Each medullary pyramid and the associated cortical tissue at its base and sides (one half of each adjacent renal column) constitutes a lobe of the kidney. The lobar organization of the kidney is conspicuous in the developing fetal kidney (Fig. 20.5). Each lobe is reflected as a convexity on the outer surface of the organ, but they usually disappear after birth. The surface convexities typical of the fetal kidney may persist, however, until the teenage years and, in some cases, into adulthood. Each human kidney contains 8 to 18 lobes. Kidneys of some animals possess only one pyramid; these kidneys are classified as unilobar, in contrast to the multilobar kidney of the human.
A lobule consists of a collecting duct and all the nephrons that it drains.
The lobes of the kidney are further subdivided into lobules consisting of a central medullary ray and surrounding cortical material (Fig. 20.6 and Plate 75, page 730). Although the center or axis of a lobule is readily identifiable, the
FIGURE 20.3 • Diagram of two types of nephrons in the kidney and their associated collecting duct systems. A long-looped nephron is shown on the left, and a short-looped nephron is shown on the right. The relative position of the cortex, medulla, papilla, and capsule are indicated. The inverted cone-shaped area in the cortex represents a medullary ray. The parts of the nephron are indicated by number: 1, renal corpuscle including the glomerulus and Bowman’s capsule; 2, proximal convoluted tubule; 3, proximal straight tubule; 4, descending thin limb; 5, ascending thin limb; 6, thick ascending limb (distal straight tubule); 7, macula densa located in the final portion of the thick ascending limb; 8, distal convoluted tubule; 9, connecting tubule; 9*, collecting tubule that forms an arch (arched collecting tubule); 10, cortical collecting duct; 11, outer medullary collecting duct; and 12, inner medullary collecting duct. (Modified from Kriz W, Bankir
L. A standard nomenclature for structures of the kidney. The Renal Commission of the International Union of Physiological Sciences (IUPS). Kidney Int 1988;33:1–7.) boundaries between adjacent lobules are not obviously demarcated from one another by connective tissue septa. The concept of the lobule has an important physiologic basis; the medullary ray containing the collecting duct for a group of nephrons that drain into that duct constitutes the renal secretory unit. It is the equivalent of a glandular secretory unit or lobule.
FIGURE 20.4 • Renal papilla and calyx. a. This scanning electron micrograph shows the conical structure that represents the renal papilla, projecting into the renal calyx. The apex of the papilla contains openings (arrows) of the collecting ducts (of Bellini). These ducts deliver urine from the pyramids to the minor calyx. The surface of the papilla containing the openings is designated the area cribrosa. (Courtesy of Dr. C. Craig Tisher.) b. Photomicrograph of an H&E–stained specimen of the papilla, showing the distal portion of the collecting ducts opening into the minor calyx. 120.
The Nephron
The nephron is the structural and functional unit of the kidney.