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Physiology 441
The Renal System, Chp. 14
Text: Human Physiology (Sherwood), 6th Ed.
Julie Balch Samora, MPA, MPH
[email protected]
293-3412, Room 3145
Review
• How is glucose transported?
• What is the renal threshold?
• Do we see this for passive transport?
• Do the kidneys regulate glucose/
phosphate plasma concentration?
Review cont…
• What is the vascular sequence in the
nephron?
• What is the tubular sequence?
Review cont…
• What happens if ↓ NaCl, ↓ ECF volume,
and ↓ arterial blood pressure?
Tubular Secretion
• Additional mechanism to eliminate certain
substances
• Secretion of H+, NH3, K+, organic anions
and cations
• Acid and ammonia secretion imp. in acidbase balance
• Ammonia is secreted during acidosis in
order to buffer the secreted H+
H+ and NH3 secretion
• Hydrogen and ammonia secretion aid with
acid-base balance
• H+ secretion provides a highly
discriminating mechanism for varying the
amt of H+ excreted in the urine, depending
on the acidity of the body fluids
• NH3 is secreted in pronounced acidosis to
buffer the H+ secreted into the urine
K+ Secretion
• K+ is actively reabsorbed in the proximal
tubule, but is also actively secreted in the
distal tubule.
• Allows fine degree of control over plasma
K+ concentration
• Secretion of K+ is variable and subject to
aldosterone control
• Even slight fluctuations in ECF [K+] can
alter nerve and muscle excitability
Organic anion and cation secretion
• The proximal tubule contains 2 different
carriers for secreting organic ions
• These systems aid in secreting foreign
organic substances
• The liver helps this process by converting
many foreign substances into anionic
metabolites
Process of Secretion
Renal Processes 1
Urine excretion
• The final quantity of urine formed
averages about 1ml/min
• This urine contains a high conc. of waste
products & low or no conc. of substances
needed by the body
• Minimum volume of urine to eliminate
wastes is 500 ml/day
Plasma Clearance
• The amount of plasma “cleared” of a
substance
• Plasma clearance for a substance that is
FILTERED, but not REABSORBED or
SECRETED == GFR == 125ml/min
• EXAMPLE = INULIN
INULIN- Filtered, NOT reabsorbed, NOT secreted
Renal Processes 2
Plasma Clearance
• Plasma clearance for a substance that is
FILTERED AND REABSORBED, but NOT
SECRETED < GFR
• EXAMPLE = GLUCOSE, UREA
• Clearance rate can be anywhere from 0 up
to normal clearance (125 ml/min)
depending on amount reabsorbed
Glucose/urea- Filtered and reabsorbed, NOT secreted
Renal Processes 2
Plasma Clearance
• Plasma clearance for a substance that is
FILTERED AND SECRETED > GFR
• EXAMPLE = PAH (Para-aminohippuric acid)
• Not only is the plasma that is filtered
cleared of that substance, but additional
amt cleared from plasma which was not
filtered
• Plasma clearance rate for PAH=RENAL
PLASMA FLOW
Plasma clearance rate for PAH = Renal Plasma Flow
Renal Processes 2
Plasma Clearance Questions
• What is the PC for a substance that is
FILTERED AND REABSORBED, but NOT
SECRETED?
• What is the PC for a substance that is
FILTERED AND SECRETED?
• What is the PC for a substance that is
FILTERED, but not REABSORBED or
SECRETED?
Osmolarity
• Measure of the concentration of individual
solute particles dissolved in fluid
• The higher the osmolarity, the higher the
concentration of solutes
• WATER moves by osmosis from an area
of lower osmolarity (higher water conc) to
an area of higher osmolarity (lower water
conc) until the concentration difference is
eliminated
Osmolarity
• Isotonic- solution is the same conc. as
normal body fluids (300 milliosmols/L)
• Hypotonic- solution is more dilute than
normal body fluids (< 300 milliosmols/L)
• Hypertonic- solution is more concentrated
than normal body fluids (> 300
milliosmols/L)
Question
• How can the kidneys put out a
concentrated urine when the body is
dehydrated?
• How can the kidneys excrete a dilute urine
when the body is overhydrated?
