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BIO 391- The Excretory System
The Structure and Function of the Kidney
Function of kidney: Filter blood, reabsorb materials & form
urine
Cross section of the kidney
• Over 1 million nephrons in each kidney
• Nephrons span both the cortex and medulla
• Renal pelvis: collects urine, passes it to ureter
• Renal artery: brings blood to kidney
• Renal vein: transports purified blood back to heart
Structure of the nephron
Functional Regions
Homeostasis by the Kidney
• Maintain volume of extracellular fluid • Maintain ionic balance in extracellular fluid• Maintain pH and osmotic concentration of the
extracellular fluid. • Excrete toxic metabolic by-products such as urea,
ammonia, and uric acid.
Steps of urine formation• Glomerular filtration:
o removes solutes from bloodo 1st step in urine formation
• Tubular reabsorption: o almost all nutrients are reabsorbed by passive or active
transporto 2nd step in urine formation
• Tubular secretion:o Solutes and waste are secreted into the tubules and
collecting ducto Last step in urine formation
Production of Urine
What is urine made of?• 96% water• 2% urea• 2% other substances• e.g.: ammonia, sodium, potassium, chloride, sulphates,
phosphates, excess vitamins, drugs
• Normal urine is acidic, but it depends on diet and other factors