1. Dr. Shaikh Mujeeb Ahmed Assistant Professor AlMaarefa College
TUBULAR REABSORPTION
URINARY BLOCK 313
Dr. Shaikh Mujeeb Ahmed
Assistant Professor
AlMaarefa College

2. Objectives Define tubular secretion
Role of tubular secretion in maintaining K+ conc.
Mechanisms of tubular secretion.

3. URINE FORMATION
Three Basic Mechanisms (Renal Processes) Of Urine Formation include:
Glomerular Filtration
Tubular Reabsorption
Tubular Secretion

4. Tubular secretion
Secretion – transfer of material from blood into tubular fluid
Helps control blood pH
Helps eliminate substances from the body

5. Tubular Secretion First step is simple diffusion from peritubular
capillaries to interstitial fluid
Enter to tubular cell can be active or passive
Exit from tubular cell to lumen can be active or
passive
Examples: potassium, hydrogen, organic acids,
organic bases, NH3

6. Calculation of Tubular Secretion
Secretion = Excretion - Filtration
H+, K+, NH3
Organic acids
and bases

7. Tubular Secretion Tubular secretion is important for:
Disposing of substances not already in the filtrate
Eliminating undesirable substances such as urea and uric acid
Ridding the body of excess potassium ions
Controlling blood pH by secreting H+

8. Tubular Secretion Most important substances secreted by the tubules:
Important in regulating acid-base balance
Secreted in proximal, distal, and collecting tubules K+
Keeps plasma K+ concentration at appropriate level to maintain normal membrane excitability in muscles and nerves
Secreted only in the distal and collecting tubules under control of aldosterone
Organic ions
Accomplish more efficient elimination of foreign organic compounds from the body
Secreted only in the proximal tubule

9. Potassium balance 98% of K+ is in ICF & 2% in ECF
ICF = 150 m Eq/L & in ECF = 4.5 mEq/L
Balance → intake = out put
Maintenance of K balance is important in normal functioning of excitable tissue

10. Importance of regulating plasma K+ concentration
K+ plays a key role in the membrane potential of excitable tissues.
Both increase and decrease in plasma K+ can change intracellular to extracellular K+ conc. Gradient which can change the RMP.
Its impact on the heart – decreased cardiac excitability
Rise in ECF K+ conc. decreases excitability of the neurons & skeletal muscle cells.
Decrease in ECF K+ lead to skeletal muscle weakness, diarrhea and abdominal distension.

11. Potassium handling by nephron

12. Potassium handling by nephron(continued)
Distal tubule & collecting ducts :
Responsible for adjustment of K+ excretion by either re absorption or secretion as dictated by need
α -Intercalated cells : absorption of potassium if person is on low K+ diet
Principle cells : if person on normal or high K+ diet potassium is excreted by principle cells
The magnitude of potassium excretion is variable depending on diet & several other factors for eg.aldosterone,acid base status ,flow rate etc

13. Effect of H+ secretion on K+ secretion
During acidosis H+ secretion is increase lead to retention of K+.

14. Principle cells in Late DCT & CT

15. Factors affecting K+secretion
Magnitude of K+ secretion is determined by the size of electrochemical gradient across luminal membrane
Diet:
High K+ diet concentration inside thus principle cells increases electrochemical gradient across membrane

16. Factors affecting K+secretion(continued)
Aldosterone :
Aldosterone Na+ re absorption by principle cell by inducing synthesis of luminal membrane Na+ channels & basolateral membrane Na+- K+ channel
more Na+ is pumped out of the cell simultaneously more K+ pumped into the cell
Thus increasing the electrochemical gradient for K+ across the luminal membrane that leads to increase K+ secretion

17. DUAL EFFECT OF ALDOSTERONE
Fall in Na+
- through RAAS
Increase in K+

18. Late Distal, Cortical and Medullary Collecting Tubules
Principal Cells
Tubular Lumen
H20 (+ ADH)
Na +
Na +
ATP
ATP K+ K+
Cl -
Aldosterone

19. Increases Na+ reabsorption - principal cells
Aldosterone Actions on Late Distal, Cortical and Medullary Collecting Tubules
Increases Na+ reabsorption - principal cells
Increases K+ secretion - principal cells
Increases H+ secretion - intercalated cells

20. Relationship between Na+ absorption & K+ secretion
High Na+ diet:
more Na+ will be delivered to principle cells ,more Na+ is available for Na+- K+ ATPase than more K+ is pumped into the cell which increases the driving force for K+ secretion
Diuretics :
loop & thiazide diuretics inhibit Na+ re absorption in part of tubule earlier to principle cells, so increases Na+ delivery to principle cells , more Na+ is reabsorbed & more K+ is excreted

21. Organic Anion and Cation secretion
Proximal tubule contains two types of secretory carriers
For organic anions
For organic cations
Organic ions such as Prostaglandin, epinephrine – after their action removed from blood
Non filterable organic ions also removed
Chemicals, food additives, non nutritive substances
Drugs – NSAID, antibiotics

22. PAH –EXAMPLE OF SECRETION
PAH is an organic acid
Used for measurement of renal plasma flow
Both filtered and secreted
PAH transporters located in peritubular membrane of proximal tubular cells.
There are parallel secretory mechanism for secretion of organic bases like quinine and morphine

23. UREA & Uric acid Urea is freely filtered – 50% reabsorbed in PCT.
Urate is freely filtered
In PCT there is reabsorption and secretion takes place.
In the initial & middle part of PCT reapsorption is more than secretion
In the distal portion of PCT moderate amount of urates are secreted.

24. References Human physiology by Lauralee Sherwood, seventh edition
Text book of physiology by Linda .s contanzo,third edition
Text book physiology by Guyton &Hall,11th edition