This image shows two adjacent renal corpuscles in a section of the rat kidney stained with hematoxylin and eosin. Only half of one of the corpuscles is shown (bottom, right).The renal corpuscle is the initial part of the nephron, where filtration of plasma takes place. The renal corpuscle is an ovoid structure with physical dimensions of approximately 150 μm to 250 μm. The renal corpuscle is composed of two distinct structures: the glomerulus and the Bowman's capsule. 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.