Urinary System

  1. What is one of the main roles of the kidney?
    • to remove metabolic waste products from blood
    • but really it does this by selecting what's useful & letting everything else pass through the convoluted tubule system
  2. Major Functions of the Kidney
    • removes metabolic waste products from blood
    • urine is yellow because it contains urobilin, a breakdown product of heme
    • regulates ion concentrations (Na+, K+, etc.)
    • regulates blood acid-base balance
    • regulates blood pressure (renin, secretion)
    • regulates erythrocyte production (erythropoietin)
    • activates vitamin D (parathyroid hormone controls 2nd hydroylation in the kidney → active vitamin D)
  3. Is the kidney an endocrine organ?
    • yes, 2 hormones are synthesized & released by the kidney
    • 1. renin: increases blood pressure (in response to low BP)
    • 2. erythropoietin: regulates RBC production in bone marrow
  4. Why would early dialysis patients become anemic?
    because patients with kidney issues were not producing erythropoietin
  5. Vitamin D
    • a 'hormone' synthesized in skin (requires UV light for synthesis) but turned into its active form in the kidney
    • it increases Ca2+ absorption in small intestine & renal tubules, as well as osteoclast (bone breakdown) activity to increase Ca2+ serum levels
  6. Kidney Anatomy
    • human kidney has multiple lobes
    • striated central region of lobe is the medulla; outside of that is the cortex
    • each area of the lobe that contains the medulla is called a renal pyramid
    • the apex/tip of the renal pyramid is the renal papilla
    • it's from the renal papilla that the fully concentrated urine drips out into calyxes
    • individual calyxes unite to form a major calyx
    • major calyxes all unite to form the renal pelvis, which is the proximal part of the ureter
    • Image Upload 1
  7. Kidney Blood Supply
    • there are multiple branches off the renal artery
    • these branches turn into interlobar arteries, which then turn into:
    • when they run between the medulla/cortex they're called arcuate arteries (follow the border between the cortex & medulla) 
    • from the arcuate arteries you have small branches that extend off in a radial fashion known as interlobular arteries
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    • lobules are the regions between interlobular arteries
    • in the center of each lobule are medullary rays, straight elongated tubules extending up from the medulla into the middle of lobules/cortex
  8. Kidney Cortex
    • center of a lobule: medullary rays
    • lobule borders: interlobular vessels
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    • it's within the medullary rays where collecting ducts receive all the urine produced in the lobular region
  9. What vessels extend off the interlobular vessels (that themselves extend off arcuate vessels which extend from the renal artery?
    • afferent arterioles: bring blood toward glomerular tuft (capillaries)
    • efferent arteriole: leave the glomerular tuft
    • for glomeruli high up in the cortex, efferent arterioles will give rise to peritubular capillary networks
    • for glomeruli closer to the cortex/medulla border, efferent arterioles will give rise to vasa recta, long/straight vessels that go deep into the medulla
  10. Peritubular Capillary Network
    • a network of capillaries that surrounds both the PCTs & DCTs
    • to go from lumen of a tubule into a BV is an extremely short distance
    • the point of this is to absorb everything useful from the tubules & transfer them back to the blood stream
  11. What is the point of glomeruli?
    they allow the plasma component of the blood to enter a system of tubules so that everything useful can be retrieved from the plasma & what isn't wanted can be passed as waste
  12. Bowman's Capsule
    • C cup shaped-structure made up of 2 layers that holds the glomerulus (tuft of capillaries)
    • simple squamous outer (parietal) layer is continuous with the proximal convoluted tubule (which eventually gives rise to the loop of Henle → distal convoluted tubule → short vessel → collecting duct) 
    • inner (visceral) layer differentiates to form podocytes
  13. Terminology
    • renal corpuscle: glomerulus + Bowman’s capsule
    • nephron: renal corpuscle + tubule (up until the DCT)
    • uriniferous tubule: nephron + collecting duct
  14. Plane of Section Explanation
    • see around corpuscles circular/oblong profiles that correspond to convoluted tubules (PCTs & DCTs)
    • the medullary rays represent collecting ducts
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  15. Why is the afferent arteriole larger than the efferent arteriole?
    • this set-up creates pressure that forces plasma OUT of the glomerulus' capillary loops into the Bowman's space
    • the contents of the space/capsule becomes the glomerular filtrate which will begin its course initially through the PCT & so forth
  16. What types of cells do the nuclei seen in the glomerulus belong to?
    • 1. endothelial cells
    • 2. mesangial cells
    • 3. podocytes
  17. Glomerular Endothelial Cells
    • endothelial cells (of afferent arteriole) are continuous as they approach the glomerulus
    • in the capillary tuft they become fenestrated (so plasma can escape blood & get into Bowman's space)
  18. Intraglomerular Mesangial Cells
    • supportive cells that sit within endothelial cells' basal lamina & support the delicate capillary loops
    • they're phagocytic (damage or dead cells can be cleared by mesangial cells)
    • they assist with cell repair
    • they're contractile (regulate BP)

