This ppt only for study purposes

1. Mechanism of Urine
formation -II
Dr. Pandian M.
Dept. of Physiology
TUBULAR REABSORPTION AND SECRETION

2. Introduction
•Small proteins & some peptide hormones are
reabsorbed in the proximal tubules by
endocytosis.
•Other substances are secreted or reabsorbed in the
tubules by passive diffusion btw cells & through
cells by facilitated diffusion
•Chemical or electrical gradients or active
transport against such gradients.

3. •Movement is by way of ion channels,
exchangers, cotransporters, & pumps.
•Many of these have now been cloned, & their
regulation is being studied.
•Like transport systems elsewhere, renal active
transport systems have a maximal rate, or
transport maximum (Tm) , at which they can
transport a particular solute

4. Continue…..
•The 180 L glomerular filtrate formed per day, about
1.5 L (i.e. less than 1%) per day is excreted as urine.
•The different segments of the renal tubule viz.
•The proximal tubule,
•The loop of henle,
•The distal tubule,
•The collecting tubule, and, finally, the collecting
duct —before it is excreted as urine.

5. •The composition and volume of the urine by process
of selective reabsorption of solutes and water and
selective secretion of solutes.
•For understanding renal tubular reabsorption
and secretion can be considered in the following
subsections:
I. General principles of renal tubular transport,
II. Transport across different segments of renal
tubule,
III. Tubular transport of common solutes and water.

6. I. General principles of renal tubular transport

7. •For a substance to be reabsorbed, it must first
be transported
(1)across the tubular epithelial membranes
into the renal interstitial fluid and then
(2)through the peritubular capillary
membrane back into the blood

8. Transport mechanisms across cell membrane
•The water moves across the cell
membrane of renal tubular cells
passively, while the solute movement
occurs by both passive and active
mechanisms.

9. •Passive transport does not need energy and
occurs spontaneously, down an
electrochemical gradient by following
mechanisms:
• Diffusion,
• Facilitated diffusion (channels, uniport,
coupled transport, uniport or symport) and
Solvent drag.

10. •Active transport requires direct input of
energy and is abolished if cell
metabolism is inhibited.
•Active transport can occur against an
electrochemical gradient.
•Most of the active transports are carrier
mediated.

11. Transepithelial transport pathways
•In the renal tubule, a substance can be reabsorbed
or secreted by two pathways
•Transcellular pathway
•Paracellular pathway

13. •Transcellular pathway refers to the transport through
the cells.
•e.g. includes transcellular Na+ reabsorption by the
proximal tubule, by two process:
• Movement of Na+ into the cell across the apical
membrane occurs down an electrochemical gradient
established by Na+−K+−ATPase.
• Movement of Na+ into the ECF across the basolateral
membrane occurs against an electrochemical gradient
via Na+−K+−ATPase.

14. •Paracellular pathway refers to the transport
between the cells.
•E.g. of paracellular pathway include:
• Reabsorption of Ca2+ and K+ across the proximal
tubule, Some of the water reabsorbed across the
proximal tubule crosses the paracellular pathway
and
• Some solutes dissolved in this water (in particular
Ca2+, and K+) are carried along with the reabsorbed
fluid across the paracellular pathway by the process
of solvent drag.

15. I. General principles of renal tubular
transport,
II. Transport across different segments
of renal tubule,
III. Tubular transport of common
solutes and water.

16. Describe the reabsorption taking place in the
proximal convoluted tubule.
Reabsorption
Active Passive
Nonreabsorption Secretion
Na+ Cl− Inulin H+
K+ HCO3− Creatinine Water
Ca2+ HPO4 Sucrose Penicillin
Mg2+ Water Mannitol Sulphonami
de
HPO4
2
− Urea Creatinine
NO3− Water

17. Reabsorption
Active Passive
Nonreabsorption Secretion
Glucose
Amino acids
Protein
Urate
Vitamins
Acetoacetate

18. 1. Sodium.
•About 65% of filtered Na+ is reabsorbed in the
proximal tubules.
•There are two mechanisms for reabsorption.
(a) Primary active transport or uniport mechanism.
This is the active transport of sodium through
basolateral surface of epithelial cell by Na+-K+
pump.

19. •This causes lowering of Na+ concentration in
the epithelial cell and allows diffusion of Na+
from lumen into the cell through luminal
surface.
•This mechanism also helps in absorption of
glucose and amino acids along with sodium in
Na+-co-transport mechanisms.
•It also causes osmotic absorption of water.

20. (b) Antiport mechanism.
•This is sodium counter-transport
mechanism.
•When sodium ion diffuses from lumen to
epithelial cell, some other ion (H+ or K+) is
transported from cell to the lumen.

21. Mechanisms of secondary active transport. The upper
cell shows the co-transport of glucose and amino
acids along with sodium ions through the apical side
of the tubular epithelial cells, followed by facilitated
diffusion through the basolateral membranes. The
lower cell shows the counter-transport of hydrogen
ions from the interior of the cell across the apical
membrane and into the tubular lumen; movement of
sodium ions into the cell, down an electrochemical
gradient established by the sodium-potassium pump
on the basolateral membrane, provides the energy for
transport of the hydrogen ions from inside the cell into
the tubular lumen.

