1. 2. The renal papilla is located at the apex of each renal pyramid. It has a perforated tip (area cribrosa) that projects into the lumen of a minor calyx.
2. The renal cortex is the superficial layer of the kidney beneath the capsule. It consists primarily of renal corpuscles and convoluted tubules.
   1. Renal columns of Bertin are extensions of cortical tissue between adjacent renal pyramids.
   2. Medullary rays are groups of tubules that extend from the base of each renal (medullary) pyramid into the cortex. These tubules are the straight portions of the proximal and distal tubules as well as the collecting ducts that drain nephrons.
3. The renal lobe consists of a renal pyramid and its closely associated cortical tissue.
4. The renal lobule consists of a central medullary ray and the closely associated cortical tissue surrounding it, bounded on each side by an interlobular artery. Its many nephrons drain into the collecting tubules of the medullary ray.
5. The renal interstitium is the connective tissue compartment of the kidney. It is scanty in the cortex, occupying less than 10% of the cortical volume, and somewhat greater in the medulla, occupying about 20% of the medullary volume. The renal interstitium consists primarily of fibroblasts and mononuclear cells (probably macrophages). In the medulla, it consists of two additional cell types:
   1. Pericytes (see Chapter VI III B) are located along the blood vessels that supply the loops of Henle.
   2. Interstitial cells have long processes that extend toward (and perhaps encircle) capillaries and tubules in the medulla. These cells manufacture medullipin I, a vasodepressor hormone that is converted to medullipin II in the liver. Medullipin II is a vasodilator that acts to reduce blood pressure.