This lecture serves as a foundation for understanding renal tubular function by providing an in-depth overview of how the kidneys process filtrate to maintain homeostasis. It covers the essential mechanisms of tubular reabsorption and secretion, explaining how different substances like sodium, glucose, urea, and potassium are handled by the renal tubules. Learners will explore the driving forces behind these processes, including electrochemical gradients, active transport mechanisms, and osmotic principles. The lecture further discusses pathophysiological implications such as glucosuria in diabetes mellitus, demonstrating how disruptions in tubular processing can lead to clinical manifestations. Through detailed diagrams, interactive explanations, and case studies, this session will help students, medical professionals, and researchers grasp the foundational principles of renal physiology and their clinical significance. Who Can Benefit? Medical and nursing students seeking a clear understanding of renal physiology. Healthcare professionals who want to reinforce their knowledge of renal function and its clinical implications. Researchers in nephrology interested in the physiological basis of kidney function and renal disorders. Pharmacology students learning about drug interactions affecting renal tubules. Key Takeaways: Understanding the roles of reabsorption and secretion in maintaining electrolyte balance. The significance of glomerular filtration, net solute movement, and excretion. The impact of renal transporters on glucose, sodium, and water reabsorption. Clinical applications such as the development of glucosuria in diabetes.

1. Renal Tubular Processing
Dr Faiza
Assistant Professor of Physiology
MBBS (Best Graduate, AIMC Lahore)
FCPS Physiology,
ICMT, CHPE, DHPE (STMU)
MPH (GC University, Faisalabad)
MBA (Virtual University of Pakistan)

2. Learning Objectives:
• Describe reabsorption and secretion by the renal tubules.
• Differentiate between filtered load, reabsorption rate, and
secretion rate.
• Describe location, functions, and importance of renal
transporters along the tubules.
• Discuss the pathophysiology of Glucosuria in poorly controlled
diabetes.
• Analyze the transport maximum (Tm) concept and renal threshold
differences.

4. Reabsorption or
Secretion Rate
• Difference between the amount
filtered across the glomerular
capillaries and the amount
excreted in urine

5. Filtrated load = Glomerular filtration rate × Plasma concentration
Excretion Rate = Urine Flow Rate × Urine concentration
Reabsorption Rate = Filtered Load - Excretion Rate
Secretion Rate = Excretion Rate - Filtered Load

6. Net Secretion or Reabsorption….?
• If the filtered load is less than excretion rate

7. A woman with untreated diabetes mellitus has a GFR of 120 mL/min, a plasma
glucose concentration of 400 mg/dL, a urine glucose concentration of 2500
mg/dL, and a urine flow rate of 4 mL/min. What is the reabsorption rate of
glucose?

8. Tubular Reabsorption
• Highly Selective
• Reabsorption of a substance may be
• Complete
• Partial
• Subjected to variation as per homeostatic needs

9. Lumen
Interstitium
Blood
Apical/Luminal
Border
Basolateral
Border

11. Identify the three
Modes of transport
(A,B,C) encountered in
reabsorption across
renal tubules.
A
B
C

12. Carefully Observe the
diagram and describe
various forms of
sodium reabsorption
across renal tubules.
Transcellular vs
Paracellular
Active/passive
Concentration gradient
Electrochemical gradient

13. Net reabsorption of Na+ from the Tubular
Lumen:
1. Diffuses across the luminal membrane into the cell down an
electrochemical gradient
2. Active transported across the basolateral membrane by the
sodium-potassium ATPase pump
3. Reabsorbed from the interstitial fluid into the peritubular
capillaries by ultrafiltration
(Ultrafiltration: a passive process driven by the hydrostatic
and colloid osmotic pressure gradients)

14. Describe the reabsorption of Glucose and
amino acids from tubular lumen to interstitial
fluid in form of a diagram.
Basolateral Border
90% SGLT2 proximal PCT
10% SGLT1 distal PCT
Luminal Border
GLUT 2 proximal PCT
GLUT 1 distal PCT
Secondary active vs
Facilitated diffusion

15. Name the cellular transport involved in
extrusion of H+ in the given diagram.

16. Characters of Actively Transported Substances
1. Transport Maximum
2. Gradient time transport

17. Transport Maximum
• For substances that are actively reabsorbed or secreted
Limit to the rate at which the solute can be transported
due to saturation of the specific transport systems
• Capacity of the carrier proteins
• Specific enzymes in the transport process

19. • Normal conc of plasma
glucose
• Filtered Load at normal
plasma conc
• Reabsorption at normal
plasma conc
• Excretion at normal plasma
conc
100mg/100ml
125mg/min
125mg/min
Nil

20. • Threshold: Glucose first
excreted in urine at which
plasma conc
• Filtered Load at threshold
conc
• Reabsorption at threshold
conc
• Excretion at threshold conc
180-200mg/100ml
250mg/min
<250mg/min
Just begins

21. • Transport Max: Plasma conc
at which Glucose
reabsorption has increased
to a maximum
• Filtered Load at Transport
Maximum
• Reabsorption at transport
max
• Excretion at transport
maximum
300-350mg/100ml
375mg/min
All nephrons saturated
All that exceeds the
reabsorptive capacity

22. Threshold
Transport max
Splay

23. Why Transport Maximum and Renal Threshold
are different?
• Heterogeneity of nephrons
• relatively low affinity of the Na+– glucose carrier

24. Transport Maximum of Actively Reabsorbed
Substances

25. Transport Maximum of Actively Secreted
Substances

26. Gradient Time Transport
• Some substances that are actively reabsorbed, but do not
demonstrate a transport maximum
• Active transport depends on;
• Electrochemical gradient for diffusion of the substance across the
membrane
• Permeability of the membrane for the substance
• Tubular Flow Rate: Time that the fluid containing the substance remains
within the tubule.

27. Gradient Time Transport
• Example:
• Na transport in proximal tubules

28. Thank You