Page 698 - Atlas of Histology with Functional Correlations

of the glomerular capsule consists of simple squamous epithelium.

The renal corpuscle is the initial segment of each nephron. The entry point of

the afferent glomerular capillaries into and the exit of efferent vessels from the
renal corpuscle is the vascular pole. On the opposite end of the vascular pole of
the renal corpuscle is the urinary pole, where the filtrate produced by the
glomerulus leaves the renal corpuscle.

Blood Filtration

Blood flowing through the kidneys is filtered in renal corpuscles through the
glomerular capillaries. The produced filtrate then enters the capsular (urinary)

space located between the parietal and visceral cell layers of the glomerular
capsule of the renal corpuscle. The filtrate leaves each renal corpuscle at the
urinary pole where the proximal convoluted tubule originates. The filtration
barrier for blood in the renal corpuscles consists of three different components:

the glomerular capillary endothelium, the underlying thicker glomerular
basement membrane, and the visceral layer of the Bowman capsule,
podocytes, and pedicles.

Filtration Barrier in Glomerulus

Blood filtration is facilitated by the glomerular endothelium of the capillaries,
which is thin, porous (fenestrated), and permeable to many substances in the
blood, except to the formed blood elements or large plasma proteins. Located

between the capillary endothelium and the visceral podocytes is the denser
glomerular basement membrane formed by the fusion of the endothelium and
the visceral layer of podocytes. The glomerular basement membrane is a
selective physical barrier that filters and restricts the movement of

macromolecules such as albumin from the blood. The semipermeable slit
diaphragms between the individual pedicles of the podocytes are highly
specialized junctional complexes containing a transmembrane protein called
nephrin that becomes the main structural and functional portion of the slit

diaphragm. The protein nephrin connects or anchors firmly with the actin
filaments in the adjacent pedicles of the podocytes forming filtration slits that act
like a fine sieve in the renal corpuscle. Thus, although each component of the
filtration barrier in the glomerulus contributes to blood filtration, the podocyte

slit diaphragms are responsible for glomerular permeability and filtration

697