transport of sodium , K ,

1. Renal Transport Mechanisms
DR KIRAN KUMAR C
MBBS.,MD.,FAGE.,C DIAB
ASSISTANT PROFESSOR
DEPARTMENT OF PHYSIOLOGY

2. • Mental integrity is a sine qua non of the free and independent
life. As intermittent rays of light blend into moving images on
the cinematographic screen, so the multiform activities within
the brain are integrated into images of consciousness and
brought into an unstable focus to form that fleeting entity
which we call personality, or self.
• But let the composition of our internal environment suffer change,
let our kidneys fail for even a short time to fulfill their task, and
our mental integrity, our personality, is destroyed.
• HOMER WILLIAM SMITH

3. HOMER WILLIAM SMITH
• Renal clearance methods,
• Non-invasive methods for the
measurement of glomerular
filtration rate, of renal blood
flow
• Tubular transport capacity,
and provided novel insights
into the mechanisms of
excretion of water and
electrolytes
January 2,1895-March 25,1962

4. GENERAL PRINCIPLES
• Solute movement across the
membrane occurs by
1. Active transport
2. Passive transport
3. Endocytosis
• Diffusion of water occurs
1. Water channels

5. Solvent Drag
• Absorption of solute
along with water

6. Transcelluar Pathway and Paracellualr pathway
• Paracellular transport refers to the transfer of substances across an
epithelium by passing through the intercellular space between the
cells.
• Transcellular transport, where the substances travel through the cell,
passing through both the apical membrane and basolateral
membrane

7. Reabsorption Along The
Nephrons

8. PROXIMAL TUBULES
• Reabsorbs 67% of the filtered water, Na+ , K+ , Cl- and other
solutes.
• Also absorbs glucose and amino acids.
• Key element in the proximal tubule absorption is Na+ K+
ATPase in the basolateral membrane.

9. Na+ absorption
• Na+ is absorbed by different mechanisms in first and second half of
the proximal tubules.
• In the first half
• Na+ is reabsorbed along with HCO3 and number of other solutes
• Na+ uptake into the cell is couples with either H+ or organic solutes

12. • Second half of the proximal tubule
• Na+ is reabsorbed along with Cl-

13. Absorption of Water
• 67%
• Trans tubular osmotic
gradient is the driving force
• PT is highly permeable to
water
• Osmosis
• Aquaporin's
• OBLIGATORY WATER
TRANSPORT

14. Absorption of proteins
• Proteins filtered across the glomerulus are reabsorbed in
the proximal tubule.
• Peptide hormones, small proteins, and small amounts of
large proteins such as albumin are filtered by the
glomerulus.

16. • It is normal to find trace amounts of protein in the urine.
• Trace amounts of protein in the urine can be derived from two
sources:
(1) filtration and incomplete reabsorption by the proximal tubule and
(2) synthesis by the thick ascending limb of the loop of Henle.
• Cells in the thick ascending limb produce Tamm-Horsfall glycoprotein
and secrete it into the tubular fluid.
• However, more than trace amounts of protein in the urine is
indicative of renal disease.

17. Transport across Henle’s Loop
• Reabsorbs 25% of the filtered Nacl
• Reabsorbs 15% of the water
• Reabsorption of NaCl in the loop of Henle occurs in
both thin and thick limbs of ascending limb
• Thin descending limb is not permeable to Nacl

18. • Water reabsorption occurs in descending limb through
aquaporin channels
• Ascending limb is impermeable to water
• Ca+2 and HCO3- are also absorbed

21. Transport across Distal tubules and Colleting
Duct
• 8% of the filtered Nacl reabsorbed
• 8%-17% of the Water reabsorbed
• Secrete variable amount of K+ and H+

23. Transport across the collecting duct
• The collecting tubule consists of 2 types of cells
1. Principle cell
2. Intercalated cell
• Principle cell reabsorbs NaCl and H2O
• Secretes K+
• Intercalated cell secretes either H+ or HCO3-
• Important in regulating acid base balance

24. ENaC

27. Absorption of water
• In the absence of ADH
• In the presence of ADH

29. Urea transport
• The kidney filters, reabsorbs, and secretes urea
• The liver generates urea from NH+4, the primary nitrogenous end
product of amino acid catabolism.
• The primary route for urea excretion is the urine, although some urea
exits the body through the stool and sweat.

