urinary anatomy

• Functions of the Urinary System

  • The kidneys produce urine
  • The ureters transport urine to the urinary bladder
  • The urinary bladder stores urine
  • The urethra transports urine to the outside of the body
• Functions performed by the kidneys

  • Excretion (eliminates waste)
  • Regulate blood volume and pressure
  • Regulation of the concentration of solutes in the blood (ion concentration)
  • Regulation of extracellular fluid pH
  • Regulation of red blood cell synthesis
  • Vitamin D production
• Kidneys


  • Lie behind the peritoneum on the posterior abdominal wall on each side of the vertebral column
  • Surrounded by a renal capsule and fat and is held in place by the renal fascia
  • The hilum, on the medial side of each kidney, is where blood vessels and nerves enter and exit the kidney
• The two layers of the kidney are the cortex and the medulla
• The renal columns extend into the medulla between the renal pyramids
• The tips of the renal pyramids project to the minor calyces
• The minor calyces open into the major calyces, which open into the renal pelvis
• The renal pelvis leads to the ureter
• Nephron


  • The functional unit of the kidney
  • The parts of a nephron are the renal corpuscle, the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule
  • The renal corpuscle is Bowman's capsule and the glomerulus
  • Fluid leaves the blood in the glomerulus and enters Bowman's capsule
  • The nephron empties through the distal convoluted tubule into a collecting duct
  • The collecting ducts empty into papillary ducts, which empty into minor calyces
• Types of nephrons


  • Cortical nephrons (85% of total nephrons, located in the cortex)
  • Juxtamedullary nephrons (located at the cortex-medulla junction, have loops of Henle that deeply invade the medulla, have extensive thin segments, are involved in the production of concentrated urine)
• Bowman's capsule


  • Has an outer parietal layer and an inner visceral layer consisting of podocytes
  • The filtration membrane consists of endothelium of glomerular capillaries (with fenestrae), basement membrane, and podocytes (with filtration slits)
• Capillary beds of the nephron

  • Every nephron has two capillary beds: glomerulus and peritubular capillaries
  • Each glomerulus is fed by an afferent arteriole and drained by an efferent arteriole
• Blood pressure in the glomerulus is high because arterioles are high-resistance vessels and afferent arterioles have larger diameters than efferent arterioles
• Fluids and solutes are forced out of the blood throughout the entire length of the glomerulus
• Arteries and veins of the kidneys

  • The renal artery enters the kidney and branches many times, forming afferent arterioles, which supply the glomeruli
  • Efferent arterioles from the glomeruli supply the peritubular capillaries and vasa recta
  • The peritubular capillaries and vasa recta join small veins that converge to form the renal vein, which exits the kidney
  • The juxtaglomerular apparatus consists of the granular cells of the afferent arteriole and the macula densa (part of the distal convoluted tubule)
• Urine production


  1. Filtration

  2. Tubular reabsorption

  3. Tubular secretion
• Glomerular filtration rate


  The amount of filtrate produced per minute
• The filtrate is plasma minus blood cells, platelets, and blood proteins
• Most (99%) of the filtrate is reabsorbed
• Filtration pressure


  Glomerular capillary pressure minus capsule pressure minus blood colloid osmotic pressure
• Regulation of glomerular filtration rate

  • Autoregulation dampens systemic blood pressure changes by altering afferent arteriole diameter
  • Autoregulation entails two types of control: myogenic (responds to changes in pressure in the renal blood vessels) and flow-dependent tubuloglomerular feedback (senses changes in the juxtaglomerular apparatus)
  • Sympathetic stimulation decreases renal blood flow and afferent arteriole diameter
• Tubular reabsorption


  A transepithelial process whereby most tubule contents are returned to the blood
• There is a medullary concentration gradient from the cortex (300 mOsm/kg) to the tip of the renal pyramids (1200 mOsm/kg)
• Transported substances move through three membranes: luminal and basolateral membranes of tubule cells, and endothelium of peritubular capillaries
• Only Ca2+, Mg2+, K+, and some Na+ are reabsorbed via paracellular pathways
• Tubular reabsorption


  1. Filtrate is reabsorbed by diffusion, facilitated diffusion, active transport, symport, and antiport from the nephron and collecting ducts into the peritubular capillaries and vasa recta

  2. The proximal convoluted tubule reabsorbs 65% of filtrate water and NaCl (solutes)

  3. The descending limb of the loop of Henle reabsorbs 15% of filtrate water

  4. The ascending limb of the loop of Henle reabsorbs 25% of filtrate NaCl

  5. The distal convoluted tubules and collecting ducts reabsorb up to 19% and 9%-10% of filtrate water respectively
• Waste products and toxic substances are concentrated in the urine
• Tubular secretion


  1. Substances are secreted in the proximal or distal convoluted tubules and the collecting ducts

  2. Hydrogen ions, K+, and some substances not produced in the body are secreted by antiport mechanisms
• The addition of solutes increases the medullary interstitial fluid concentration
• The ascending limb of the loop of Henle adds NaCl, but not water
• Urea cycles between the collecting ducts and the thin segments of the loop of Henle
• The vasa recta uses a countercurrent exchange mechanism
• Port


  Transport mechanism that moves substances from the nephron and collecting ducts into the peritubular capillaries and vasa recta
• Symport


  Transport mechanism that moves two substances in the same direction
• Antiport


  Transport mechanism that moves two substances in opposite directions
• Proximal convoluted tubule


  Reabsorbs 65% of filtrate water and NaCl (solutes)
• Descending limb of the loop of Henle

  Reabsorbs 15% of filtrate water
• Ascending limb of the loop of Henle

  Reabsorbs 25% of filtrate NaCl
• Distal convoluted tubules and collecting ducts

  Reabsorb up to 19% of filtrate water and 9%-10% of filtrate water respectively
• Substances are secreted in the proximal or distal convoluted tubules and the collecting ducts