Kidney

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The kidneys are paired retroperitoneal organs of the urinary system. Their function is to filter blood and produce urine. Each kidney consists of a cortex, medulla and pelvis.

• The renal cortex is the outer layer of the kidney tissue and contains ovoid and coiled parts of the nephrons (renal corpuscles and convoluted tubules).

• The renal medulla consists of renal pyramids separated by the renal columns. The apices of the pyramids project towards the renal pelvis and open into the minor calyces via perforated plates on their surfaces (area cribrosa). Each renal pyramid, with its surrounding cortical tissue, forms a renal lobe. Renal lobes are further divided into renal lobules. Each lobule consists of a group of nephrons emptying into one collecting duct.

The nephron is the main functional unit of the kidney, responsible for removing metabolic waste and excess water from the blood. Depending on their distribution and morphology, there are two main types of nephrons in the kidney; cortical and juxtamedullary. 

The renal corpuscle is the filtration apparatus of the nephron. Each corpuscle consists of two main elements; the glomerulus and glomerular (Bowman's) capsule.

• The glomerulus is a network of capillaries formed by branches of the renal artery (afferent and efferent arterioles). 

• The glomerular capsule surrounds the glomerulus. It consists of two layers (parietal and visceral), which bound a cavity called the glomerular capsular space (Bowman’s / urinary space). The inner visceral layer is made of special cells called podocytes. The outer parietal layer is made of simple squamous epithelium and is continuous with the nephron tubules. The afferent and efferent arterioles enter the renal corpuscle at the vascular pole, while the site where the glomerular capsule narrows and continues as the proximal thick segment of the nephron is called the urinary pole.

At the vascular pole of the nephron, a collection of cells called the juxtaglomerular apparatus (JGA) can be found. This is formed by 3 types of cells; macula densa, juxtaglomerular granular (JG) cells and extraglomerular mesangial (Lacis) cells.

Watch the following video to learn more about the histological appearance of the kidney:

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Kidney

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Renal corpuscle and juxtaglomerular apparatus