Histology and endocrinological function of the Juxtaglomerular apparatus
Abbreviations used: DCT- Distal convoluted tubule, JGA-Juxtaglomerular apparatus, MD-Macula Densa, TGF-tubular glomerular feedback, NO-Nitric Oxide, NOS-Nitric Oxide Synthase
The nephron, the functional unit of the kidney, is made of 2 parts: the renal corpuscle and the renal tubule. The renal corpuscle is composed of the glomerulus and Bowman’s capsule while the renal tubule extends from the Bowman’s capsule to the collecting duct.
The glomerulus is a globular network of capillaries surrounded by Bowman’s capsule. Filtration occurs under pressure through the fenestrations in the capillary wall and through gaps between the podocytes of the Bowman’s capsule into the Bowman’s space. The filtrate passes into the proximal convoluted tubule which is the longest and most convoluted part of the renal tubule where 65% of ions and water is reabsorbed. Next the filtrate travels through the loop of Henle which is surrounded by the vasa recta capillaries. Here further absorption of NaCl occurs by the counter-current multiplier system (Young et al, 2006).
The last part of the DCT passes close to the afferent artery at the vascular pole of the same Bowman’s capsule before joining the collecting duct. Together with the afferent and efferent arterioles, and the extraglomerular mesangium cells, it forms the juxtaglomerular apparatus that is involved in the regulation of systemic blood pressure via the renin-angiotensin –aldosterone, salt and water regulation (Young et al, 2006). Specialised tubular epithelial cells on the juxtaglomerular apparatus called Macula Densa cells increase in volume with increase in concentration of NaCl in filtrate. NaCl is absorbed into the MD cells via Na+-K+-2Cl- cotransporters present on the apical membrane of the cells (Obermuller et al, 1996). Absorption of NaCl by MD cells has the effects of regulating resistance of the glomerular arterioles and renin secretion...
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