Features of PCTPCT have high capacity of active & passive re-absorption. This is due to special cellular features of epithelial cells. They have increased no. of mitochondria due to high metabolic activity. brush border on luminal (apical) side. Brush border contains protein carrier molecules to transport Na+ by co-transport mechanism with other substances (a.acids, glucose etc). Additional sodium is transported by COUNTER-TRANSPORT that reabsorb sodium while secreting hydrogen. About 65 % of filtered load of Na+ & water is reabsorbed in PCT. A lower % age of Cl- is also absorbed. In 1st half of PC tubules, Na+ is re-absorbed by co-transport along with glucose, a.acids and other solutes. In 2nd half of PC tubules, mainly Na+ is reabsorbed with Cl- and some of glucose + a.acids remain un-absorbed. 2nd half of PCT has high conc of Cl- (140 mEq/L) as compared to 1st half (105 mEq/L).

1. REABSORPTION IN RENAL TUBULE

2. PROXIMAL
CONVULATED
TUBULE

3. Reabsorb about 65 percent of the filtered sodium, chloride, bicarbonate, and potassium
and essentially al the filtered glucose and amino acids.
Secrete organic acids, bases, and hydrogen ions into the tubular lumen.

4. Features of PCT
• PCT have high capacity of active & passive re-absorption.
• This is due to special cellular features of epithelial cells.
• They have increased no. of mitochondria due to high
metabolic activity.
• brush border on luminal (apical) side.
• Brush border contains protein carrier molecules to transport
Na+ by co-transport mechanism with other substances
(a.acids, glucose etc).
• Additional sodium is transported by COUNTER-
TRANSPORT that reabsorb sodium while secreting
hydrogen.
• About 65 % of filtered load of Na+ & water is reabsorbed in
PCT.
• A lower % age of Cl- is also absorbed.
• In 1st half of PC tubules, Na+ is re-absorbed by co-
transport along with glucose, a.acids and other
solutes.
• In 2nd half of PC tubules, mainly Na+ is reabsorbed
with Cl- and some of glucose + a.acids remain un-
absorbed.
• 2nd half of PCT has high conc of Cl- (140 mEq/L) as
compared to 1st half (105 mEq/L).

5. Secretion of substances in PT
• Important substances resulting from metabolism, drugs, toxins etc are secreted in PT.
• These substances include organic acids & bases like bile salts, oxalate, urate,
catecholamines.
• These substances need rapid excretion in urine.
• Para-amino-hippuric acid (PAH) is rapidly secreted by PT.
• 90 % of PAH is cleared from plasma flowing through kidneys and excreted in urine.
• Due to this, rate of PAH clearance may be used to estimate renal plasma flow.

6. Re-absorption of HCO3:
• HCO3 is mainly absorbed in PC tubules.
• CO2 present in tubular cells under
carbonic anhydrase goes following
reaction.
• CO2 + H2O → H2CO3
• H2CO3 ionizes into H+ and HCO3.
• H+ passes into tubular lumen and
HCO3 diffuses into peritubular fluid
with the help of specific HCO3
transporter.
• When H+ passes into tubular
lumen, it is exchanged with Na+ by
counter-transport system.

7. LOOPS OF HENLE
Functionally consists of
i- thin descending segment,
ii- thin ascending limb and
iii- thick ascending segment.

8. Loops of Henle
• Thin descending segment and thin ascending
segments have thin epithelial membranes with no
brush borders, few mitochndria. Function is : is to
permit simple diffusion of various substances
through its walls.
 Thin descending segment is highly permeable to
water & moderate permeable to other solutes
(including urea, Na+).
 20 % of filtered is re-absorbed in thin descending
limb
 Thin & thick parts of ascending limb
are impermeable to water.
 This characteristic feature is of
great significance for concentrating
urine.

10.  Thick segment has high metabolic activity and
capable of active re-absorption of Na+, K+, Cl-.
 25 % of filtered load of these ions are absorbed
in LOH & mainly in thick ascending limb.
• Some amount of Ca++, HCO3- & Mg++ are also
absorbed here.
• An important part of solute re-absorption in thick
ascending limb is Na+K+ ATP ase pump present in
baso-leteral memb epithelial cells.
• Re-absorption of solute in thick ascending limb
related to re-absorptive capability of Na+K+ ATP
ase pump.
• Movement of Na+ across luminal membrane in thick
ascending loop is mainly mediated by a co-
transporter,1-Na, 2CI, 1 K.
• Thick ascending limb of LOH is the site of action of
loop diuretics (frusamide, bumetanide etc).
• All these diuretics inhibit the action of Na+ 2Cl- K+
co-transporter.
• There is slight back leak of K+ into lumen which
creates a +ve charge of about 8 mv in tubular lumen.
• This +ve charge forces cations such as Mg++, Ca++
to diffuse from tubular lumen through para-cellular
space into interstitial fluid.

