 Excretion  The removal of metabolic wastes from the body  Kidneys are the primary organ of this process

 1. Excretion of Metabolic Wastes  Removing metabolic waste products, notably nitrogenous waste from breakdown of amino acids (aa):  Urea  Breakdown of amino acids releases ammonia which is toxic to cells)  Liver converts the ammonia to urea so it be safely transported in the blood to the kidney where it is eliminated in urine  Creatinine  Uric acid  if too much uric acid crystals can form in joints  This is known as gout

 2. Maintenance of Water-Salt Balance  Blood volume is associated with salt balance in body  Salt (NaCl) can cause water diffusion into blood  ↑ salt = ↑ blood volume = ↑ blood pressure

 3. Maintenance of Acid-Base Balance  Kidneys regulate acid-base in blood  Excretion of hydrogen ions and reabsorption of bicarbonate to keep pH around 7.4  Urine often has a pH of 6

 4. Secretion of Hormones  Renin  Leads to secretion of aldosterone  Reabsorption of sodium ions  Erythropoetin  Stimulates red blood cell production  Activates vitamin D from the skin  Helps promote calcium absorption from the digestive tract.

16-9  Fig. 16.1

 Kidneys  Two bean-shaped organs  Located near lumbar region on either side of vertebral column  Has a depression called the hilum where the Renal artery Renal vein are located

 Ureters  Conduct urine from the kidneys to the bladder  Small, muscular tube 25cm long, 5 mm thick

 Urinary Bladder  Stores urine until it is expelled from the body  Located in the pelvic cavity  3 openings (orifices):  2 for the ureters  1 for urethera

 Urinary Bladder con’t...  Expandable  Detrusor Muscle – bladder wall  Sphincter Muscles – surround bladder neck and urethra

 While the bladder is filling:  the detrusor muscle is relaxed so the bladder wall can expand and the sphincter muscles are contracted to keep the urethra closed  When the bladder empties:  the detrusor muscle contracts to squeeze the urine out, and the sphincter muscles relax so the urethra can open up ying/pages/default.aspx

 Urethra  Small tube that extends from the urinary bladder to an external opening  females  4 cm long  Increases likelihood of urinary tract infection  Not connected to reproductive system  Males  20 cm long  Common pathway for reproductive and urinary tracts

16-17  Micturition  When the bladder fills to 250ml  Stretch receptors in the bladder wall send sensory impulse to the spinal cord  Motor impulses from spinal cord send a signal the bladder  Bladder contraction and sphinter to relax  This is called an involuntary reflex  Present in infants and young children

 In older children and adults the brain can control this reflex and delay urination until a suitable time  Stretch receptors in the bladder send a sensory signal to micturition center in the pons  If not appropriate it will send an inhibitory signal to keep the sphinter closed  This is called a voluntary response 