urine formation

1. Urine formation III
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
R.D. Gardi Medical College, Ujjain, Madhya Pradesh.
Email: dr.goothy@gmail.com

2. The student should be able to
Describe urine formation
Explain the mechanism of reabsorption
Explain the mechanism of secretion

3. Question
Oedema of nephrotic syndrome is due to
1. Protein into urine
2. Urea into urine
3. Electrolytes into urine
4. Glucose into urine

4. Question
Anaemia in chronic renal failure is due to
1. Decreased secretion of renin
2. Decreased secretion of erythropoietin
3. Decreased secretion of catecholamines
4. Increased secretion of prostaglandlins

5. Question
One of the following substance is absorbed by secondary active
transport
1. Chloride
2. H2O
3. HCO3-
4. Amino acids

6. Introduction
DCT and CD together are called distal nephron
About 20% of hypotonic fluid enters the DCT
Early DCT sodium reabsorption continues and dilutes the fluid
In late DCT about 5% of the filtrate is reabsorbed
About 15% of filtrate enters the CD
14.7% is absorbed
0.3% is excreted
At the end of CD urine is 4 times concentrated than plasma
CD descends down into the inner medulla to join the duct of Bellini

7. Na+ reabsorption
Depends on body requirement
Aldosterone is required for sodium reabsorption from DCT and CD
Any factor that stimulates the release of aldosterone from adrenal
cortex increases sodium absorption
Sodium absorption occurs by principal cells
Principal cells – sodium, chloride, water absorbed
Principal c ells- K+ secreted

8. K+ reabsorption
Depends on dietary intake, aldosterone, acid-base status and urine
flow rate
Intercalated cells- absorb K+ and secrete H+

9. HCO3- reabsorption
HCO3- is reabsorbed by I cells along with K+

10. Chloride and HCO3- reabsorption
Chloride is absorbed passively along with the K+ by principal cells
Paracellular pathway
There is a competition between chloride and bicarbonate depending
on acid base balance
In acidosis HCO3- is reabsorbed
In alkalosis chloride is reabsorbed

11. Water reabsorption
About 15% of water is absorbed from late DCT and CD
The absorption of water is by P Cells
Depends on presence or absence of ADH
Any factor that increase ADH secretion increases the water
reabsorption from late DCT and CD
The absorption of water under the influence of ADH is called as
facultative reabsorption of water
The urine concentration depends on how much water absorbed in CD

12. Water intoxication
Absence of ADH
Or excess of water intake
Urine out put increases to 23 lit/day
If excess water is taken, water accumulates in the body and produces
the water intoxication
Neurons swell results in convulsions and coma

13. Calcium reabsorption
PTH facilitates active absorption of calcium from DCT and CD
Thiazides increase calcium reabsorption from DCT
Amiloride increases calcium reabsorption from CD
Both require the presence of PTH
Loop diuretics decreases calcium reabsorption from LH

14. Magnesium reabsorption
Absorbed actively from DCT (Passively from PCT and LH)

15. Urea reabsorption
Absorbed by inner medullary part of CD
Urea transporter -1 (facilitated diffusion)
As water is absorbed from CD, urea concentration increases
So it diffuses out into medullary interstitium
From here small amounts are absorbed into vasa recta and AL of LH
At normal flow rates, 20-30% of filtered urea is excreted into the urine

16. Hormones that influence reabsorption
Aldosterone
Acts on DCT and Cd of nephrons
Increases NaCl absorption
By activating Na+-K+ ATPase
Increases K+ or H+ secretion from CD
Glucocorticoids have a similar effect but minimum

18. Hormones that influence reabsorption
Angiotensin II
Increases sodium and chloride absorption in two ways
Direct action on PCT
By stimulating the secretion of aldosterone
Stimulates H+ secretion in exchange for sodium

19. Hormones that influence reabsorption
ADH
Increases water reabsorption from the later part of DCT and CD
Mediated through CAMP
Increases absorption of sodium and chloride from thick AL of LH and
urea from the inner medullary collecting duct

20. Hormones that influence reabsorption
Atrial natriuretic peptide (ANP)
Inhibits sodium and chloride absorption from PCT and CD
Increase in excretion of NaCl and water in urine

21. Hormones that influence reabsorption
Para thyroid hormone
Inhibits phosphates absorption from PCT
Increase reabsorption of calcium from LH and DCT and Magnesium
from LH

22. Hormones that influence reabsorption
Calcitonin
Inhibits reabsorption of calcium from LH
Increases calcium excretion into urine

23. Hormones that influence reabsorption
Prostaglandlins
12 and E2, Dopamine, Bradykinin inhibits sodium absorption

24. Hormones that influence reabsorption
Cortisol
Similar action as of cortisol

25. Secretion of substances into urine
Mechanism resembles the process of reabsorption by active
transport
Only difference is in the orientation of the transport mechanism
Substances are transported from the blood into the lumen
H+, K+, NH3, uric acid, drugs and creatinine are secreted
What is the importance of secretion??

26. Secretion of substances into urine
Substances that can not be filtered are excreted by the secretion
Secretion of H+ and ammonia help in the regulation of blood pH
Drugs that are not filtered are also excreted by the secretion

27. Renal erythropoietin
Peritubular capillary interstitial cells of cortex and outer medulla
produce 85% of erythropoietin
Hepatocytes and Kupffer cells may produce 15% of erythropoietin
Erythropoietin also produced to some extent by JG cells and
mesangial cells
Hypoxia stimulates the release of erythropoietin from the kidney
Erythropoietin binds to EP-sensitive stem cell receptors
Renal failure- no EP- leads to anemia

28. Drugs that block renin-angiotensin system
Renin release is blocked by B blocker- propranolol and inhibitor
of PG synthesis - Indomethacin
Pepstatin inhibits the conversion of angiotensinogen to
angiotensin 1
Captopril prevents the conversion of angiotensin I to angiotensin
II
Saralasin blocks the effect of angiotensin II on the target organ
These drugs are used in the management of renal hypertension

29. Drugs that block renin-angiotensin system
The liver plays an important role in helping rid the body of foreign
compounds.
Many foreign organic chemicals are not ionic in their original form, so
they cannot be secreted by the organic ion systems.
The liver converts these foreign substances into an anionic form that
facilitates their secretion by the organic anion system and thus
accelerates their elimination.
Many drugs, such as penicillin and nonsteroidal anti-inflammatory
drugs (NSAIDs), are eliminated from the body by the organic ion
secretory systems.

30. Unwanted waste products are not reabsorbed
The other filtered waste products besides urea, such as uric acid, creatinine,
and phenols (derived from many foods) are likewise concentrated in the
tubular fluid as H2O leaves the filtrate to enter the plasma.
But urea molecules, being the smallest of the waste products, are the only
wastes passively reabsorbed by this concentrating effect.
The other wastes cannot leave the lumen down their concentration gradients
to be passively reabsorbed because they cannot permeate the tubular wall.
 Therefore, these waste products generally remain in the tubules and are
excreted in the urine in a highly concentrated form.