The Urinary System

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Chapter 19

• The Urinary System

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Objectives

• Identify the major organs of the urinary system
  and give the generalized function of each
• Name the parts of a nephron and describe the role
  each component plays in the formation of urine
• Explain the importance of filtration, tubular
  reabsorption, and tubular secretion in urine
  formation

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Objectives

• Discuss the mechanisms that control urine volume
• Explain how the kidneys act as vital organs in
  maintaining homeostasis
• List the major renal and urinary disorders and
  explain the mechanism of each

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Kidneys

• Location
• Kidneys lie under back muscles, behind parietal
  peritoneum, just above waistline
• Right kidney is usually a little lower than left
• Internal structure
• Renal cortexouter layer of kidney
• Renal medullainner portion of kidney

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Kidneys

• Internal structure
• Renal pyramidstriangular divisions of medulla
• Renal columnscortical tissue that dips down
  between renal pyramids
• Renal papillanarrow, innermost end of pyramid
• Renal pelvisexpansion of upper end of ureter
  lies inside kidney
• Renal calycesdivisions of renal pelvis

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Kidneys

• Microscopic structure
• Interior of kidney composed of more than 1
  million microscopic nephron units
• Unique shape of nephron well suited to function
• Principle components are renal corpuscle and
  renal tubule

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Kidneys

• Microscopic structure
• Renal corpuscle
• Bowman capsulecup-shaped top of nephron
• Glomerulusnetwork of blood capillaries
  surrounded by Bowman capsule
• Cortical nephrons85 of total located mostly in
  renal cortex
• Juxtamedullary nephronshave important role in
  concentrating urine located near junction
  between cortex and medullary layers

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Kidneys

• Microscopic structure
• Renal tubule
• Proximal convoluted tubule (PCT)first segment
• Henle loopextension of proximal tubule consists
  of descending limb, loop, and ascending limb
• Distal convoluted tubule (DCT)extension of
  ascending limb of Henle loop
• Collecting duct (CD)straight extension of distal
  tubule

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Kidneys

• Functions
• Excrete toxins and nitrogenous wastes
• Regulate levels of many chemicals in blood
• Maintain water balance
• Help regulate blood pressure and volume
• Regulate red blood cell production by secreting
  erythropoietin (EPO)

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Formation of Urine

• Filtration
• Goes on continuously in renal corpuscles
• Glomerular blood pressure causes water and
  dissolved substances to filter out of glomeruli
  into the Bowman capsule
• Normal glomerular filtration rate 125 mL/min

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Formation of Urine

• Reabsorption
• Movement of substances OUT of renal tubules into
  blood in peritubular capillaries
• Water, nutrients, and ions are reabsorbed
• Water is reabsorbed by osmosis from proximal
  tubules

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Formation of Urine

• Secretion
• Movement of substances into urine in the distal
  and collecting ducts from blood in peritubular
  capillaries
• Hydrogen ions, potassium ions, and certain drugs
  are secreted by active transport
• Ammonia is secreted by diffusion

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Formation of Urine

• Control of urine volume
• ADHsecreted by posterior pituitary promotes
  water reabsorption by collecting ducts reduces
  urine volume
• Aldosteronesecreted by adrenal gland, triggered
  by RAAS process promotes sodium and water
  reabsorption in nephron reduces urine volume
• ANHone of the peptide hormones (ANPs) secreted
  by atrial cells in heart promotes loss of sodium
  and water into kidney tubules increases urine
  volume

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Ureters

• Structure
• Narrow long tubes with expanded upper end (renal
  pelvis) located inside kidney
• Lined with mucous membrane and muscular layer
• Functiondrain urine from renal pelvis to urinary
  bladder
• Urinalysis
• Examination of the physical, chemical, and
  microscopic characteristics of urine
• May help determine the presence and nature of a
  pathological condition

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Urinary Bladder

• Structure
• Elastic muscular organ, capable of great
  expansion
• Lined with mucous membrane arranged in rugae,
  like stomach mucosa
• Functions
• Storage of urine before voiding
• Voiding

