Urinary System

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Urinary System

• Dr. Michael P. Gillespie

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Major Components

• 2 kidneys.
• 2 ureters.
• 1 urinary bladder.
• 1 urethra.

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Functions Of Kidneys

• Excrete wastes in urine.
• Regulate blood volume.
• Regulate blood composition.
• Regulate blood pressure.
• Synthesize glucose.
• Release erythropoietin.
• Participate in vitamin D synthesis.

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Functions Of Urinary System Continued

• Ureters transport urine from the kidneys to the
  urinary bladder.
• Urinary bladder stores urine.
• Urethra discharges urine from the body.

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Kidneys

• The paired kidneys are reddish,
  kidney-bean-shaped organs.
• They are located above the waist between the
  peritoneum and the posterior wall of the abdomen.
• They are retroperitoneal.
• The right kidney is slightly lower than the left
  due to the presence of the liver.

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External Anatomy Of The Kidneys

• A typical kidney is 10-12 cm long, 5-7 cm wide
  and 3 cm thick.
• The concave medial border of the kidneys faces
  the vertebral column.
• The renal hilus is a deep vertical fissure
  through which the ureter, blood vessels,
  lymphatic vessels, and nerves pass.

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External Anatomy Of The Kidneys

• Layers of tissue around the kidneys.
• Renal capsule deep layer. Smooth transparent
  sheet of dense irregular connective tissue.
  Maintains the shape of the kidney.
• Adipose capsule mass of fatty tissue. Protects
  the kidney from trauma and holds it in place.
• Renal fascia superficial layer. Anchors the
  kidney to the surrounding structures and to the
  abdominal wall.

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Nephroptosis

• Nephroptosis or floating kidney is an inferior
  displacement or dropping of the kidney.
• It occurs when the kidney slips from its normal
  position.

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Nephroptosis

• This happens when it is not securely held in
  place by adjacent organs or its covering of fat.
• It occurs most often in very thin people who have
  a deficient adipose capsule or renal fascia.
• The ureter may kink and block urine flow.

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Internal Anatomy Of The Kidneys

• Renal cortex superficial layer.
• Renal medulla inner region.
• 8-18 cone-shaped renal pyramids which taper to a
  renal papilla.
• The renal columns are portions of the cortex that
  extend between the pyramids.
• A renal lobe consists of a renal pyramid,
  overlying renal cortex, and one-half of each
  adjacent renal column.

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Internal Anatomy Of The Kidneys

• Together, the renal cortex and renal pyramids
  constitute the parenchyma (functional portion) of
  the kidney.
• The nephrons (functional units) of the kidney are
  within the parenchyma.
• Urine formed by the nephrons drains into the
  papillary ducts, which drain into the minor and
  eventually major calyces.

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Internal Anatomy Of The Kidneys

• Each kidney has 8 to 18 minor calyces and 2 to 3
  major calyces.
• The major calyces drain urine into a large cavity
  called the renal pelvis.
• The hilus expands into the renal sinus (a cavity
  within the kidney).

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Blood Nerve Supply Of The Kidneys

• The kidneys have abundant blood vessels.
• The kidneys remove wastes from the blood and
  regulate its volume and ionic composition.
• Right and left renal arteries supple the kidneys.
• They branch into segmental arteries, which branch
  into interlobar arteries.

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Blood Nerve Supply Of The Kidneys

• The interlobar arteries arch between the renal
  medulla and cortex and are referred to as arcuate
  arteries here.
• Afferent arterioles branches come off the
  interlobar arteries and one supplies each
  nephron.
• The afferent arteriole divides into a ball of
  capillaries called a glomerulus.

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Blood Nerve Supply Of The Kidneys

• The glomerular capillaries reunite to form the
  efferent arteriole.
• The efferent arteriole divides to form the
  peritubular capillaries.
• These reunite to form peritubular venules, then
  interlobar veins and eventually the renal vein.

