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ANATOMY OF THE KIDNEY

  1. The paired kidneys are bean-shaped retroperitoneal organs, each about 12-cm long and located on the posterior abdominal wall.

  2. The renal artery, the renal vein, and the renal pelvis pass through the renal hilus; the renal nerves and lymphatic vessels are associated with the renal blood vessels (Figure 6-1).

  3. The cut surface of the kidney has a pale outer region, the cortex, and a darker inner region, the medulla.

    1. The medulla is divided into conical areas called renal pyramids.

    2. The apex of each pyramid tapers toward the renal pelvis, forming the renal papillae.

    3. The renal pelvis has a funnel-shaped upper end and is continuous with the ureter below it, which conveys urine to the bladder.

  4. image Renal colic is pain in the flank that radiates toward the groin. Colicky pain is usually severe and most commonly caused by renal calculi (“kidney stones”). The three common sites of obstruction leading to the clinical presentation of renal colic are:

    1. At the junction of the renal pelvis and ureter.

    2. At the site where the ureter passes over the pelvic brim.

    3. At the junction between the ureter and bladder. image

  5. Structure of the nephron.

    1. The functional unit of the kidney is a nephron, a tube that consists of different transporting epithelia in sequence; each kidney has between 500,000 and 800,000 nephrons.

    2. A nephron has several functionally and histologically distinct segments (Figure 6-2).

      1. The glomerulus is the site of primary urine formation where blood plasma is filtered into the nephron; the glomerular capillaries are enveloped by the blind-ended upper part of the nephron, known as Bowman's capsule.

      2. Filtrate flows from Bowman's space into the proximal tubule, which consists of convoluted and straight portions.

      3. The proximal tubule is continuous with the loop of Henle, which is a U-shaped tubule that descends into the medulla before turning back toward the cortex.

        • The loop of Henle has three functional parts: the thin descending limb, the thin ascending limb, and the thick ascending limb (TAL).

      4. The macula densa is a short segment that passes close to the glomerulus and connects the TAL to the distal tubule in the cortex.

      5. Distal tubules from approximately six nephrons converge with a single collecting duct.

      6. As the collecting ducts descend through the renal medulla, they converge to form the larger ducts of Bellini, which deliver final urine into the renal calyces.

      7. Urine proceeds to the renal pelvis and then to the lower urinary tract.

    3. There are two types of nephrons: about 85% are cortical nephrons and 15% are juxtamedullary (JM) nephrons (Figure 6-3).

      1. Glomeruli of cortical nephrons are located in the outer cortex and the loops of Henle are short.

      2. JM nephrons have glomeruli located deep in the cortex and have long loops of Henle, many extending to the tip of the renal papilla.

        • JM nephrons are “salt conserving” and are important for urine concentration.

        • When effective circulating blood volume is reduced, a higher proportion of renal blood flow ...

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