The number of lobes in a kidney equals the number of medullary pyramids
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 organ ization of the kidney is conspicuous in the developing fetal kidney (Fig. 19.5). 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. 19.6). Although the center or axis of a lobule is readily identified, the 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
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