Chapter 12: The Excretory System

• Elimination of Waste Products

  1. In cellular respiration, CO2 is produced and is eliminated by the lungs (exhalation)

  2. The liver eliminates ingested toxins (i.e alcohol, heavy metals) by converting them into soluble compounds

  3. The kidneys eliminate dangerous products of protein metabolism using the liver to convert the products into chemicals

  4. The kidneys play an important role in removing waste, balancing blood pH, and maintaining water balance

  5. The excretory mainly regulates the composition and volume of body fluids by removing wastes and returning needed substances to the body for reuse
• Deamination


  Removal of an amino group from an organic compound
• Deamination occurs in the liver
• Deamination converts any extra protein consumed into carbohydrates
• Ammonia (NH3)


  Very toxic byproduct of the removal of the amino group from amino acids
• Conversion of ammonia to urea

  In the liver, two NH3 molecules combine with CO2 to form urea
• Urea


  Nitrogen waste formed from two molecules of ammonia and one molecule of carbon dioxide, 100 000 times less toxic than ammonia
• Uric Acid


  A waste product formed from the breakdown of nucleic acids
• Water Balance


  1. Regulated by the kidneys

  2. The average adult loses about 2 L of water through urine, exhaled air, and perspiration

  3. Water loss is increased during exercise

  4. Maintaining fluid balance requires a consumption of 2 L of fluids/day
• Waste Products


  • Ammonia (NH3)
  • Urea
  • Uric Acid
  • Carbon Dioxide (CO2)
  • Bile Pigments
  • Lactic Acid
• Organs of Excretion


  • Kidneys
  • Liver
  • Lungs
  • Skin
• Renal Arteries


  Carry blood to the kidneys, branch from abdominal aorta
• Kidneys


  Hold as much as 25% of the body's blood at any given time, filter waste from the blood, transfer waste (urine) via the ureters to the bladder
• Ureter


  A tube that conducts urine from the kidney to the bladder
• Bladder


  Stores the urine, has a urinary sphincter muscle that acts as a valve to help store urine, stretch receptors send a signal to the brain when the bladder is full
• Urethra


  The tube that carries urine from the bladder to the exterior of the body
• Parts of the Kidney

  • Cortex
  • Medulla
  • Renal Pelvis
• Nephron


  A functional unit of the kidney, long slender tubes where urine is formed and collected, about 1 million nephrons in the kidneys
• Afferent Arterioles


  Small branches of the renal artery that carry blood to the glomerulus, located in the cortex of the kidney
• Glomerulus


  The high-pressure capillary bed that is the site of filtration
• Efferent Arteriole


  Small branches of the renal artery that carry blood away from the glomerulus to the peritubular capillaries, located in the cortex of the kidney
• Peritubular Capillary


  Network of small blood vessels that surround the tubule of the nephron, wrap around the kidney tubule
• Bowman's Capsule


  Cuplike structure that surrounds the glomerulus, located in the cortex of the kidney, fluid to be processed into urine enters the Bowman's capsule from the blood
• Proximal Tubule


  Section of the nephron joining the Bowman's capsule with the loop of Henle, urine is carried from the proximal tubule to the loop of Henle
• Loop of Henle


  Section of the tubule that carries filtrate from the proximal tubule to the distal tubule, descends into the medulla of the kidney
• Distal Tubule


  Conducts urine from the loop of Henle to the collecting duct, last segment of the nephron
• Collecting duct


  Tube that carries urine from nephrons to the renal pelvis, collect urine from many nephrons that merge in the pelvis of the kidney
• Urine Formation


  1. Filtration

  2. Reabsorption

  3. Secretion
• Filtration


  Movement of fluid from the blood into Bowman's capsule, blood moves from the afferent arterioles into the glomerulus, dissolved solutes pass through the walls of the glomerulus into the Bowman's capsule
• Reabsorption


  The transfer of essential solutes and water from the nephron back into the blood, 1 mL of urine is formed for every 120 mL of fluid that is filtered, selective reabsorption occurs by passive and active transport, water reabsorption occurs by osmosis
• Threshold Level


  The maximum amount of a substance that can be moved across the nephron
• Selective Reabsorption: Passive Transport


  Movement of substances across a membrane from where they are in high concentration to where they are in low concentration
• Selective Reabsorption: Facilitated Diffusion


  Movement of substances using carrier molecules, larger molecules and charged ions may use a carrier protein to help their passage across the membrane
• Selective Reabsorption: Active transport


  Movement of substances against the concentration gradient (from low to high concentration), mitochondria are present to supply ATP for active transport
• Water Reabsorption: Osmosis


  Movement of water across a semipermeable membrane from where it is in high to low concentration, when solutes move from the nephron back into the capillary, water will follow
• Regulation of Water Reabsorption

  Osmoreceptors in the hypothalamus send impulses to the pituitary gland that causes the release of antidiuretic hormone (ADH), ADH travels in the blood to the kidneys where it increases the permeability of the distal tubule and the collective duct to reabsorb more water into the blood, alcohol and caffeine are diuretics that decrease ADH release
• The kidney regulates salt balance in the blood by controlling the excretion and reabsorption of substances
• When blood plasma is too concentrated

  1. Osmotic pressure is increased

  2. Osmoreceptors in the hypothalamus send impulses to the pituitary gland

  3. Pituitary gland releases antidiuretic hormone (ADH)

  4. ADH travels to kidneys

  5. ADH increases permeability of distal tubule and collecting duct

  6. More water reabsorbed into blood

  7. Blood diluted, osmotic pressure lowered
• When blood plasma is too dilute

  1. Osmotic pressure is decreased

  2. Osmoreceptors prevent release of ADH

  3. Distal tubule and collecting duct become less permeable to water

  4. More water excreted in urine

  5. Blood solutes concentrated, osmotic pressure returns to normal
• Diuretics


  Substances that decrease ADH release, increasing water loss to urine