Peritubular capillaries

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). 

Figure 19.2 Basic renal processes. These processes include filtration, reabsorption, and secretion. (1) Filtration is the movement of fluid and solutes from the glomerular capillaries into Bowman s capsule. (2) Reabsorption, which takes place throughout the nephron, is the movement of filtered substances out of the tubule and into the surrounding peritubular capillaries. (3) Secretion is the movement of selected unfiltered substances from the peritubular capillaries into the renal tubule for excretion. Any substance that is filtered or secreted, but not reabsorbed, is excreted in the urine.

Throughout its length, the tubule of the nephron is composed of a single layer of epithelial cells. Furthermore, the tubule is close to the peritubular capillaries, so reabsorption involves movement of a substance along the following pathway ... 

Tubular reabsorption is considered passive when each of the steps in transepithelial transport takes place without the expenditure of energy. In other words, the movement of a given substance is from an area of high concentration to an area of low concentration by way of passive diffusion. Water is passively reabsorbed from the tubules back into the peritubular capillaries. 

Formation of Na+, K+-ATPase carrier molecules in the basolateral membrane of the tubular epithelial cells (promotes extrusion of Na+ ions from the cells and their movement into plasma by way of peritubular capillaries enhances the concentration gradient for passive diffusion through Na+ channels in the luminal membrane)... 

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. 

Tubular secretion is the transfer of substances from the peritubular capillaries into the renal tubule for excretion in urine. This process is particularly important for the regulation of potassium and hydrogen ions in the body it is also responsible for removal of many organic compounds from the body. These may include metabolic wastes as well as foreign compounds, including drugs such as penicillin. Most substances are secreted by secondary active transport. 

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. 

Sympathetic stimulation also increases the resistance of the efferent arteriole, leading to a decrease in blood pressure in the peritubular capillaries. This fall in pressure facilitates movement of sodium and water from the tubules into these capillaries. 

The third mechanism is peritubular extraction of peptides and proteins from postglomerular capillaries and intracellular metabolism. Experiments using iodi-nated growth hormone (125I-rGH) have demonstrated that, while reabsorption into endocytic vesicles at the proximal tubule is still the dominant route of disposition, a small percentage of the hormone may be extracted from the peritubular capillaries [79, 86]. Peritubular transport of proteins and peptides from the baso-lateral membrane has also been shown for insulin [87] and the mycotoxin ochra-toxin A [88]. 

Figure 8. Transplanted kidney stained for C4d using (a) an ultrasensitive polymer detection system (EnVision) and (b) standard strepavidin-biotin peroxidase system. There was clear staining of peritubular capillaries in (b) compared to the polymer system where the stain extended into the intertubular interstitium masking the capillaries.

The capillaries of the nonglomerular renal cortex (e.g., peritubular capillaries) represent a second capillary bed that derives from ramifications of the efferent glomerular arteriole. The vascular sequence is renal artery—arcuate arteries/arcuate arterioles—cortical arterioles—afferent (glomerular) arteriole—glomerular capillaries—efferent (glomerular) arteriole—peritubular capillaries [11,12]. 

Each kidney receives its blood supply from a renal artery, two of which branch from the abdominal aorta. Upon entering the hilum of the kidney, the renal artery divides into smaller arteries which in turn give off still smaller branches. Branching off these are the afferent arterioles supplying the glomerular capillaries, which drain into efferent arterioles. Efferent arterioles divide into peritubular capillaries that provide an extensive blood supply to the renal cortex. Blood from these capillaries collects in renal venules and leaves the kidney via the renal vein. Blood supply is intimately linked to blood pressure. 

Upon gross examination, three major anatomical areas of the kidney are apparent cortex, medulla, and papilla (Figure 29.1). The cortex is the outermost portion of the kidney and contains proximal and distal tubules, glomeruli, and peritubular capillaries. Cortical blood flow is high relative to cortical volume and oxygen consumption the cortex receives about 90% of total renal blood flow. A blood-borne toxicant will be delivered preferentially to the renal cortex and therefore has a greater potential to influence cortical, rather than medullary or papillary, functions. 

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. 

Blood flow to the two kidneys is approximately 22-25% of the cardiac output. The kidneys are supplied by the renal artery which enters the kidneys through the hilum and then branches progressively to form the interlobar arteries, arcuate arteries, interlobular arteries (also called radial arteries), and afferent arterioles, which lead to the glomerular capillaries. The distal ends of each glomerulus coalesce to form the efferent arteriole, which leads to a secondary capillary network, the peritubular capillaries which surround the renal tubules. The cortex receives approximately 90% of the blood flow compared to the medulla or papillae so blood-borne toxic molecules reaching the kidneys have a more toxic effect on the cortex, as compared to the medulla or renal papillae. The interstitial space is occupied by the fenestrated peritubular capillaries and a small number of fibroblast-like cells. Increase in thickness of interstitial space in pathological conditions is due to edema, proliferation of fibrous tissue, or infiltration of inflammatory cells (Guyton and Hall, 2006). 

Richman AV, Gerber LI, Balls JU Peritubular capillaries. A major target site of endotoxin-induced vascular injury in the primate kidney. Lab Invest 43 327-332,1980... 

Martinez-Maldonado Cordova 1990, Rose 1989, 1991, Wilcox 1991). In the absence of a loop diuretic, sodium ions transported into the cell are translocated into the peritubular capillary by the action of the Na, K -ATPase pump. Chloride ions are translocated out of the cell by two pathways a selective chloride channel and an electroneutral K, Cr-cotransporter. These processes maintain low intracellular sodium and chloride ion concentrations and favor continued entry of sodium and chloride from the tubular lumen. In contrast, potassium ion concentrations in the tubular fluid and within the cell are lower and higher, respectively, compared with sodium and chloride. Although these potassium ion concentrations would seem to inhibit the action of the Na, K, 2Cr-cotrans-porter, this problem is overcome by the recycling of much of the reabsorbed potassium (that does... 

Reclamation of bicarbonate. The filtered Na is reabsorbed by the proximal tubular cell in exchange for H". The filtered HCOj is converted to H2O and CO2 catalyzed by the luminal carbonic anhydrase IV (CAIV). CO2 diffuses in the tubular cell where it is hydrated to H2CO3 by carbonic anhydrase II and dissociated to H+ and HCOJ. Three molecules of HCO3 and one of Na" are transported to the peritubular capillary by the basolateral cotransporter. 

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