- The excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
- Kidneys are located in the abdomen, one on either side of the backbone.
- Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra.
- How is urine produced? The purpose of making urine is to filter out waste products from the blood.
- Just as CO2 is removed from the blood in the lungs, nitrogenous waste such as urea or uric acid are removed from blood in the kidneys.
- It is then no surprise that the basic filtration unit in the kidneys, like in the lungs, is a cluster of very thin-walled blood capillaries.
- Each capillary cluster in the kidney is associated with the cup-shaped end of a tube that collects the filtered urine .
- Each kidney has large numbers of these filtration units called nephrons packed close together.
- Some substances in the initial filtrate, such as glucose, amino acids, salts and a major amount of water, are selectively re-absorbed as the urine flows along the tube.
- The amount of water re- absorbed depends on how much excess water there is in the body, and on how much of dissolved waste there is to be excreted.
- The urine forming in each kidney eventually enters a long tube, the ureter, which connects the kidneys with the urinary bladder.
- Urine is stored in the urinary bladder until the pressure of the expanded bladder leads to the urge to pass it out through the urethra.
- The bladder is muscular, so it is under nervous control, as we have discussed elsewhere. As a result, we can usually control the urge to urinate.