Homeostasis and Excretion

HOMEOSTASIS:Maintenance of a constant and stable internal environment

THE URINARY SYSTEM: Two kidneys at back of abdominal cavity Excrete waste nitrogen as urea Eliminate excess salts Osmoregulate to maintain water content of blood

Blood supplied to kidney through renal artery From aorta so blood enter at high pressure Blood filtered and passes out through renal vein to vena cava Urine passes out through ureters and stored in bladder Bladder has tube leading to outside called urethra Wall of urethra has sphincters, 2 ring-like muscles which contract to hold back urine

THE KIDNEYS:

ULTRAFILITRATION: Arterioles branch into network of capillaries in glomerulus Small molecules E.G. water, sugar, amino acids, vitamins, minerals, urea pass into Bowman’s capsule through selectively permeable membrane from glomerulus to form the glomerular filtrate Larger substances E.G. RBC, WBC, proteins, platelets, too large to be filtered or pass through membrane

SELECTIVE REABSORPTION: Efferent wraps around proximal convoluted tubule Some substances diffuse back into blood stream Since higher glucose in tube than blood, half diffused back into bloodstream Then, through active transport, most substances go back into the blood, except for urea Osmosis also takes place

SECRETION: Excessive quantities of certain dissolved substances secreted in distal convoluted tubule E.G. creatinine, hydrogen and potassium ions, some hormones and drugs Blood pH level maintained If body needs water reabsorbed back into blood from collecting duct

CONTROL OF THE BODY’S WATER CONTENT:

CONTROL OF BODY TEMPERATURE: