Excretory System

- lots of chemical reactions take place in cells all the time

- some of these reactions produce waste products

- these waste products would collect in the blood if not removed

- it is the job of the kidneys to remove wastes from the blood

- each minute about 1 L of blood is filtered by the kidneys

- each kidney has millions of tiny tubes called nephrons

- these are the filters of the kidney

- cleaning the blood happens in three steps:

Filtration

         - blood enters the kidney

         - blood passes into part of the nephron that is a ball-like structure

         containing lots of capillaries

         - inside the ball, water, salt, glucose, vitamins, and urea (a nitrogen

          waste from protein) are filtered out of the blood

         - these all move into the tube part of the nephron

         - the liquid that forms inside the tube is called urine

Reabsorption

         - some of the things filtered out are useful to the body so they must

          be recovered

         - as the urine continues through the tube, the useful nutrients are

          reabsorbed back into the blood

         - lots of water is also reabsorbed so that the urine becomes more

          concentrated

         - this is an important way for organisms to avoid losing water

Secretion

         - sometimes not all the wastes are removed during filtration

         - these excess wastes must be specially moved out of the blood into

         the nephron

- urine flows through two tubes called ureters to the bladder where it is stored

- the urine then passes through the urethra and out of the body

Disorders

Kidney stones

- some wastes like calcium that are not excreted by the kidney can build up and form solid stones

- these are called kidney stones

- the most common cause is not drinking enough water

- small kidney stones can pass through the ureter but it is extremely painful

- larger stones can block the ureter and cause urine to back up into the kidney

- some of the chemicals in urine are toxic and can kill the cells of the kidneys

- medication or ultrasonic treatment would be required to treat this condition

Kidney infection

- infections in the urinary tract can sometimes move into the kidney

- serious infections can cause the kidney to fail which is very dangerous

- wastes would not be removed from the blood and death can result in a matter of days

- in cases of kidney failure, dialysis is used to artificially filter the blood

- some dialysis patients spend several hours each week connected to a dialysis machine to clean their blood