Lobe/Lobule Lidney Concept

Another way to divide the kidney is into lobes and lobules. 
A lobe consists of a medullary pyramid and the cortex overlying it. Interlobar arteries and veins mark them off. 
 

The lobe can be divided into lobules, which consist of a medullary ray, the nephrons draining into it, and the section of the medullary pyramid continuous with it. In the cortex, interlobular arteries and veins mark them off, but there is no such clear division in the medulla. In this photograph, a lobe is outlined with a white line and a lobule with a black line. In this low power view of the cortex note that the interlobular vessels are not a part of the medullary rays.

Cortex and Medulla

The parenchyma of the kidney is divided into an outer cortex and an inner medulla. In a fresh, hemisected kidney, the cortex appears darker than the medulla, primarily because the cortex contains about 90% of the blood passing through the kidney. However, specimens with little or no blood can have a dark medulla and a lighter-colored cortex.

In the gross view, the cortex has a striated appearance. This series of vertical striations is known as medullary rays, because they appear to emanate from the medulla. Medullary rays consist of one or more collecting tubules, as well as the straight portion of the proximal tublules, distal tubules, and the loop of Henle. The medullary rays are surrounded by the rest of the cortical tissue: the renal corpuscles and the convoluted portions of the tubules. In this low power micrograph, medullary rays are quite evident, and at higher power, the main components of the medullary ray in the cortex can be compared: the collecting duct, distal tubule, and proximal tubule. These will be discussed further below.

The medulla consists of 10-18 pyramids. These pyramids are separated by inward extensions of cortical tissue called the renal columns of Bertin. One or two pyramids come together near the hilum, and drain into a minor calyx. (7-10 minor calyces per kidney.) These then gather into a major calyx (3-5 per kidney), which then joins the other major calyces to form the renal pelvis.