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Blood Nerve Supply Of The Kidneys

• Most renal nerves originate in the celiac
  ganglion and pass through the renal plexus.
• Renal nerves are part of the sympathetic division
  of the autonomic nervous system.
• They regulate blood flow through the kidney
  causing vasodilation and vasoconstriction.

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Nephron

• Nephrons are the functional units of the kidneys.
• Two main parts
• Renal corpuscle where blood plasma is filtered.
• Renal tubule into which the filtered fluid
  passes.

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Renal Corpuscle

• Two components
• Glomerulus a capillary network.
• Glomerular (bowmans) capsule a double walled
  epithelial cup that surrounds the glomerular
  capillaries.

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Renal Tubule

• 3 main sections
• Proximal convoluted tubule.
• Loop of Henle (nephron loop).
• Distal convoluted tubule.

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Nephron Continued

• The distal convoluted tubules of several nephrons
  empty into a single collecting duct.
• Collecting ducts then unite and converge into
  papillary ducts, which drain into minor calyces.
• 1 kidney has approximately 1 million nephrons.

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Loop Of Henle

• The loop of Henle connects the proximal and
  distal convoluted tubules.
• It consists of a descending limb and an ascending
  limb.

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Types Of Nephrons

• About 80-85 of the nephrons are cortical
  nephrons.
• They have short loops of Henle and the renal
  corpuscles lie mainly in the renal cortex.
• They extend only slightly into the medulla.

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Types Of Nephrons

• The other 15-20 of the nephrons are
  juxtamedullary nephrons.
• They have long loops of Henle and extend into the
  deepest regions of the medulla.
• These allow the kidneys to excrete either very
  dilute or concentrated urine.

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Histology Of The Nephron Collecting Duct

• A single layer of epithelial cells forms the
  entire wall of the glomerular capsule, renal
  tubule, and ducts.

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Glomerular Capsule

• The glomerular (bowmans) capsule consists of
  visceral and parietal layers with a capsular
  (bowmans) space in between.
• Visceral layer modified simple squamous
  epithelial cells called podocytes.
• Parietal layer simple squamous epithelium.
• Fluid filtered from the glomerular capillaries
  enters the capsular space.

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Renal Tubule Collecting Duct

• Proximal convoluted tubule cells are simple
  cuboidal epithelial cells with a brush border of
  microvilli.
• The descending limb and the first part of the
  ascending limb are composed of simple squamous
  epithelium.
• The thick ascending limb is composed of simple
  cuboidal epithelium to low columnar epithelium.

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Number Of Nephrons

• The number of nephrons is constant from birth.
• Growth in kidney size is due to growth in size of
  the nephrons, not increase in number.
• Signs of kidney dysfunction do not usually become
  apparent until function declines to less than 25
  of normal.

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Number Of Nephrons

• The remaining functional nephrons adapt to form a
  larger than normal load.
• Surgical removal of one kidney stimulates
  hypertrophy of the other kidney.
• One kidney can eventually filter blood at a rate
  of 80 of two normal kidneys.

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Functions Of Nephrons Collecting Ducts

• Glomerular filtration water and most solutes
  move across the wall of glomerular capillaries
  into the glomerular capsule and into the renal
  tubule.
• Tubular reabsorption tubule cells reabsorb
  about 99 of the water and many useful solutes
  into the peritubular capillaries.
• Tubular secretion the tubule cells secrete
  wastes, drugs, and excess ions into the fluid as
  it moves through the tubule and collecting duct.

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Glomerular Filtration

• Glomerular filtrate the fluid that enters the
  capsular space.
• Filtration fraction the fraction of blood
  plasma in the afferent arterioles of the kidneys
  that becomes filtrate (typically 16-20).

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Filtration Membrane

• The endothelial cells of the glomerular
  capillaries and the podocytes, which encircle the
  capillaries, form a leaky barrier known as the
  filtration membrane.

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Filtration Membrane

• Fenestrations (pores) in the glomerular
  epithelial cells cause them to be quite leaky.

