The Urinary System

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The Urinary System
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The Urinary System

• Paired kidneys
• A ureter for each kidney
• Urinary bladder
• Urethra

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Main Functions of Urinary System

• Kidneys filter blood to keep it pure
• Toxins
• Metabolic wastes
• Excess water
• Excess ions
• Dispose of nitrogenous wastes from blood
• Urea
• Uric acid
• Creatinine
• Regulate the balance of water and electrolytes,
  acids and bases

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• Kidneys are retroperitoneal organs (see next
  slide)
• Superior lumbar region of posterior abdominal
  wall
• Lateral surface is convex
• Medial surface is concave
• Hilus is cleft vessels, ureters and nerves
  enter and leave
• Adrenal glands lie superior to each kidney
• (the yellow blob in pic)

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Note layers of adipose (fat), capsule, fascia
Transverse sections show retroperitoneal position
of kidneys
Note also liver, aorta muscles on CT
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• Kidney has two regions
• Cortex outer
• Columns of cortex divide medulla into pyramids
• Medulla inner
• Darker, cone-shaped medullary or renal pyramids
• Parallel bundles of urine-collecting tubules

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• The human kidney has lobes
• Pyramid and cortical tissue surrounding it
• 5-11 per kidney
• Renal pelvis (basin)
• Expanded, funnel shaped, superior part of ureter
• Branches to form two or three major calices (seen
  best on right pic below)
• Each of these divides again, minor calices
  collect urine from papillae of pyramids

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The Arteries

• Aorta gives off right and left renal arteries
• Renal arteries divides into 5 segmental arteries
  as enters hilus of kidney

Segmentals branch into lobar arteries Lobars
divide into interlobars Interlobars into arcuate
in junction of medulla and cortex Arcuates send
interlobular arteries into cortex Cortical
radiate arteries give rise to glomerular
arterioles
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Vasculature of the kidney

• The glomerular capillary bed is unusual in having
  arterioles going both to it and away from it
  (afferent and efferent), instead of a vein going
  away as most
• It is also unusual in having two capillary beds
  in series (one following the other)

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• Uriniferous tubule is the main structural and
  functional unit

• To left is a single, generalized uriniferous
  tubule
• More than a million of these tubules act together
  to form the urine
• Three main mechanisms
• Glomerular filtration
• Tubular reabsorption
• Tubular secretion
• Two major parts
• A urine-forming nephron
• A collecting duct which concentrates urine by
  removing water from it

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Outline

• Uriniferous tubule (anatomical unit for forming
  urine)
• Nephron
• Renal corpuscle (in cortex)
• Glomerulus (tuft of capillaries)
• Glomerular (Bowmans) capsule
• Tubular section
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting duct

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Outline

• Uriniferous tubule (anatomical unit for forming
  urine)
• Nephron
• Renal corpuscle (in cortex)
• Glomerulus (tuft of capillaries)
• Glomerular (Bowmans) capsule
• Tubular section
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting duct

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Understand at least this much

• Filtration
• a. Fluid is squeezed out of the glomerular
  capillary bed
• Resorption
• b. Most nutrients, water ad essential ions are
  returned to the blood of the peritubular
  capillaries
• Secretion
• c. Moves additional undesirable molecules into
  tubule from blood of peritubular capillaries

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Nephron
Uriniferous tubule (anatomical unit for forming
urine) Nephron Renal corpuscle (in
cortex) Glomerulus (tuft of
capillaries) Glomerular (Bowmans)
capsule Tubular section
Proximal convoluted tubule Loop of
Henle Distal convoluted tubule
Collecting duct

• Renal corpuscle
• Tubular section
• Renal corpuscle only in cortex
• Tuft of capillaries called glomerulus
• Surrounded by cup-shaped, hollow glomerular
  (Bowmans) capsule

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(refer to this pic as we go)

• Visceral layer of capsule has podocytes
• Unusual branching epithelial cells
• Foot processes with slit processes between them

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• Scanning EM of podocytes clinging to capillaries
  (left) and filtration membrane diagram (right)

The capillary pores (fenestrations) restrict the
passage of the largest elements such as blood
cells The basement membrane and slit diaphragm
hold back all but the smallest proteins while
letting through small molecules such as water,
ions, glucose, amino acids, and urea
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Nephron
Uriniferous tubule (anatomical unit for forming
urine) Nephron Renal corpuscle (in
cortex) Glomerulus (tuft of
capillaries) Glomerular (Bowmans)
capsule Tubular section
Proximal convoluted tubule Loop of
Henle Distal convoluted tubule
Collecting duct

• Renal corpuscle
• Tubular section
• (processes the filtrate)
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• (ends by joining
• collecting duct)

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Proximal convoluted tubule
Uriniferous tubule (anatomical unit for forming
urine) Nephron Renal corpuscle (in
cortex) Glomerulus (tuft of
capillaries) Glomerular (Bowmans)
capsule Tubular section
Proximal convoluted tubule Loop of
Henle Distal convoluted tubule
Collecting duct

• Confined to renal cortex
• Cuboidal epithelial cells with long microvilli
  (fuzzy appearance in pics)
• Resorption of water, ions and solutes

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Loop of Henle
Uriniferous tubule (anatomical unit for forming
urine) Nephron Renal corpuscle (in
cortex) Glomerulus (tuft of
capillaries) Glomerular (Bowmans)
capsule Tubular section
Proximal convoluted tubule Loop of
Henle Distal convoluted tubule
Collecting duct

• Descending limb
• Thin segment
• Thick ascending limb

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Distal convoluted tubule
Uriniferous tubule (anatomical unit for forming
urine) Nephron Renal corpuscle (in
cortex) Glomerulus (tuft of
capillaries) Glomerular (Bowmans)
capsule Tubular section
Proximal convoluted tubule Loop of
Henle Distal convoluted tubule
Collecting duct