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Urethra

• Structure
• Narrow tube from urinary bladder to exterior
• Lined with mucous membrane
• Opening of urethra to the exterior called urinary
  meatus
• Functions
• Passage of urine from bladder to exterior of the
  body
• Passage of male reproductive fluid (semen) from
  the body

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Micturition (urination)

• Passage of urine from body (also called urination
  or voiding)
• Regulatory sphincters
• Internal urethral sphincter (involuntary)
• External urethral sphincter (voluntary)
• Bladder wall permits storage of urine with little
  increase in pressure

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Micturition

• Emptying reflex
• Initiated by stretch reflex in bladder wall
• Bladder wall contracts
• Internal sphincter relaxes
• External sphincter relaxes, and urination occurs
• Enuresisinvoluntary urination in young child

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Micturition

• Urinary retentionurine produced but not voided
• Urinary suppressionno urine produced but bladder
  is normal

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Micturition

• Incontinenceurine is voided involuntarily
• Urge incontinenceassociated with detrusor
  overactivity or involuntary detrusor muscle
  contractions
• Stress incontinenceassociated with weakened
  pelvic floor muscles
• Overflow incontinenceassociated with urinary
  retention and overdistended bladder
• Reflex incontinence occurs in absence of any
  sensory warning or awarenesscommon following a
  stroke or spinal cord injury
• Neurogenic bladderperiodic but unpredictable
  voiding related to paralysis or abnormal
  function of the bladder

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Renal and Urinary Disorders

• Obstructive disorders interfere with normal urine
  flow, possibly causing urine to back up and cause
  hydronephrosis or other kidney damage
• Renal calculi (kidney stones) may block ureters,
  causing intense pain called renal colic
• Tumorsrenal cell carcinoma (kidney cancer) and
  bladder cancer are often characterized by
  hematuria (blood in the urine)
• Hydronephrosisenlargement of renal pelvis and
  calyces caused by blockage of urine flow

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Renal and Urinary Disorders

• Urinary tract infections (UTIs) often caused by
  gram-negative bacteria
• Urethritisinflammation of the urethra
• Cystitisinflammation or infection of the urinary
  bladder
• Pyelonephritisinflammation of renal pelvis and
  connective tissues of the kidney may be acute
  (infectious) or chronic (autoimmune)
• Hydronephrosisenlargement of renal pelvis and
  calyces

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Renal and Urinary Disorders

• Glomerular disorders result from damage to the
  glomerularcapsular membrane of the renal
  corpuscles
• Nephrotic syndrome accompanies many glomerular
  disorders
• Proteinuriaprotein in the urine
• Hypoalbuminemialow plasma protein (albumin)
  level caused by loss of proteins to urine
• Edematissue swelling caused by loss of water
  from plasma as a result of hypoalbuminemia

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Renal and Urinary Disorders

• Glomerular disorders (contd)
• Acute glomerulonephritis caused by delayed immune
  response to a streptococcal infection
• Chronic glomerulonephritis is a slow inflammatory
  condition caused by immune mechanisms and often
  leads to renal failure

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Renal and Urinary Disorders

• Kidney failure or renal failure occurs when the
  kidney fails to function
• Acute renal failureabrupt reduction in kidney
  function that is usually reversible
• Chronic renal failureslow, progressive loss of
  nephrons caused by a variety of underlying
  diseases
• Polycystic kidney disease (PKD)numerous
  fluid-filled cysts destroy kidney tissue as they
  grow hereditary

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Renal and Urinary Disorders

• Progression of kidney failure
• Stage 1early in this disorder, healthy nephrons
  often compensate for the loss of damaged nephrons
• Stage 2often called renal insufficiency loss of
  kidney function ultimately results in uremia
  (high BUN levels) and its life-threatening
  consequences
• Stage 3called uremia or uremic syndrome
  complete kidney failure results in death unless a
  new kidney is transplanted or an artificial
  kidney substitute is used

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