The Urinary System

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The Urinary System

• Chapter 18
• Pgs 547-573

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Overview

• Introduction
• The Organization of the Urinary System
• The Kidneys
• Superficial and sectional anatomy
• The nephron
• Blood supply to the kidneys
• Basic Principles of Urine Production
• Filtration at the glomerulus
• Reabsorption and secretion along the renal tubule
• Control of kidney function

• Urine Transport, Storage, and Elimination
• The ureters and urinary bladder
• The urethra
• The micturition reflex and urination
• Fluid, Electrolyte, and Acid-Base Balance
• Fluid and electrolyte balance
• Acid-base balance

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Functions of the Urinary System

• Remove organic wastes generated by cells
• Regulates blood volume and blood pressure
• Regulates plasma concentrations of ions
• Helps to stabilize blood pH
• Controls valuable nutrients

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Basic Principles of Urine Formation

• Process involves excretion and elimination of
  dissolved solutes (3 metabolic wastes)
• Urea
• Most abundant organic waste (21 grams/day)
• Produced during break down of amino acids
• Creatinine
• Generated during breakdown of creatine phosphate
  (1.8 g/day)
• Uric acid
• Breakdown and recycling of RNA (480 mg/day)

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Three Distinct Processes of Urine Production

• Filtration
• Bp forces water across filtration membrane
• Depends on solute size
• Renal corpuscle across cap walls of glomerulus
• Reabsorption
• Removal of water and solute molecules from
  filtrate after enters renal tubule
• Selective process
• Simple diffusion or carrier proteins
• Water passive (osmosis)
• Water and solutes reenter circ at peritubular
  caps and vasa recta
• Primarily at PTC
• Secretion
• Transport of solutes across tubular epith into
  filtrate
• Necessary because
• Filtration does not force all dissolved materials
  out of plasma
• Blood entering peritubular caps may still contain
  undesirable substances
• Loop of Henle and collecting system (water,
  sodium, potassium lost to urine)
• All processes create fluid very different from
  other body fluids

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Filtration at the Glomerulus Filtration Pressure

• Net force promoting filtration is filtration
  pressure
• Higher than capillary blood pressure elsewhere in
  body
• Result of difference in diameter of afferent and
  efferent arterioles
• Which one do you think would have a smaller
  diameter?

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Filtration Pressure

• Very low (10 mm Hg)
• If glomerular blood pressure drops, kidney
  filtration will stop
• Minor changes in blood pressure
• Reflexive vasodilation/constriction of arterioles
• Automatic or due to SNS
• Serious drop in bp can reduce or stop filtration
• Kidneys most sensitive to bp than any other organ
• Control many homeostatic mechanisms for
  regulating blood pressure and blood volume

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Filtration at the Glomerulus The Glomerular
Filtration Rate

• Glomerular filtration
• Process of filtrate production at the glomerulus
• Glomerular filtration rate (GFR)
• Amount of filtrate produced in the kidneys each
  minute
• Averages 125 mL/min
• 99 of filtrate reabsorbed
• Very important process
• Inability to reclaim water can quickly cause
  death by dehydration

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DCT and Aldosterone

• DCT cells actively transport sodium ions out of
  tubular fluid in exchange for potassium or
  hydrogen ions
• Pumps regulated by aldosterone
• Aldosterone secretion occurs
• In response to circulating ACTH from anterior
  pituitary
• In response to elevated potassium ion
  concentrations in extracellular fluid
• The higher the aldosterone levels, the more
  sodium that is reclaimed and the more potassium
  that is lost

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DCT and Antidiuretic Hormone (ADH)

• Controls the amount of water that is reabsorbed
• Absence of ADH
• DCT and collecting ducts impermeable to water
• Higher the ADH, the greater the water
  permeability and the more concentrated the urine

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Properties of Normal Urine

• pH 4.5-8
• Water content 93-97
• Volume 1200 mL/day
• Color clear yellow
• What does dark yellow urine indicate?
• Odor varies with composition
• Bacterial content sterile

