Blood flow is provided by a single afferent arteriole and is drained by a single efferent arteriole.
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The endothelial cells are perforated with relatively large fenestrae (70-100 nm), but the epithelial cells interdigitate tightly with one another, leaving relatively small filtration slits (about 25 nm).
The two cell types with their basement membranes provide an effective filtration barrier to cells and large-molecular-weight substances.
This barrier has multiple anionic sites that give it a net negative charge, favoring filtration of cations relative to anions. A third cell type, called intraglomerular mesangial cells, is located between the basement membrane and epithelial cells near adjacent capillaries.
These contractile cells regulate glomerular blood flow and also exhibit phagocytic activity. They secrete various substances, absorb immune complexes, and contain contractile proteins that respond to vasoactive substance.
Mesangial cells contract, reducing glomerular filtration, in response to angiotensin II, vasopressin, norepinephrine, histamine, endothelins, thromboxane A2, leukotrienes (C4 and D4), prostaglandin F2, and platelet-activating factor.
They relax, thereby increasing glomerular filtration, in response to atrial natriuretic peptide (ANP), prostaglandin E2, and dopaminergic agonists.
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The glomerulus contains the glomerular capillaries as well as the mesangial cells (not readily identified in this image). The glomerular capillary tuft in each glomerulus arises from the afferent arteriole that brings blood to the glomerulus. In turn, toward the end of the glomerular capillaries, these capillaries coalesce to form the efferent arteriole that takes blood away from the glomerulus. Mesangial cells are modified smooth muscle cells, and lie in between the glomerular capillaries. The outer layer of the Bowman's capsule is the outer boundary of the renal corpuscle. The inner layer of the Bowman's capsule is composed of cells known as podocytes. Podocytes have foot-like processes that wrap themselves tightly around endothelial cells of the glomerular capillaries. The association of the glomerular capillary endothelial cells and podocytes leads to the formation of the glomerular filtration barriers (also referred to as the blood-to-urine barriers) (see figure showing the components of the glomerular filtration barriers). The space between the outer and inner layers of the Bowman's capsule is known as the Bowman's space, which is continuous with the lumen of the nephron proximal tubule (not readily identified in this image). Plasma filtration in the kidney nephrons leads to the movement of water, ions, small molecules, and even larger molecules of up to 40 kDa (see glomerular permselectivity figure) out of the lumen of the glomerular capillary and into the Bowman's space, and ultimately into the lumen of the proximal tubule for further movement along the loop of Henle and other distal segments of the nephron. Note the presence of red blood cells in the afferent and efferent arterioles, as well as in the glomerular capillaries.
Each renal corpuscle contains a glomerulus, which is composed of tufts of capillaries that jut into Bowman’s capsule, providing a large surface area for the filtration of blood.
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