Excretion – Section 39.3

Excretion n Process that rids the body of substances: –toxic chemicals –excess water –salts –carbon dioxide n Maintains osmotic (water) and pH balance

Excretion n Lungs –Excrete CO 2 and water vapor in exhaled air n Kidneys –Excretes nitrogen wastes, salts, water, and other substances in urine n Skin –Excretes water, salts, small amounts of nitrogen wastes, and other substances in sweat

Excretion n Urea –Main nitrogenous waste of body –Produced in liver from ammonia (NH 3 –Formed during metabolism of proteins and nucleic acids

How the Kidney Works

The Kidneys n Maintains balance of salt, water, minerals, vitamins, pH, and volume of blood n Each Kidney composed of ~ 1 million microscopic units called a nephron that filter the blood n Three phases occur as blood flows through a nephron: –Filtration –Reabsorption –Secretion 6

Nephron Structure

Filtration n Occurs in Bowman’s capsule where there is a network of arteries called a glomerulus n Blood pressure inside the capillaries forces fluid containing water, salt, glucose, amino acids, and urea into the hollow interior of Bowman’s capsule. n This fluid is called filtrate. n Blood cells, proteins, and other molecules too large to cross the membrane remain in the blood.

Filtration

Reabsorption n Filtrate passes from the Bowman’s capsule into the renal tubules. n As the filtrate passes through the renal tubules useful molecules; glucose, ions, and some water, is reabsorbed. n These substances reenter the bloodstream through capillaries that wrap around the tubule n About 99% of water is returned to blood by osmosis

Reabsorption

Secretion n When substances pass from the blood into the filtrate n Eliminate toxic substances from blood

Some Interesting Facts! n Blood passes through kidneys 300 times a day. n Nephrons clean all your blood in 45 minutes n Every day the nephrons send about six cups of urine to the bladder

Urine Formation n Filtrate contained in nephron is urine n Formed from the water, urea, and salts that are left after the absorption and secretion processes n Urine leaves the kidneys through the ureters and travels to the urinary bladder n Urinary bladder stores urine and expands as more urine enters 17

Excretion n Stretch receptors send nerve impulses to spinal cord n Spinal cord returns impulses to bladder and urethra simultaneously n Impulses cause contractions of bladder’s muscular wall and relaxes muscle of urethra n Urine is emptied through the urethra

Animation n Click here for Urinary System Animation Click here for Urinary System Animation

Review Questions 1. Identify how the carbon dioxide in your body is produced and excreted. 2. Relate the following terms to the formation of urine: filtration, reabsorption, and secretion. 3. Summarize how urine is stored and eliminated from the body. 4. Concept Generalization: circulatory system, respiratory system, and excretory system