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The Control of Kidney Function

• Regulated in 3 ways
• Local, automatic adjustments
• in glomerular pressures
• through changes in diameters of afferent and
  efferent arterioles
• Activities of SNS
• Effects of hormones
• Make complex, long-term adjustments in bp and
  blood vol
• Stabilize GFR by regulating transport mechanisms
  and water permeabilities in DCT and collecting
  duct

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Local Regulation of Kidney Function

• Change in diameter of afferent and efferent
  arterioles and glomerular capillaries
• Can compensate for minor changes in bp
• Ex ? blood flow and ? glomerular pressure will
  trigger
• ________ of the afferent arteriole and
  glomerular capillaries and
• ________ of the efferent arteriole

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Sympathetic Activation and Kidney Function

• Autonomic regulation primarily through SNS
• Serves to shift blood away from kidneys
• Affect on GFR?
• Direct effects on kidney function
• Powerful constriction of afferent arterioles
• ? GFR, slows production of filtrate
• Why is that important?
• Can override local regulation in sudden crisis
• Acute fall in bp, heart attack
• When done, GFR returns to normal

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Sympathetic Activation

• Indirect effects
• When changes region pattern of blood circulation,
  blood flow to kidneys affected
• Ex dilation of bv in hot weather shunts blood
  away from kidneys
• Glomerular filtration declines temporarily

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Hormonal Control of Kidney Function

• Angiotensin II
• ADH
• Aldosterone
• Atrial Natriuetic Peptide (ANP)
• Secretion of angiotensin II, ADH, aldosterone
  integrated by renin-angiotensin system

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Renin-Angiotensin System

• Glomerular pressures can remain low due to
• Decrease in blood volume
• Fall in systemic bp
• Blockage of renal artery
• Then juxtaglomerular apparatus releases enzyme
  renin
• Renin ? angiotensinogen ? angiotensin I ?
  angiotensin II
• Angiotensin II is a powerful vasoconstrictor

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Renin-Angiotensin System

• Angiotensin II has following effects
• Peripheral capillary beds
• Brief but powerful vasoconstriction
• Elevates bp in renal arteries
• Nephron
• Triggers contraction of efferent arterioles
• Elevates glomerular pressures and filtration
  rates
• CNS
• Triggers release of ADH
• Simulates reabsorption of water and sodium ions
• Stimulates hypothalamus
• Thirst sensation
• Adrenal gland
• Stimulates secretion of aldosterone
• Stimulates sodium reabsorption along DCT and
  collecting system
• Stimulates secretion of epinephrine and
  norepinephrine
• Sudden, dramatic increase in systemic bp

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ADH

• Increases water permeability of DCT and
  collecting duct
• Stimulates reabsorption of water from tubular
  fluid
• Causes thirst sensation
• Release occurs
• Under angiotensin II stimulation
• Independently
• Hypothalamus neurons stimulated by ? in bp or ?
  in solute concentration of circulating blood

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Aldosterone

• Stimulates reabsorption of sodium ions and
  secretion of potassium ions in DCT and collecting
  duct
• Primarily occurs
• Under angiotensin II stimulation
• In response to rise in potassium ion
  concentration of blood

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Atrial Natriuretic Peptide (ANP)

• Oppose renin-angiotensin system
• Released by atrial cardiac muscles when bp and
  blood volume too high
• Affects on kidney
• Decrease in rate of sodium ion reabsorption in
  DCT
• Increased sodium ion loss in urine
• Dilation of glomerular capillaries
• Increased filtration and urinary water loss
• Inactivation of renin-angiotensin II system
• Inhibition of renin, aldosterone, ADH secretion
• Net result
• Increased loss of sodium ions
• Increase in vol of urine produced
• Combination lowers blood vol and bp

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The Micturition Reflex and Urination

