Re absorption and secretion by nephron

urinary processing

1. DR. AMIR BAHADUR
Re-absorption and Secretion by
Nephron

2. Important to remember
Different types of cells and different
structures at different parts of
nephron causes different changes to
the filtrate
Difference of osmolality in various
parts of nephron causes changes to
filtrate
Different hormones/chemicals exert
their actions

3. Few basic facts
120mL/min-----GFR
1mL/ min--------Urine formed
119mL/min------Re-absorbed
172L/day---- filtered
1.5L/day------urine formed
Approx 170L---- re-absorbed

4. Filtrate
Water
Glucose
Amino acids
Bicarbonates
Sodium
Chloride
Phosphates

5. PCT structure

6. PCT--- series of events
On baso-lateral membrane are 3Na/
2K ATPase. 3 Na out to interstitium,
2K inside to PCT cell
K- leaky channels---- K outside to
insterstitium
Above two events, set the start by
Negative electro-chemical state inside
the PCT cell

7. Sodium in PCT
Concentration gradient for sodium
Electric gradient for sodium
Sodium transporters (co-transporters)
Facilitated transport
Other arm of co-transporter can attach
glucose, chloride or amino acid.
Sodium is transported from lumen to PCT
cells via facilitated diffusion
65% sodium is re-absorbed in PCT

8. Glucose & Amino Acids in PCT
Co-transporters in the luminal brush border
with one arm for sodium and other for
glucose and amino acid.
Glucose and amino acids are transported via
secondary active transport from lumen to the
PCT cells.
100% of glucose and amino acids are re-
absorbed in PCT.

9. Chloride in PCT
Chloride is re-absorbed by same way as
glucose and amino acids.
Only difference is that preference is given to
glucose and amino acid and later chlorine is
take up.
Chloride re-absorption from lumen to PCT is
secondary active transport.
65% of chloride is re-absorbed in PCT

11. Bicarbonate
Not really re-absorbed
Rather replaced
 hydrogen ions are transported from inside of
PCT to lumen of PCT via Na/H counter-transport
H combines with HCO3 in lumen to form
carbonic acid which dis-associates to CO2 and
H2O.
CO2 diffuse into PCT and combines with H2O to
form H and HCO3 ions.
HCO3 are transported to interstitium.

13. Water
Trans-cellular by the solutes
Para-cellular through tight junctions
Also solutes go along with water through
tight junctions
65% water is re-absorbed in PCT

14. Plasma proteins
Usually no plasma proteins are
filtered.
If filtered, PCT brush border has
affinity to get them attached
PINO-CYTOSIS and re-absorbed to
PCT cell

15. Secretion in to PCT lumen
Oxalates
Urates
Bile salts
Catecholamines
Toxins
Drugs
Secreted through special channels in the baso-
lateral surface. Transported across
concentration gradient from capp to
interstitium to PCT cell to Lumen.

16. Bit more in PCT
Minimal urea re-absorption across
concentration gradient.
PTH---- phosphate trashing hormone---
inhibits phosphate re-absorption.
Angiotensin II acts on PCT and increases
sodium re-absorption.
PCT cells also activates 25-hydroxy
cholecalciferol to 1,25 di-hydroxy
cholecalciferol

17. Leaving PCT
Same proportion of solutes and water are re-
absorbed in PCT hence; the osmolality of
filtrate entering and leaving PCT is SAME.
Though osmolality of the filtrate remains the
same, quantity of water and solutes
decreases.

18. Descending Loop of Henle
Cells are freely permeable to water
Cells are minimally active metabolically
No brush border and not much of ATPases
Descending limb is freely permeable to water
and interstitium is hyper-osmolar so water
re-absorbed.
20% of water here.

19. Descending Loop of Henle
Water going out, solutes retained and further
solutes coming in---- all across conentration
gradient.
Filtrate is concentrated so much that filtrate
osmolality equals that of interstitium.

