Excretion

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Excretion Section 39.3
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Excretion

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

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Excretion

• Lungs
• Excrete CO2 and water vapor in exhaled air
• Kidneys
• Excretes nitrogen wastes, salts, water, and other
  substances in urine
• Skin
• Excretes water, salts, small amounts of nitrogen
  wastes, and other substances in sweat

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Excretion

• Urea
• Main nitrogenous waste of body
• Produced in liver from ammonia (NH3
• Formed during metabolism of proteins and nucleic
  acids

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How the Kidney Works
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The Kidneys

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

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Nephron Structure
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Filtration

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

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Filtration
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Filtration
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Reabsorption

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

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Reabsorption
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Reabsorption
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Secretion

• When substances pass from the blood into the
  filtrate
• Eliminate toxic substances from blood

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Some Interesting Facts!

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

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Urine Formation

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

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Excretion

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

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Animation

• Click here for Urinary System Animation

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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