Excretory System

Excretion• Rids the body of toxic chemicals,

excess water, salts, and carbon dioxide

• Most important substances excreted:–carbon dioxide -

transported to lungs by circulatory system and exhaled

–nitrogen wastes - urea

–water - sweating and urine

Urea

• Kidneys - regulate amount of water and salt in blood plasma–complex organ

composed of about 1 million microscopic blood filtering units called nephrons

• Each nephron contains 3 elements:–filtration - the filtration device at the top

of each nephron is the Bowman’s capsule•an arteriole enters and splits into a network of capillaries called a glomerulus

•The blood pressure inside the glomerulus forces fluid through the capillary walls

•blood cells, proteins, and other large molecules cannot pass through the wall

•water and small molecules (urea, sodium, and glucose) pass through and enter the hollow interior of the Bowman’s capsule - filtrate

–Reabsorption•Bowman’s capsule is connected to a long narrow tube called the renal tubule

•the tubule bends back on itself forming a loop – loop of Henle

•extracts useful molecules (glucose, ions, and some water)

•these molecules reenter bloodstream by passing into capillaries that wrap around the tubule

Kidney cross-section

–Urine formation (secretion)

•renal tubule empties into a larger tube called a collecting duct

•collecting duct removes much of the water from the filtrate

•the filtrate that remains after salts, nutrients, and water have been removed is urine

Urine Removal from the Body

• Urine produced in kidneys passes from kidneys to urinary bladder through the ureters

• Urinary bladder - hollow, muscular sac that expands as it fills

• Urine leaves bladder and exits body through the urethra

Kidney Disease• Kidney failure is life threatening• Most common causes:

–infection–long-term diabetes–untreated long-term high blood

pressure–autoimmune kidney disease

Kidney stones

Kidney Infection

• Hemodialysis

–tubes are surgically inserted into an artery and a vein

–blood passes from artery to machine to vein

–machine filters blood

• Kidney transplant

–problem with transplant is rejection

–histocompatibility antigens - markers on surface of cells that identify them so they are not attacked by the immune system

–the more closely related, the more likely the histocompatibility antigens are common