Cortical nephrons

Distinguish between a cortical nephron and a juxtamedullary nephron... 

The glomerulus of each cortical nephron is located in the outer region of the cortex. Furthermore, the Loop of Henle in these nephrons is short and does not penetrate deeply into the medulla. In humans, 70 to 80% of the nephrons are cortical. 

The terminal portion of the proximal tubule leads into the descending thin limb of Henle in the region corresponding to the demarcation of outer and inner stripes of the outer zone of the medulla. The thin limb loops upward to form the ascending thin limb of Henle, which differentiates into the ascending thick limb of Henle at the intersection of the inner zone of medulla and the inner stripe of the outer medullary zone. This is characteristic of juxtamedullary nephrons that have long loops of Henle. In contrast, as noted earlier, cortical nephrons have short loops of... 

Potassium-Sparing Diuretics. Potassium-sparing diuretics act on the aldosterone-sensitive portion of cortical collecting tubules, and partially in the distal convoluted tubules of the nephron. The commonly used potassium-sparing diuretics are triamterene, amiloride, and spironolactone (Table 3). Spironolactone is a competitive aldosterone receptor antagonist, whereas triamterene and amiloride are not (44,45). 

Sodium reabsorption Much less than 10% of the filtered load of NaCl reaches the distal nephron. Regulation of Na uptake, occurring mainly in the principal cells of the cortical collecting tubule, is controlled by the steroid hormone aldosterone (see Section 4.4). The net effect of aldosterone is the reclamation of NaCl and potassium excretion in to the luminal fluid. 

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. 

The main action of thiazides is exerted on the early segment of distal tubule or cortical diluting segment. They inhibit reabsorption of sodium and chloride. The thiazides enter the tubule partly by glomerular filtration and partly by active secretion into the proximal tubule. At usual therapeutic doses, the major portion of the diuresis is due to an inhibition of reabsorption in the more distal parts of the nephron. 

Each kidney also contains over one million nephrons, which are the functional units of the kidney. Nephrons originate in the cortex where an afferent arteriole forms a specialized capillary bed known as the glomerulus (Figure 1). Some nephrons originate near the surface of the kidney and are called superficial nephrons, while other nephrons originate near the cortical-medullary region and are called juxtamedullary nephrons. 

Isolated perfused kidney/tubule Renal cortical slices Nephron segments Isolated renal epithelial cells Renal cell cultures... 

Under normal conditions, approximately 180 L of glomerular ultrafiltrate are produced per day, the vast majority of which must be reabsorbed by the renal tubules to maintain homeostasis. Clinically, the renal tubule can be divided into three major sections the proximal tubule, Henle s loop, and the distal nephron, which includes the distal tubule, the cortical collecting tubule, and the medullary collecting ducts. In the proximal tubule, approximately 60% to 70% of the filtered load of water and solute is isovolemicaUy reabsorbed, as is the vast majority of filtered amino acids, glucose, and bicarbonate. 

Cl carrier in the distal tubule. Einally, potassium-sparing diuretics inhibit the sodium channel in the cortical collecting duct either directly (triamterene and amiloride), or by interfering with aldosterone activity (spironolactone and eplerenone). The efficacy of a diuretic depends on the presence of several factors, including the amount of filtered solute normally reabsorbed at the site of action, the amount of solute reabsorbed distal to the site of action, and adequate delivery of drug to the site of action in the nephron. 

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