Page 707 - Atlas of Histology with Functional Correlations

capillaries, numerous proteinaceous macromolecules are trapped in the

glomerular basement membrane and filtration slit diaphragms. Mesangial
cells function as macrophages in the intraglomerular regions by removing
the trapped material from filtration slits and glomerular basal membrane, thus
preventing its clogging and keeping the glomerular filter free of debris. They
also phagocytose antigen–antibody complexes and produce several

interleukins in response to glomerular injury or damage. Mesangial cells are
also contractile and regulate glomerular blood flow and pressure changes in
the vascular pole region, between the afferent and efferent arterioles. Here,

they are called the extraglomerular mesangial cells, also called lacis cells,
and form part of the juxtaglomerular apparatus.

KIDNEY CELLS

The kidneys maintain body’ s stable internal environment, or homeostasis.
This function is performed by regulating the body’s blood pressure; blood
composition; and pH, fluid volume, and acid–base balance. The kidneys also

produce urine, which is formed by three main functions: (1) filtration of
blood in the glomeruli, (2) reabsorption of nutrients and other valuable
substances from the ultrafiltrate that enters the proximal and distal

convoluted tubules, and (3) secretion, or excretion, of metabolic waste
products or unwanted chemicals or substances into the filtrate that become
urine. Approximately 99% of the glomerular ultrafiltrate produced by the
kidneys is reabsorbed into the system; the remaining 1% of the filtrate is
voided as urine.

In addition, kidneys have endocrine functions by producing hormones
erythropoietin and renin. Endothelial cells of the peritubular capillary
network in the renal cortex produce the glycoprotein hormone erythropoietin,
which is a growth factor that stimulates erythrocyte production in red bone

marrow in response to hypoxia, a decrease in oxygen concentration in blood
or tissues. Renin is produced by kidney cells to regulate blood pressure and
to maintain proper filtration pressure in the glomeruli.

KIDNEY TUBULES

Proximal Convoluted Tubules

All nephrons participate in the formation of urine. As the ultrafiltrate from

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