    (specialized smooth muscle cells; anchor capillary loops to the vascular pole)
  19. Lacis (Extraglomerular Mesangial) Cells
    • light-staining cells found outside the glomerulus near the vascular pole & macula densa that monitor blood pressure (direct contraction or expansion of capillary vessels)
    • are part of the juxtaglomerular apparatus
  20. Podocytes
    • specialized cells that make up the visceral (inner) layer of the Bowman's capsule
    • at all points where Bowman tubule cells make contact with glomerular capillary cells, they differentiate into Podocytes
    • Image Upload 5
    • they completely invest the glomerular capillaries & their multiple foot processes form filtration slits through which blood plasma must pass to get into the Bowman's Space
  21. Filtration barrier through which plasma must pass to enter the Bowman's Space from Blood
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    • looking at sections of 3 capillary loops
    • 1. from inside capillaries of the glomerulus
    • 2. through the fenestrated capillary epithelium
    • 3. through the glomerular basement membrane (a double basement membrane synthesized by both the capillary endothelial cells & podocytes)
    • 4. through the podocyte filtration silts
    • finally INTO the Bowman's space
    • only molecules smaller than ~68 MW (nothing bigger than albumin) can pass through these filters
    • plasma is now free to enter the PCT

    Image Upload 7
  22. PCTs & DCTs
    • anywhere in the cortex there are both PCTs & DCTs
    • there are 3x more PCTs than there are DCTs
    • PCTs are analogous to the small intestine: most resorption takes place in the PCT (80% of ultrafiltrate reabsorbed) 
    • DCTs are analogous to the large intestine: most water & ion re-uptake takes place there
  23. Histology of PCTs & DCTs
    PCT: contains a brush border (fuzzy stuff extending from apical end of cells) & abundant mitochondria (why the cell cytoplasm stains very eosinophilically)

    DCT: fewer, contain no brush border, has a very smooth apical surface, fewer mitochondria (cytoplasm is less eosinophilic)

    Image Upload 8

    structures with RBCs in them surrounding the convoluted tubules are peritubular capillaries (formed from efferent arterioles high up in the cortex)
  24. PCT Cell
    • PCT is the primary site for resorption of useful sugars, small proteins & ions
    • cuboidal cells of the PCT have a rich brush border (apical microvilli with digestive enzymes) & mitochondria-rich basal infoldings essential for rapid resorption

    Image Upload 9
  25. Resorption in the PCT
    this is basal end of a PCT cell (can see infoldings of the basal plasma membrane & mitochondria)

    Image Upload 10

    1. pump materials into ECM (active movement of non-waste products into the interstitium at the cells’ basal surface)

