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• 2 Anatomy of the Urinary system

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• Explain the structures of the urinary system

  • Urine is formed in the kidneys, then conducted to the bladder through the ureters. Bladder stores the urine and then excretes it via the urethra out of the body. 
    
  • The kidneys do not lie in the peritoneum. They are retroperitoneal.
  • Kidneys receive blood via renal arteries, which branch off from the aorta and enter each kidney at the renal hilus. 
  • Kidney receives 20% of the cardiac output.
  • Blood returns to the circulation via the renal veins, which drain into the inferior vena cava. 

• What is the renal corpuscle?

  • The renal corpuscle consists of 2 parts: a spherical structure at the inflow end of the renal tubes called Bowman's capsule, and a bundle of capillaries called glomerulus. 
  • Blood is filtered in the renal corpuscle and the tubular fluid, or filtrate, originates here.
  • Blood comes in via the afferent arteriole, then flows through the glomerular capillaries into Bowman's capsule. Blood leaves via the efferent arteriole.  

• What are the renal tubes and what kinds are there?

  1. Proximal convoluted tubule - is highly folded or convoluted.
  2. Proximal straight tube.

  
  These 2 together form the proximal tubule, which empties into the loop of Henle, the hairpin loop. Loop of Henle is divided into 3 parts:
  

  1. Descending limb
  2. Thin ascending limb
  3. Thick ascending limb

  
  Fluid then flows into the distal convoluted tubule, then the connecting tubule and then joins the nephron with the collecting duct. 

• What types of nephrons are there?

  • Cortical nephrons - Majority of the nephrons. Almost entirely in the renal cortex. Only the tip of the loop of Henle dips into the medulla.
  • Juxtamedullary nephrons - 15-20% of all nephrons. Renal corpuscle is located near the border between cortex and medulla. Loop of Henle dips deep into the renal medulla.

• What is the Juxtaglomerular apparatus?

  It is a site where the initial portion of the distal tubule comes in contact with a nephron's afferent and efferent arterioles. Contains 2 components:
  

  1. Macula densa - specialized cluster of epithelial cells of the distal tubule.
     
  2. Granular cells (or juxtaglomerular cells) - specialized cells in the wall of the afferent arterioles that have granular cytoplasms due to the presence of secretory cells containing renin. 

• Explain the blood supply to the kidney.

  • Branching of artery within the kidney: renal artery --> segmental arteries --> interlobal arteries --> arcuate arteries --> interlobular arteries --> arterioles that lead into the glumerular capillary beds.

  1. Peritubular capillaries branch from the efferent arterioles of cortical nephrons and are located close to renal tubules.
     
  2. Vasa recta branch off from the efferent arterioles of juxtamedullary nephrons and are networks of blood vessels forming hairpin loops that run along the loops of Henle. 

• 3 Basic Renal Exchange Processes

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• What 3 exchange processes occur in the renal nephrons?

  1. Glomerular filtration
  2. Reabsorption
  3. Secretion

• What is glomerular filtration?

  Filtration in the renal corpuscle. Driven by Starling forces (hydrostatic and osmotic pressure gradients). The filtrate resembles plasma except in lacking proteins present in plasma. The glomerular filtrate passes three barriers to enter Bowman's capsule.
  

  1. Capillary endothelial layer
  2. Surrounding epithelial layer
  3. Basement membrane that is sandwiched between them
     

  3 layers together are called the glomerular membrane or filtration barrier.

• What is the glomerular filtration rate?

  The volume of the plasma filtered per unit time is the glomerular filtration rate, which is about 125mL/min --> 180L per day!! Total plasma volume filters through the body every 22 minutes.

• What is the filtration fraction?

  Filtration fraction is the fraction of the renal plasma that is filtered. Equal to the glomerular filtration rate divided by the renal plasma flow (625mL/min). So it is 125/625 = 0.20 = 20%.
  Thus: 20% of the plasma that flows through the kidneys is filtered and enters Bowman's capsule.