Download The Urinary System

The Urinary System
Chapter 18
Pgs 547-573
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
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
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)
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
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?
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
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
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
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
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
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
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
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
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
Hormonal Control of Kidney
Function
•
•
•
•
•
Angiotensin II
ADH
Aldosterone
Atrial Natriuetic Peptide (ANP)
Secretion of angiotensin II,
ADH, aldosterone integrated
by renin-angiotensin system
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
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
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
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
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
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
Micturition Reflex and Urination
• Both internal and external sphincters must
be relaxed
– External under voluntary control
• When external relaxes so does internal
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
Fluid Balance
• Water Loss
• Water Gain
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
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
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)
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
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
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?
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
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
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+)
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)
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)
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
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
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 acidbicarbonate 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
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