    2. below the basal cell surface & ECM is a capillary; step 2 is to transport contents from ECM via pinocytosis into the capillary lumen (passive movement into peritubular capillaries)
  26. DCT Cell
    • low cuboidal cells:
    • Image Upload 11
    • don't have a brush border
    • have clear cytoplasm (because fewer MTs)
    • have Na+/K+ ATPases because the DCT serves mainly to resorb Na+ into the ECM & bloodstream (H2O follows Na+ passively down the osmotic gradient)
  27. Which region of the uriniferous tubule is sensitive to Aldosterone?
    • the DCT
    • Aldosterone is a mineralocorticoid produced in the zona glomerulosa of the Adrenal Cortex that ↑ reabsorption of ions & water in the kidney → ↑ BP
  28. What purpose does the renal medulla serve?
    • the renal medulla concentrates the urine by creating a Na+ gradient that facilitates H2O resorption
    • there's a strong osmotic gradient in the medulla (stronger the deeper you go)
    • also plays a role in acid/base balance
    • 4 things can be visualized in the medulla: thin & thick loops of Henle, vasa recta, & the collecting ducts
  29. Medulla X-SectionImage Upload 12
    • Largest: collecting ducts
    • 2nd Largest: thick (ascending) loop of Henle
    • thin (descending) loop of Henle: very thin circle filled with provisional urine (clear
    • vasa recta: very thin circle filled with blood (colored, not clear)

