The Excretory System

• Excretion


  The removal of wastes of metabolism from the body
• Excretory functions of organs


  • Lungs
  • Sweat glands
  • Liver
  • Kidneys
• Lungs


  • Involved in excretion of carbon dioxide that is produced in all body cells during cellular respiration
• Sweat glands


  • In the skin, secrete sweat (mostly water) for cooling which also contains metabolism by-products such as salts, urea + lactic acid
• Liver


  • Processes many substances so that they can be excreted
• Kidneys


  • Principal excretory organs, maintain constant concentration of materials in body fluids and remove toxic wastes including urea, uric acid & creatinine
• While chemical processes produce essential products, they also produce wastes which must be removed from the body
• Excretory system


  • Kidneys
  • Skin
  • Liver
  • Lungs
• Deamination


  The process where the amino group (NH2) is removed from an amino acid, converting it to ammonia (NH3) which is then converted to less toxic urea
• Excess amino acids need to be converted to urea as they cannot be stored
• Substances excreted in sweat


  • Water
  • Sodium chloride
  • Lactic acid
  • Urea
  • Some drugs (e.g. salicylic acid)
• Kidney structure


  • Renal capsule
  • Renal cortex
  • Renal medulla
  • Renal pelvis
  • Renal pyramids
  • Renal columns
  • Renal hilum
• Nephron


  • The functional unit of the kidney, there are about 1.2 million in each kidney
• Formation of urine


  1. Glomerular filtration

  2. Selective reabsorption

  3. Secretion

  4. Excretion
• Glomerular filtration


  • Fluid is forced out of the blood of the glomerulus and is collected in the glomerular capsule
  • Afferent arteriole is wider than efferent arteriole, producing high blood pressure and increased fluid filtration
  • Filtrate consists of all components of blood except RBCs and WBCs
• Selective reabsorption


  • Many components of the filtrate are reabsorbed back into the blood in the peritubular capillaries
  • Substances reabsorbed include water, glucose, amino acids, ions
  • Reabsorption is regulated by hormones to control water balance
• Tubular secretion


  • Adds material from the blood to the filtrate as a "last chance" to excrete unwanted substances
• Blood plasma


  Differs in composition from urine
• Ration


  Fluid is forced out of the blood of the glomerulus and is collected in the glomerular capsule
• Afferent arteriole


  • Leading in is wider than the efferent arteriole leading out – produces a high blood pressure and increases fluid forced out of arteriole
• Blood in glomerulus


  • Separated from cavity in glomerular capsule by only two cells – one of the capillary wall and one of the glomerular capsule (which has thin, flat cells)
• Filtrate


  Resulting fluid from glomerular filtration
• Components of filtrate


  • Water, salts, amino acids, fatty acids, glycerol, glucose, hormones, ions, etc.
• Only 20% of plasma is filtered into the glomerular capsule due to blood continually being pushed through the capillaries
• Selective Reabsorption


  Many of the components of filtrate are of use to the body, therefore, some selective reabsorption of filtrate must take place to return them to the blood in the peritubular capillaries
• Substances reabsorbed


  • Water, glucose, amino acids, ions (sodium, potassium, calcium, chloride, bicarbonate)
• Tubular Secretion


  While reabsorption removes substances from filtrate into the blood, tubular secretion adds material from the blood to the filtrate - "last chance" for substances we want removed from body
• Materials secreted into filtrate


  • Potassium and hydrogen ions, creatinine, and drugs such as penicillin
• Effects of Tubular Secretion


  Maintains blood pH by removing hydrogen and ammonia ions, Maintains urine pH by adding hydrogen and ammonia ions to make it slightly acidic
• Regions of Nephron


  • Collecting Duct
  • Loop of Henle
  • Proximal convoluted tubule
  • Distal convoluted tubule
• Transport Processes


  • Active Reabsorption
  • Passive Reabsorption
  • Formation of filtrate
  • Secretion
• Substances Transported


  • Water
  • K+, Cl- and HCO3- ions
  • Glucose, Na+ ions and Amino acids
  • Na+ and Cl- ions
  • H+, K+, NH4+, urea, creatinine and some drugs
• Urine Formation


  1. Glomerular Filtration

  2. Selective reabsorption

  3. Tubular Secretion
• Glomerular Filtration


  Fluid forced from blood and into the glomerular capsule as it passes through the glomerulus. All components of blood, except RBCs and WBCs, are filtered out of the blood.
• Selective Reabsorption


  So the 'useful' substances are not lost via excretion, substances are selectively reabsorbed from the filtrate into the blood. Occurs all along the renal tubule – proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.
• Tubular Secretion


  Materials are added from the blood to the filtrate, to be excreted. Allows control over pH of blood and urine.
• Activities in Regions of Nephron

  • Renal corpuscle: Filtration of blood from capillaries of glomerulus and formation of filtrate in glomerular capsule
  • Proximal convoluted tubule: Reabsorption of water, sodium, potassium, chloride, bicarbonate ions, glucose
  • Loop of Henle: Reabsorption of water, sodium and chloride ions
  • Distal convoluted tubule: Reabsorption of sodium ions, water
  • Collecting duct: Reabsorption of water
• Transport Processes


  Passive: Reabsorption of water, sodium, potassium, chloride and bicarbonate ions

  Active: Reabsorption of glucose, secretion of hydrogen and potassium ions, creatinine and certain drugs
• Excretion


  Water and other substances not reabsorbed drain from the collecting ducts into the renal pelvis. Now called urine, drains into ureters (one from each kidney) and to the urinary bladder. Bladder is a hollow muscular organ from which the urethra exits. Urethra carries urine from the bladder to the exterior of the body.
• Components of Urine


  • Water (96%), Urea (2%), Various ions (1.5%), Other (0.5%)