Blood-gas interface

According to Pick s law of diffusion, the amount of gas that moves across the blood-gas interface is proportional to the surface area of the interface and inversely proportional to thickness of the interface. In other words, gas exchange in the lungs is promoted when the surface area for diffusion is 

More specifically, the blood-gas interface consists of the alveolar epithelium, capillary endothelium, and interstitium. The alveolar wall is made up of a single layer of flattened type I alveolar cells. The capillaries surrounding the alveoli also consist of a single layer of cells — endothelial cells. In between the alveolar epithelium and capillary endothelium is a very small amount of interstitium. Taken together, only 0.5 pm separates the air in the alveoli from the blood in the capillaries. The extreme thinness of the blood-gas interface further facilitates gas exchange by way of diffusion. 

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Describe the blood-gas interface and explain why the lungs are ideally suited for gas exchange... 

Oxygen and carbon dioxide cross the blood-gas interface by way of diffusion. The factors that determine the rate of diffusion of each gas are described by Pick s law of diffusion ... 

Diffusion is proportional to the surface area of the blood-gas interface (A) the diffusion constant (D) and the partial pressure gradient of the gas (AP). Diffusion is inversely proportional to the thickness of the blood-gas interface (T). 

Oxygen and carbon dioxide are small molecules with low molecular weights however, carbon dioxide is 20 times more soluble than oxygen. Therefore, the value of the diffusion constant for carbon dioxide is larger than that of oxygen, which facilitates the exchange of carbon dioxide across the blood-gas interface. 

The thickness of the blood-gas interface is normally less than 0.5 (im. This extremely thin barrier promotes the diffusion of gases. The thickness may increase, however, under conditions of interstitial fibrosis, interstitial edema, and pneumonia. Fibrosis involves the excess production of collagen fibers by fibroblasts in the interstitial space. Edema is the movement of fluid from the capillaries into the interstitial space. Pneumonia causes inflammation and alveolar flooding. In each case, the thickness of the barrier between the air and the blood is increased and diffusion is impaired. 

Bubble and disk oxygenators share a number of disadvantages. Most importantly, one must remember that they can be lethal devices. Bubble or disk oxygenators are relatively safe only for a small number of hours after which patients (or animals) will not survive. The reasons for this appear to include the denaturation of blood proteins and protein complexes at the blood-gas interface (29, 30). Bubble and film oxygenation of dog plasma has been shown to denature a-lipoprotein with accumulation of an abnormal insoluble phospholipid, cholesterol, triglyceride, and... 

Blood gas exchange devices have either a gas-blood interface, as found in the disk and bubble oxygenators, or a membrane interposed be-... 

In order for any inhaled solvent vapors to enter the blood circulation, they need to reach the alveoli, cross the gas-blood interface and dissolve in the blood. Gas is carried to the gas-blood interface by airways, while blood is carried by blood vessels. Since these functions take place in the lung, the essentials of respiratory physiology will be briefly reviewed. 

Fig. 18-3. The GE DuaLung employs a plastics membrane. Interposed between the sweep gas and blood is a membrane which prevents the blood from interfacing with the gas. (Courtesy General Electric Medical Systems Division)...

Using nasal or facial interfaces, NIV alleviates respiratory failure of various origins. Its mechanisms of action include resting the respiratory muscles, increasing thoracic comphance, and resetting the respiratory centers (20). In OHS, nocturnal NW has been shown to be clinically effective because of a rapid and sustained improvement in daytime arterial blood gas levels (9) and a net reduction of daytime sleepiness. However, mechanisms of improvement remain unclear, as well as the pathophysiology and the natural history of the disease. 

The culture of stem cells is traditionally and usually performed on fiat two-dimensional surfaces such as tissue culture flasks (T-flasks), well plates, or gas-permeable blood bags consisting of a single unstirred compartment where nutrients diffuse to cells. Gas exchange (e.g., oxygen and carbon dioxide) occurs at the medium/gas interface. These systems are widely used for research purposes because of their simplicity, ease of handling, and relatively low cost. 

Reaction of Gases or Vapors at Various Levels of the Gas-Blood Interface... 

To model sulfur dioxide absorption by the blood through the walls of the upper airways, as demonstrated by Frank et one must include the transport rates of sulfur dioxide across a mucus-tissue interface, a tissue layer, and a tissue-blood interface (Figure 7-2). For the case of release of dissolved gas back into the exhaled air, which is depleted of gas in the lower lung, the mucus layer would still represent the greatest resistance to transfer. Consequently, the overall transfer coefficient, kg, would still be given by ki/H. 

Recently developed blood oxygenators are disposable, used only once, and can be presterilized and coated with anticoagulant (e.g., heparin) when they are constructed. Normally, membranes with high gas permeabilities, such as silicone rubber membranes, are used. In the case of microporous membranes, which are also used widely, the membrane materials themselves are not gas permeable, but gas-liquid interfaces are formed in the pores of the membrane. The blood does not leak from the pores for at least several hours, due to its surface tension. Composite membranes consisting of microporous polypropylene and silicone rubber have also been developed. 

It is interesting that the adult human lung has an enormous gas tissue interface, approximately 90 m2, 70 m2 alveolar space. This large surface, together with the blood capillary network surface of 140 m2, with its continuous and profuse blood flow, offers an extremely rapid and efficient medium for the absorption of chemicals from the air, into the alveolar portion of the lungs, and into the bloodstream. 

Interfacing the TEA to both a gas and a HPLC has been shown to be selective to nitro-based explosives (NG, PETN, EGDN, 2,4-DNT, TNT, RDX and HMX) determined in real world samples, such as pieces of explosives, post-blast debris, post-blast air samples, hand swabs and human blood, at picogram level sensitivity [14], The minimum detectable amount for most explosives reported was 4-5 pg injected into column. A pyrolyser temperature of 550°C for HPLC-TEA and 900°C for GC/TEA was selected. As the authors pointed out, GC uses differences in vapour pressure and solubility in the liquid phase of the column to separate compounds, whereas in HPLC polarity, physical size and shape characteristics determine the chromatographic selectivity. So, the authors reported that the use of parallel HPLC-TEA and GC-TEA techniques provides a novel self-confirmatory capability, and because of the selectivity of the technique, there was no need for sample clean-up before analysis. The detector proved to be linear over six orders of magnitude. In the determination of explosives dissolved in acetone and diluted in methanol to obtain a 10-ppm (weight/volume) solution, the authors reported that no extraneous peaks were observed even when the samples were not previously cleaned up. Neither were they observed in the analysis of post-blast debris. Controlled experiments with handswabs spiked with known amounts of explosives indicated a lower detection limit of about 10 pg injected into column. 

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