The kidney is a bean-shaped organ with renal arteries and veins entering with the ureter at the hilum. It consists of a cortex containing renal corpuscles and their associated convoluted and straight tubules and a medulla containing mainly straight tubules and ducts. The renal corpuscles and their associated tubules constitute the nephron, the basic functional unit of the kidney involved in blood filtration and urine production.

In the medulla, the straight tubules and ducts are organized into 8-15 renal pyramids. Each renal pyramid is positioned with its base at the corticomedullary junction and its tip (renal papillae) near the renal pelvis in association with a minor calyx that leads into the ureter. The pyramid is surrounded by cortical tissue (a renal column of Bertin) that dips down into the medulla. The renal pyramid with its associated cortical tissue constitutes a renal lobe. Medullary tissue also extends up into the cortex, forming medullary rays that are interspersed between cortical tissue containing the renal corpuscles and their associated convoluted tubules.

Study the diagrams at the right showing the overall organization of the kidney and the arrangement of structures within the nephrons. Refer to Figure 19-3 in Junqueira’s Histology 14e to review the blood supply to the kidneys noting the relationships between the vasculature and structures of the kidney. Click ere for a larger, printable image.

Examine the overall histological organization of the kidney, using this section of rat kidney and the image to the right. Unlike the human kidney, the rat kidney is unilobular, consisting of one large renal lobe with all of its ducts converging in the direction of the hilum.

• Identify the capsule and adipose-rich connective tissue at the hilum surrounding the renal pelvis.
• Identify the cortex with its renal corpuscles and the medulla with its straight tubules and collecting ducts.
• Near the corticomedullary junction, identify arcuate arteries and veins cut transversely.
• Identify the renal papilla, noting that the largest collecting ducts (the ducts of Bellini) converge here.
• Examine the calyx located near the renal papilla. Note that the calyx is lined by transitional or urinary epithelium.
• Click here for a large, printable image.

Clinical note: Kidney stones (nephrolithiasis) are concretions of calcium salts and uric acid that can form in the renal pelvis when urine contains high concentrations of substances such as calcium and uric acid. Small stones may pass down the ureters (with considerable pain) and larger stones can be destroyed by focused sound waves in a procedure called lithotripsy. (Photo courtesy of Dr. C.F. Verkoelen)
 

Now let's take a look at the renal cortex.