that of the proximal convoluted tubule. cells lining the tubule have numerous mitochondria to produce enough energy (atp) for active transport to take place. much of the ion transport taking place in the distal convoluted tubule is regulated by the endocrine system. in the presence of parathyroid hormone, the distal convoluted tubule reabsorbs more calcium and secretes more phosphate. when aldosterone is present, more sodium is reabsorbed and more potassium secreted. ammonia is also absorbed during the selective reabsorption. atrial natriuretic peptide causes the distal convoluted tubule to secrete more sodium. connecting tubule a part of distal nephron. this is the final segment of the tubule before it enters the collecting duct system. water, some salts and nitrogenous waste like urea and creatinine are passed out to collecting tubule. collecting duct system main article: collecting duct system fig.6) cross-sectional histologic preparation showing (b)small connecting tubules with simple columnar epithelium and (a) large connecting tubules with simple cuboidal epithelium. each distal convoluted tubule delivers its filtrate to a system of collecting ducts, the first segment of which is the connecting tubule. the collecting duct system begins in the renal cortex and extends deep into the medulla. as the urine travels down the collecting duct system, it passes by the medullary interstitium which has a high sodium concentration as a result of the loop of henle's countercurrent multiplier system.: 67 because it has a different origin during the development of the urinary and reproductive organs than the rest of the nephron, the collecting duct is sometimes not considered a part of the nephron. instead of originating from the metanephrogenic blastema, the collecting duct originates from the ureteric bud.: 50–51 though the collecting duct is normally impermeable to water, it becomes permeable in the presence of antidiuretic hormone (adh). adh affects the function of aquaporins, resulting in the reabsorption of water molecules as it passes through the collecting duct. aquaporins are membrane proteins that selectively conduct water molecules while preventing the passage of ions and other solutes. as much as three-quarters of the water from urine can be reabsorbed as it leaves the collecting duct by osmosis. thus the levels of adh determine whether urine will be concentrated or diluted. an increase in adh is an indication of dehydration, while water sufficiency results in a decrease in adh allowing for diluted urine.: 406 fig.7) cross-sectional diagram of the juxtaglomerular apparatus and adjacent structures: 1) top, yellow - distal convoluted tubule; 2) top, brown -macula densa cuboidal cells surrounding arterioles; 3) small blue cells - juxtaglomerular cells; 4) large blue cells - mesangial cells; 5) tan - podocytes lining bowman's capsule adjacent to capillaries, and parietal layer of capsule, 6)center - five glomerular capillaries, and the 6)bottom, purple - exiting tubule. structures (2), (3), and (4) constitute the juxtaglomerular apparatus. lower portions of the collecting organ are also permeable to urea, allowing some of it to enter the medulla, thus maintaining its high concentration (which is very important for the nephron).: 73–74 urine leaves the medullary collecting ducts through the renal papillae, emptying into the renal calyces, the renal pelvis, and finally into the urinary bladder via the ureter.: 406–407 juxtaglomerular apparatus main article: juxtaglomerular apparatus the juxtaglomerular apparatus (jga) is a specialized region associated with the nephron, but separate from it. it produces and secretes into the circulation the enzyme renin (angiotensinogenase), which cleaves angiotensinogen and results in the ten amino acid substance angiotensin-1 (a-1). a-1 is then converted to angiotensin-2, a potent vasoconstrictor, by removing two amino acids: this is accomplished by angiotensin converting enzyme (ace). this sequence of events is referred to as the renin–angiotensin system (ras) or renin-angiotensin-aldosterone system (raas). the jga is located between the thick ascending limb and the afferent arteriole. it contains three components: the macula densa, juxtaglomerular cells, and extraglomerular mesangial cells.: 404 clinical significance patients in early stages of chronic kidney disease show an approximate 50% reduction in the number of nephrons, comparable to the nephron loss that occurs with aging (between ages 18-29 and 70-75). diseases of the nephron predominantly affect either the glomeruli or the tubules. glomerular diseases include diabetic nephropathy, glomerulonephritis and iga nephropathy; renal tubular diseases include acute tubular necrosis, renal tubular acidosis, and polycystic kidney disease. additional images distribution of blood vessels in cortex of kidney. (although the figure labels the efferent vessel as a vein, it is actually an arteriole.) glomerulus is red; bowman's capsule is white. kidney tissue glomerulus this image shows the types of cells present in the glomerulus part of a kidney nephron. podocytes, endothelial cells, and glomerular mesangial cell are present.