excretion

• what is excretion?

  process of removal of metabolic wastes
• In unicellular organisms the waste product is removed by simple diffusion from the body surface into surrounding water.
• excretory system in humans includes:

  a pair of kidneys, a pair of ureters, urethra and urinary bladder
• path of flow of urine

  kidneys -> ureters -> urinary bladder -> urethra
• Nephrons are the structural and functional unit of the kidney.
• filtration unit of the human body

  the kidney
• location of kidneys

  in the abdomen, one on either side of the backbone
• steps of urine formation:

  glomerular filtration, selective reabsorption and tubular secretion
• The amount of water absorption depends upon amount of excess water present in the body and the amount of dissolved waste to be excreted.
• Tubular secretion occurs mostly in the distal convoluted tubule (DCT).
• Vasopressin is a hormone secreted only when concentrated urine is to be passed out.
• Absence of vasopressin causes dilute urine.
• Normally, in a healthy adult the initial filtrate in the kidneys is about 180 l daily. However, the volume actually excreted is only 1-2 l a day because the remaining filtrate is reabsorbed in the kidney tubules.
• In case of kidney failure, an artificial kidney can be used which is a device to remove nitrogenous waste products from the blood through dialysis.
• Artificial kidney contains a number of tubes with a semi-permeable lining suspended in a tank filled with dialysing fluid which has the same osmotic pressure as blood except that it is devoid of nitrogenous wastes.