The document summarizes the key processes involved in urine formation: - Glomerular filtration, tubular reabsorption, and tubular secretion are the three major renal processes. Substances are reabsorbed or secreted across tubular cells via transcellular or paracellular routes. - The proximal convoluted tubule reabsorbs the majority of filtered sodium, water, bicarbonate, chloride and potassium. - The loop of Henle and distal convoluted tubule further regulate reabsorption and secretion of ions and water, facilitated by hormones like aldosterone and antidiuretic hormone. - Tubular secretion disposes of substances not in the original filtrate like drugs,

1. Tubular Processing of
Glomerular Filtrate
By Dr. Karishma R. Pandey
Assistant Professor
BPKIHS, Nepal

2. Overview
 Steps in urine formation
 Processing of glomerular filtrate by Reabsorption and
secretion
 Substances reabsorbed in different parts of renal tubule
and its mechanism
 Substances secreted in different parts of renal tubule and
its mechanism
 Regulation of renal processing

3. Steps in Urine Formation
Cortical
radiate
artery
Afferent arteriole
Glomerular capillaries
Efferent arteriole
Glomerular capsule
Rest of renal tubule
containing filtrate
Peritubular
capillary
To cortical radiate vein
Urine
Glomerular filtration
Tubular reabsorption
Tubular secretion
Three major
renal processes:

4. Processing of glomerular filtrate by
Reabsorption and secretion
• Reabsorption is defined as movement of a substance from the tubular
fluid to the blood, and this process occurs either via the tubular cells ”the
transcellular route” or between the cells” the paracellular route .
• Tubular secretion is defined as movement of a substance from the blood
into the tubular fluid.
• The reabsorption and secretion that occur via the transcellular route are
largely the result of secondary active transport of solutes by the tubular
cells.
• Paracellular reabsorption occurs as a result of concentration or electrical
gradients that favor movement of solutes out of the tubular fluid.

5. Routes of Water and Solute Reabsorption

6. Cells at renal tubules and collecting duct

7. Proximal Convoluted Tubule (PCT); Reabsorption
Figure 25.4b
• PCT is the most active in reabsorption
• All glucose, lactate, & amino acids
• Most Na+, H2O, HCO3
- , CL-
• and K+
– 65% Na+ and H2O
– 90% HCO3
-
– 50% CL-
– 55% K+

11. Protein absorption

14. Reabsorption of water / ions / nutrients
• Passive tubular reabsorption:
– Na+ ions establish an electro-
chemical gradient favoring anions (Cl-
& HCO3
-)
– Na+ establishes an osmotic gradient
allowing water (via aquaporins) to
leave water permeable region (PCT
& Loop)
As water leaves the tubules the remaining solutes become more
concentrated & follow their diffusion gradient out of the filtrate
(cations, fatty acids, urea)

16. Transport maximum for substances that are actively reabsorbed
For most substances that are
actively reabsorbed or secreted,
there is a limit to the rate at
which the solute can be
transported, often referred to as
the transport maximum.
This limit is due to saturation of
the specific transport systems
involved when the amount of
solute delivered to the tubule
exceeds the capacity of the
carrier proteins and specific
enzymes involved in the
transport process.

17. Non-Reabsorbed Substances
• Non-reabsorption due to:
– Lack of carriers
– Lipid insolubility
– Molecules too large to pass through membrane pores
• Non-reabsorbed substances are usually nitrogenous wastes
– Urea (50-60% reclaimed)
– Creatinine: not reabsorbed

18. Loop of Henle: Reabsorption
• Descending limb:
– H2O reabsorbed by osmosis

19. Loop of Henle: Reabsorption
Ascending limb:
Na+, Cl-, K+ active transport
Ca2+, Mg2+ passive transport
H2O : impermeable

20. Distal Convoluted Tubule: Reabsorption
Early tubule:
• Na+ : symporter mediated
• Ca2+ : PTH mediated
• Cl- : diffusion
• H2O : impermeable

21. Distal Convoluted Tubule & Collecting Tubule
: Reabsorption
Principal cells:
• Na+ : Aldosterone mediated
• Ca2+ : PTH mediated
• H2O : ADH mediated
Intercalated cells
• H2CO3

23. Mechanism of Action of Aldosterone

24. Mechanism of Action of Antidiuretic Hormone

25. Mechanism of Action of Parathyroid Hormone

27. Medullary Collecting Duct: Reabsorption
HCO3
-
H2O : ADH dependent
Urea: facilitated diffusion

28. Tubular Secretion
• Tubules also secrete substances into the filtrate.
• H+, K+, NH4
+, creatinine
• Important functions:
– Disposes of substances not in original filtrate (certain drugs
and toxins)
– Bile salts, oxalate, urate and catecholamines
– Disposes of excess K+
• Urine consists of filtered & secreted substances

29. Regulation of renal processing

31. Regulation of renal processing

32. Regulation of renal processing

33. Mechanism of Action of Angiotensin II

34. Mechanism of Action of Atrial Natriuretic Peptide

35. References
1. Textbook of Medical Physiology-12th
edition(Guyton and Hall)
2. Ganong’s Review of Medical Physiology-23rd
edition
3. Textbook of Physiology-6th edition(Berne and
Levy)
4. Textbook of Medical Physiology-2nd
edition(Walter F. Boron, Emile L. Boulpaep)

36. Thank you!!!