Lecture 44: Tubular Reabsorption and Secretion

Lecture 44: Tubular Reabsorption and Secretion

• Instructor: Dr. Kevin Tipper, ND

• Slide Credits: UBC BIOLOGY 155

Learning Objectives

• LO1: Describe the three basic functions of the renal tubule in urine formation; identify the two physiological processes occurring in the renal tubule.

• LO2: Discuss the major functions of the Proximal Convoluted Tubule (PCT) and how its anatomy supports these functions.

• LO3: Explain the reabsorption process of organic nutrients (e.g., glucose) and plasma proteins by PCT, detailing the types and locations of transmembrane transporter proteins involved.

• LO4: Describe paracellular water and ion reabsorption in PCT, predicting effects of solute reabsorption changes on water reabsorption.

• LO5: Explain the association between diabetes mellitus, polyuria, and polydipsia.

• LO6: Discuss secretion of H+, NH3/NH4+, and organic ions by PCT and DCT concerning transporter proteins and cellular enzymes.

Overview of Renal Physiology

• Filtration: Water and small solutes from plasma are filtered from the glomerulus to the nephron based on Net Filtration Pressure (NFP).

  • Normal Glomerular Filtration Rate (GFR) is approximately 120 ext{ mL/min} or 180 ext{ L/day}.

• Reabsorption and Secretion: After glomerular filtration, nephron processes transform filtrate into urine.

  • Reabsorption: Removal of nutrients and water back into circulation.

  • Secretion: Active transport of wastes from blood into filtrate.

Functions of the Renal Tubule

1. Remove nutrients and proteins from tubular fluid.

2. Reabsorb filtered fluid (water and ions).

3. Concentrate waste products in tubular fluid.

Proximal Convoluted Tubule (PCT)

• Located in renal cortex, closely associated with peritubular capillaries.

• Reabsorption Capacity:

  • Reabsorbs 60-70% of water, 99-100% of organic substrates (e.g., glucose), 60-70% of sodium and chloride ions.

• Key Processes:

  • Active Reabsorption: Glucose, amino acids, vitamins, ions (e.g., Na+, K+).

  • Passive Reabsorption: Water, urea, lipid-soluble materials.

  • Secretion: H+, NH4+, creatinine, drugs, and toxins.

Structure of PCT Epithelium

• Type: Leaky simple cuboidal epithelium with microvilli (brush border) to increase surface area for absorption.

• Transport Mechanisms:

  • Paracellular Route: Flow between cells through tight junctions.

  • Transcellular Route: Movement through epithelial cells facilitated by channels and transporters.

Transport Mechanisms for Organic Nutrients & Ions

• Co-Transporters:

  • Apical surface has co-transporters that bind sodium (Na+) and glucose or amino acids (e.g., SGLT1, SGLT2).

  • Basolateral membrane contains transporters for glucose/amino acids to enter blood (e.g. GLUT2).

• Glucose Reabsorption:

  • Up to 99% of filtered glucose reabsorbed via secondary active transport.

  • Glucose transport saturates when blood glucose exceeds ≥200 mg/dL, leading to glucosuria (glucose in urine).

Paracellular Water and Ion Reabsorption

• Occurs alongside solute reabsorption.

• If solute reabsorption changes (e.g., glucose remains in filtrate), it affects osmolarity, possibly reducing water reabsorption leading to diuresis.

Diabetes Mellitus and its Effects

• Leads to polyuria and polydipsia due to high plasma glucose levels.

• Pathophysiology:

  • High glucose in glomerular filtrate overwhelms PCT transport capacity, leading to glucose and osmotic diuresis.

  • Symptoms include frequent urination, increased thirst, blurred vision, and unexplained weight loss.

Secretion Mechanisms in PCT and DCT

• Secretion of Wastes:

  • Nitrogenous wastes, metabolic acids, toxins, and drugs are secreted from blood into filtrate.

• Importance of Transporters:

  • Organic cation (OC+) and anion (OA-) transporters in renal cells selectively facilitate the excretion of various substances.

Summary of Key Points

• Anatomy of PCT: Leaky epithelium with microvilli enhances absorption.

• Fluid Reabsorption: Approx. 108-116 L/day (~60-70% of total filtrate) reabsorbed via osmotic processes.

• Diabetes and Renal Consequences: Elevated glucose leads to polyuria and glycosuria through saturated reabsorption mechanisms; can affect overall renal function.

• Exam Preparation: Focus on understanding transport processes, effects of diabetes, and kidney filtration mechanics for thorough comprehension.