23. 2. Chloride and bicarbonate ions.
•Active absorption of positively charged
ions causes passive absorption of
negatively charged ions (anions) mainly
chloride (Cl-) and to a lesser extent
bicarbonate ions (HCO-
3).

24. 3. Water.
•Absorption of water is secondary, mainly to
sodium (osmotic absorption).
•About 65% of water is absorbed in the
proximal tubules.
•This is termed obligatory water reabsorption
as it cannot be changed according to the needs
of the body.

25. 4.Glucose. Filtered glucose is almost completely
reabsorbed in proximal tubules by sodium co-transport
mechanism.
5. Phosphate. It is actively absorbed in proximal
tubules.
6. Proteins. Very small amount of protein is filtered, &
is completely absorbed by proximal tubules by
pinocytosis.

26. 7.Potassium. Almost all K+ filtered is
reabsorbed by proximal tubules (potassium
excreted in urine comes from secretion in the
distal segment of nephron).
8. Sulphates, urates and lactates. These are
reabsorbed by active transport mechanisms.

27. 9. Uric acid and urea.
•Its mainly reabsorbed in proximal tubules; only
10% of filtered uric acid is excreted.
•Large amount of filtered load of urea is reabsorbed
in proximal tubules, by passive absorption.
•When water is reabsorbed, there is increase in
concentration of urea in the tubular fluid.

28. •This creates a high concentration gradient for urea
between tubular fluid & peritubular fluid.
•This causes reabsorption of urea.
•Thus, urea reabsorption depends on water and salt
reabsorption.
•Greater is the salt and water reabsorption, greater is
the gradient produced and greater is the reabsorption
of urea.
•Urea reabsorption also depends on rate at which
tubular fluid flows in the tubule.

29. *Cellular ultrastructure and
primary transport
characteristics of the
proximal tubule.
*The proximal tubules
reabsorb about 65 per cent
of the filtered sodium,
chloride, bicarbonate, and
potassium and
essentially all the filtered
glucose and amino acids.
*The proximal
tubules also secrete
organic acids, bases, and
hydrogen ions into the
tubular lumen.

30. Transport across loop of Henle
•The loop of Henle consists of three functionally
distinct segments:
•the thin descending segment,
•the thin ascending segment, and
•the thick ascending segment.
•The thin descending and thin ascending segments, as
their names imply, have thin epithelial membranes
with no brush borders, few mitochondria, and
minimal levels of metabolic activity

32. Transport across loop of Henle
Q. Name substances which are reabsorbed in loop of Henle.
• In the descending limb of LOH,
there is passive reabsorption of
water (because of hypertonic
interstitial fluid).
• It is accompanied by diffusion of
Na+ions from interstitial fluid into
tubular lumen.
• Ascending limb of LOH is
impermeable to water and
therefore fluid leaving
• ascending limb is hypotonic
relative to plasma.

33. •In thick part of ascending
limb, Na+ions are actively
absorbed along with Cl-
& K+ ions.
•In thin part of ascending
limb, movement of the
salt is passive and it
occurs passively down a
concentration gradient.
•Urea may be passively
absorbed in the loop.

35. • Mechanisms of Na+, Cl-, and K+ transport in the thick ascending loop of
Henle.
• The Na+ - K+ ATPase pump in the basolateral cell membrane maintains a
low intracellular Na+ concentration and a negative electrical potential in
the cell.
• The 1-sodium, 2-chloride, 1-potassium co-transporter in the luminal
membrane transports these three ions from the tubular lumen into the
cells, using the potential energy released by diffusion of sodium down an
electrochemical gradient into the cells.
• Na+ is also transported into the tubular cell by Na+ - H+ counter-
transport.
• The positive charge (+8 mV) of the tubular lumen relative to the
interstitial fluid forces cations such as Mg++ and Ca++ to diffuse from the
lumen to the interstitial fluid via the paracellular pathway.

36. Approximately 7% of the filtered NaCl and about
8–17% of water is reabsorbed and K+ and H+ are
secreted in these segments.
Transport across distal tubules
and
collecting duct

40. Medullary Collecting Duct
• Medullary collecting ducts are
permeable to urea.
• Urea is therefore reabsorbed
which helps to raise osmolarity
of medullary interstitium.
• Permeability of these tubules
to water is controlled by
hormone ADH.
• These ducts secrete hydrogen
ions against a large
concentration gradient as in
case of cortical collecting
ducts.
• This is important in regulation
of pH of body fluids.

42. Hormonal Control of Tubular Reabsorption

43. Referred :-
• Text book of Medical Physiology
• Guyton, 14th edition,
• Text book of Medical Physiology
• Indu khurana,
• Text book of Medical Physiology
• Vander’s
• Text book of Medical Physiology
• Sembulingam &
• LPR

44. THANK YOU . . .