Ans. is 'a' i.e. Juxtaglomerulus apparatus Physiological anatomy of the Nephron:The nephron is the functional unit of the kidney. Each kidney has approx. 1.3 million nephrons.Each nephron consists of 1) the glomerulus through which large amounts of fluid are filtered from the blood and 2) a long tubule in which the filtered fluid is converted into urine on its way to the pelvis of the kidney.The glomerulus is formed by the invagination of a tuft of capillaries into Bowman's capsule. The capillaries are supplied by an afferent arteriole and drained by efferent arterioles.Fluid filtered from the glomerular capillaries flows into Bowman's capsule and then into the proximal convoluted tubule(PCT). The proximal convoluted tubule and the glomerulus lie in the cortex of the kidney.From the proximal convoluted tubule, fluid flows into the loop of Henle, which dip into the renal medulla. Each loop of Henle consists of a descending and an ascending limb.The ascending limb of loop of Henle reaches the glomerulus of the nephron from which the tubule arose and lies between its afferent and efferent arterioles.At the end of the ascending limb of loop of Henle is a plaque like structure in the wall k/a macula densa. The macula densa along with extraglomerular mesangial cells (lacis cells), and juxtaglomerular cells forms the juxtaglomerular apparatus which regulates the function of each nephron.Beyond the macula densa the fluid enters the distal convoluted tubule (DCT), which like the PCT lies in the cortex.The distal convoluted tubules coalesce to form collecting ducts (cortical collecting ducts and medullary collecting ducts) that pass through the renal cortex and medulla to empty into the pelvis of the kidney at the apices of the medullary pyramids.Physiological anatomy of renal circulation:The renal artery enters the kidney through the renal hilum and then divides progressively into the interlobar arteries, arcuate arteries, interlobular arteries and afferent arterioles. The afferent arterioles divide into glomerular capillaries. The distal ends of the glomerular capillaries coalesce to form the efferent arteriole. The efferent arteriole divides into a second capillary network, the peritubular capillaries which surround the renal tubules.The peritubular capillaries empty into the vessels of the venous system, which run parallel to the arteriolar vessels.Arrangement of the vascular structures differ between the cortical nephrons and the juxtamedullary nephrons. The capillaries draining the tubules of the cortical nephrons form the peritubular capillary network around the tubules, whereas the efferent arterioles from the juxtamedullary glomeruli drain not only into a peritubular capillary network but also into vessels that form hairpin loops called vasa recta. These loops dip into the medullary pyramids alongside the loops of Henle.