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Lecture – 3
Dr. Zahoor
1
TUBULAR REABSORPTION

All plasma constituents are filtered in the
glomeruli except plasma protein.

After filtration, essential material and
electrolytes needed are reabsorbed but
waste products are eliminated.

Tubular reabsorption is highly selective
process.
2
3
TUBULAR REABSORPTION
4
TUBULAR REABSORPTION
GFR is 125ml/min, out of this 124ml/min
is reabsorbed.
 1ml/min is excreted.

IMPORTANT
 99% of water is reabsorbed.
 99.5% Na+ is reabsorbed.
 100% glucose is reabsorbed.
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TUBULAR REABSORPTION

In tubule, there is passive and active
reabsorption of different substances.
What is Passive Reabsorption?
 Passive Reabsorption occurs from tubular
lumen to the plasma (trans epithelial
transport), when no energy is spent.
Movement occurs due to electro-chemical or
osmotic gradient.
6
TUBULAR REABSORPTION
What is Active Reabsorption?
 Active Reabsorption is, when energy is
required for trans epithelial transport i.e.
when there is movement of substance from
tubular lumen to plasma against electro
chemical gradient.

E.g. Na+ , Glucose, Amino acid, Phosphate
(PO4-3)
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Steps of Transepithilial transport
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TUBULAR REABSORPTION
What is Trans epithelial transport of substance?
 It involves following 5 steps:
1). Substance must leave tubular fluid by crossing
luminal membrane of tubular cell.
2). Substance must pass through one side of tubular
cell to the other.
3). Substance must cross basolateral membrane of
tubular cell to enter interstitial fluid.
4). Substance must diffuse through the interstial fluid.
5). It must penetrate the capillary wall to enter blood
plasma.
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TUBULAR REABSORPTION
We will discuss Na+ reabsorption.

Na+ reabsorption is active process i.e.
requires Na+ - K+ ATPase pump in the
basolateral membrane.
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Sodium Reabsorption
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Na+ REABSORPTION
Na+ is filtered and 99.5% of Na+ is
reabsorbed in the tubule.
 Na+ reabsorption in different areas of
tubule.
- Proximal convoluted tubule-- 65-67%
- Loop of Henle (thick ascending limb) –
25%
- Distal and Collecting Tubule – 8%

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OVERALL
HANDLING
OF NA+
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Na+ REABSORPTION

Na+ Reabsorption in proximal
convoluted tubule helps in reabsorption
of glucose, amino acid, H2O, Cl-, urea

Na+ Reabsorption in Loop of Henle
occurs with Cl- reabsorption.

Na+ Reabsorption in DCT and CT is
under control of hormone Aldosterone.
14
Na+ REABSORPTION
IMPORTANT
 Na+ is reabsorbed throughout the tubule
except descending limb of Loop of
Henle, because it is impermeable to Na+
15
Na+ REABSORPTION
Na+ reabsorption in DCT and CT is
under control of hormone Aldosterone.
 With Na+, Cl- is passively absorbed
down its concentration gradient.

We will study Renin-AngiotensinAldosterone System (RAAS) and
Atrial Natriuretic Peptide in regulation
of sodium.
16
Dual Control
of
Aldosterone
Secretion By
K+ and Na+
17
Renin
Angiotensin
Aldosterone
System
(RAAS)
18
Na+ REABSORPTION IN DCT & CT

There are 2 types of cells located in DCT and CT
i. Principal Cells
ii. Intercalated Cells
Where Aldosterone acts in DCT and CT?
Aldosterone acts on Principal Cells
 They are in large number and Aldosterone acts
on them and causes Na+ reabsorption and K+
secretion.
 Main site of action of Aldosterone is DCT and
CT.
19
Na+ REABSORPTION
Intercalated Cells
 They are concerned with acid base
balance and we will discuss later.
ii.
IMPORTANT
 In DCT and CT, 8% of filtered Na+ depends
on Aldosterone for reabsorption.
 If no aldosterone, 20g of NaCl maybe lost
per day.
20
APPLIED
Role of Renin Angiotensin Aldosterone System
(RAAS) in various diseases

RAAS activity if abnormally increased can cause
hypertension.

RAAS is also responsible for fluid retention and
EDEMA occurring in congestive heart failure.

Angiotensin Converting Enzyme inhibitor – ACE
inhibitor drugs are used for hypertension and
congestive heart failure.
21
APPLIED

Drugs that affect Na+ reabsorption
1. Diuretics
- They cause diuresis (increased urinary
output) by inhibiting tubular reabsorption of
Na+.
- As Na+ is lost, more water is lost, therefore,
they help to remove excess extra cellular
fluid.
22
ATERIAL NATRIURETIC PEPTIDE
(ANP)

Hormone ANP causes Na+ loss, therefore,
decreases BP.

