week 8 Renal System

The Renal System

• The renal system consists of paired kidneys, paired ureters, a bladder, and a urethra.
• The kidneys filter our entire plasma volume (~3 L) 60 times each day.

The Role of the Kidneys

• Primarily concerned with filtration, secretion, and reabsorption.
• Maintaining water balance, proper osmolarity of body fluids, and regulating the quantity and concentration of most extracellular fluid (ECF) ions.
• Maintaining plasma volume, acid-base balance, and excreting end products of bodily metabolism.
• Excreting foreign compounds, producing erythropoietin and renin, and converting vitamin D into its active form.
• The kidneys play a crucial role in glucose homeostasis.

Renal Processes

• Glomerular Filtration: Protein-free plasma (20%) is filtered through the renal corpuscle (Bowman capsule & glomerulus) at a rate of 125 mL min-1, known as the 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 at the proximal convoluted tubule.

Renal Glucose Handling

• Reabsorption: Glucose is reabsorbed via Sodium Glucose coTransporters (SGLTs).
• Release: Glucose is released into circulation via gluconeogenesis.
• Uptake: Glucose is taken up from circulation to satisfy its energy needs.
• Gluconeogenesis: The synthesis of new glucose from non-carbohydrate sources, e.g., lactate, amino acids, and glycerol. Mediated by glucose-6-phosphatase (G-6-P) and primarily occurs in the liver.

Filtration Rate of Glucose

• All plasma glucose is filtered by the kidneys.
• The filtration rate of glucose describes the amount of glucose filtered by the kidneys.
• Glucose filtration is proportional to the plasma glucose concentration - higher plasma glucose concentration = higher glucose filtration rate.

Glucose Titration Studies

• Glucose is the single biochemical fuel source for both neurons and the retina.
• Renal glucose filtering must be balanced by renal glucose reabsorption.
• In uncontrolled diabetes, plasma glucose concentrations exceed reabsorption capacity leading to glucosuria (glucose in urine), polyuria (increased urine volume), and polydipsia (increased thirst).

Glucose Reabsorption by SGLT's

• 6 members of the SGLT family: SGLT1, SGLT2, SGLT4, SGLT5, SGLT6, and SMIT1.
• SGLT1: Located in the intestine and kidney (late proximal tubule, S3). High glucose affinity (Km= 0.4mM) and low glucose transport capacity. Responsible for ~ 10% of renal glucose reabsorption, primarily involved in dietary absorption of glucose and galactose.
• SGLT2: Located in the kidney (early proximal tubule, S1). Low glucose affinity (Km= 2mM) and high glucose transport capacity. Responsible for ~ 90% of renal glucose reabsorption. Plays a crucial role in renal glucose reabsorption.

This quiz covers the essential components of the renal system, including the structure and function of the kidneys, ureters, bladder, and urethra. It also explores renal processes such as glomerular filtration and tubular reabsorption, emphasizing the kidneys' roles in maintaining homeostasis and regulating body fluids.