The Urinary System

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The Urinary System
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What does the urinary system do?

• Eliminates wastes
• Remove toxins, poisons
• Maintains homeostasis
• Regulate blood pH, electrolytes, blood pressure
  (volume), oxygen/carbon dioxide content
• Closely linked to reproductive system
• e.g. in males, urethra is transporter for both
  urine and sperm

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What are the parts?

• Kidneys
• Ureters
• Bladder
• Recall what type of cells line the bladder?
• Urethra

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What do the kidneys do?

• Filter plasma to remove wastes
• Detoxify free radicals and drugs
• Control blood volume/pressure
• Secrete renin
• Activates aldosterone, angiotensin to control bp
  and sodium balance

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What do the kidneys do?

• Secretes erythropoitin
• Help lungs regulate CO2 balance and blood pH
• Gluconeogenesis during starvation
• Remove NH2 (secrete as ammoniaNH3) and make
  glucose from remainder of amino acid

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What kinds of wastes do the kidneys remove?

• Metabolic waste metabolic processes produce
• Compare to feces (elimination)
• Nitrogenous waste
• Urea (50 of total) protein catabolism byproduct
• Liver converts ammonia (v. toxic) to urea (less
  toxic)
• Uric acids nucleic acid catabolism
• Creatinine creatinine phosphate catabolism
• Recall when does this happen and where?

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What if the kidneys dont remove wastes?

• Azotemia nitrogenous wastes in blood
• Uremia progression of azotemia
• Vomiting, dyspnea (shortness of breath),
  arrhythmia
• Convulsions, coma, death

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What is excretion?

• Separating wastes from body fluids and
  eliminating these wastes
• Compare to digestive elimination
• Skin water, salts, lactic acid, urea
• Lungs carbon dioxide, water
• Digestive tract water, bile, salts, cholesterol
• But eliminates unused food
• Kidneys urea, toxins, drugs, hormones, salts,
  water, H

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What are the gross parts of the kidney?

• Renal fascia (under parietal peritoneum)
  outermost
• Then adipose capsule
• Then renal capsule innermost
• Review cortex, medullary pyramids, renal
  papilla, major calyx, minor calyx, renal columns,
  renal vein, renal artery, lobar and interlobar
  vessels, arcuate vessels, renal pelvis

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What is the path of blood through the kidney?

• renal a., lobar a, interlobar a., arcuate a.,
  interlobular a., afferent arteriole, glomerulus,
  efferent arteriole, peritubular capillaries,
  interlobular v., arcuate v., interlobar v., lobar
  v., renal v., inf vena cava

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What are the microscopic working units called?

• Nephron
• 1.2M in each kidney
• Two types
• Cortical (most common)
• Juxtamedullary (only 15 of all nephrons)
• Maintain salinity of renal pyramid
• Contains glomerular capsule, capillaries and
  collecting tubule
• Vasa recta

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What is the glomerular capsule?

• AKA Bowmans capsule
• Two layers
• Parietal simple squamous
• Visceral podocytes (wrap around capillaries)
• Capsular space between two layers
• Glomerular filtrate collects here
• Capsule surrounds glomerular capillaries

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What is the renal tubule?

• Connects to capsular space and contains
• Proximal tubule (also proximal convoluted tubule)
• Simple cuboidal with microvilli
• What does this mean?
• Nephron loop (Loop of Henle)
• Portions conserve water (thin segment), other
  parts reabsorb electrolytes (thick segment)
• Distal tubule (distal convoluted tubule)
• Juxtaglomerular apparatuswhere distal tubule
  touches afferent and efferent arterioles
• Collecting duct to papillary duct to minor calyx
  to major calyx to renal pelvis to ureter to
  bladder to urethra

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How is urine formed?

• Glomerular filtration
• GFR how much filtrate formed per min.
• Tubular reabsorption
• Tubular secretion
• Water conservation

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What is glomerular filtration?

• Blood plasma filtered out of glomerular
  capillaries
• High blood pressure from afferent arterioles
• Hypertension can destroy g. capillaries, cause
  scarring
• All but proteins, cells enter capsular space

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What is glomerular filtration?

• Glomerular capillaries
• Fenestrated all but cells
• Basement membrane of epithelial cells
• Filters out large molecules
• proteins not allowed b/c of neg. charge of
  basement membrane
• Podocytes have negatively charged filtration
  slits
• Pedicels (plasma extensions wrapped around
  capillaries)
• repel large, negatively charged moleculessuch
  as?

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What makes it into the capsular space?

• Small molecules
• Water, sodium, potassium, amino acids, glucose,
  urea (and other wastes), water-soluble vitamins
• Proteinuria if protein (albumin) in urine
• Hematuria if blood in urine
• Kidneys filter about 180 L of blood plasma/day!
• Produce about 2 L of urine/day

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What is reabsorbed?

• Occurs in proximal tubule
• Returns substances to peritubular capillaries
• Via solvent drag and osmosis
• About 65 of all filtered
• Transcellular route
• Paracellular route
• Sodium
• Na/K pumps on basal side pump Na into ECF and
  then to peritubular caps
• Creates concentration gradient (for collecting
  duct)
• Glucose hitchhikes with Na
• Sodium-glucose transport proteins (SGLTs)

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What is reabsorbed?

