Download Formation of concentrated urine

Aims
• Regulation of extracellular fluid osmolarity
• Readings; Guyton, Chapter 28
Osmolarity
• The osmole is the
unit of measure of the
# of particles of a
molecule in a solution
– Osmole is used in
place of grams
• Osmolality
– # of osmoles/ kg of H2O
– Measurement not used
to often due to difficulty
in weighing H2O in a
solution.
• Osmolarity
•1 osmole is 1g mol. wt. of an
undissociated solute.
•180g of glucose= 1 osmole of
glucose
•58.5g of NaCl= 2 osmoles of
NaCl because it dissociates into
2 ions (Na and Cl)
– # of osmoles/ L of H2O
– Most frequently used
– For dilute solutions in the
body the difference
between osmolality and
osmolarity is less than
1%.
Antidiuretic Hormone (ADH) Control
Excess H2O in the body
Extracellular fluid osmolarity is
reduced.
The secretion of ADH is
reduced.
This reduces the permeability of
the distal tubules and collecting
tubules for water and results in a
dilution of the urine.
Guyton’s Textbook of Medical Physiology 28-2
Formation of Dilute Urine
• When there is excess water in the body, the
extracellular fluid osmolarity is reduced.
• In response the kidney excretes the excess water
in the form of diluted urine.
• The kidney can reduce the osmolarity of the urine
(dilute the urine) 6-fold.
• Conversely, when there is a deficit of water the
kidneys can increase the osmolarity of the urine
4-fold.
• The kidneys can also adjust the volume of urine
secreted.
Formation of Dilute Urine
• Importantly, the
kidneys can adjust
the osmolarity and
volume of urine
without impacting
the rate of
excretion of
solutes such as
Na+.
Guyton’s Textbook of Medical Physiology 28-1
Formation of Concentrated Urine
• Obligatory urine volume
– The minimum volume of urine which must be
secreted in order to secrete the necessary
amount of solute.
• Example
– Each day Dr. Donati must excrete 600
milliosmoles of solute.
– Dr. Donati’s kidneys can concentrate urine up
to 1200 milliosmoles/L.
600 milliosmoles
1200 milliosmoles/L = 500 ml (Obligitory Urine
Volume)
• Thus, Dr. Donati must eliminate a minimum of
500 ml per day
Why is it bad to drink seawater?
• Question
• If Dr. Donati got stranded on a desert
island with his pet Australian hopping
mouse Wilbur (who by the way can
concentrate it’s urine to 10,000
milliosmoles/L.) who should drink the
water?
• Answer
• The salinity of sea water is ~2400
milliosmoles/L.
• Since Dr. Donati can only concentrate
his urine to 1200 milliosmoles/L, he will
have to urinate out 2L for every 1L he
drinks. A recipe for disaster.
• Wilbur on the other hand can
concentrate his urine to 10,000
milliosmoles/L. Thus he can drink to his
hearts delight as he will have to urinate
only ~250 ml per L of sea water drunk.
Requirements for the concentration
of urine
• High osmolarity of the renal medullary
interstitial fluid.
• High levels of ___________________
High osmolarity of the renal medullary
interstitial fluid.
Sherwood’s Human Physiology 14-27 5th Ed. & 14-24 6th Ed.
High osmolarity of the renal medullary
interstitial fluid.
• The Countercurrent
mechanism
– Descending limb of the
loop of Henle.
• Permeable to Water
– Thin Descending limb of
the loop of Henle
• Passive reabsorption of
sodium chloride
• ____________ to water.
Guyton’s Textbook of Medical Physiology table 28-1
High osmolarity of the renal medullary
interstitial fluid.
• The Countercurrent mechanism
– Thick ascending limb of the loop of Henle.
Active transport of Na+ and co-transport of K+ and
Cl-.
Can establish a 200 milliosmole concentration
gradient between the tubular lumen and the
interstitial fluid.
Impermeable to water.
Guyton’s Textbook of Medical Physiology table 28-1
• The process of the Countercurrent Multiplier
involves the repetitive reabsorption of
sodium chloride from the thick ascending
limb and the continued inflow of new sodium
chloride from the proximal tubules.
Sherwood’s Human Physiology 14-28 5th Ed. & 14-25 6th Ed.
• H2O continues to be
reabsorbed in the
descending limb of the
loop of Henle and Na+
continues to be
reabsorbed in the thick
ascending limb.
• This creates a high
osmolarity in the
medullary interstitium
and a lower osmolarity as
we ascend to the cortical
interstitium.
