Tubular fluid

The macula densa is a dense aggregation of cells in the distal tubule of nephrons facing the glomerular tuft of capillaries. These cells sense the salt content of the distal tubular fluid and adjust glomerular perfusion and renin secretion accordingly. 

G, E. O Connor and T. W. E Russell, Heat Transfer in Tubular Fluid-Fluid Systems P. C. Kapur, Balling and Granulation... 

This chapter has two goals, to provide a critical review of the current state of the art in the field of two-phase flow with heat transfer and to provide procedures which can be used for the design of tubular fluid-fluid systems. We hope that this work will help point out areas in which further theoretical and experimental research is critically needed, and that it will motivate design engineers to test out our procedures (in combination with details from the original references) in solving pragmatic problems. 

HEAT TRANSFER IN TUBULAR FLUID-FLUID SYSTEMS... 

Potassium ion secretion. Potassium ions are secreted in the distal tubule and the collecting duct. These ions diffuse down their concentration gradient from the peritubular capillaries into the interstitial fluid. They are then actively transported up their concentration gradient into the tubular epithelial cells by way of the Na+, K+ pump in the basolateral membrane. Finally, potassium ions exit the epithelial cells by passive diffusion through K+ channels in the luminal membrane and enter tubular fluid to be excreted in the urine. 

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. 

D.Z. Levine, M. Iacovitti, K.D. Burns, and XJ. Zhang, Real-time profiling of kidney tubular fluid nitric oxide concentrations in vivo. Am. J. Physiol.-Renal Physiol. 281, FI 89-194 (2001). 

D.Z. Levine and M. Iacovitti, Real time microelectrode measurement of nitric oxide in kidney tubular fluid in vivo. Sensors 3, 314 (2003). 

The answers are 370-c, 371-d, 372-c. (Hardman, pp 697, 701, 705.) The loop diuretic ethacrynic acid has its site of action in the ascending limb of the loop of Henle. This drug inhibits the reabsorption of Na and Cl" by interfering with the Na+, K+, 2CL co-transport system. In addition, loop diuretics block the reabsorption of Mg and Ca from the renal tubular fluid into the blood in this segment of the nephron unit. 

Hydrochlorothiazide has its proposed site of action at the distal convoluted tubule or, more specifically, at the early portion of the distal tubule. Hydrochlorothiazide inhibits the reabsorption of Na and Cl. It also promotes the reabsorption of Ca back into the blood, but inhibits the re absorption of Mg from the renal tubular fluid. The K-sparing diuretic agents (spironolactone, triamterene, and amiloride) have their site of action in the nephron at the late distal tubule and the collecting duct. These diuretic agents only cause a mild natriuretic effect... 

Expressed in the simplest terms, the glomeruli are filters and the tubules execute active and passive transport between the tubular fluid (glomerular filtrate) and the blood. The combined and coordinated function of the glomeruli and tubules constitutes the renal waste disposal and nutrient recycling system. 

Apart from glomerular filtration (B), drugs present in blood may pass into urine by active secretion. Certain cations and anions are secreted by the epithelium of the proximal tubules into the tubular fluid via special, energyconsuming transport systems. These transport systems have a limited capacity. When several substrates are present simultaneously, competition for the carrier may occur (see p. 268). 

The enzyme is used in tubule cells to generate H+, which is secreted into the tubular fluid in exchange for Na. ... 

These active secretory systems are important in drug excretion because charged anions and cations are often strongly bound to plasma proteins and therefore are not readily available for excretion by filtration. However, since the protein binding is usually reversible, the active secretory systems can rapidly and efficiently remove many protein-bound drugs from the blood and transport them into tubular fluid. 

D) Decreasing renal tubular fluid pH will increase eliminafion of weakly acidic drugs. 

A nephron, showing the major sites and percentage (in braces) of sodium absorption along with other features of solute transport. The filtered load = GFR (180 L/day) Xplasma Na+ (140 mEq/L) or 25,200 mEq/day. About 1% of this amount is excreted in voided urine. Sites where tubular fluid is isosmotic, hypertonic, or hypotonic relative to plasma are shown. POT, proximal convoluted tubule LH, loop of Henle DOT, distal convoluted tubule CCD, cortical collecting duct TAL, thick ascending loop. 

An important functional characteristic of the proximal tubule is that fluid reabsorption is isosmotic that is, proximal reabsorbed tubular fluid has the same osmotic concentration as plasma. Solute and water are transported in the same proportions as in the plasma because of the high water permeability of the proximal tubule. Thus, the total solute concentration of the fluid in the proximal convoluted tubule does not change as the fluid moves toward the descending loop of Henle. The corollary of this high water permeability is that unabsorbable or poorly permeable solutes in the luminal fluid retard fluid absorption by proximal tubules. This is an important consideration for understanding the actions of osmotic diuretics. 

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