• Process of urination or micturition coordinated
  by micturition reflex
• Stretch receptors stimulated as bladder fills
• Increased impulses in afferent sensory fibers
• Brings parasympathetic motor neurons in sacral
  spinal cord to threshold
• Stimulates interneurons to relay sensation to
  cerebral cortex (conscious awareness)
• Urge to urinate when bladder contains 200 mL of
  urine

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Micturition Reflex and Urination

• Both internal and external sphincters must be
  relaxed
• External under voluntary control
• When external relaxes so does internal

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Fluid, Electrolyte, and Acid-Base Balance

• Fluid Balance
• Amount of water gained each day to amount lost
• Involves regulating content and distribution of
  water in ECF and ICF
• Cells and tissues cannot transport water so
  reflects control of electrolyte balance
• Electrolyte Balance
• Gain electrolytes from food and drink lose in
  urine, sweat, feces
• Balance exists when net gain net loss
• Involves balancing absorption rates
• Acid-Base Balance
• Production of H loss
• pH of body fluids within normal limits
• Body produces acids so prevention in reduction
  primary problem
• Lungs and kidneys

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Fluid Balance

• Water Loss

• Water Gain

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Fluid Shifts

• Water movement between ECF and ICF
• Occur rapidly, reach equilibrium within min to
  hrs
• Occur in response to changes in osmotic
  concentration (osmolarity) of ECF
• ECF more concentrated (hypertonic) than ICF
• Water moves from cells to ECF until equil reached
• ECF more dilute (hypotonic) than ICF
• Water moves from ECF into cells and vol of ICF
  will increase accordingly

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Electrolyte Balance

• Important because
• A gain or loss of electrolytes can cause a gain
  or loss in water
• The concentrations of individual electrolytes
  affect a variety of cell functions
• Will discuss sodium and potassium b/c
• They are major contributors to osmotic
  concentration of ECF and ICF
• Most common problems with electrolyte balance
  caused by imbalance between sodium gains and
  losses
• Have direct effects on normal functioning of
  living cells
• Problems with potassium balance less common but
  more dangerous

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Sodium Balance

• Amount of Na in ECF represents balance between
  absorption in digestive tract and excretion
• Excretion in
• Urine
• Primary
• Kidneys most important site (aldosterone and ANP)
• Sweat
• If intake or output rate changes, corresponding
  gain or loss of water occurs
• Water follows salt!!!
• Ex
• High salt meal will not raise sodium ion of
  bodily fluids
• Sodium chloride crosses digestive epith and
  osmosis brings additional water into ECF
• Reason why people with ? bp not supposed to eat
  high salt diet (dietary salt will be absorbed and
  blood vol and bp will increase)

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Potassium Balance

• Primary cation of ICF (98 of potassium in body)
• Concentration in ECF represents balance between
• Rate of potassium ion entry across diges epith
• Proportional to amount in diet
• Rate of loss into urine
• Strongly affected by aldosterone
• Reabsorption of sodium from filtrate in exchange
  for potassium ions from ISF
• High potassium levels in ECF high aldosterone
  additional loss of potassium in urine

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Acid-Base Balance

• pH of body fluids represent balance between
  acids, bases, and salts in solution
• Maintained at 7.35-7.45
• Any deviation dangerous
• H changes
• Disrupt stability of cell membranes
• Alters protein structure
• Changes activities of important enzymes
• Cannot survive with pH below 6.8 or above 7.7

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Acid-Base Balance

• pH below 7.35 acidosis
• pH above 7.45 alkalosis
• Affect all systems but nervous system and
  cardiovascular very sensitive to fluctuations
• Severe acidosis deadly b/c
• CNS function deteriorates
• Individual becomes comatose
• Cardiac contractions grow weak and irregular
• Symptoms of heart failure
• Peripheral vasodilation
• Dramatic drop in bp circulatory collapse
• Problems with acidosis more common
• Why?

