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The Urinary System SUMMARY OF KEY POINTS Kidney

  • Each kidney has a thick outer cortex, surrounding a medulla that is divided into 8-12 renal pyramids; each pyramid and its associated cortical tissue comprises a renal lobe.

  • The apical papilla of each renal pyramid inserts into a minor calyx, a subdivision of two or three major calyces extending from the renal pelvis.

  • The ureter carries urine from the renal pelvis and exits the renal hilum, where the renal artery and vein are also located.

Renal Vasculature
  • Renal arteries branch to form smaller arteries between the renal lobes, with interlobular arteries entering the cortex to form the microvasculature; venous branches parallel the arterial supply.

  • In the cortex, afferent arterioles enter capillary clusters called glomeruli, which are drained by efferent arterioles, instead of venules, an arrangement that allows higher hydrostatic pressure in the capillaries.

  • The efferent arterioles from cortical glomeruli branch diffusely as peritubular capillaries, while those from juxtamedullary glomeruli branch as long microvascular loops called vasa recta in the medulla.

  • Functional units of the kidney are the nephrons, numbering about 1.25 million, each with a renal corpuscle and a long renal tubule, and a system of collecting ducts.

  • The renal corpuscle has a simple squamous parietal layer of the glomerular (Bowman) capsule, continuous with the proximal tubule, and a specialized visceral layer of podocytes surrounding the glomerular capillaries.

  • Podocytes extend large primary processes that curve around a capillary and extend short, interdigitating secondary processes or pedicels, between which are narrow spaces called slit pores.

  • The elevated pressure in the capillaries forces water and small solutes of blood plasma through the glomerular filter into the capsular (or urinary) space inside the glomerular capsule.

  • In each glomerulus the filter has three parts: the finely fenestrated capillary endothelium; the thick (330 nm) fused basal laminae of type IV collagen and other proteins produced by the endothelial cells and podocytes; and the slit pores between the pedicels, covered by thin filtration slit diaphragms.

  • From the renal corpuscle, filtrate enters the long nephron tubule that extends through both the cortex and medulla, with epithelial cells for both reabsorption and secretion of substances into the filtrate.

  • The first tubular part, the PCT, is mainly cortical, has simple cuboidal cells with long microvilli in the lumen, abundant mitochondria, and large, interdigitating basolateral folds.

  • In the PCT, all glucose and other organic nutrients, all small proteins and peptides (which are degraded to amino acids), and much water and electrolytes are reabsorbed from the filtrate and transferred to the peritubular capillaries.

  • From the PCT filtrate flows into the loop of Henle, located in the medulla, which has squamous thin descending and ascending limbs; the latter extends as a TAL back into the cortex.

  • In the cortex, the TAL (also known as the distal straight tubule...

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