Kidney: Gross Structure
The adult kidney is about 11 cm long, 2.5 cm thick, 5 cm wide, and weighs between 120 and 170 g. The lateral border of each kidney is convex, whereas the medial border is concave. The superior and inferior poles are rounded. Both the anterior and posterior surfaces of the kidney are also convex, although the posterior surface may be relatively ﬂattened.
The renal artery and vein, as well as the urine collecting system, enter and exit the medial aspect of each kidney at the hilum. This indented region leads to a spacious cavity within each kidney known as the renal sinus. Within the renal sinus, a matrix of perinephric fat surrounds branches of the renal artery and vein, as well as the large branches of the urinary collecting system. The veins are generally the most anterior and the branches of the collecting system most posterior, with the arteries coursing in between.
The entire outer rim of the renal parenchyma consists of a brownish pink region known as the renal cortex. Deep to the cortex, numerous darker-colored renal pyramids, with bases directed peripherally and apices directed centrally, collectively form the renal medulla. The apices of the renal pyramids are known as the renal papillae. Two or more pyramids may fuse at their papillae; thus there are more pyramids than papillae in each kidney.
The areas of cortex overlying the bases of the pyramids, separating them from the outer surface of the kidney, are known as cortical arches. The areas of cortex projecting between pyramids are known as renal (cortical) columns (of Bertin). The term “column” refers to their appearance on section; in fact, they are more like walls, which surround and separate the pyramids.
Although the borders between pyramids and renal columns are sharply deﬁned, the pyramids project striations into the cortical arches, known as medullary rays.
These striations largely represent collecting ducts (see Plate 1-26), which extend from the cortex to the renal papillae, merging along the way into papillary ducts. The papillary ducts drain urine to 20 or more small pores at each papilla’s cribriform area (area cribrosa). One to three papillae drain into each minor calyx; two to four minor calices join to form a major calyx; and two or three major calices join to form the funnel-shaped renal pelvis, which becomes the ureter after leaving the hilum. The ureter, in turn, conveys urine to the bladder for storage.
The parenchyma served by a single papilla is known as a renal lobe, and in the fetus and infant these lobes are evident as grossly visible convexities separated by deep grooves on the kidney surface. Such lobulation persists in some mammalian species throughout life, and vestigial demarcations of lo ulation are occasionally present in the human adult.