11. • Thick ascending limb of LOH also bears a Na+ - H+ counter transport system in
luminal cell membrane.
• This mechanism mediates Na+ re-absorption and H+ secretion in thick limb of LOH.
• Thick segment of ascending limb of LOH is impermeable to water.
• Tubular fluid in ascending limb is very dilute when it flows towards distal tubule.

12. Mechanism of transport of Na+, Cl- & K+ in
thick ascending LOH

14. DISTAL
CONVULATED
TUBULES

15. DISTAL TUBULE
 EARLY DISTAL TUBULE
 LATE DISTAL TUBULE

16. EARLY DISTAL TUBULE
• Forms macula densa (juxtaglomerular complex)
• Reabsorbs sodium, potassium and chloride.
• 5 % of filtered load of NaCl is re-absorbed in
early distal tubule.
• NaCl co-transporter moves NaCl from tubular
lumen into cell.
• Na+K+ ATP ase pump moves Na+ out of cell
across baso-leteral membrane.
• There is a group of diuretics known as
thiazide diuretics, commonly used to treat
conditions like heart failure, hypertension
etc.
• These diuretics act by inhibiting NaCl co-
transporter.
• Early distal tubule is impermeable to water and
urea. For this reason it is called as “diluting
segment”.

17. Mechanism of transport of NaCl in early distal
tubule.

18. LATE DISTAL TUBULE
• The second half of the distal tubule and the subsequent cortical collecting
tubule have similar features….
• They are impermeable to urea.
• Consists of two types of cells.
1. principal cells
2. Intercalated cells

19. • PRINCIPAL CELLS:
• Re-absorb Na & water from lumen & secrete K+
into lumen.
• Na+ re-absorption & K+ secretion depends on
activity of Na-K ATP ase pump.
• Principle cells are main sites of action of a
group of diuretics known as potassium-sparing
diuretics (spironolactone, amiloride etc).
• POTASSIUM SPARING DIURETICS
1. ALDOSTERONE ANTAGONISTS
* Spironolactone
* Eplerenone
inhibit NA – K ATPASE PUMP
2. SODIUM CHANNEL BLOCKERS
* Amiloride
* Triamterene
inhibit sodium channel blockers

20. • INTERCALATED CELLS
• ii- intercalated cells which reabsorb potassium
and bicarbonate ions and secrete H+ into tubular lumen.
• H+ secretion is mediated by H+ - ATP ase transport
mechanism.
• pLAY IMPORTANT ROLE IN ACID-BASE REGULATION.

21. COLLECTING
DUCTS

22. Medullary Collecting Duct
• The medullary collecting ducts reabsorb less than 10 per cent of the filtered water and sodium, they are the
final site for processing the urine .
• Features:
1. ADH level controls permeability of medullary collecting ducts to water.
↑ ADH, ↑ water re-absorption, ↓ urine volume & conc of most of solutes in urine.
2- Medullary collecting ducts are permeable to urea, re-absorbing some of tubular urea into medullary
interstitium.
• 3- Medullary collecting ducts are capable of secreting H+ against a large conc gradient.
• So med. collecting ducts play very important role in regulating acid-base balance.

23. Cellular ultra-structure & features of
medullary collecting ducts
Medullary collecting ducts actively
re-absorb Na+ and secrete H+ and
permeable to urea.

24. • Urea permeability is high in inner medullary CT but no permeability in cortical or outer
medullary ducts.
• ↑ % age of water re-absorbed, solute becomes more concentrated.
• ↑ % age of solute re-absorbed, substance becomes more diluted

25. Permeability of different parts of
renal tubules to water & urea
Part of renal tubule Permeability to urea Permeability to water
PC tubule high High
Descending limb of
LOH
slight High
Thin ascending limb of
LOH
Decreases gradually Decreases gradually
Thick ascending limb
of LOH
NIL NIL
DC tubule Either absent or very
low
Low but more if ADH
present
Collecting ducts High if ADH present High if ADH present