30. • The kidney freely filters urea at the glomerulus, and then it both
reabsorbs and secretes it.
• Because the tubules reabsorb more urea than they secrete, the
amount of urea excreted in the urine is less than the quantity filtered
• The primary sites for urea reabsorption are the proximal tubule and
the medullary collecting duct,
• The primary sites for secretion are the thin limbs of the loop of Henle.

32. • Urea transport depends primarily on urea concentration differences
across the tubule epithelium,
• Changes in urine flow unavoidably affect renal urea handling
• At low urine flow, when the tubule reabsorbs considerable water and
therefore much urea, the kidneys excrete only ~15% of filtered urea
• The kidneys may excrete as much as 70% of filtered urea at high urine
flow, when the tubules reabsorb relatively less water and urea

33. Glucose transport
• The fasting plasma glucose concentration
70 to 100 mg/dL and is regulated by insulin
and other hormones.
• The kidneys freely filter glucose at the
glomerulus and then reabsorb it, so that
only trace amounts normally appear in the
urine

34. • The proximal tubule reabsorbs nearly all the
filtered load of glucose,
• In the proximal tubule, luminal [glucose] is
initially equal to plasma [glucose].
• As the early proximal tubule reabsorbs
glucose, luminal [glucose] drops sharply,
falling to levels far lower than those in the
interstitium.
• Accordingly, glucose reabsorption occurs against a concentration gradient
and must therefore be active.

36. Glucose excretion in the urine occurs only when
the plasma concentration exceeds a threshold
The glucose titration curve is
obtained experimentally by infusing
glucose and measuring its rate of
reabsorption as the plasma
concentration is increased

37. •Filtered load.
•Glucose is freely filtered across glomerular capillaries, and the
filtered load is the product of GFR and plasma glucose concentration
•
• filtered load = GFR × [P]x
•
•Thus, as the plasma glucose
concentration is increased, the filtered
load increases linearly.

38. •Reabsorption.
•At plasma glucose concentrations less than 200 mg/dL, all of the
filtered glucose can be reabsorbed because Na+-glucose
cotransporters are plentiful.
•In this range, the curve for reabsorption is
identical to that for filtration; that is,
reabsorption equals filtration.
•The number of carriers is limited

39. • At plasma
concentrations
above 200 mg/dL,
the reabsorption
curve bends
because some of the
filtered glucose is
not reabsorbed.

40. •Excretion.
•Below plasma glucose concentrations of 200 mg/dL, all of the
filtered glucose is reabsorbed, and none is excreted.
•At plasma glucose concentrations
above 200 mg/dL, the carriers are
nearing the saturation point.
•Most of the filtered glucose is
reabsorbed, but some is not; the
glucose that is not reabsorbed is
excreted

41. •The plasma concentration at which glucose is first excreted in
the urine is called threshold, which occurs at a lower plasma
concentration than does Tm.
•Above 350 mg/dL, Tm is reached and the carriers are fully
saturated.
•The curve for excretion now increases linearly as a function of
plasma glucose concentration, paralleling that for filtration

42. • The Tm for glucose is approached gradually, rather than
sharply , a phenomenon called splay.
• Splay is that portion of the titration curve where reabsorption is
approaching saturation, but it is not fully saturated.
• Because of splay, glucose is excreted in the urine (i.e., at
threshold) before reabsorption levels off at the Tm value

43. • Low affinity of the Na+-glucose cotransporter
• Tm for the whole kidney reflects the average Tm of all
nephrons,
• All nephrons do not have exactly the same Tm.
• Some nephrons will reach Tm at lower plasma concentration
than others, and glucose will be excreted in the urine before
the average Tm is reached

44. Diuretics

45. • Diuretics, as the name implies, are drugs that cause an increase in
urine output
• All diuretics have as their common mode of action the primary
inhibition of Na+ reabsorption by the nephron.
• Consequently, they cause an increase in the excretion of Na+, termed
natriuresis