The kidneys regulate fluid volume through urine production and reabsorption. They can conserve water by producing concentrated urine or eliminate excess water by producing dilute urine. Hormones like ADH, aldosterone, and ANH control this. Sodium levels are regulated by the renin-angiotensin-aldosterone system and ADH. When sodium is retained, potassium is excreted and vice versa. Reabsorption and secretion occur through different mechanisms along the nephron, determining what passes into urine.

1. Mechanisms Of Regulation
Of Fluid Volume Through
Kidney

2. Mechanisms Of Regulation Of
Fluid Volume Through Kidney
 One way the kidneys can directly
control the volume of bodily fluids is
by the amount of water excreted in the
urine. Either the kidneys can
conserve water by producing urine
that is concentrated relative to plasma,
or they can rid the body of
excess water by producing urine that
is dilute relative to plasma.

4. Mechanisms Of Regulation Of
Fluid Volume Through Kidney
 The major hormones regulating
body fluids are ADH, aldosterone and
ANH.
 Mechanisms for controlling
Na+ concentration in the blood include
the renin angiotensin aldosterone
system and ADH. When Na+ is
retained, K+ is excreted; when Na+ is
lost, K+ is retained.

5. Hormonal Control Of Tubular
Reabsorption

6. Mechanisms Of Regulation Of
Fluid Volume Through Kidney
 Filtration: Glomerular capillaries 
Bowman’s capsule.
 Secretion: Peritubular capillaries 
tubular fluid.
 Reabsorption: Tubular fluid 
peritubular capillaries.
 Excretion = Filtration + Secretion –
Reabsorption.

7. Reabsorption And
Secretion
 Facilitated diffusion.
 Osmosis.
 Active transport.
 Co transport.
 Counter transport.

8. Reabsorption Of Sodium
 Of the sodium filtered ~99.5% is
reabsorbed, 0.5% excreted. Sodium
reabsorption:
 ~70% from proximal tubule.
 ~20% from ascending limb of loop of
Henle.
 ~5% from distal tubule.
 3-5% from collecting duct (regulated by
aldosterone & atrial natriuretic peptide-
less important).

9. Water Transport
 Transport mechanism: Passive diffusion
through aquaporin channels down osmotic
gradient.
 Reabsorption: ~99% of filtered water is
reabsorbed.
 Sites of reabsorption:
 ~70% from proximal tubule.
 ~15% from descending limb of loop of Henle.
 0% from Henle’s ascending limb & distal tubule.
 0-15% from collecting duct depending on plasma
ADH level.

10. Reabsorption Of Urea
 Urea is freely filtered.
 As water is reabsorbed, the tubular
urea rises.
 Urea is passively reabsorbed down its
concentration gradient.
 About half the filtered urea is excreted.

11. Potassium Excretion
 Reabsorption:
1. Proximal tubule.
2. Henle’s ascending limb.
 Secretion: Collecting duct (regulated
process).

12. Thick Ascending Limb Of
Loop Of Henle
 Reabsorption:
1. Reabsorption of sodium.
2. Reabsorption of chloride.
3. Reabsorption of potassium.
4. Reabsorption of calcium.
5. Reabsorption of magnesium.
6. Reabsorption of bicarbonate.
 Secretion: Secretion of hydrogen ions.

13. Reabsorption And Secretion At
Cortical Collecting Tubule
 Reabsorption:
1. Reabsorption of water (depends on
ADH).
2. Reabsorption of sodium and chloride.
 Secretion:
1. Secretion of potassium.
2. Secretion of hydrogen ions.

14. Reabsorption And Secretion Along
The Medullary Collecting Duct
 Water loss is regulated by ADH.
 Reabsorption: Sodium ion,
bicarbonate, and urea are reabsorbed.
 Secretion: Secretion of hydrogen
ions.

15. Role Of ALLH
As salt (NaCl) leaves the ALLH, the
osmolarity of the tubular fluid
decreases. This happens because the
ALLH is impermeable to water.
The net effect of this is to remove salt
from the kidney filtrate and transfer it
into the medulla.

16. Role Of Urea
 Medullary collecting duct is
permeable to urea.
 Via passive diffusion urea is
transported from medullary collecting
tubule to medullary interstitium making
it more hyperosmotic.

17. Role Of Vasa Recta
 As blood flows through the vasa recta
it picks up water and leaves behind
salt. Thus, the vasa recta maintain the
hyperosmotic medulla.