Renal Function and Urine Formation

Renal Function and Urine Formation

• Renal Corpuscle: Composed of the glomerulus and Bowman's capsule.
• Glomerulus Filtration: Filters small molecules (water, glucose, amino acids, waste) but not large ones (proteins, blood cells).
• Podocytes: Located in Bowman's capsule; their foot processes wrap around glomerular capillaries, regulating filtration.
• Proximal Convoluted Tubule (PCT): Primary site for reabsorption; takes back water, electrolytes, and nutrients (glucose, amino acids, bicarbonate). Waste (urea, creatinine) is not reabsorbed.
• Sodium-Glucose Co-Transporters (SGLTs): In the PCT; responsible for glucose and sodium reabsorption.
• Transport Maximum (Tmax): Maximum rate of substance reabsorption by renal tubules; determined by transporter availability.
• Loop of Henle: Establishes a concentration gradient in the kidney.
  • Ascending Limb: Impermeable to water, reabsorbs electrolytes.
  • Descending Limb: Permeable to water, impermeable to electrolytes.
• Distal Convoluted Tubule (DCT) and Collecting Duct: Regulated by hormones.
• Hormonal Regulation:
  • Aldosterone: Increases sodium reabsorption in DCT and collecting duct.
  • ADH (Antidiuretic Hormone): Increases water reabsorption in the collecting duct.
  • ANP/BNP: Promote salt and water excretion.
  • PTH (Parathyroid Hormone): Increases calcium reabsorption in the DCT.
• Nephron Parts: Glomerulus, Bowman's capsule, PCT, Loop of Henle, DCT, collecting duct.
• Hormones Impact: DCT and collecting duct.
• Most Reabsorption Occurs In: The PCT.
• Loop of Henle Permeability: Permeable to salt; Descending limb is permeable to water.
• Vasa Recta and Peritubular Capillaries: Assist in reabsorbing substances and maintaining the concentration gradient.
• Urine Formation Sequence: Filtration, reabsorption, secretion, excretion.
• Ureter and Bladder: Transport and store urine. Peristalsis moves urine.
• Urethral Sphincters: Internal (involuntary), external sphincter (voluntary).

Urine Constituents and Disorders

• Abnormal Urine Constituents (cause for concern): Protein (proteinuria), glucose (glycosuria), blood (hematuria).
• Other Urine Constituents: Ketones (uncontrolled diabetes or fasting).
• Pyelonephritis: Kidney inflammation, often bacterial.

Transport Mechanisms

• Passive Transport: Substances move down their concentration gradient, no energy required.
• Facilitated Diffusion: Uses carrier proteins to move substances across a membrane.
• Secondary Active Transport: Uses a gradient established by primary active transport.

Kidney Function Regulation

• Intrinsic Controls: Local mechanisms within the kidney.
• Extrinsic Controls: Hormonal and neural regulation from outside the kidney.
• Countercurrent Multiplier: Mechanism by which loop of Henle establishes kidney concentration gradient.
• Juxtaglomerular Apparatus: Regulates blood pressure and filtration rate.
  • Macula Densa: Monitors sodium levels in distal tubule.
  • Granular Cells: Release renin in response to low blood pressure.
• Kidney Hormones: Erythropoietin (stimulates red blood cell production) and Calcitriol (regulates calcium levels).
• Calcium Regulation: Primarily by PTH and Calcitonin

Diuretics and Considerations

• Diuretics: Can lead to electrolyte imbalances and dehydration.
• Blood in Urine: Most concerning abnormality.