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Filtration Membrane

• Filtration slits are spaces between the pedicels
  (footlike processes from the podocytes), which
  allow passage of molecules smaller than 6-7 nm.
• Water, glucose, vitamins, amino acids, very small
  plasma proteins, ammonia, urea, and ions can pass
  through.
• Albumin is too large to easily pass through the
  slits.
• Filtration utilizes pressure to drive fluids and
  solutes through a membrane.

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Factors That Affect Filtration

• Glomerular capillaries present a large surface
  area for filtration because they are long and
  extensive.
• The filtration membrane is thin and porous.
  Glomerular capillaries are about 50 times leakier
  than other capillaries due to fenestrations.
• Glomerular capillary blood pressure is high due
  to a small diameter of the efferent arteriole
  resulting in backflow of blood.

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Net Filtration Pressure

• 3 main pressures determine the level of
  glomerular filtration.
• Glomerular blood hydrostatic pressure (GBHP)
  promotes filtration. 55 mmHg.
• Capsular hydrostatic pressure (CHP) opposes
  filtration. Hydrostatic pressure exerted by
  fluid already in the capsular space (back
  pressure). 15 mmHg.
• Blood colloid osmotic pressure (BCOP) opposes
  filtration. Plasma proteins (albumin,
  fibrinogen, globulins) draw fluid into
  capillaries. 30 mmHg.

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Net Filtration Pressure

• Net filtration pressure (NFP) GBHP CHP
  BCOP.
• NFP 55mmhg 15mmhg 30mmhg 10mmhg.

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Loss Of Plasma Proteins

• Kidney disease can cause damage to glomerular
  capillaries allowing them to be permeable to
  plasma proteins.
• As plasma proteins filter out, the osmotic
  pressure of the blood decreases, allowing water
  to be drawn from the blood plasma into
  interstitial tissues. This results in edema.

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Glomerular Filtration Rate

• The amount of filtrate formed in all the renal
  corpuscles of both kidneys each minute is the
  glomerular filtration rate (GFR).
• If the GFR is too high, substances may pass too
  quickly through the tubules that they are not
  reabsorbed.
• If the GFR is too low, nearly all the filtrate
  may be reabsorbed resulting in inadequate
  excretion.

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Reabsorption

• Primary and secondary active transport mechanisms
  are utilized to pump a substance across a
  membrane.
• Obligatory water reabsorption occurs due to the
  solute reabsorption and corresponding osmotic
  pressure created. Water follows the solutes.

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Glucosuria

• When the blood concentration of glucose is above
  200 mg/ml, the renal symporters cannot work fast
  enough to reabsorb all of the glucose that enters
  the glomerular filtrate.
• Some of the glucose remains in the urine
  (glucosuria).
• Diabetes mellitus is the most common cause of
  glucosuria.

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Hormonal Regulation Of Tubular Reabsorption
Secretion

• Angiotensin II increases reabsorption of Na,
  other solutes, and water, which increases blood
  volume.
• Aldosterone increases secretion of K and
  reabsorption of Na, Cl-. This increases
  reabsorption of water and increases blood volume.

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Hormonal Regulation Of Tubular Reabsorption
Secretion

• Antidiuretic hormone (ADH) or vasopressin
  increases facultative reabsorption of water.
• Atrial natriuretic peptide (ANP) increases
  excretion of Na in urine (natriureses),
  increases urine output (diuresis) and decreases
  blood volume.

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Diuretics

• Diuretics are substances that slow renal
  reabsorption of water and thereby causes
  diuresis, an elevated urine flow rate, which in
  turn reduces blood volume.
• Diuretics are prescribed to treat hypertension.
• Naturally occuring diuretics include caffeine
  which inhibits Na reabsorption, and alcohol
  which inhibits secretion of ADH.

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Evaluation Of Kidney Function

• The kidneys are evaluated by assessing the
  quantity of urine, the quality of urine, and the
  level of wastes in blood.
• Urinalysis, blood urea nitrogen (BUN) test,
  plasma creatinine, and renal plasma clearance
  tests are utilized to assess kidney functioning.

