Medical Physiology Chapter 5 & 36 Quiz

Renal Function and Glucose Homeostasis

• The renal system consists of paired kidneys, ureters, a bladder, and a urethra. Its primary function is filtration, secretion, and reabsorption.
• Kidneys maintain water balance, proper osmolarity of body fluids, and regulate ions in extracellular fluid (ECF).
• Key functions also include maintaining plasma volume, acid-base balance, excretion of bodily metabolic products and foreign compounds, erythropoietin production, renin production, vitamin D conversion, and glucose homeostasis.
• Kidneys filter the entire plasma volume (~3 liters) approximately 60 times daily.

Learning Outcomes

• Demonstrate the function of the renal system in homeostasis.
• Demonstrate the role of the kidneys in glucose handling.
• Describe how renal glucose reabsorption is quantified.

Recommended Reading

• Medical Physiology, Chapter 5: Transport of Solutes and Water
• Medical Physiology, Chapter 36: Transport of Urea, Glucose, Phosphate, Calcium, Magnesium, and Organic Solutes
• BS31013 Lecture 1: Membrane transport
• Osmosis - Tubular Reabsorption of Glucose (YouTube video link provided).

Renal Processes

• Glomerular filtration: Protein-free plasma (20%) is filtered through the renal corpuscle (Bowman capsule and glomerulus) at a rate of 125 mL/min (glomerular filtration rate (GFR)).
• Tubular reabsorption: Valuable substances like glucose are reabsorbed in the renal tubules.
• Tubular secretion: Organic acids and bases are secreted into the urine in the proximal convoluted tubule.

Renal Glucose Handling

• Glucose reabsorption occurs through Sodium Glucose coTransporters (SGLTs).
• Glucose release into circulation happens via gluconeogenesis.
• Glucose is taken up from circulation to meet energy demands.

Gluconeogenesis in the Kidney

• Gluconeogenesis synthesizes new glucose from non-carbohydrate sources (lactate, amino acids, glycerol).
• It is mediated by glucose-6-phosphatase (G-6-P).
• Primarily occurs in the liver, but kidneys contribute (20-25% of gluconeogenesis).

Filtration Rate of Glucose

• All plasma glucose is filtered by the kidneys.
• Glucose filtration rate reflects the amount of glucose filtered.
• Glucose filtration is directly proportional to plasma glucose concentration. A higher plasma glucose concentration leads to a higher filtration rate.

Glucose Titration Studies

• Glucose is a critical biochemical fuel for neurons and retina.
• Renal glucose filtering must be balanced by reabsorption.
• Renal glucose handling can be measured clinically through glucose titration studies.
• In uncontrolled diabetes, plasma glucose levels exceeding reabsorption capacity lead to glucosuria (glucose in urine), polyuria (increased urine volume), and polydipsia (increased thirst).

Glucose Reabsorption by SGLTs

• Six members (1, 2, 4, 5, 6, and SMIT1) of the SGLT family reabsorb glucose.
• SGLT1 is predominantly located in the intestine and late proximal tubule (S3), with high glucose affinity (Km = 0.4 mM).
• SGLT2 is primarily in the early proximal tubule (S1), with lower affinity (km = 2 mm), but reabsorbs 90% of glucose.
• SGLT2 inhibitors are used in treating type 2 diabetes (e.g., canagliflozin, dapagliflozin, empagliflozin, ertugliflozin). These inhibitors reduce glucose reabsorption, leading to glucosuria and lower blood glucose.

Summary

• Kidney's role in glucose homeostasis involves glucose reabsorption, gluconeogenesis, and glucose uptake.
• SGLT2 inhibitors exploit the kidney's role in glucose reabsorption to lower blood glucose in type 2 diabetes.

Test your understanding of renal function and its role in glucose homeostasis with this quiz based on Medical Physiology. Explore the intricate processes of filtration, secretion, and regulation performed by the kidneys. You'll assess your knowledge on how these functions contribute to overall homeostasis and glucose management.