The Cause of Pressure Sores
J G Webster in Prevention of Pressure Sores, 2019
The capillary wall is a thin membrane made up of endothelial cells. Substances pass through the junctions between endothelial cells and some also pass through the cells by either vesicular transport, filtration, or diffusion for lipid-soluble substances. Diffusion is the most important in terms of the exchange of nutrients and waste materials between blood and tissue. Glucose and oxygen are higher in concentration in the bloodstream than in the interstitial tissues so they diffuse into them. CO2 diffuses in the opposite direction. The rate of filtration at any point along a capillary depends upon a balance of forces called Starling forces. One of these forces, the hydrostatic pressure gradient, is the hydrostatic pressure in the capillary minus the hydrostatic pressure in the interstitial fluid. Due to these forces, substances tend to leave blood capillaries where they begin by the arterioles and enter them as they end near the venules.
Maternal Fetal Circulation and Respiration
Miriam Katz, Israel Meizner, Vaclav Insler in Fetal Well-Being, 2019
Placental respiratory function between the mother and the fetus depends on the area of exchange, the permeability of placental membrane and the concentration gradients. The latter phenomenon determines the route of diffusion of respiratory gases. According to Fick’s equation15 the rate of transfer may be calculated from the above three parameters. The main limiting factor for effective placental transfer is the thickness of placental membrane; the differences in concentration of different materials on two sides of this membrane must, however, also be considered. Due to the special character of the blood flow in the intervillous space it is very unlikely that the concentration gradient is identical at each point in the intervillous space. It is most probable that the blood circulating in the periphery of the intervillous space will leave it with a higher oxygen content than that which circulates in direct contact with the villi.16 The system by which blood flows in the intervillous space is described as a multivillous stream system,17 therefore any calculation of concentration gradient on both sides of exchange surfaces is extremely difficult and may be inaccurate.
Maturation, Barrier Function, Aging, and Breakdown of the Blood–Brain Barrier
Shamim I. Ahmad in Aging: Exploring a Complex Phenomenon, 2017
Before entering the brain, the molecules have to cross the EC membrane. There are different routes for the molecules to go through this barrier. Simple diffusion transports molecules through an aqueous channel formed within the membrane. Several channels are expressed at high levels at the CNS barriers, for example, aquaporin (AQP)-1 (Nazari et al. 2015) and ion channels. The presence of specific ion channels at the BBB and blood-cerebrospinal fluid barrier (BCSFB) ensures that the ionic composition is optimal for synaptic signaling. The ions are also important for the formation of the brain fluids: interstitial fluid (ISF) and cerebrospinal fluid (CSF). ECs are an important source of ISF (Abbott 2004), while ion transport across the choroid plexus epithelium (CPE) drives CSF secretion (Redzic 2011).
Merits and advances of microfluidics in the pharmaceutical field: design technologies and future prospects
Published in Drug Delivery, 2022
Amr Maged, Reda Abdelbaset, Azza A. Mahmoud, Nermeen A. Elkasabgy
Fabrication of nanoparticles in a continuous flow (single-phase) system involves the combination of fluids by diffusion in laminar flow streams that uses single or multiple solvents (Rhee et al., 2011; Clark et al., 2017). Diffusion is described as the process of molecules spreading from a higher concentration region to a lower concentration region through Brownian motion, resulting in gradual material mixing. Diffusion is explained mathematically using Fick’s law (Fick, 1855): j is the diffusive flux (mol /m2s1), D is the diffusion coefficient (m2 /s1), /m3), and x is the position of the species (m).
Anti-ageing peptides and proteins for topical applications: a review
Published in Pharmaceutical Development and Technology, 2022
Mengyang Liu, Shuo Chen, Zhiwen Zhang, Hongyu Li, Guiju Sun, Naibo Yin, Jingyuan Wen
The transcellular pathway refers to the transportation of solutes through a cell, including transcellular passive diffusion, transcellular active transport, and transcytosis (Kasting et al. 2019). Diffusion is the movement of chemicals from a region of higher concentration to a region of lower concentration. Active transport, also known as carrier-mediated transport, involves using energy to help specific molecules move across the barrier and against the concentration gradient (Fung et al. 2018). Since the cell membrane is lipophilic, it might resist the passive diffusion of hydrophilic or charged compounds. Transcytosis is another type of transcellular route, where macromolecules are carried across the cell membranes (Liu et al. 2019). These macromolecules are captured in vesicles on the side of the cell, drawn across the cell, and then ejected on the other side (Liu et al. 2019). However, most experimental studies suggest that the primary pathway across SC is the intercellular pathway, as described below.
Liposomes as vehicles for topical ophthalmic drug delivery and ocular surface protection
Published in Expert Opinion on Drug Delivery, 2021
José Javier López-Cano, Miriam Ana González-Cela-Casamayor, Vanessa Andrés-Guerrero, Rocío Herrero-Vanrell, Irene Teresa Molina-Martínez
The ability of drugs to cross the cornea is conditioned by the size and the distribution coefficient of the active substance. The higher the diffusion coefficient, the greater the importance of the transcellular pathway. For values of distribution coefficient between 0,01–10, the pass through the lipophilic epithelium and endothelium becomes more viable. When the value is higher than 10, almost all the passage occurs through the transcellular route and the stroma becomes the limiting barrier. This is the reason why when the distribution coefficient is too large the permeability stops increasing. However, in the case of solutes with a low distribution coefficient, that is, substances with a hydrophilic nature, the main impediment is the epithelium and the main passage through the cornea is the paracellular route. In this sense, the passage of hydrophilic substances depends on their size or molecular weight, being this process easier for small solutes with a molecular weight less than 500 Da, and especially difficult for macromolecules [71–74]. After penetration through the cornea, the drug will reach the intraocular tissues. First, the drug reaches the aqueous humor, from where it will pass to the intraocular tissues of the anterior segment. By this way, the drug will have to go through the anterior segment to reach the posterior segment [75].
Related Knowledge Centers
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