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Acids in the Body

• Carbonic acid (H2CO3) important
• Lungs carbonic acid breaks down into CO2 H2O
• CO2 diffuses into alveoli
• Peripheral tissues CO2 in solution interacts
  with H2O
• Forms H2CO3 which dissociates into hydrogen ion
  and bicarbonate
• CO2 H2O ? H2CO3 ? H HCO3-
• Reaction occurs spontaneously and rapidly
• Carbonic anhydrase

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Buffers and Buffer Systems

• Metabolic acids must be controlled by buffers
• Buffers
• Dissolved compounds that can provide or remove
  hydrogen ions
• Stabilize pH of solution
• Include weak acids (hydrogen ion donors) and weak
  bases (hydrogen ion acceptors)
• Buffer system
• Consists of combination of weak acids and its
  dissociated products
• H and an anion
• 3 major systems
• Protein buffer system
• Carbonic acid-bicarbonate buffer system
• Phosphate buffer system

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Protein Buffer System

• Contributes to regulation of pH in ECF and ICF
• Depend on ability of amino acids to respond to
  changes in pH by accepting or releasing hydrogen
  ions
• ? pH, carboxyl group (--COOH) of a.a. dissociates
  and releases a hydrogen ion
• ? pH, amino group (--NH2) accepts additional
  hydrogen ions (forms NH3)

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Protein Buffer System

• Plasma proteins and hemoglobin contribute to
  buffering capabilities of blood
• ISF contains extracellular protein and amino
  acids that help regulate pH
• ICF contains structural and functional proteins
• Prevent change in pH when organic acids produced
  by cellular metabolism (lactic acid)

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Carbonic Acid-Bicarbonate Buffer System

• Important buffer system in ECF
• Carbonic acid acts as weak acid bicarbonate acts
  as weak base
• Net effect
• CO2 H2O ? H HCO3-
• Hydrogen ions removal will be replaced through
  combo of water and carbon dioxide
• Hydrogen ions added will be removed through
  formation of water and carbon dioxide
• Primary role is to prevent pH changes caused by
  metabolic acids
• Hydrogen ions released through dissociation of
  the acids combine with bicarbonate and form water
  and carbon dioxide
• Carbon dioxide excreted at lungs
• Can cope with large amounts of acids
• Body fluids contain an abundance of bicarbonate
  ions (bicarbonate reserve)

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Phosphate Buffer System

• Weak acid (anion) dihydrogen phosphate (H2PO4-)
• H2PO4- ? H HPO42-
• In ECF plays supporting role in regulating pH
• Many more bicarbonate ions than phosphate ions
• Very important in ICF
• High concentration of phosphate ions

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Maintaining Acid-Base Balance

• Buffer systems only provide temporary solution
• Hydrogen ions have been tied up but not
  eliminated
• Must be removed from body fluids
• maintenance of acid-base balance involves
  controlling hydrogen ion losses and gains
• Respiratory and renal mechanisms support buffer
  systems by
• Secreting or absorbing hydrogen ions
• Controlling excretion of acids and bases
• Generating additional buffers when necessary

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Respiratory Contributions to pH Regulation

• Respiratory compensation
• Change in respiratory rate that helps to
  stabilize pH
• Occurs when ph outside normal limits
• Respiratory activity has direct effect on
  carbonic acid-bicarbonate buffer system
• Increasing or decreasing rate of respiration
  alters pH by lowering or raising PCO2
• Changes in PCO2 have direct effect on
  concentration of hydrogen ions in plasma
• ? PCO2, ? pH
• ? PCO2 stimulates carotid and aortic bodies
  (chemoreceptors)
• Increase in resp rate, more carbon dioxide loss
  at lungs, ? PCO2 returns to normal

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Renal Contributions to pH Regulation

• Renal compensation
• Change in rates of hydrogen ion and bicarbonate
  ion secretion or absorption by kidneys in
  response to change in plasma pH
• Normal conditions body generates hydrogen ions
  through production of metabolic acids
• Hydrogen ions released must be excreted in urine
  to maintain balance
• Glomerular filtration puts hydrogen ions and
  carbon dioxide into filtrate
• Kidney tubules modify pH of filtrate by secreting
  hydrogen ions or reabsorbing bicarbonate ions