Proximal tube transport

What is also interesting about the HTCVD is the material transport. Why does the silane not decompose and grow directly on the walls of the inlet There are two reasons for this. First, the injector is a coaxial injector, as illustrated in Figure 1.9 [40]. The silane and ethylene are transported in the inner tube, whereas there is only carrier gas in the outer tube. Thus the silane will not be in close proximity with a wall when it decomposes and will preferentially form nuclei through homogeneous nucleation. Second, the ethylene will make these particles very stable and hence they will not be prone to growth on the walls at all. 

To understand the mechanism of urine concentration, one must retrace the fate of the fluid in the various segments of the kidney. In the glomerulus, the membranes of the Bowman s capsule cells allow passage of all plasma components except protein. The ultrafiltrate is markedly reduced in volume as it passes through the proximal tubule. In fact, only 20% of the original volume reaches the distal portion of the proximal convoluted tube. The volume of the ultrafiltrate is reduced due to passive water reabsorption. Passive means that no known molecular mechanism exists for the transport of water from the lumen of the proximal tubule to the interstitial tissue. However, the movement of water follows that of sodium. In the proximal tubule, sodium is excreted actively into the interstitial tissue, and as a result, the osmotic pressure of the interstitial tissues increases. This draws water from the lumen of the tubule into the interstitial environment of the medulla because the tubule is highly permeable to water. 

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