The Urinary System

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The Urinary System

• Functions
• Excretion of wastes from the blood (kidneys)
• Elimination of wastes into environment (ureters,
  bladder, urethra)
• Regulation of volume and solute concentration of
  blood plasma

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Organs of the Urinary Tract

• Kidneys
• reddish brown, bean shaped, located just below
  the ribs on posterior wall of abdominal cavity
• regulate volume, composition, and pH of body
  fluids (remove metabolic wastes in the process)
• secrete hormone erythropoetin (controls rate of
  RBC formation)
• secrete enzyme renin (regulates blood pressure)
• functional unit is the nephron (will discuss
  later)

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Organs cont.

• Ureters
• transport urine from kidneys to bladder
• 25 cm long tubes
• muscular walls help move urine by peristalsis
• kidney stones can block this tube

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Organs cont.

• Urinary bladder
• temporarily stores urine
• hollow, muscular organ composed of transitional
  epithelium (stretches)
• internal urethral sphincter - muscle that
  surrounds the neck of the bladder remains
  contracted to prevent the bladder from emptying
  until pressure increases to a certain level
  autonomic controlled
• External urethral sphincter voluntarily
  controlled, ring of muscles where urethra exits
  body. How you control when you use the restroom.

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Female Male
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Organs cont.

• Urethra
• conveys urine out of body
• about 4 cm long in females
• about 19 cm long in males

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Urethral Sphincters
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Micturition

• bladder can hold as much as 600 ml of urine
• urge to urinate occurs when it contains 200 ml
• the reflex to urinate is inhibited by the
  conscious control of the external urethral
  sphincter
• Urine leaves the bladder by micturition (a.k.a.
  urination)

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Micturition Reflex

• bladder distends as it fills with urine and
  stimulates stretch receptors in bladder wall
• when stretch receptors are triggered, they
  stimulate muscular bladder wall to contract
• external urethral sphincter is relaxed
  voluntarily, and the internal urethral sphincter
  relaxes also
• urination occurs
• if ext. urethral sphincter does not relax, int.
  urethral sphincter remains closed

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Micturition Reflex cont.

• What happens if you hold it too long?
• Once volume exceeds 500 ml, pressure may force
  open internal sphincter which causes reflexive
  relaxation of external sphincter, and urination
  occurs

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Urinary Diseases and Disorders

• UTI - urinary tract infection
• more common in women than men b/c the urethra is
  shorter in females
• symptoms burning pain during urination
• cause bacteria spread from rectum to urethra
• Kidney Stones (a.k.a. renal calculi)
• result from chronically concentrated urine which
  causes uric acid to clump and form stones
• Possible causes oxalate found in green, leafy
  vegetables, coffee, and chocolate
• Gout uric acid build up in joints

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Gout Kidney Stones
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Worlds Largest Kidney Stone22 oz. 5.5 in across
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The Nephron

• each kidney contains 1 million nephrons
• they filter 7.5 liters of plasma each hour
• each nephron consists of a renal corpuscle (blood
  vessels) and a renal tubule (tube)
• the renal corpuscle is divided into 3 parts.
  Renal artery, glomerulus, and peritubular
  capillary.
• renal artery becomes a cluster of capillaries
  called a glomerulus (surrounded by Bowmans
  capsule)
• Then goes to a network of capillaries called
  peritubular capillaries which surround the renal
  tubule
• the renal tubule is divided into 4 parts
  Bowmans capsule, the proximal convoluted tubule,
  loop of Henle, and the distal convoluted tubule

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Urine Formation

• Urine formation occurs in the nephrons
• Consists of 3 processes
• 1. glomerular filtration (collects good and bad
  stuff)
• 2. tubular reabsorption (removes good stuff from
  filtrate and put back into body)
• 3. tubular secretion (puts in extra bad stuff
  into filtrate which is going to the toilet)

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1. Glomerular Filtration (collecting good and bad
stuff)

• Occurs when the renal artery delivers blood to
  the glomerulus
• blood pressure forces water and small molecules
  from blood plasma into the Bowman's capsule
• this fluid is now known as filtrate and moves
  into the renal tubule
• ex. of molecules in filtrate water, glucose,
  amino acids, sodium, urea, uric acid, other
  minerals
• Filters both good and bad stuff for example
  water and toxic substances

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2. Tubular Reabsorption (removes good stuff from
filtrate)

• occurs in the proximal convoluted tubule, loop of
  Henle, distal convoluted tubule, and collecting
  duct
• molecules from the filtrate (in the tubule) are
  returned by diffusion and active transport to the
  blood in the peritubular capillaries
• ex. of molecules that are reabsorbed glucose,
  amino acids, and almost all of the water and
  sodium that enters the tubule is reabsorbed
• Make a really salty place so that on the next
  pass by we can remove almost all the water. (good
  stuff)

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3. Tubular Secretion (Puts in extra bad stuff
into filtrate)

• occurs mainly at the distal convoluted tubule
• molecules from the peritubular capillaries (blood
  vessels) are actively transported into the renal
  tubule (tubes)
• ex. of molecules that are secreted uric acid,
  hydrogen ions, ammonia, penicillin (and other
  drugs)

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Notice hormones in orange and green on right
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Urine Formation cont.

• the distal convoluted tubule empties the fluid
  (urine) into a collecting duct which carries it
  away from the nephron
• Each collecting duct empties many nephrons
• Many collecting ducts empty the medullas into the
  funnel-shaped central chamber of the kidney
  (renal pelvis)
• the ureters drain this chamber
• the peritubular capillaries merge to form venules
  that carry the filtered blood away from the
  nephron and eventually out of the kidney

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Urine

• Physical characteristics
• The yellow color in urine is due to chemicals
  called urobilins. These are the breakdown
  products of the bile pigment bilirubin. Bilirubin
  is itself a breakdown product of the heme part of
  hemoglobin from worn-out red blood cells. Most
  bilirubin is partly broken down in the liver,
  stored in the gall bladder, broken down some more
  in the intestines, and excreted in the feces (its
  metabolites are what make feces brown), but some
  remains in the bloodstream to be extracted by the
  kidneys where, converted to urobilins, it gives
  urine that familiar yellow tint.
• pH slightly acidic (6.0)
• Chemical composition
• 95 water, 5 solutes

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Urine Components

• Urea is the bodys main waste product - formed
  when amino acids are broken down into energy is
  toxic to cells in high levels
• Uric acid a by-product of the breakdown of
  nucleic acids (RNA DNA)
• Creatinine a by-product of muscle metabolism
• Ammonia waste formed when bacteria break down
  proteins
• Ketones waste produced when fats are digested
  (diabetes and Atkins diet)

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Urine

• Composition affected by lifestyle and diet
• Volume varies from 0.6 to 2.5 liters/day
• Diuretics inhibit reabsorption of water so
  increase urine volume ex. coffee, tea,
  alcoholic beverages

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Hormonal Regulation

• Aldosterone
• stimulates both reabsorption of Na and the
  secretion of K
• Na reabsorption increases water reabsorption,
  leading to a rise in blood volume and therefore a
  rise in blood pressure

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Hormonal Regulation

• ADH (antidiuretic hormone) causes increased water
  absorption
• high solute conc. in blood ? increased ADH
  secretion
• alcohol and caffeine inhibit ADH secretion
  causing excess excretion of dilute urine and
  dehydration

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Diuretics in orange