Download kidney 2

Renal lecture 2
- Reabsorption
- Secretion
- Countercurrent multiplier
- Micturition (urination)
Reabsorption
From the tubules to PTC:
99% H2O
100% glucose, amino acids
99.5% Na+
50% urea.
Most of this occurs at proximal convoluted tubule.
Tubular Reabsorption
•  Involves the transfer of substances from tubular lumen into
peritubular capillaries
–  Reabsorbed substance must cross five barriers
•  Must leave tubular fluid by crossing luminal membrane of
tubular cell
•  Must pass through cytosol from one side of tubular cell to the
other
•  Must cross basolateral membrane of the tubular cell to enter
interstitial fluid
•  Must diffuse through interstitial fluid
•  Must penetrate capillary wall to enter blood plasma
Steps in Transepithelial Transport
Tubular Reabsorption
•  Passive reabsorption
–  No energy is required for the substance’s net
movement
–  Occurs down electrochemical or osmotic
gradients
•  Active reabsorption
–  Occurs if any one of the steps in
transepithelial transport of a substance
requires energy
–  Movement occurs against electrochemical
gradient
Na+ Reabsorption
Tubule area
% of Na+
reabsorbed
Role of Na+
reabsorption
Proximal
tubule
67%
Plays role in
reabsorbing
glucose, amino
acids, H2O, Cl-,
and urea
Ascending
limb of the
loop of Henle
25%
Plays critical role
in kidneys’
ability to produce
urine of varying
concentrations
Distal and
collecting
tubules
8%
Variable and
subject to
hormonal control;
plays role in
regulating ECF
volume
•  An active Na+ - K+ ATPase pump in basolateral
membrane is essential for Na+ reabsorption
•  Of total energy spent by kidneys, 80% is used for
Na+ transport
•  Water follows reabsorbed sodium by osmosis which
has a main effect on blood volume and blood
pressure
Sodium Reabsorption
Glucose Reabsorption
Glucose Reabsorption
Amino Acid Reabsorption
Amino acid Reabsorption
Tubular Secretion
•  Transfer of substances from peritubular
capillaries into the tubular lumen
•  Involves transepithelial transport
•  Kidney tubules can selectively add some
substances to the substances already
filtered
Tubular Secretion
•  Most important secretory systems are for
–  H+
•  Important in regulating acid-base balance
•  Secreted in proximal, distal, and collecting tubules
–  K+
•  Keeps plasma K+ concentration at 4.3 mmol/L to maintain
normal membrane excitability in muscles and nerves
•  Secreted only in the distal and collecting tubules under
control of aldosterone (lecture 3)
–  Organic ions
•  Accomplish more efficient elimination of foreign organic
compounds from the body
•  Secreted in the proximal tubule
K+ secretion occurs in cortical collecting tubule
and relies upon active transport of K+ across basolateral
membrane and passive exit across apical membrane into
tubular fluid.
Urine Excretion
•  Depending on the body’s state of hydration, the kidneys secrete
urine of varying concentrations."
"
•  Too much water in the ECF establishes a hypotonic ECF."
•  A water deficit establishes a hypertonic ECF."
•  A large, vertical osmotic gradient is established in the
interstitial fluid of the medulla (from 100 to 1200-1400 mOsm). This
increase follows the hairpin loop of Henle deeper into the medulla."
"
Countercurrent Multiplication
•  The medullary vertical osmotic gradient is established by
countercurrent multiplication"
"
•  The descending limb of loop of Henle is highly permeable to water
but impermeable to sodium for reabsorption."
•  The ascending limb actively transports NaCl out of the tubular lumen
into the surrounding interstitial fluid. It is impermeable to water.
Therefore, water does not follow the salt by osmosis."
•  There is a countercurrent flow produced by the close proximity of the
two limbs."
•  The ascending limb produces an interstitial fluid that becomes
hypotonic to the descending limb. It does this by pumping out
sodium ions. Water does not follow. This interstitial fluid faces
against the flow of fluid (countercurrent) in the descending limb,
attracting the water by osmosis for reabsorption"
Countercurrent Multiplier
•  fluid in ascending limb
becomes hypotonic as
solute is reabsorbed
•  fluid in descending limb
becomes hypertonic as it
loses water by osmosis
UREA
Plasma conc is 2.5-7.5 mmol/l
Contributes to interstitial osmotic pressure
Freely filtered
50% passively reabsorbed at PCT
Recycled
Urea recycling
• 
• 
• 
• 
Urea toxic at high levels,
but can be useful in small
amounts.
Urea recycling causes build
up of high [urea] in inner
medulla.
Under control of ADH (see
lecture 3)
This helps create the
vertical osmotic gradient at
loop of Henle so H2O can
be reabsorbed.
1400 mOsm
Hormonal regulation of
urine composition - tomorrow!
Urine Transport,
Storage, and Elimination
Takes place in the urinary tract:
–  ureters
–  urinary bladder
–  Urethra
Normal Urine
-  Is a clear, sterile solution
-  Yellow color (pigment urobilin) generated in kidneys
from urobilinogens
Urinary Bladder
•  hollow, muscular organ
•  Functions as temporary reservoir urine storage
•  Full bladder can contain 1 liter of urine
Sympathetic
Hypogastric nerve
L2
Inhibit detrusor
Contracts int. sphincter
Parasympathetic
Pelvic nerve
S2-4
Activates detrusor
Inhibits int. sphincter
Motor
Pudendal nerve
Activates ext sphincter
Sensory afferents
Stretch receptors in mucosa
Micturition Reflex
See also Lecture 5
3 Micturition Reflex Problems
1. 
Sphincter muscles lose tone:
–  leading to incontinence
2. 
Control of micturition can be lost due to:
–  a stroke
–  Alzheimer’s disease
–  CNS problems affecting cerebral cortex or
hypothalamus
3. 
In males, urinary retention may develop if enlarged
prostate gland compresses the urethra and restricts urine
flow
Renal system and ageing
•  kidneys appear scarred and grainy
•  kidney cells die
•  by age 80, kidneys have lost a third of their mass
•  kidney shrinkage due to loss of glomeruli
•  proteinuria may develop
•  renal tubules thicken
•  harder for kidneys to clear certain substances
•  bladder, ureters, and urethra lose elasticity
•  bladder holds less urine