Tubular segments

The response of the axial dispersion model to step or pulse tracer inputs can be determined by writing a material balance over a short tubular segment and then solving the resultant differential equations. A transient material balance on a cylindrical element of length AZ gives... 

Levine and coworkers first reported on the real-time profiling of kidney tubular fluid nitric oxide concentration in vivo [89, 91], In the 2001 publication, a modified version of a combination NO electrode (WPI, ISONOP007) was successfully used to measure NO concentration profiles along the length of a single nephron of a rat kidney tubular segment. Since it was shown that the electrode is sensitive to NO in the rat tubule it was used to detect NO concentration differences in rat kidney tubules before and after 5/6 nephrectomy. The results clearly showed that the NO concentration was much higher in nephrectomized rats vs unnephrectomized rats. 

After administration of mercuric chloride to adult rat kidneys, several changes were found, for example, pars recta of the proximal tubular segment showing fragmentation and disruption of the plasma membrane, basophilic staining, vesiculation and disruption of endoplasmic reticulum and other cytoplasmic membranes, mitochondrial swelling, loss of mitochondrial dense granules and condensation of nuclear chromatin [223-225]. 

After administration of mercuric chloride to mice, cell necrosis was found to be severest in the S2 and proximal S3 [228] segments of the proximal tubules corresponding to the preferential accumulation of mercury, engaging the convoluted part of the proximal tubular segment [229]. Very large mercury-containing lysosomes developed in the distal S3 segment. 

Both prerenal factors (dehydration, blood loss, altered vasomotor tone, age-related decreases in renal blood flow in rats) and postrenal factors (obstruction or extravasation of urine to the peritoneal cavity) may cause elevations of the commonly measured analytes that do not reflect primary kidney injury. Plasma analytes also cannot be used to determine the location of renal injury (glomerular versus tubular, or tubular segment affected) (Baum et al. 1975 Corman and Michel 1987 Finco 1997 Newman and Price 1999). 

Both classical transport systems are located exclusively in the proximal tubule of the nephron. Several techniques such as visual observations, stop-flow experiments, tubular micropuncture, in vivo and in vitro tubular microperfusions have demonstrated this particular transport capacity of the proximal tubular segment of the nephron [19]. Secretion entails an increase of proximal cell concentration of transported substrates that may become higher than in interstitium, and in some case may result in nephrotoxicity [40-42]. 

Various in vivo and in vitro models have been described to study the elimination of drugs from the kidneys. Glomerular filtration and clearance rates require in vivo studies or are performed on perfused whole kidneys.121,122 Tubular secretion and reabsorption of substances have been studied in kidney slices, tubular segments, tubular suspensions, primary cultures, and established cell lines. 

Approximate Percentage of Sodium Reabsorption in Renal Tubular Segments... 

For example, at low and high rates of urine flow, the minimal and maximal values of the may vary from 30% to 60% of the glomerular filtration rate. This occurs because various tubular segments are permeable to urea and allow passive reabsorption to occur under conditions of antidiuresis. The fractional excretion of urea (FE ,pa) is calculated as [(urine urea/ plasma urea)/(urine creatinine/plasma creatinine) x... 

Figure 8.7, Schematic representation of a capillary electrochromatography column consisting of a packed bed and open tubular segment separated by a bed retaining frit (top) and the generation of electroosmotic flow in a packed bed (bottom).

Most sodium and water reabsorption occurs throughout the renal tubular segments. Diuretics affect one or more of these segments. Every one and one-half hours the kidneys (glomeruli) clean the body s extracellular fluid (ECF). 

In rats orally administered 2 g/kg of annatto powder (27% bixin) 20 times over a 4-week period, no changes in hematological or plasma biochemical parameters were observed. A decrease in body weight gain was observed in male rats, and kidney apoptosis in restricted areas without proliferation or tubular segments modification was observed in 20% of female rats. The precise nature of apoptosis was not investigated. The authors concluded that annatto was not toxic to the rat (Bautista et al. 2004). 

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