how urine is made

• Production of urine

  1. Glomerular filtration

  2. Fluid forced out of blood into glomerular capsule

  3. Fluid forced out of capillaries into tissue due to pressure differences

  4. Narrowing of efferent arteriole increases resistance and pressure in glomerulus
• Glomerular filtration

  • Blood in capillaries separated from capsule cavity by two single layers of thin, flat cells
  • Filtrate consists of all materials in blood except red/white blood cells and plasma proteins
• In a healthy person, the filtrate consists of water, salts, amino acids, fatty acids, glucose, urea, uric acid, creatinine, hormones, toxins and various ions
• 20% of the plasma is filtered through the capillary walls into the glomerular capsule
• The total filtrate produced by all the renal corpuscles of both kidneys is about 125 mL per minute, or 180 L in a day
• Only about 1% of the filtered fluid actually leaves the body as urine, the rest is reabsorbed back into the blood
• Reabsorption

  1. Selective reabsorption of filtrate components back into blood in peritubular capillaries

  2. Materials reabsorbed include water, glucose, amino acids, ions like sodium, potassium, calcium, chloride, bicarbonate, and some wastes like urea
• Reabsorption

  • Large surface area achieved by long length of kidney tubule and huge number of nephrons
  • Permeability of tubule cell membranes can be regulated to control water reabsorption based on body's needs (facultative reabsorption)