The urinary system and the kidneys — Edexcel GCSE Biology

Revision notes

5 short notes, in the order of the specification. Each one in short:

1. The urinary system

   The urinary system removes waste from the blood as urine. The renal artery brings blood to each kidney, where it is filtered, and the renal vein takes it away. Urine passes from the kidneys down the ureters to the bladder, where it is stored, and leaves the body through the urethra.

2. Urea

   Urea is a waste substance made in the liver when excess amino acids are broken down, because the body cannot store them. It is carried in the blood to the kidneys, which remove it from the blood in the urine. Remember that urea is made from amino acids, not from glucose or fat.

3. The nephron: filtering and reabsorption

   A nephron is a tiny tube in the kidney that filters the blood and forms urine. At the glomerulus, high pressure forces small molecules such as water, glucose, urea and ions into the Bowman's capsule. All the glucose is then reabsorbed by active transport, and water is reabsorbed along the loop of Henle and collecting duct.

4. ADH and the collecting duct

   Higher tier

   ADH is a hormone released by the pituitary gland that controls how much water the kidney reabsorbs. It makes the collecting duct more permeable to water. When the blood water content is too low, more ADH is released, more water is reabsorbed, and a small volume of concentrated urine is made. Less ADH gives a large volume of dilute urine.

5. Treating kidney failure

   Kidney failure lets waste substances such as urea build up in the blood, and it is treated by kidney dialysis or a kidney transplant. In dialysis, urea and excess ions and water pass from the blood across a partially permeable membrane into dialysis fluid. A donor kidney must be a good tissue match, and drugs prevent rejection.