Natriuretic means inducing Na+ loss in
urine.


Site of production of ANP – Atria of heart
Site of action of ANP – distal part of
nephron (DCT and CT), causes decreased
Na+ reabsorption, therefore, increased Na+
and water loss in urine.
23
ATERIAL NATRIURETIC PEPTIDE
(ANP) (cont)

Other action of ANP are
- inhibits aldosterone secretion from adrenal
cortex
- inhibits renin secretion, therefore, has negative
effect on RAAS
- inhibits vasopressin secretion and its action,
therefore causes decreased water reabsorption
- dilates afferent arteriole and constricts efferent
arteriole, therefore, increases GFR
- relaxes glomerular mesangial cells, therefore,
increased Kf – increase GFR
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25
WATER REABSORPTION &
EXCRETION
Normal GFR 125ml/min
or 180 liters/day
 99 – 99.7% water is reabsorbed
 Average urine volume – 1 liter/day
 Minimum urine needed per day to get rid
of waste products – 500ml/day

26
WATER REABSORPTION
Water is reabsorbed through water
channels, made up of proteins called
Aquaporins.
Water reabsorption in the tubule
 Proximal convoluted tubule (PCT) – 60-70%
It is passive, due to osmotic gradient due to
active reabsorption of solutes e.g. Na+

27
WATER REABSORPTION

Loop of Henle – 15% of water is
reabsorbed
IMPORTANT
 Descending limb of Loop of Henle [LH] is
permeable to water but ascending limb of
Loop of Henle is impermeable to water
 Because of this fluid in the descending limb
of LH becomes hypertonic and fluid in
ascending limb of LH becomes hypotonic
28
Osmolari
ty of fluid
in
different
segment
s of
tubule
29
WATER REABSORPTION
Distal Convoluted Tubule (DCT) and
Collecting Tubule (CT) – 20% of filtered
water is reabsorbed.
 DCT – 5% water reabsorbed
 CT – 15% water reabsorbed

IMPORTANT
 In DCT and CT, water is reabsorbed under
the action of ADH (AntiDiuretic Hormone)
or Vasopressin
 ADH main site of action is CT
30
WATER REABSORPTION
ADH acts on DCT and CT
 There are Aquaporin – 2 (protein water
channels) in DCT and CT, principal cells
 Aquaporin – intracellular protein are
stored in vesicles in the cytoplasm of
principal cells
 Vasopressin causes rapid insertion of
these vesicles in luminal membrane of
principal cells
31
WATER REABSORPTION
This action of vasopressin (ADH) is
mediated by binding of ADH to V2
receptors
 V2 receptors are G-protein which
activate cAMP – second messenger
system
 As ADH causes water reabsorption in
collecting tubules, fluid becomes
hypertonic and urine passed is
concentrated.

32
Mechanism
of action of
Vasopressin
33
ADH OR VASOPRESSIN
ADH is hormone synthesized in
hypothalamus, supraoptic nuclei mainly,
but also in para- ventricular nuclei
 They have axonal connection to
posterior pituitary
 ADH once synthesized in hypothalamus
is transported via axon to posterior
pituitary and stored there in posterior
pituitary.

34
WATER REABSORPTION
Q. If ADH is absent, what will happen?
 If no ADH, collecting tubule epithelium is
relatively impermeable to water in
absence of ADH, therefore, large
amount of dilute urine will be excreted.
 Urine flow may increase to 15ml/min or
22liters/day.
35
APPLIED
DIABETES INSIPIDUS
Diabetes Insipidus [DI] is of 2 types:
1. Central DI - occurs due to deficiency of
ADH
2. Nephrogenic DI – occurs when V2
receptors in collecting tubule fail to
respond to ADH

In both cases, person will pass dilute
urine up to 22 liters/day
36
WATER
DIURESIS
37
BODY
RESPONSE
TO
DECREASED
WATER
INTAKE
38
IMPORTANT POINTS
Water reabsorption is by osmosis and is
dependent upon Na+ reabsorption, but
water absorption depends on ADH in DCT
and CT
2. Na+ is absorbed actively by all tubular
segments except descending limb of Loop
of Henle, which is impermeable to Na+.
3. Descending limb of LH is permeable to
water, but ascending limb of LH is
impermeable to water
1.
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THANK YOU
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