The respiratory system
Laurie K. McCorry, Martin M. Zdanowicz, Cynthia Y. Gonnella in Essentials of Human Physiology and Pathophysiology for Pharmacy and Allied Health, 2019
Fick’s Law of Diffusion states the amount of gas that moves across the blood-gas interface is proportional to the surface area of the interface and inversely proportional to the thickness of the interface. In other words, gas exchange in the lungs is promoted when the surface area for diffusion is maximized and the thickness of the barrier to diffusion is minimized. In fact, anatomically, the lungs are ideally suited for the function of gas exchange. There are 300 million alveoli in the lungs. Furthermore, the walls of each alveolus are surrounded by a network of capillaries. There are as many as 280 billion pulmonary capillaries or almost 1,000 capillaries per alveolus. This results in a vast surface area for gas exchange of approximately 70 m2, which is roughly the size of a tennis court.
Oxygen Transport
James N. Cobley, Gareth W. Davison in Oxidative Eustress in Exercise Physiology, 2022
At the first step of oxygen transport, the atmospheric air is channeled to the lungs through ventilation at the alveolar-capillary region, to replace oxygen and remove carbon dioxide (Figure 4.2). The gas exchange between the alveoli and pulmonary capillaries is facilitated via passive gas diffusion, from a high to low-pressure gradient. In human lungs, the number of alveoli is ≈4.8 × 108 (Ochs et al., 2004) and is estimated to occupy a large surface area of ≈60–80 m2 (Pittman, 2016). The alveoli are in close contact with the pulmonary capillaries, separated only by a thin air-blood barrier ≈0.2–0.6 μm thick (Hall, 2016). Each alveolus has a wide diameter of ≈200 μm (Ochs et al., 2004), whereas a pulmonary capillary could be as small as ≈3–5 μm (Hall, 2016; Pittman, 2016; Kuck, Peart and Simmonds, 2020). The erythrocytes are evidently larger cells (i.e., ≈7–8 μm diameter) than capillaries and have to change their shape and mechanical properties to traverse through the pulmonary capillaries and bind oxygen (Kuck, Peart and Simmonds, 2020) (Figure 4.3). Overall, the large surface area of the alveolar region, the thin air-blood barrier, the difference in gas pressure gradients and the erythrocyte’s ability to deform greatly enhance gas diffusion at this step.
Skin and Organs with Epidermoid Mucosae
George W. Casarett in Radiation Histopathology, 2019
One or more sebaceous glands are connected with each hair follicle and excrete their products through short ducts into the follicular canals or necks of the hair follicles. The wall of the alveolus or secretory portion in the sebaceous gland is formed by a basement membrane upon the inside of which is a single layer of thin cells with round nuclei. Toward the center of the alveolus are a few cornified cells, but most of the cells are larger, polyhedral cells which gradually become filled with fat droplets. The central part of an alveolus is filled with large cells distended with fat droplets. The nuclei gradually shrink and disappear, and the cells break down into fatty detritus mixed with horny scales, constituting the oily secretion of the gland; this is an holocrine type of secretion.
Understanding the mechanisms for COVID-19 vaccine’s protection against infection and severe disease
Published in Expert Review of Vaccines, 2023
Huijie Yang, Ying Xie, Changgui Li
The pulmonary alveolus is a specialized structure in the lung and is responsible for most of its functions, including a gas exchange between the lung and the blood. Pulmonary arteries, airways, and veins constitute the largest vascular bed in the body [30,31]. Circulating blood in the lung accounts for approximately 9% of the whole body, which is meaningful for gas exchange, and indicates the vast antibody repertoire in this area. In addition, the proximity of the vasculature to the epithelium in the terminal airways and alveoli sets the stage for potential crosstalk, facilitating the transport of antibodies to the lumen of pulmonary airways and its subsequent detection in bronchoalveolar lavage fluid [12]. Therefore, after immunization with vaccines, the epithelial cells of alveoli can be readily protected by specific neutralizing IgG antibodies on the cell surface; the latter can be continuously supplied by tightly adjacent capillaries.
The latest advances in high content screening in microfluidic devices
Published in Expert Opinion on Drug Discovery, 2023
Weiyu Liu, Jingyu Wang, Huibo Qi, Qisen Jiao, Lei Wu, Yu Wang, Qionglin Liang
The lung is a crucial respiratory organ in human, with alveolus being the main site for gaseous exchange and considered the functional unit of the lung. Comprising a single layer of epithelial and pulmonary capillary endothelial cells, the alveolus possesses a complex physiological structure. However, traditional cell culture-based HCS assays have failed to accurately mimic the morphological and mechanical features of the lung that are fundamental to the function of this organ. Existing model systems have been unable to replicate the active tissue-tissue interface between the microvascular endothelium and surrounding parenchymal tissues, where essential transport processes such as fluid, nutrients, immune cells, and other regulatory factor exchange occur. Moreover, these systems cannot apply dynamic mechanical forces critical for the growth and function of living organs [114].
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].
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