Urinary 2

• Functions of the urinary system

  • maintain homeostasis by keeping fluid electrolyte and acid base balance through blood plasma filtering and urine production
  • regulate the rate of ADH and aldosterone
  • synthesize active form of vitamin D and erythropoietin
• There are approximately 1 million nephrons in each kidney. They are all used to process blood plasma and form urine through filtration, tubular reabsorption and tubular secretion
• Filtration is the first step of urine formation. It occurs in the renal corpuscules (glomerulus and bowman’s capsule)
• As blood flows during filtration, water and small solutes filter out of the blood into the Bowman’s capsule. Proteins and cells do not move out of the blood.
• 180 liters of filtrate pass through the kidneys but 99% is reabsorbed into the blood
• Reabsorption is the second step in urine formation. It takes place in renal tubules by passive and active transport.
• Reabsorption mostly occurs in the proximal convoluted tubule where water electrolytes and reabsorbed. Sodium and other ions are actively transported out of the tubule and into the blood
• Active transport of sodium and passive transport of chlorine go into the interstitial fluid and into the blood by diffusion. This results in the blood being hypertonic to the renal tubule and therefore water diffuses out from the tubule into the blood through osmosis
• Glucose and amino acids are reabsorbed in the PCT by a sodium cotransport mechanism meaning that a carrier binds to sodium and glucose and passively transports both substances into the cell
• After the carrier transports sodium and glucose through the cell, sodium is transported actively to the interstitial fluid and glucose is transported passively
• Glucose in the urine happens in glycosuria and diabetes mellitus
• Glycosuria is when glucose appears in the urine even though blood glucose levels are normal. This is due to the lack of glucose sodium carriers from congenital defect
• Diabetes mellitus is when glucose appears in the urine because blood glucose levels are high in the blood and not all the glucose is reabsorbed because there are not enough glucose sodium carriers
• Urea is a waster product from protein catabolism
• After all of the sodium, chloride and water have moved the tubule is high in urea concentration. The urea then passively diffuses into the blood (around half) and the other half moves to the loop of Henle