Proximal
tubule
Distal
tubule
Distal
tubule
Glomerulus
Bowman’s
capsule
Proximal
tubule
Cortex
Medulla
Collecting
duct
Descending
limb of
loop of
Henle
Loop of Henle
Other nephrons emptying into
the same collecting duct
Vasa recta
Ascending
limb of
loop of
Henle
To renal
pelvis
Fig. 14-5, p. 505
Countercurrent System
• There is a vertical osmotic gradient in the
interstitial fluid of the medulla
• This gradient, with the help of ADH
(vasopressin) allows the kidney to vary the
concentration of the urine depending on
the body’s needs
• Countercurrent multiplication- loops of
Henle and Juxtamedullary nephrons
• Countercurrent exchange- vasa recta of
the juxtamedullary nephrons
Countercurrent System
• At the end of the proximal tubule and
beginning the loop of Henle, the filtrate is
isotonic (300 mosm/L)
• Na+ is actively reabsorbed against its
concentration gradient in the proximal
tubule, creating an osmotic gradient,
pulling water across to maintain osmotic
equilibrium
Countercurrent Multiplication
• The loops of Henle of the juxtamedullary
nephrons are responsible for establishing
the concentration gradient in the renal
medulla
• Desc. limb is freely permeable to H2O, but
NOT NaCl – so H2O goes into IF
• The ascending limb actively transports out
NaCl
• Asc. Limb IMPERMEABLE to water
Medullary vertical
osmotic gradient
• There is a countercurrent flow produced
by the close proximity of the two limbs
• The ascending limb produces an interstitial
fluid that becomes hypertonic to the
ascending limb.
• This interstitial fluid faces against the flow
of fluid (countercurrent) in the descending
limb, attracting the water by osmosis for
reabsorption
Countercurrent Multiplication
• The fluid in the descending limb becomes
progressively more concentrated
• At the bottom, its concentration is 1200
mosm/L!!
• As the asc. Limb is impermeable to water,
when NaCl is pumped out, water remains,
causing the remaining liquid in the loop to
become hypotonic (100 mosm/L)
Countercurrent Multiplication
• Therefore, a vertical osmotic gradient
varying from 300 to 1200 mosm/L exists in
the medulla
• The filtrate as it leaves the loop of Henle to
enter the distal tubule is HYPOTONIC
• Gets its name due to the flow in opposing
directions through the two limbs and the
stepwise multiplication of concentration
due to transport and permeabilities
Countercurrent
Countercurrent exchange
• The vasa recta are responsible for
preserving the concentration gradient in
the medulla that has been established by
the loop of Henle
• The blood in the vasa recta follows the
same course as the loops of Henle of the
juxtamedullary nephrons.
• The vasa recta are exposed to the same
gradient in the IF-blood concentration is
similar
Countercurrent exchange
• This process is a passive, osmotic
exchange b/t the IF and two closely
opposed vessels with fluid flowing in
opposite directions
Importance of the Vertical Gradient
• The collecting ducts are normally
impermeable to water
• However, if the body needs to conserve
H2O, ADH (vasopressin) is secreted
• ADH increases the permeability of the
distal tubules and collecting tubules to
H2O
ADH- Saving Water
• Due to the vertical osmotic gradient, the
tubular fluid loses more water into the IF,
which eventually enters the plasma
• At the end of the collecting tubule, it is
possible to have urine concentrated to
1200 mosm/L (very small volume
excreted)
Role of ADH
• ADH is produced in the hypothalamus and stored in
the posterior pituitary. The release of this substance
signals the distal tubule and collecting duct,
facilitating the reabsorption of water.
• ADH works on tubule cells through a cyclic AMP
mechanism.
• During a water deficit, the secretion of ADH
increases. This increases water reabsorption.
• During an excess of water, the secretion of ADH
decreases. Less water is reabsorbed. More is
eliminated.
Elimination of excess H2O
• If overhydrated, no ADH released,
therefore distal tubules and collecting
ducts remain impermeable to H2O
• Even though water wants to leave (due to
osmotic forces), it cannot
• Therefore, a large volume of dilute urine is
excreted
Urine Concentration
Summary
• The medullary vertical osmotic gradient
and ADH allow the kidney to excrete urine
of varying concentrations
• Without this osmotic gradient, we could
not produce a concentrated urine, and
would be unable to conserve water
• We would likewise be unable to rid the
body of excess water w/o this system