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Characteristics Of Normal Urine

• Volume 1 to 2 liters / 24 hours (varies).
• Color yellow or amber, but varies with
  concentration and diet. Concentrated urine is
  darker. Diet (reddish color from beets),
  medications, and diseases may affect color.
  Kidney stones can produce blood in urine.
• Turbidity transparent when freshly voided, but
  becomes turbid (cloudy) upon standing.

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Characteristics Of Normal Urine

• Odor mildly aromatic but becomes ammonia-like
  upon standing. Urine of diabetics has a fruity
  odor due to ketone bodies.
• pH ranges between 4.6 and 8.0 (average 6.0).
  High protein diets increases acidity, vegetarian
  diets increase alkalinity.
• Specific gravity (density) ranges from 1.001 to
  1.035. Greater concentration of solutes yields
  greater specific gravity.

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Blood Urea Nitrogen (BUN)

• This test measures the blood nitrogen that is
  part of the urea resulting from catabolism and
  deamination of amino acids.
• BUN rises as the glomerular filtration rate
  decreases due to renal disease or obstruction of
  the urinary tract.
• Decreasing protein intake decreases urea
  production.

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Plasma Creatinine

• Plasma creatinine results from the catabolism of
  creatinine phosphate from skeletal muscle.
• Creatinine levels above 1.5 mg/dL indicate poor
  renal function. Decreased levels indicated
  decreased muscle mass (I.e. muscular dystrophy).

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Renal Plasma Clearance

• Renal plasma clearance is the volume of blood
  that is cleaned or cleared of a substance per
  unit of time.
• High renal plasma clearance indicates efficient
  excretion of a substance in the urine low
  clearance indicates inefficient clearance.

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Dialysis

• If a persons kidneys are so impaired by disease
  or injury that they are uable to function, the
  blood must be cleansed artificially by dialysis.
• Dialysis is the separation of large solutes from
  smaller ones through the use of a selectively
  permeable membrane.

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Dialysis

• An artificial kidney machine performs
  hemodialysis. It directly filters a patients
  blood.
• After passing though the dialysis tubing, the
  cleansed blood flows back into the patients body.

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Urine Transportation, Storage, Elimination

• The urine drains from collecting ducts through
  papillary ducts into the minor calyces, which
  join the major calyces, that unite to form the
  renal pelvis.
• From the renal pelvis, the urine drains into the
  ureters and then into the urinary bladder.
• Urine is discharged from the body through a
  single urethra.

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Ureters

• Each of the two ureters transport urine from the
  renal pelvis of one kidney to the urinary
  bladder.
• Peristaltic contractions of the muscular walls of
  the ureters push the urine towards the bladder.
• No anatomical valve exists between the ureters
  and bladder however, a physiological one exists.
  Pressure from the filling bladder compresses the
  openings of the ureters preventing backflow of
  urine and microbes.

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Urinary Bladder

• The urinary bladder is a hollow, distensible
  muscular organ.
• It resides in the pelvic cavity posterior to the
  pubic symphysis.

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Urinary Bladder

• When the bladder is empty, it is collapsed. When
  it is full, it becomes spherical in shape.
• The muscularis, also called the detrusor muscle,
  consists of smooth muscle.
• An internal urethral sphincter of smooth muscle
  and an external urethral sphincter of skeletal
  muscle exist.

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Micturition

• Micturition is discharge of urine from the
  urinary bladder. It is also known as urination
  or voiding.
• The micturition reflex occurs when volume within
  the bladder exceeds 200 400 mL and causes
  stretch of the bladder wall.

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Urethra

• The urethra is a small tube leading from the
  internal urethral orifice in the floor of the
  urinary bladder to the exterior of the body.
• It is the terminal portion of the urinary system.
• In males, it discharges semen from the body as
  well as urine.

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Urinary Incontinence

• A lack of voluntary control over micturition is
  called urinary incontinence.
• Stress incontinence physical stresses that
  increase abdominal pressure such as coughing,
  sneezing, laughing, exercising, pregnancy, or
  walking can cause leakage of urine from the
  bladder.
• Those who smoke have twice the risk or developing
  urinary incontinence.