URINARY SYSTEM

• Kidneys dispose of waste products in urine:

  • Nitrogenous wastes
  • Toxins
  • Drugs
  • Excess ions
• Kidneys’ regulatory functions include:

  • Production of renin to maintain blood pressure
  • Production of erythropoietin to stimulate red blood cell production
  • Conversion of vitamin D to its active form
• The kidneys are situated against the dorsal body wall in a retroperitoneal position (behind the parietal peritoneum)
• The kidneys are situated at the level of the T12 to L3 vertebrae
• The right kidney is slightly lower than the left (because of position of the liver)
• An adult kidney is about 12 cm (5 in) long and 6 cm (2.5 in) wide
• Renal hilum

  • A medial indentation where several structures enter or exit the kidney (ureters, renal blood vessels, and nerves)
• An adrenal gland sits atop each kidney
• Three protective layers enclose the kidney:

  • Fibrous capsule encloses each kidney
  • Perirenal fat capsule surrounds the kidney and cushions against blows
  • Renal fascia is the most superficial layer that anchors the kidney and adrenal gland to surrounding structures
• Three regions revealed in a longitudinal section

  • Renal cortex—outer region
  • Renal medulla—deeper region
  • Renal pelvis—medial region
• Renal (medullary) pyramids—triangular regions of tissue in the medulla
• Renal columns—extensions of cortexlike material that separate the pyramids
• Renal pelvis—medial region that is a flat, funnel-shaped tube
• Calyces form cup-shaped “drains” that enclose the renal pyramids
• Calyces collect urine and send it to the renal pelvis, on to the ureter, and to the urinary bladder for storage
• Blood supply - One-quarter of the total blood supply of the body passes through the kidneys each minute
• Renal artery provides each kidney with arterial blood supply
• Renal artery divides into segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries
• Venous blood flow:

  Cortical radiate veins → arcuate veins → interlobar veins → renal vein
• There are no segmental veins
• Renal vein returns blood to the inferior vena cava
• Nephrons - structural and functional units of the kidneys
• Nephrons - each kidney contains over a million nephrons
• Nephrons - each nephron consists of two main structures
• Two main structures of nephron:

  • Renal corpuscle
  • Renal tubule
• Renal corpuscle consists of:

  o Glomerulus, a knot of capillaries made of podocytes
• Renal corpuscle consists of:

  Podocytes make up the inner (visceral) layer of the glomerular capsule

  • Foot processes cling to the glomerulus
  • Filtration slits create a porous membrane—ideal for filtration
• Glomerular (Bowman’s) capsule is a cup-shaped structure that surrounds the glomerulus

  • First part of the renal tubule
• Renal tubule

  • Extends from glomerular capsule and ends when it empties into the collecting duct
• From the glomerular (Bowman’s) capsule, the subdivisions of the renal tubule are:

  • Proximal convoluted tubule (PCT)
  • Nephron loop (loop of Henle)
  • Distal convoluted tubule (DCT)
• Cortical nephrons

  • Located entirely in the cortex
  • Include most nephrons
• Juxtamedullary nephrons

  • Found at the cortex-medulla junction
  • Nephron loop dips deep into the medulla
  • Collecting ducts collect urine from both types of nephrons, through the renal pyramids, to the calyces, and then to the renal pelvis
• Two capillary beds associated with each nephron

  • Glomerulus
  • Peritubular capillary bed
• Glomerulus

  • Fed and drained by arterioles
• Afferent arteriole — arises from a cortical radiate artery and feeds the glomerulus
• Efferent arteriole — receives blood that has passed through the glomerulus
• Glomerulus

  • Specialized for filtration
  • High pressure forces fluid and solutes out of blood and into the glomerular capsule
• Peritubular capillary beds

  • Arise from the efferent arteriole of the glomerulus
  • Low-pressure, porous capillaries
• Peritubular capillary beds

  • Adapted for absorption instead of filtration
  • Cling close to the renal tubule to receive solutes and water from tubule cells
  • Drain into the interlobar vein
• Urine formation is the result of three processes

  1. Glomerular filtration 2. Tubular reabsorption 3. Tubular secretion