 Synthesis and secretion of the acid protease renin, an enzyme involved in control of blood pressure and blood volume. Renin is produced by juxtaglomerular cells and cleaves circulating angiotensinogen to release angiotensin I (see page 713).
 Hydroxylation of 25-OH vitamin D3, a steroid precursor produced in the liver, to hormonally active 1,25-(OH)2 vitamin D3. This step is regulated primarily by parathyroid hormone (PTH), which stimulates activity of the enzyme 1-hydroxylase and increases the production of the active hormone (see Folder 20.1).
The kidneys are large, reddish, bean-shaped organs located on either side of the spinal column in the retroperitoneal space of the posterior abdominal cavity. They extend from the 12th thoracic to the 3rd lumbar vertebrae, with the right kidney positioned slightly lower. Each kidney measures approximately 10 cm long  6.5 cm wide (from concave to convex border)  3 cm thick. On the upper pole of each kidney, embedded within the renal fascia and a thick protective layer of perirenal adipose tissue, lies an adrenal gland. The medial border of the kidney is concave and contains a deep vertical fissure, called the hilum, through which the renal vessels and nerves pass and the expanded, funnel-shaped origin of the ureter, called the renal pelvis, exits. A section through the kidney shows the relationship of these structures as they lie just within the hilum of the kidney in a space called the renal sinus (Fig. 20.1). Although not shown in the illustration, the space between and around these structures is filled largely with loose connective tissue and adipose tissue.
The kidney surface is covered by a connective tissue capsule. The capsule consists of two distinct layers: an outer layer of fibroblasts and collagen fibers, and an inner layer with a cellular component of myofibroblasts (Fig. 20.2). The contractility of
FIGURE 20.1 • Diagram of kidney structure. The diagram represents a hemisection of a kidney, revealing its structural organization.
the myofibroblasts may aid in resisting volume and pressure variations that can accompany variations in kidney function. Its specific role, however, is unknown. The capsule passes inward at the hilum, where it forms the connective tissue covering of the sinus and becomes continuous with the connective tissue forming the walls of the calyces and renal pelvis (see Fig. 20.1).
Examination with the naked eye of the cut face of a fresh, hemisected kidney reveals that its substance can be divided into two distinct regions:  Cortex, the outer reddish brown part  Medulla, the much lighter-colored inner part
The color seen in the cut surface of the unfixed kidney reflects the distribution of blood in the organ. Approximately 90% to 95% of the blood passing through the kidney is in the cortex; 5% to 10% is in the medulla.
The cortex is characterized by renal corpuscles and their associated tubules.
The cortex consists of renal corpuscles along with the convoluted tubules and straight tubules of the nephron, the collecting tubules, collecting ducts, and an extensive vascular supply. The nephron is the basic functional unit of the kidney and is described in a following section. The renal corpuscles are spherical structures, barely visible with the naked eye. They constitute the beginning segment of the nephron and contain a unique capillary network called a glomerulus.
Examination of a section cut through the cortex at an angle perpendicular to the surface of the kidney reveals a series of
FIGURE 20.2 • Photomicrograph of human kidney capsule.