The kidneys form the first part of the urinary system and their principal function is to maintain electrolyte homeostasis and the acid-base balance. Kidney function is vital for regulating blood pressure and the kidneys are also a source of several important hormones such as erythropoietin, which regulates the production of red blood cells. Histologically, the renal parenchyma consists of four parts: glomeruli, tubules, interstitium and blood vessels. Glomeruli are complex vascular structures composed of a tuft of capillaries comprised of specialized endothelial, epithelial and mesangial cells arranged around a relatively thick basement membrane. The afferent arteriole divides into 4-8 branches in the glomerulus. Each branch spread into networks of capillaries that form lobules and then rejoin the vascular pole to drain into the efferent arteriole. The tuft of capillaries lies within the lumen of the expanded proximal end of the nephron, or Bowman's space, which is lined with a layer of attenuated epithelial cells overlying a thick basement membrane. Together the epithelial cells and basement membrane comprise the Bowman's capsule. The function of the glomeruli is filtration of the blood that leads to the formation of urine.
A complex tubular system begins at the urinary pole (where urine is first formed in the Bowman's space) that extends to the renal papilla. The system comprises of the proximal tubule, the loop of Henle, distal tubule and collecting duct. The proximal tubule consists of convoluted and straight portions, lined with tall columnar cells with abundant, acidophilic cytoplasm rich in structures for active fluid transport. The loop of Henle has thin descending and thick ascending portions covered with cuboidal and columnar cells. The distal tubule is narrower and shorter than the proximal tubule and lined with low cuboidal cells that do not display the deeply acidophilic, granular cytoplasm characteristic of the proximal tubule. Cuboidal cells with pale acidophilic cytoplasm and central nuclei line the collecting ducts.
The interstitium contains specialized interstitial cells and connective tissue elements. The larger renal blood vessels are structurally similar to those in other parts of the body.