• Amino acids also hitchhikes with sodium
• Water follows sodium b/c ICF and ECF are
  hypertonic
• Aquaporins--H2O channel proteins
• ADH causes their addition in CD
• Blood is hypertonic to this, so water enters and
  carries sodium, etc. with it solvent drag
• Chloride follows positive charges
• Other electrolytes solvent drag
• 40-60 of urea reabsorbed
• Also reabsorbs almost all uric acid
• What didnt get reabsorbed?

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What is the transport maximum?

• Limits amount can be reabsorbed
• If all transport proteins occupied, cant
  reabsorb more
• Glycosuria sugar levels too high

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What happens in the nephron loop?

• Thin segment (descending)
• Permeable to water, but not to electrolytes
• Thick segment (ascending)
• Impermeable to water, but permeable to
  electrolytes

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What is secreted?

• In proximal tubule and nephron loop
• secretes urea, uric acid, bile salts, ammonia
• Also penicillin, aspirin, other drugs
• Secretes hydrogen and bicarbonate ions
• In distal tubule and collecting duct
• Water reabsorption via hormonal control
  (principal cells)
• Aldosterone absorb more Na, secrete K
• ADH makes collecting duct more water permeable
• Alcohol inhibits ADH secretionresult?
• PTH
• encourages Ca2 absorption in ascending nephron
  loop
• inhibits P absorption in PCT

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How is water conserved?

• Happens in collecting tubule w/help from
  juxtamedullary nephron loops
• In medulla, tissues surrounding reabsorb water to
  concentrate urine
• Walls water but not NaCl permeable
• See next slide!
• Medullary portion has high salt concentration
• b/c 15 of nephrons are juxtamedullary nephrons
• Very long nephron loops extend length of medulla
  and reabsorb salts into medullary tissue

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Kidney regulation and diseases
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How do the kidneys regulate urine output?

• Renal autoregulation
• If bp rose without this, too much urine made
• High bp afferent arterioles constrict
• Myogenic mechanism --gt smooth muscles constrict
• Less blood to glomerular capsule
• Or dilate efferent arterioles for faster flow
• Reverse if bp drops
• Note caffeine also causes afferent dilation

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How do the kidneys regulate urine output?

• Juxtaglomerular apparatus
• Dilate/constrict afferent/efferent, secrete renin
  (raises bp)
• vasoconstrictor by proxy
• Macula densa monitors salinity of tubular fluid
• Sympathetic NS causes afferent constriction

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How does renin work?

• If bp drops, juxtamedular cells secrete renin
• Splices angiotensinogen to angiotensin I
• Lung protein (angiotensin converting enzyme ACE)
  converts angiotensin I to angiotensin II causing
• Systemic vasoconstriction
• Increased tubular reabsorption (via ADH)
• Adrenal cortex to produce aldosterone
• Stimulates thirst response

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What are properties of urine?

• Color due to urochrome
• Cloudy (pyuria) pus, possible kidney infection
• Red (hematuria) blood, poss. UTI, kidney stones,
  trauma
• Volume 1-2L/day
• Polyuria gt2L/day
• Oliguria lt500 ml/day
• Anuria 0-100 ml/day
• Kidney disease, shock, dehydration
• Also causes azotemia
• Diuretics
• Caffeine dilates afferent arterioles --gt
  ____________ GFR
• Alcohol inhibits ADH secretion --gt
  _______________

From http//www.bbc.co.uk/health/images/300/urine
_sample.jpg
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What are renal calculi?

• Hard granule of calcium, phosphate, protein, uric
  acid
• Form in renal pelvis and can block r. pelvis or
  ureter
• passing causes painful contractions
• Causes hypercalcemia, frequent UTIs, dehydration
• lithotripsy

From http//www.medicinenet.com/images/STONES.JPG
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What is hemodialysis?

• If renal insufficiency, need to filter blood
  mechanically
• Blood pumped from radial artery and returned via
  a vein
• Dialysis machine passes blood through
  semipermeable cellophane tube surrounded by
  dialysis fluid
• 3 sessions/week for 6-8 hrs each
• Alternative continuous ambulatory peritoneal
  dialysis

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How is urine voided?

• AKA micturition reflex
• Detrusor muscle forms middle layer of bladder
• Thick near urethra to form internal urethral
  sphincter
• Smooth muscle, involuntary control
• External urethral sphincter where urethra passes
  through pelvic wall
• Skeletal muscle, voluntary control

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What is the micturition reflex?

• As bladder fills, stretch receptors fire
• Parasympathetic reflex arc
• detrusor contraction
• Internal urethral sphincter relaxation
• Micturition center in pons
• Does same as parasympathetic reflex arc
• Also assesses appropriateness of urination
• Inappropriate constant nerve stimuli to external
  urethral sphincter
• Appropriate nerve stimuli inhibited
• Valsalva maneuver