The document describes urine formation and mechanisms of reabsorption and secretion in the kidney. It discusses how the glomerular filtration rate is normally 125-250 ml/min. It explains that the proximal convoluted tubule reabsorbs the maximum amount of filtrate, around 65%. Modes of reabsorption include active transport, passive transport, pinocytosis and osmosis. Glucose, amino acids, sodium, and other substances are actively transported, while urea and water diffuse passively.

1. Urine formation II
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
Normal GFR from both the kidneys is
1. 50 ml/min
2. 125 ml/min
3. 250 ml/min
4. 75 ml/min

4. Question
Maximum percentage of filtrate is reabsorbed at
1. DCT
2. LH
3. PCT
4. CD

5. Question
Angiotensin II causes the following effects except
1. vasoconstriction
2. Stimulates aldosterone secretion
3. Stimulates ADH secretion
4. Inhibits thirst centre

6. Introduction
Two important basic functions of kidney tubules
1. Secretion
2. Reabsorption

7. Modes of reabsorption
The following mechanisms are available
1. Active transport – primary and secondary
2. Passive transport – Diffusion, facilitated diffusion
3. Pinocytosis
4. Osmosis
5. Solvent drag

8. Modes of reabsorption
The following mechanisms are available
1. Active transport – glucose, amino acids, sodium etc
2. Passive transport – urea and water
3. Pinocytosis – Traces of albumin and other proteins
4. Substances can be absorbed transcellularly or paracellularly

10. Transport maximum (TM)
Maximum amount of the substance that is absorbed by tubules (both
the kidneys) in one minute
TM refers to the limitation of the tubular capacity to transport or
reabsorb substances actively
TMG – transport maximum for glucose – normal value is 375 mg/min
in males and 300 mg/min in females

11. Threshold substances
These gets excreted in urine when its concentration in the plasma
reaches a critical level
High threshold substances and low threshold substances
High threshold substances – glucose, sodium, calcium, potassium,
magnesium, amino acids etc.
Low threshold substances – urea, uric acid, phosphates

12. Non threshold substances
These do not exhibit any threshold value
e.g., Sulphur, creatinine

13. Reabsorption from PCT
Most substances absorbed in PCT to the maximum extent
65% of the fluid and other substances are absorbed in PCT
At the end of PCT, the fluid remains isotonic to plasma

14. Glucose absorption
Glucose is completely absorbed by PCT by secondary active transport
Sodium and glucose are co-transported from the lumen into the cell
Sodium-glucose transporter-2 (SGLT-2)
From the cells sodium is pumped into lateral intercellular space in
exchange for potassium
Glucose is absorbed either into the lateral intercellular space or
directly passes into the blood through the basal region by facilitated
diffusion by the GLUT-2 transporter (Glucose transporter-2).

16. Glucose absorption
Energy required for glucose transport is provided by sodium pump
Sodium and glucose are co-transported from the lumen into the cell
At a blood glucose level of 100 mg/dl, filtered glucose load is 180
g/day and the entire load is reabsorbed
Glucose exhibits a transport maximum
Normal – 375 mg/min
The excess appears in the urine

17. Renal splay
Glucose exhibits a transport maximum
Normal – 375 mg/min
Theoretically the excess appears in the urine
But practically, glucose appears in the urine when blood concentration
exceeds 180 mg/100ml
This anomaly between the predicted value and practical value is called
renal splay
Why renal splay?

19. Renal splay
Why renal splay?
All nephrons may not have the same TMG. Some have low TMG
Heterogenicity in size, length of PCT, and number of carrier
proteins for glucose absorption

20. Amino acids absorption
All amino acids are essential for the body
All are absorbed by PCT
Absorbed by co-transport with sodium

21. Oligo peptides and proteins absorption
99% of filtered oligo peptides are reabsorbed in PCT
Absorbed by co-transport with H+
Oligo peptide/ H+ cotransporter
Other proteins like albumin, lysozyme glucagon, etc are absorbed
by endocytosis

22. Creatine absorption
Completely absorbed from PCT

23. Sulphates absorption
Absorbed by active transport mechanism

24. Phosphates absorption
Absorbed by the active transport mechanism
A small amount is absorbed passively
80% absorbed by PCT by cotransport
10% participate in acid-base balance
15-20% excreted in urine

25. Uric acid absorption
Normal plasma level 3-7 mg/100ml
Absorbed and also secreted in the PCT
10% of the filtered uric acid is excreted in urine

26. Vitamins and ketone bodies absorption
Absorbed by active transport mechanism

27. Water absorption
Water absorbed passively along with ions
This type of absorption is called obligatory water reabsorption
67% of water is absorbed in PCT
K+ and Ca++ are absorbed by solvent drag along with water

28. Water absorption
Water absorbed passively along with ions
This type of absorption is called obligatory water reabsorption
67% of water is absorbed in PCT
K+ and Ca++ are absorbed by solvent drag along with water
 Two-thirds of water reabsorption occurs through the
transcellular pathway and one-third by the paracellular pathway

29. Sodium absorption
67% of filtered sodium is reabsorbed from PCT
Rest 33% pass through other parts of the nephron
25% is absorbed in LH
8% from DCT and CD
PCT lumen has large amount of sodium
The interior of cell contains less sodium
So there is a diffusion of sodium into the cell by concentration
and electrical gradient

30. Sodium absorption
Sodium is co-transported into the cell along with glucose, amino
acids, lactate and in exchange with H+ secretion.
Some amount of sodium is absorbed by solvent drag along with
water paracellularly
From the cell sodium is pumped into the lateral inter cellular
space by active transport
Sodium –potassium pump is responsible

32. Bicarbonate absorption
1. CO2 is hydrated in the epithelial cells to H2CO3
2. This splits up into H+ and HCO3-
3. HCO3- is absorbed along with sodium
4. H+ secreted into the lumen
5. In lumen it combines with HCO3- and forms H2CO3
6. This is converted to CO2 and H2O
7. Both are absorbed

34. Potassium absorption
K+ is absorbed by both active and passive transport mechanisms
In PCT 60% of the filtered potassium is absorbed actively and 7%
passively by solvent drag
20% absorbed in LH
Rest is excreted in urine

35. Urea absorption
Blood urea level is 20-40 mg/100mL
50% of filtered urea is absorbed from PCT passively
Rest passes to collecting ducts

36. Calcium absorption
65% of filtered absorbed by active and passive mechanisms

37. Calcium absorption
65% of filtered absorbed by active and passive mechanisms

38. Reabsorption from LH
LH is interposed between PCT and DCT
Mainly concerned with the development of osmotic gradient in the
medullary interstitium
About 35% of filtered fluid enters LH
About 15% is absorbed in LH
LH is arranged like a hairpin with both the limbs close and parallel
Fluid flows in two opposite directions
Fluid gets concentrated in the descending limb due to passive
absorption of water

39. Water absorption
Fluid gets concentrated in the descending limb due to passive
absorption of water
Absorption is due to presence of peritubular hypertonic fluid
This is called osmotic water reabsorption

40. Absorption of Na+, Ca++, K+, Cl-, Mg++
Sodium and chloride are absorbed passively from the thin
ascending limb of LH
Sodium, Potassium, and chloride are absorbed by active
transporters through symporter in the thick ascending limb of LH
Sodium-Hydrogen anti-porter also helps in the absorption of the
sodium from the thick ascending limb of LH
Calcium, magnesium, and small amounts of sodium and
potassium are absorbed passively by the paracellular pathway