Download Chap 28 - Renal and Associated Mechanisms for Controlling

Renal and Associated Mechanisms for Controlling
Extracellular Fluid Osmolality and Sodium Concentration
Physiology III, Tri 4
Guyton & Hall, Chapter 28
Dr. Robyn Strader
OBJECTIVES:
1. To understand how the body reduces the amount of excess water.
2. To understand how and why the body produces a dilute urine.
3. To understand how the kidney uses the countercurrent mechanism to concentrate urine.
4. To understand the role of ADH in the kidney.
5. To understand the role of aldosterone in maintaining osmolality.
6. To understand the importance of and regulation of sodium concentration.
I.
The Mechanism for Excreting Excess Water: Excretion of a Dilute Urine.
A.  Osmolality,   Urine   body water
B.  Osmolality,  excretion of solutes
C. with decreased osmolality:
1. increased urine output
2. urine may have osmolarity as low as 50 mosm/liter
3. sodium and potassium concentrations may not be effected
D. Renal mechanism
1.
the kidney has the ability to regulate water excretion independently of
2.
necessary for survival
3.
absorb solutes
4.
distal tubules
II.
The Mechanism for Excreting Excess Solutes: the Countercurrent Mechanism for
Excreting a Concentrated Urine
A. Antidiuretic hormone (ADH) = Vasopressin
1.
posterior pituitary
2.
distal tubule
3.
most powerful feedback system for regulating plasma osmolarity and
B. Counter current mechanism
1.
Juxtamedullary nephrons
2.
Vasa recta
C. Hyperosmolality of the medullary interstitial fluid, and mechanisms for achieving it
1.
 osmotic pressure of medullary fluid
a.
active/passive transport of Na+, active transport of Cl- and co-transport
of K, etc. out of the thick portion of the ascending limb
b.
carried downward into inner medulla
c.
active transport of Na from the collecting ducts and passive absorption
of Cl ions with Na ions
 ADH   urea absorbed into the fluid of the inner medulla from
the collecting ducts
e.
 medullary interstitial fluid osmolality is due to:
(1)
active transport of the ions into the interstitium by the thick
portion of the ascending limb of loop of Henle
(2)
active transport of ions from the collecting duct into the
interstitium
(3)
passive diffusion of large amounts of urea from the collecting
duct into the interstitium
 Water Reabsorption via ADH
1.
ADH acts on basolateral membrane
2.
ADH activates adenyl cyclase in membrane to form cyclic adenosine
monophosphate (cyclic AMP) in the cell membrane
d.
D.
E.
Countercurrent Multiplier = repetitive reabsorption of Na Cl by the thick ascending
segment of the loop of Henle, along with the continual inflow of new NaCl from the
proximal tubule into the loop of Henle
F.
Counter Current Exchange Mechanism in the Vasa Recta - A Mechanism for
Holding Solutes in the Medulla
1.
medullary blood flow maintains high solute concentration in medullary
interstitial fluids
2.
"sluggish" blood flow minimizes solute removal
3.
vasa recta function as a counter current exchanger
G.
Mechanism for Excreting a Concentrated Urine - Role of ADH
1.
increase permeability of cortical collecting duct, collecting duct and distal
tubule
2.
water enters medullary interstitium via osmosis
3.
 concentration in collecting duct
H.
Summary of the Osmolal Concentration Changes in the Different Segments of the
Tubules.
1.
proximal tubule - highly permeable to water
2.
Loop of Henle - osmolality rises rapidly due to counter current mechanism
3.
In distal tubule, cortical collecting duct, and collecting duct - osmolality is
dependent on ADH
I.
Osmolar clearance; Free Water Clearance
1.
C osm = Osmoles entering urine per minute
Plasma Osmolar Concentration
2.
Free water = excess water that is excreted
CH2O = Urine volume per minute - C osm
III.
Control of Extra Cellular Fluid Osmolality and Sodium Concentration
A. Extracellular fluid osmolality = 300 + 3% mosm/L
Na ion concentration = 142 + 3% mEq/L
B. Relationship between extra cellular fluid osmolality and sodium
concentration is determined almost entirely by the extracellular fluid Na
concentration
C. Control systems regulating extracellular osmolality and sodium
concentration
1.
osmoreceptor - antidiuretic hormone
2.
thirst mechanism
3.
salt appetite mechanism
IV.
Sodium Excretion and its Control by Aldosterone
A. Role of tubular system is to reabsorb Na
1.
Proximal tubule = 65%
2.
Ascending limb of Loop of Henle = 27%
B.
Variable Reabsorption of Sodium in the late distal tubules and Cortical
Collecting Ducts - Role of Aldosterone
1.
Na excretion in the late distal tubules is dependent on aldosterone
2.
Aldosterone activates the DNA molecule to Form (M) RNA to cause
formation of carrier proteins or protein enzymes necessary for Na
transport process
3.
Stimulation of Aldosterone secretion
a.
 Angiotensin II
b.
 extracellular fluid potassium ion concentration
c.
 extracellular fluid Na ion concentration
V. Control of sodium intake
A. Stimuli
1. decreased sodium concentration in the extracellular fluid
2. circulatory insufficiency
B. Thirst - elicited immediately
C. Salt appetite - progressively builds after several hours