Sherwood’s Human Physiology 14-28 5th Ed. & 14-25 6th Ed.
High osmolarity of the renal medullary
interstitial fluid.
Urea’s contribution
• Urea is passively
absorbed.
• Urea gets trapped in the
medullary interstitium.
• Absorbed in the
collecting ducts.
• Secreted into
_________________.
Guyton’s Textbook of Medical Physiology 28-5
High osmolarity of the renal medullary
Urea’s contribution
interstitial fluid.
Sherwood’s Human
Physiology 14-5
High osmolarity of the renal medullary
interstitial fluid.
Vasa recta’s role
• Medullary blood flow is
low (~1-2% of total
renal blood flow).
• Countercurrent
exchange.
– Due to U shape.
– Thus, no net dilution or
“wash out” of interstitial
fluid.
– Does not create High
osmolarity, just
preserves it.
Sherwood’s Human Physiology 14-31 5th Ed. & 14-28 6th Ed.
High levels of ADH
• The early and late distal
tubule
– Actively transports NaCl
(early).
– Relatively impermeable
to H2O (both).
• Cortical & Medullary
collecting tubule
– In absence of ADH
mostly impermeable to
H2O.
– With high levels of ADH
late distal and
collecting tubules are
very permeable to
H2O.
Guyton’s Textbook of Medical Physiology 28-4
Osmoreceptor-ADH feedback system
• Net effect is that
H2O is conserved
while Na+ and
other solutes
continue to be
excreted in the
urine.
Guyton’s Textbook of Medical Physiology 28-8
Osmoreceptor-ADH feedback system
• Osmoreceptor
– Senses increased
osmolarity.
• Hypothalamus
– Synthesizes ADH.
• Posterior pituitary
– Releases ADH.
• Kidney
– Responds to ADH by
reabsorbing H2O and
increasing the
concentration of
solutes in the urine.
Guyton’s Textbook of Medical Physiology 28-9
ADH Secretion
• Other stimulators of ADH secretion.
– Arterial baroreceptor reflex.
• Increased signal ______________________________
ADH secretion.
– Vice versa is true.
– Cardiopulmonary reflex (volume reflex).
• Decreased blood volume and decreased arterial
pressure (decreased stretch of atria) results in an
increase in ADH secretion.
– Vice versa is also true.
ADH Secretion
• ADH secretion is
more sensitive to
small changes in
osmolarity than
similar changes in
blood volume.
Guyton’s Textbook of Medical Physiology 28-10
Summary of Excretion of Urine in
the Presence and Absence of ADH
ADH is present = Concentrated urine
Sherwood’s Human Physiology 14-30 5th Ed. & 14-27 6th Ed.
Summary of Excretion of Urine in
the Presence and Absence of ADH
ADH is absent = Dilute urine
Sherwood’s Human Physiology 14-30 5th Ed. & 14-27 6th Ed.
Thirst
• Conscience desire for water.
Thirst Center
• In the preoptic nucleus and anteroventral
wall of the third ventrical.
– When stimulated electrically or with salt
injections causes immediate drinking.
Control of Thirst
• Osmolarity
– When the extracellular fluid Na+ concentration increases
only 2 mEq/L above normal the thirst mechanism is
activated.
Guyton’s Textbook of Medical Physiology table 28-3
Renin-Angiotensin System
Decreased arterial pressure
Renin released from Kidneys
Angiotensinogen
Angiotensin I (mild vasoconstriction)
Converting enzyme (in lung)
Angiotensin II
Angiotensinase
(inactive)
Increase Thirst
Renal retention of
salt and water
Strong vasoconstriction
Control Extracellular Osmolarity
• When either the ADH
or thirst mechanisms
fail the other normally
can still control
extracellular
osmolarity and sodium
concentration.
• If both fail regulation is
lost.
Guyton’s Textbook of Medical Physiology 28-11
Next Time
• Potassium, phosphate, calcium, and
magnesium excretion
• Readings: Sherwood, Chapter 14.
Objectives
1. Understand how dilute and concentrated urine is
formed.
1.
2.
3.
4.
Role of ADH
Role of interstitial fluid osmolarity
Countercurrent multiplier
Obligatory urine volume
2. Understand the regional renal tubular
permeability to H2O, NaCl, and urea.
3. Understand the role of the vasa recta regarding
interstitial fluid osmolarity.
4. Understand the function of hypothalamic
osmoreceptors and other stimulators of ADH
secretion.
5. Understand the regulators of thirst.