Figure 191

Diagram of kidney structure. The diagram represents a hemisection of a kidney, revealing its structural organization.

arcuate vessel renal column hilum renal artery renal vein renal pelvis medullary ray minor calyx medulla mately 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 and straight tubides 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 below. 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 vertical striations that appear to emanate from the medulla (see Fig 19.1). These striations are the medullary rays (of Ferrein). Their name reflects their appearance, as the striations seem to radiate from the medulla. Approximately 400 to 500 medullary rays project into the cortex from the medulla.

Each medullary ray is an aggregation of straight tubules and collecting ducts

Each medullary ray contains straight tubules of the nephrons and collecting ducts. The regions between medullary rays contain the renal corpuscles, the convoluted tubules of the nephrons, and the collecting tubules. These areas are referred to as cortical labyrinths. Each nephron and its collecting tubule (which connects to a collecting duct in the medullary ray) form the uriniferous tubule.

The medulla is characterized by straight tubules, collecting ducts, and a special capillary network, the vasa recta

The straight tubules of the nephrons and the collecting ducts continue from the cortex into the medulla. They are accompanied by a capillary network, the vasa recta, that runs in parallel with the various tubules. These vessels represent the vascular part of the countercurrent exchange system that regulates the concentration of the urine.

The tubules in the medulla, because of their arrangement and differences in length, collectively form a number of conical structures called pyramids. Usually 8 to 12 but as many as 18 pyramids may be present in the human kidney. The bases of the pyramids face the cortex, and the apices face the renal sinus. Each pyramid is divided into an outer medulla (adjacent to the cortex) and an inner medulla. The outer medulla is further subdivided into an inner stripe and an outer stripe. The zonation and stripes are readily recognized in a sagittal section through the pyramid of a fresh specimen. They reflect the location of distinct parts of the nephron at specific levels within the pyramid (Fig. 19.3).

papilla FIGURE 19.3

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: 7, 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); 70, cortical collecting duct; 77, outer medullary collecting duct; and 72, inner medullary collecting duct. (Modified from Kriz W, Bankir L. Kidney Int 1988;33:1-7.)

The renal columns represent cortical tissue contained within the medulla

The caps of cortical tissue that lie over the pyramids are sufficiently extensive that they extend peripherally around the lateral portion of the pyramid, forming the renal columns (of Bertin). Although renal columns contain the cortex outer medulla inner medulla

outer stripe inner stripe cortex capsule outer medulla inner medulla outer stripe inner stripe same components as the rest of the cortical tissue, they are regarded as part of the medulla. In effect, the amount of cortical tissue is so extensive that it "spills" over the side of the pyramid much as a large scoop of ice cream extends beyond and overlaps the sides of an ice cream cone.

The apical portion of each pyramid, which is known as the papilla, projects into a minor calyx, a cup-shaped structure that represents an extension of the renal pelvis. The tip of the papilla, also known as the area cribrosa, is perforated by the openings of the collecting ducts (Fig. 19.4). The minor calyces are branches of the two or three major calyces that in turn are major divisions of the renal pelvis (see Fig. 19.1).

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