• Confined to the renal cortex
• Simple cuboidal epithelium
• Selective secretion and resorption of ions

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Classes of nephrons

• Cortical nephrons
• 85 of all nephrons
• Almost entirely within cortex
• Juxtamedullary nephrons
• Renal corpuscles near cortex-medulla junction

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Collecting Ducts
Uriniferous tubule (anatomical unit for forming
urine) Nephron Renal corpuscle (in
cortex) Glomerulus (tuft of
capillaries) Glomerular (Bowmans)
capsule Tubular section
Proximal convoluted tubule Loop of
Henle Distal convoluted tubule
Collecting duct
Collecting Duct

• Each receives urine from several nephrons
• Run straight through cortex into the deep medulla

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Collecting Ducts
Uriniferous tubules 1. Nephron 2. Collecting
ducts

• At papilla of pyramid ducts join to form larger
  papillary ducts
• Empty into minor calices
• Role conserve body fluids

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The collecting ducts

• The most important role is to conserve body
  fluids
• When the body must conserve water, the posterior
  pituitary gland secretes ADH (antidiuretic
  hormone)
• ADH increases the permeability of the collecting
  tubules and distal tubules to water so more is
  reabsorbed
• This decreases the total volume of urine
• Alcohol inhibits the release of ADH, so less
  water is reabsorbed producing copious amounts of
  dilute urine (can cause dehydration)

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Vessels

• Afferent and efferent arterioles associated with
  glomerular capillaries
• Allows high pressure for forcing filtrate out of
  blood
• About 20 of renal plasma flow is filtered each
  minute (125 ml/min) this is the glomerular
  filtration rate (GFR), an important clinical
  measure of renal function  
• This is about one liter every 8 minutes (only 1
  ends up as urine)
• Peritubular capillaries arise from efferent
  arterioles
• Absorb solutes and water from tubule cells

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(vessels, continued)

• The Vasa recta is a portion of the peritubular
  capillary system which enters the medulla where
  the solute concentration in the interstitium is
  high. It acts with the loop of Henle to
  concentrate the urine by a complex mechanism of
  counter current exchange using urea. If the vasa
  recta did not exist, the high concentration of
  solutes in the medullary interstitium would be
  washed out.

The Vasa recta
____vasa recta
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Histology
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Juxtaglomerular apparatus

• Regulation of blood pressure
• Granule (jg cells) modified muscle cells
  secreting renin in response to falling blood
  pressure in afferent arteriole
• Macula densa chemoreceptors which secrete renin
  if solute concentration falls

Renin-angiotensin mechanism Sequence of
reactions resulting in aldosterone secretion from
adrenal cortex increases sodium resorption from
distal convoluted tubules water follows, blood
volume increases and blood pressure increases
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For studying
Parts of the kidney1. Renal pyramid2. Efferent
vessel3. Renal artery4. Renal vein5. Renal
hilum6. Renal pelvis7. Ureter8. Minor calyx9.
Renal capsule10. Inferior renal capsule11.
Superior renal capsule12. Afferent vessel13.
Nephron14. Minor calyx15. Major calyx16. Renal
papilla17. Renal column
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The Ureters

• Slender tubes about 25 cm (10 ) long leaving
  each renal pelvis
• One for each kidney carrying urine to the bladder
• Descend retroperitonealy and cross pelvic brim
• Enter posterolateral corners of bladder
• Run medially within posterior bladder wall before
  opening into interior
• This oblique entry helps prevent backflow of
  urine

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Ureters play an active role in transporting urine
(its not just by gravity)

• Three basic layers
• Transitional epithelium of mucosa stretches when
  ureters fill
• Muscularis
• Inner longitudinal, outer circular layers
• Inferior 3rd with extra longitudinal layer)
• Stimulated to contract when urine in ureter
  peristaltic waves to propel urine to bladder
• Adventitia (external)

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Urinary Bladder

• Collapsible muscular sac
• Stores and expels urine
• Lies on pelvic floor posterior to pubic symphysis
• Males anterior to rectum
• Females just anterior to the vagina and uterus

See also brief atlas
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• If full bladder is spherical and extends into
  abdominal cavity (holds about 500 ml or 1 pt)
• If empty bladder lies entirely within pelvis
  with shape like upside-down pyramid
• Urine exits via the urethra
• Trigone is inside area between ureters and
  urethra prone to infection (see slide 38)

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• Bladder wall has three layers (same as ureters)
• Mucosa with distensible transitional epithelium
  and lamnia propria (can stretch)
• Thick muscularis called the detrusor muscle
• 3 layers of highly intermingled smooth muscle
• Squeezes urine out
• Fibrous adventitia

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The Urethra

• Smooth muscle with inner mucosa
• Changes from transitional through stages to
  stratified squamous near end
• Drains urine out of the bladder and body
• Male about 20 cm (8) long
• Female 3-4 cm (1.5) long
• Short length is why females have more urinary
  tract infections than males - ascending bacteria
  from stool contamination

urethra
Urethra____
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• Urethral sphincters
• Internal involuntary sphincter of smooth muscle
• External skeletal muscle inhibits urination
  voluntarily until proper time (levator anni
  muscle also helps voluntary constriction)

Males urethra has three regions (see right)
_________trigone
1. Prostatic urethra__________
2. Membranous urethra____
3. Spongy or penile urethra_____
female
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With all the labels
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• Micturition
• AKA
• Voiding
• Urinating
• Emptying the bladder
• (See book for diagram
• explanation p 701)
• KNOW
• Micturition center of brain pons
• (but heavily influenced by higher centers)
• Parasympathetic to void
• Sympathetic inhibits micturition