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The urinary system is composed of the kidneys, ureters, urinary bladder, and urethra. The kidney is a homeostatic filtering mechanism responsible for regulating body water, ions, and pH, as well as for excreting many soluble wastes from the blood. These functions center on the process of urine being produced by the kidneys followed by its elimination from the body via the ureters, bladder, and urethra.

  • The functional units of the kidney are nephrons and collecting ducts. The nephron starts at a capillary bed, the glomerulus, which is located inside a renal corpuscle. Urine begins as a filtrate of blood plasma, which is driven into the renal corpuscle by blood pressure.

  • The filtrate flows down a series of tubules in the nephron and the collecting duct system, which recovers solute and water in a process known as concentrating the urine.

  • The continuous adjustment of the volume and ionic composition of blood is linked to the processes responsible for urine concentration.

  • Two countercurrent systems are found in the kidney. The countercurrent multiplier of the loop of Henle is important for urine concentration, and the countercurrent exchanger of the vasa recta supplies blood to the kidney medulla.

  • Regulation of flow through the tubules is accomplished in part by components of the nephron known as the juxtaglomerular apparatus.

  • The kidneys serve as endocrine organs. They produce renin, a protease that affects systemic blood pressure, and erythropoietin, a cytokine that stimulates red blood cell production. The kidneys also provide the final hydroxylation reaction required to produce active vitamins D2 and D3, hormones involved in Ca2+ regulation and bone physiology.


The body contains one pair of kidneys, located retroperitoneally in the abdominal cavity on either side of the spine. A coronal section of the kidney shows the gross anatomic features, the cortex, medulla, and calyces, which provide the framework and nomenclature used in the description of the microanatomy and function of the kidney (Figure 13-1A and B).

Figure 13-1:

Anatomy of the kidney and the renal corpuscle. A. Section of the kidney with the medullary pyramids surrounded by cortex. Urine production begins in the cortex and ends when fluid exits a medullary papilla and enters a minor calyx. B. Kidney tubules. The nephron begins at the renal corpuscle in the cortex, where the initial filtration of blood occurs. The nephron continues as a series of tubules, the proximal convoluted tubule (PCT), proximal straight tubule (PST), thin descending and ascending limbs of the loop of Henle (tDLH, tALH), thick ascending limb (TAL), and distal convoluted tubule (DCT). The collecting tubule (CT) and collecting duct (CD) have a separate embryonic origin. C. Renal corpuscle. The initial filtrate of blood occurs inside Bowman's capsule. The glomerulus filter unit has three components: (1) the fenestrated ...

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