Plasma renal filtration

The loop diuretics must be present in the tubular fluid before they can become effective. Because of their extensive binding to plasma proteins, filtration across the glomerular capillaries is restricted. Like the thiazides, however, the loop diuretics are weak organic acids that are substrates for the organic acid secretory system in the proximal tubule. A consequence of this active secretion is that the presence of other organic acids or certain forms of renal disease may impair the therapeutic usefulness of the loop diuretics. 

Although diastereoisomers, both quinine and quinidine, have similar physical properties (Fig. 18). In clinical studies, the renal clearance of quinidine was fourfold greater than that of quinine (57). No stereoselective differences in plasma protein binding were observed. The renal filtration and passive reabsorption of these two diastereoisomers should be similar since the compounds have similar octanol-water partition coefficients and pKa values (57). Therefore, stereoselective active renal secretion may be the mechanism responsible for the observed differences in the renal clearances of quinine and quinidine. 

Creatinine, an endogenous end product of muscle metabolism, is often measured in plasma and urine to determine creatinine clearance. Since, creatinine is freely filtered at the glomerulus and is not reabsorbed or secreted by the proximal tubule of most species, creatinine clearance provides a good measure of the GFR. Another endogenous compound, urea, is also cleared mainly by renal filtration and excretion. Increases in the blood or serum concentration of urea are indicative of decreased GFR. However, increases in BUN concentration occur only after substantial renal damage has been established. Thus, BUN concentration is not a sensitive indicator of nephrotoxicity and changes usually occur later than changes in other parameters (e.g., enzymuria). 

RBP and retinol exist in the circulation bound as a ternary complex to TTR. TTR consists of a tetramer made up of four identical subunits, with a total molecular weight of 55 kDa. Binding of RBP-ROH to TTR prevents renal filtration of the smaller RBP (MW 21 kDa) and retinol [61]. It was originally proposed, based on the observation that plasma RBP levels drop in vitamin A-deficiency whereas those of TTR do not, that RBP must complex with TTR in the circulation after both proteins are independently secreted from the hepatocyte [63]. However, recent studies by several independent research groups have suggested that the RBP-TTR complex is formed within the ER of hepatocytes prior to secretion [55, 56, 64-68]. 

Renal replacement by an artificial device providing continuous filtration of plasma based on the physical principle of convection. 

Glomerular filtration rate (GFR) The volume of plasma that is filtered by the glomerulus per unit time, usually expressed as mL/minute or mL/minute/1.73 m2, which adjusts the value for body surface area. GFR is the primary index used to describe overall renal function. 

A substance that fulfills these criteria is para-aminohippuric acid (PAH). All of the PAH not filtered at the glomerulus is secreted by the proximal tubule. The net effect is that all of the plasma flowing through the nephrons is completely cleared of PAH. It is important to note that about 10 to 15% of the total renal plasma flow supplies regions of the kidneys that are not involved with filtration or secretion. Consequently, this plasma cannot be cleared of PAH. Therefore, the plasma clearance of PAH provides a measurement of the effective renal plasma flow, that is, the volume of plasma that actually flows through the nephrons. The ERPF is normally about 625 ml/ min. (This value is based on a renal blood flow of about 1.1 1/min and a hematocrit of about 42.)... 

In some cases, the amino acid pattern on a paper chromatogram is very similar to that found in diseases of the renal tubules, such as cystinosis, where there is a failure to reabsorb all amino acids from the glomerular filtrate and, in consequence, the urinary amino acid pattern resembles that of plasma (W6, W9). In other cases the aminoaciduria is less marked and the amino acids found in greatest excess are glycine, alanine, serine, threonine, and glutamine. In some cases no aminoaciduria has been detected. 

Blood is supplied to the kidneys via the renal vein, a branch of the descending vena cava, at relatively high pressure to ensure rapid filtration of plasma across the membranes of the blood vessels in the glomeruli and the epithelial cells of the Bowman s capsule. The net filtration pressure of about 5-6 kPa, is the difference between the blood pressure forcing plasma water across the filtration barrier and the opposing osmotic and... 

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