JUXTA GLOMERULAR APPARATUS
Each nephron has a region called a juxtaglomerular apparatus, where the most distal portion of the ascending limb of the loop of Henle lies against the afferent arteriole feeding the glomerulus (and sometimes the efferent arteriole). Both the ascending limb and the afferent arteriole are modified at the point of contact.
The juxtaglomerular apparatus includes two cell populations that play important roles in regulating the rate of filtrate formation and systemic blood pressure. In the arterioles walls are the granular cells, also called juxtaglomerular cells, which are enlarged, smooth muscle cells with prominent secretory granules containing renin. Granular cells act as mechanoreceptors that sense the blood pressure in the afferent arteriole. The macula densa is a group of tall, closely packed cells of the ascending limb of the loop of Henle that lies adjacent to the granular cells. The macula densa cells are chemoreceptors that respond to changes in the NaCl content of the filtrate. A third population of cells, the extraglomerular mesangial cells, is also part of the juxtaglomerular apparatus. These cells are interconnected by gap junctions and may pass signals between macula densa and granular cells.
THE FILTRATION MEMBRANE
The filtration membrane lies between the blood and the interior of the glomerular capsule. It is a porous membrane that allows free passage of water and solutes smaller than plasma proteins.
The filtration membrane layers are:
- The fenestrated endothelium of the glomerular capillaries
- The visceral membrane of the glomerular capsule, made of podocytes which have filtration slits between their foot processes
- The basement membrane composed of the fused basal laminae of the two other layers
The fenestrations (capillary pores) allow passage of all plasma components but not blood cells. The basement membrane restricts all but the smallest protein while permitting most other solutes to pass. The structural makeup of the gel-like basement membrane also confers electrical selectivity on the filtration process. Most of the proteins in the membrane are negatively charged glycoproteins that repel other macromolecular anion and hinder their passage into the tubule. Because most plasma proteins also bear a net negative charge, this electrical repulsion reinforces the plasma protein blockage imposed by molecular size.
Almost all macromolecules that do manage to make it through the basement membrane are prevented from traveling further by thin membranes (slit diaphragms) that extend across the filtration slits. Macromolecules that get “hung up” in the filtration membrane are engulfed and degraded by mesangial cells can also contract, changing the total surface area of the capillaries available for filtration.