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Understanding Tubular Reabsorption Mechanisms for Renal Health Optimization

Learn about tubular reabsorption processes in the renal system, including types of transport, renal threshold, and transport maximum values for key substances like glucose and amino acids.

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Understanding Tubular Reabsorption Mechanisms for Renal Health Optimization

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  1. Tubular Function

  2. Specific learning objectives What is tubular reabsorption? Types of transport Reabsorption of glucose Transport maximum Real splay Applied aspects

  3. Tubular Function • (Selective) Reabsorption(of essential Constituents) • (Selective) Secretion (of waste products)

  4. PROXIMAL CONVOLUTED TUBULE • GLUCOSE • Amino acids • Sodium • Potassium • Calcium • Bicarbonates • Chloride • Phosphate • Uric acid • Water

  5. LOOP OF HENLE • Sodium • Chloride DISTAL CONVOLUTED TUBULE • SODIUM • BICARBONATE • WATER

  6. Tubular Load • Tubular load ( filtered load ) of a substance is “the total amount of the substance that enter the tubule through glomerular filtrate" per unit time. Tubular load = GFR × Plasma con.of the substance = ml/min x mg/ml=mg/min

  7. Excretion rate • The amount of substance that appears in the urine per unit time Excretion rate= urine flow rate X con.of the substance in urine = ml/min x mg/ml=mg/min

  8. Renal TubularTransport maximum (Tm) • Tm is defined as “the maximum quantity of a substance that can be transportedby the renal tubule per minute”.

  9. Tubular maximum of substances Reabsorbed • Tm glucose = 375mg/min for male = 300mg/min for female • Tm Lactate = 75mg/min • Tm Amino acid = 1.5m.molar/min • Tm Phosphate = O.1m.molar/min

  10. Tubular Maximum-For reabsorption • If the tubular load is less than Tm; then the substance is completely reabsorbed • If the tubular load is more than Tm; then the entire excessis excreted

  11. The Renal threshold This is the concentration of the solute in the plasma at or above which the solute first appears in the urine. • Example : Renal threshold for glucose is 180mg% of venous plasma

  12. Overview of Renal Tubular transport mechanism Active Transport • Primary Active transport Na+-K+ Pump at the basolateral membrane • Secondary Active transport – Reabsorption of most substances, glucose, Amino acids

  13. Primary active transport

  14. Secondary active transport • “the transport of the substance does not require energy; but to create a favorable condition for the transportenergy is required”.

  15. Different types of Transporter • Different types of transport mechanisms based on properties of carrier proteins (Transporters) are -- • Uniport –Transport only one direction -transport of sodium • Symport (co-transport)—Transport more than one substance along with Na+ --PCT & Thick Ascending limb of LOH • Antiport (counter-transport) –Transport Na+ and other substance in opposite direction –PCT, DCT, Cortical CT

  16. Overview of Renal Tubular transport mechanism Diffusion/Passive Transport Simple diffusion Water, Urea, Facilitated diffusion Transport of glucose through GLUT-2

  17. Overview of Renal Tubular transport mechanism Transepithelial Transport Transcellular pathway- Paracellular pathway

  18. Glucose Reabsorption

  19. lumen

  20. Glucose reabsorption • Glucose is reabsorbed along with the Na+ in the proximal convoluted tubule. • Reabsorbed by secondary active transport.

  21. Na+/Glucose co-transport • Transporter protein at luminal side –SGLT2 (Sodium Glucose Transport-2)—Carry one Na+ ion & one glucose molecule • Transporter protein at basal membrane –GLUT-2 (Glucose Transporter-2) –only glucose

  22. Glucose reabsorption The Glucose reabsorption and excretion processes depend onplasma glucose consentration (PG) • Normal filtered load of glucose= 125x80/100=100 mg/min

  23. Glucose reabsorption • At low PG -complete glucose Reabsorption. • PG above 180-200mg/dl glucose reabsorption is not complete and glucose passes out in the urine- • Glycosuria • This plasma glucose level at which glucose first appears in urine is called as- Renal threshold for glucose

  24. Glucose reabsorption • Actual Renal threshold for Glucose –180mg/dl of venous blood; • Equivalent to 200mg/dl of arterial blood; • TmG for glucose – 375mg/min • Predicted renal threshold= 375×100/125=300mg/dl

  25. Glucose reabsorption

  26. Glucose reabsorption • Actual renal threshold is less than predicted renal threshold from TmG & Deviation is known as Splay • Reason for splay – • Variation in Tmg of the nephrons • Heterogeneity in the size, length of PCT and number of carrier molecules

  27. Summary

  28. References Comprehensive Textbook of Medical physiology (Vol 2, 1stedition) G K Pal Text book of medical physiology (Vol 2, 6thedition) A K Jain Essentials of medical physiology (6thedition) K Sembulingam and PremaSembulingam https://www.researchgate.net/figure/Glucose-reabsorption-from-the-glomerular-filtrate-through-a-proximal-tubule-epithelial_fig2_24267825 https://www.researchgate.net/figure/TmG-Transport-maximum-reabsorption-of-glucose-N-Normal-plasma-glucose-concentration_fig1_255988083

  29. THANK YOU

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