Vasa recta

The vasa recta are modified peritubular capillaries. As with the peritubular capillaries, the vasa recta arise from efferent arterioles. However, these vessels are associated only with the juxtamedullary nephrons and are found only in the medullary region of the kidney. The vasa recta pass straight through to the inner region of the medulla, form a hairpin loop, and return straight toward the cortex. This structure allows these vessels to lie parallel to the Loop of Henle and collecting ducts. 

The vasa recta perform several important functions, including  

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The main function of the Loop at Henie (LoH) in the context of diuretics is further reabsorption and conservation of water and eiectroiytes from the remaining 30% of the fiitrate emerging from the PCT. This is achieved by a combination of counter-current muitipiier and counter-current exchange mechanisms in the LoH and the vasa recta respectiveiy which first concentrate and then diiute the filtrate during its passage. 

Degenerative changes occur in many organs of rats after administration of aziridine by various routes, including inhalation (lARC, 1975). Acute renal papillary necrosis is produced in rats and dogs administered aziridine. At low doses in rats, there was necrosis of interstitial cells, thin limbs of the loops of Henle and vasa recta, while collecting ducts were spared. At higher doses, there was total papillary necrosis (Ellis et al., 1973 Ellis Price, 1975 Axelsen, 1978). 

The renal medulla is the middle portion of the kidney and consists of the loops of Henle, vasa recta, and collecting ducts. Medullary blood flow (about 6% of total renal blood flow) is considerably lower than cortical flow. However, by virtue of its countercurrent arrangement between tubular and vascular components, the medulla may be exposed to high concentrations of toxicants within tubular and interstitial structures. 

The papilla is the smallest anatomical portion of the kidney. Papillary tissue consists primarily of terminal portions of the collecting duct system and the vasa recta. Papillary blood flow is low relative to cortex and medulla less than 1% of total renal blood flow reaches the papilla. However, tubular fluid is maximally concentrated and the volume of luminal fluid is maximally reduced within the papilla. Potential toxicants trapped in tubular lumens may attain extremely high concentrations within the papilla during the process of urinary concentration. High intraluminal concentrations of potential toxicants may result in diffusion of these chemicals into papillary tubular epithelial and/or interstitial cells, leading to cellular injury. 

The renal vasculature serves to (1) deliver waste and other materials to the tubule for excretion, (2) return reabsorbed and synthesized materials to the systemic circulation, and (3) deliver oxygen and metabolic substrates to the nephron. Vascular components of the nephron include afferent and efferent arterioles, glomerular capillaries, peritubular capillary network, and the vasa recta (Figure 29.2). 

Blood enters the glomerulus in the afferent arteriole. As it passes through the glomerular capillaries, fluid filters across the capillary wall into the renal tubules. Blood leaves the glomerulus in the efferent arteriole, which then gives rise to peritubular capillaries surrounding the renal tubules, and the vasa rectae which follow the loops of Henle down into the medulla. 

As the ascending limb of the loop re-enters the renal cortex, sodium continues to be removed from the tubular fluid by the sodium pump, accompeinied electrostatically by chloride. Both ions pass into the interstitial tissue (site 3) from which they are rapidly removed because cortical blood flow is high and there are no vasa recta present consequently the urine becomes more dilute. Thiazides act principally at this cortical diluting segment of the ascending limb, preventing sodium reabsorption. They inhibit the NaCl co-transporter (called NCCT). 

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