    Image Upload 13
  30. Loop of Henle
    • region of the uriniferous tubule between the PCT & DCT necessary for establishing & maintaining an osmotic gradient in the interstitium of the medulla
    • this is necessary for the resorption of water from provisional urine
    • the loops are surrounded by interstitial tissue, vasa recta & collecting ducts
  31. Which parts of the Loop of Henle are permeable/impermeable to what?
    • Thick Ascending Limb: pumps Na+ ions into the interstitium; is NOT permeable to water
    • Thin Descending Limb: IS permeable to water
  32. Collecting Ducts (CD)
    • receive partially concentrated urine from the distal end of the DCT
    • are clumped within the the medullary rays (aka the center of the cortical renal lobule)
    • once something has entered the CD, it can't leave, EXCEPT FOR H2O
    • as CDs pass through the deep medulla where there's a strong osmotic gradient, water will be reabsorbed in the presence of ADH
  33. CD Longitudinal Section
    Image Upload 14
    • Intercalated Dark Cells: monitor blood pH
    • Light Cells (majority): have cilia that serve as mechanosensors monitoring urine flow; respond to ADH
    • (can see a Thick Ascending Loop of Henle limb to the L of the CD)
  34. Intercalated Dark Cell Types
    • α: secrete H+ if the blood pH is too high
    • β: secreted HCO3- (bicarbonate) if the blood pH is too low
    • 2 types can't be distinguished unless a special stain is used
  35. What can mutations in Light Cell cilia lead to?
    • polycystic kidney disease
    • can be caused by mutations in polycystin-1 & -2 genes which code for cilia on light cells of the collecting ducts
    • non-functioning cilia are unable to serve as mechanosensors & intracellular Ca2+ transport is compromised
    • characterized by bilaterally enlarged kidneys with multiple cysts
  36. What is required for CD cells to be permeable to water (aka release it into the interstitium for reabsorption via vasa recta)?
    • ADH (antidiuretic hormone, released from posterior pituitary)
    • CD cells are impermeable to water in the absence of ADH
    • with ADH, aquaporins open, allow water to exit, concentrating the provisional urine
    • alcohol inhibits ADH → aquaporins stay closed, water isn't reabsorbed, pee out more water (dehydration)
  37. Diabetes Insipidus
    • results from an inability to make or respond to ADH
    • CDs remain impermeable to water despite a strong medullary osmotic gradient
    • a larger volume of urine passes through kidneys & into bladder
  38. Diabetes Mellitus
    • insulin deficiency or lack of response → large volumes of glucose in the blood & eventually the urinary filtrate (provisional urine)
    • this large amount of glucose exceeds glucose transporter capacity
    • not all can be resorbed
    • glucose is osmotically active & therefore negates the Na+ osmotic gradient in the renal medulla interstitium
    • water doesn't leave CDs → osmotic diuresis
  39. Kidney Stones (Renal Calculi)
    • opposite: overly concentrated urine
    • crystals may form from Ca2+ oxalate or uric acid precipitates
    • precipitation may be caused by low fluid intake, high protein diets, or refined sugars
    • hydronephrosis: when large precipitates clog any area & prevent urine from exiting the kidney (dangerous)
    • pelvic rim is a place they could get stuck because there's a sharp bend
  40. Juxtaglomerular Apparatus
    • where the vascular pole & DCT touch
    • area with 3 types of cells that regulate BP (raise BP when low)
    • 1. Juxtaglomerular sensory cells (JG cells, in the afferent arteriole)
    • 2. Macula Densa blood chemistry cells (in the DCT)
    • 3. Lacis cells
  41. Macula Densa
    • an area of closely packed specialized cells lining the wall of the DCT where it returns to the vascular pole of its parent glomerulus
    • MD cells are chemoreceptors that chemically monitor the salt concentration of the blood
    • when there's ↓ ion concentration there's ↓ pressure; these cells signal nearby JG cells to release Renin, initiating a BP ↑ cascade
  42. Juxtaglomerular Cells (JG Cells)
    • specialized smooth muscle baroreceptor cells that physically monitor the BP
    • release Renin, a protease that cleaves Angiotensinogen in the blood → Angiotensin I
    • are located in the wall of afferent arterioles
  43. What stimulates the release of Renin?
    • 1. ↓ BP in the afferent arteriole (by JG cells themselves)
    • 2. ↓ Na+ in the DCT (sensed by macula densa chemoreceptors)
  44. Angiotensin II
    • a vasoconstrictor made by cleavage of angiotensin I by angiotensin
    • converting enzyme (ACE, made in the lung)
    • causes an immediate ↑ in BP by vasoconstriction & stimulates release of Aldosterone (produced in the zona glomerulosa of the Adrenal Cortex that ↑ reabsorption of ions & water in the DCT → ↑ BP)
    • Image Upload 15
  45. What cells detect ↓ Na+ levels & stimulate Renin release?
    macula densa cells - chemoreceptors monitor the salt concentration
  46. Papillary Ducts
    • the name given to CDs when they get close to the renal papilla
    • the renal papilla is lined by a simple columnar epithelium
    • as soon you get to the calyx, it's lined by a transitional epithelium (urothelium)
    • Image Upload 16
    • urothelium is found from the calyxes, renal pelvis, ureter, bladder
  47. What type of epithelium lines the calyces, ureter & bladder?
    • transitional epithelium (urothelium)
    • it's pseudostratified & usually has large, dome-shaped, often binuclate cells at the luminal surface
    • Image Upload 17
    • stretched when you have to pee
    • relaxed when you don't
  48. Ureter
    Image Upload 18
    • passageway from the renal pelvis to the bladder
    • lumen is typically closed
    • has a urothelium overlying a lamina propria with no muscularis mucosa; goes straight into a submucosa & a muscularis externa 
    • muscularis externa contains 2 layers of SM muscle except for in the distal 1/3rd, where there are 3 layers of SM muscle
    • the innermost layer is longitudinal while the outer layer is circular
    • when present distally, the outermost 3rd layer is longitudinal
    • the mucosa is usually folded upon itself but expands during peristalsis
    • works via peristalsis: gravity is not needed to move urine through/out
  49. Bladder
    • has a mucosal wall with rich layers of smooth muscle but poorly organized
    • it has NO distinct muscularis mucosa deep to the lamina propria
    • it's epithelial lining = transitional
  50. What is the region with the * called?
    Image Upload 19
    • Urinary Pole
    • this is the very beginning of the PCT system
    • on the opposite end is the Vascular Pole, where the afferent & efferent arterioles enter the renal corpuscle
Card Set
Urinary System
Histology Exam 4