20. Ascending Loop of Henle
Ascending Loop of Henle is totally
impermeable to water up to half of distal
convoluted tubule.
Hyper-osmotic filtrate reach thick part of
ascending Loop of Henle

21. Thick part of ALOH
On the baso-lateral membrane of this part,
3Na/2K ATPases and they create negative
gradient inside the cell.
The luminal membrane has Na-K-2Cl co-
transporter which pumps 1 sodium, 1
pottasium and 2 chlorides in to the cells from
lumen.
This sodium is pumped out in to the
interstitium by the ATPases.
Chloride moves to interstitium via chloride
channels.

22. Thick part of ALOH
K is getting in to the cell via Na/K ATPases
and also by Na-K-2Cl co-transport.
Cell is TOO RICH in potassium.
Potassium leaks to lumen via leaky channels
and make the lumen electro-positive.
This electro-positivity of lumen created by
potassium excess repels the calcium and
magnesium ions of the filtrate.
This repulsive force cause re-absorption of
Ca and Mg through tight junctions.

23. Thick part of ALOH
Processes of re-absorbing solutes with out
water causes…….
a)Increase of osmolality in interstitium
b)Decrease of osmolality in the tubules.
So this part is also called DILUTING SEGMENT

24. Thick part of ALOH
25% sodium is re-absorbed
25% chloride is re-absorbed
How much water?

25. Loop diuretics
Frusemide
Blocks Na-K-2Cl co-transport.
More sodium going next part
More Ca and Mg going next part
Less K going next part

26. Early Distal Convoluted Tubules
First half of EDCT have same roles as that of
ALOH
Diluting segment
At start of EDCT is present-------?
Mechanism of solutes re-absorption is bit
different.

27. EDCT
3Na/2K ATPases at baso-lateral membrane
Na/Cl co-transport channels at luminal
membrane instead of Na-K-2Cl co-
transporters
EDCT cell is negative inside, Na poor cell
5% Na is re-absorbed in DCT

28. Thiazide diuretic
Thiazide diuretics can block this Na/Cl co-
transporter and causes diuresis
Weak diuretic
Calcium conserving diuretic

29. EDCT---- calcium re-absorption
Para-thyroid hormonal acts on EDCT for re-
absorption of calcium.
PTH increases the activity of calcium pump
and Ca/Na exchanger at baso-lateral
membrane and make cell calcium poor
hence; increase calcium re-absorption via
calcium channels.

30. Late DCT & Cortical Collecting tubules
Discussed together due to same properties
Principal cells and inter-calated cells

31. Inter-calated Cells
Inter-calated cells are special cells meant for
hydrogen secretion.
H ions are produced in the inter-calated cells
by decomposition of carbonic acid and H ion
is transported to lumen by H-ATPases on
luminal membrane.
HCO3 produced is supplied to circulation
hence; called bicarbonate factory

32. Principal Cells
Baso-lateral membrane has 3Na/2K ATPases,
making cells Na poor.
Luminal membrane has Na and K channels.
Na moves to the principal cells across
electrical gradient.
Potassium moves out from cell to lumen
through K channels across electrical
gradient.

33. Principal Cells
Aldosterone acts on principal cells
Increases the ATPases activity on baso-lateral
membrane, making cells more poor in Na
and more rich in K
Also K channels are increased on luminal
membrane
Na channels are also increased and Na re-
absorption is increased.
Increased Na re-absorption, drag extra water
as well.

34. K- sparing Diuretics
Spironolactone
Blocks the action of aldosterone
Less K is secreted to the lumen.
Amiloride/ triamterine
Block Na channels
Less K is secreted to the lumen

35. Principal Cells
Anti-diuretic Hormone (ADH)
In absence of ADH, no water pores on
luminal membrane, water not absorbed.
In presence of ADH, water pores on luminal
side apparent and water re-absorption
increases.

36. Medullary Collecting Tubles
Some of the cells act as principal cells and
some act as inter-calated cells.
Final tunning of urine osmolality.
Special receptors for urea.
Urea is transported to inerstitium via
concentration gradient which is taken in to
the loop of Henle again and put in to the cycle
again.