.e. bronchioles), which are largely composed of secretory cells and ciliated cells, facilitate airflow distribution to the alveoli, thereby aiding in the essential process of breathing.
Related Knowledge Centers
- Bronchus
- Epithelium
- Gas Exchange
- Respiratory Tract
- Smooth Muscle
- Submucosa
- Lung
- Cilium
- Respiratory System
- Pulmonary Alveolus