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Oxygen Transport
Published in James N. Cobley, Gareth W. Davison, Oxidative Eustress in Exercise Physiology, 2022
P.N. Chatzinikolaou, N.V. Margaritelis, A.N. Chatzinikolaou, V. Paschalis, A.A. Theodorou, I.S. Vrabas, A. Kyparos, M.G. Nikolaidis
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.
Functions of the Respiratory System
Published in Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal, Principles of Physiology for the Anaesthetist, 2020
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal
Alveolar-capillary unit. There are 200–600 million (average 300 million) alveoli in the lungs, and their diameters vary with lung volume; at functional residual capacity (FRC), the mean diameter is 0.2 mm. Exchange of oxygen and carbon dioxide takes place across the alveolar wall between the gas in the respiratory zone and the pulmonary capillary blood. The alveolar walls form septae between adjacent alveoli, which are polyhedral in shape rather than spherical as the septae are flat. The air-facing surfaces of the walls are lined by a one-cell-thick layer of flat squamous epithelial cells, type I alveolar cells. Large cuboidal type II alveolar cells are also present and produce surfactant, which lines the alveoli.
Acute Alveolar Injury: Experimental Models
Published in Joan Gil, Models of Lung Disease, 2020
The above observations support the following hypothesis. In the earliest phase of AAI, decreased lung compliance and volume are primarily due to alveolar collapse caused by increased surface tension. The contribution of increased surface tension is maximized by 5-7 days and remains constant through the following week. Beginning at 5-7 days and increasingly thereafter, irreversible closure of alveoli effects lung distensibility. As the number of irreversibly closed alveoli increases, the distensibility of the lung progressively decreases. The number of irreversibly closed alveoli must be important in determining the degree to which lung compliance remains abnormal after recovery.
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].
Mathematical analysis of oxygen and carbon dioxide exchange in the human capillary and tissue system
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2023
Ahsan Ul Haq Lone, M. A. Khanday
The human respiratory system has two main functions: oxygen intake from the surrounding air to the body, and to exhale carbon dioxide from the blood to outside air. Those transfers are achieved through passive diffusion across a membrane which separates the gaseous air and the liquid blood, at an instantaneous rate by means of the difference in partial pressures, the area of the exchange surface, and its properties in terms of diffusion (Guyton and Hall 2011; West 2011). As this diffusion tends to reduce the partial pressure difference, a constant renewal must be made on both sides of the membrane. Renewal of air is achieved by the ventilation process, which consists of in periodic inspiration-expiration cycles that provide the inside of the lung with fresh air, whereas venous blood is periodically pumped onto the exchange zone by the heart. The exchange area is the boundary of a huge collection of small cavities (around 300 million units), called alveoli, which makes an exchange area of about 100 m2 (Guyton and Hall 2011; West 2011; Tortora and Derrickson 2012; Nunn 2013). Each of this alveolus is surrounded by a network of very small blood vessels, called capillaries, whose diameter is about 5–10 μm (Guyton and Hall 2011; West 2011). Gas exchange occur through the alveolar-capillary membrane, which is less than a micrometre wide (West 2011; Tortora and Derrickson 2012). The alveoli are connected to the outside world through the respiratory tract, which is an assembling of interconnected pipes following a dyadic-tree structure.