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