Excretion

• Humans


  • Have organs specialised for the removal of certain excretory products
  • These include the lungs and kidneys
  • The liver also has a vital role in excretion
• Excretion


  The removal from the body of waste products of metabolic reactions, toxic substances and substances in excess of requirements

  
• Egestion


  The expulsion of undigested food waste from the anus

  
• Parts of the human urinary system

  • Kidneys
  • Renal artery
  • Renal vein
  • Ureter
  • Bladder
  • Urethra

  
• Kidneys


  • Remove waste products and excess substances from the blood
  • Regulate the water content of the blood (vital for maintaining blood pressure)
  • Excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts)

  
• Renal artery


  • Brings blood containing waste products and substances in excess from the heart to the kidneys

  
• Renal vein


  • Returns blood to the heart after waste products and substances in excess have been removed

  
• Ureter


  • Directs the removed waste products and substance in excess from the kidney to the bladder

  
• Bladder


  • It stores the waste products and substances in excess as urine
• Urethra


  • It directs the urine out of the body
• The colour and quantity of urine produced in the body can change quickly
• Water intake increases


  Large quantities of pale yellow, dilute urine will be produced
• Temperature increases


  Smaller quantity of dark yellow, concentrated urine will be produced
• Exercise increases


  Smaller quantity of dark yellow, concentrated urine will be produced
• Waste substances


  • Urea from deamination of excess amino acids by the liver
  • Used hormones
  • Drugs

  
• Removal of waste substances by the kidneys

  1. Blood transports waste substances to the kidneys

  2. Kidneys remove waste substances from blood through ultra-filtration and selective reabsorption to form urine

  3. Urine is then excreted out of the body

  
• Ultra-filtration


  Filtration of substances made of small molecules (e.g. glucose, amino acids, minerals salts, urea and water) by the kidney

  
• Reabsorption


  Process whereby the body absorbs all useful molecules (e.g. all glucose, all amino acids, some mineral salts and some water) back to the blood
• Nephron


  • Tiny tubules in the kidney responsible for removal of unwanted substances from blood

  
• Functions of the nephron

  1. In the glomerulus there is ultra-filtration

  2. Useful substances are reabsorbed back into the blood further down the nephron

  
• Components of filtrate: glucose, water, urea, salts
• Selective reabsorption in the nephron

  1. Glomerular filtrate flows through the tubule

  2. Useful substances (such as all glucose molecules, amino acids) are reabsorbed back to the blood by the capillaries coiled around the tubule

  
• Proximal convoluted tubule


  • Glucose is the first substance to be reabsorbed here by active transport
  • Nephron is adapted for this with many mitochondria to provide energy
  • Reabsorption of glucose cannot take place anywhere else in the nephron
• Person with normal blood glucose


  All glucose filtered out can be reabsorbed into the blood
• Person with diabetes


  Not all glucose filtered out can be reabsorbed, so it continues in the filtrate and ends up in urine
• Proximal (first) convoluted tubule


  Where glucose is reabsorbed by active transport
• Nephron


  • Adapted for reabsorption of glucose by having many mitochondria to provide energy for active transport
• Reabsorption of glucose can only take place in the proximal convoluted tubule as the gates that facilitate active transport are only found there
• In a person with normal blood glucose, there are enough gates to remove all glucose from the filtrate
• People with diabetes cannot control their blood glucose, so not all glucose can be reabsorbed in the proximal convoluted tubule
• Unabsorbed glucose continues in the filtrate and ends up in urine, which is why doctors test urine for glucose to check for diabetes
• Loop of Henle


  Principal function is to recover water and mineral salts from urine

  
• Reabsorption in the Loop of Henle

  1. Salts are reabsorbed by diffusion

  2. Water is reabsorbed by osmosis as salts are reabsorbed
• Urea and other unabsorbed substances collect in the collecting duct as urine
• The collecting duct transports urine to the ureters
• Kidney failure


  Occurs when kidneys do not work properly, leading to a build-up of toxic wastes in the body

  
• Humans can survive with one functioning kidney, but total kidney failure is fatal if not treated

  
• Kidney dialysis


  An artificial method of filtering the blood to remove toxins and excess substances

  
• How dialysis works


  1. Blood is taken from an artery, pumped into the dialysis machine, and returned to a vein

  2. Blood and dialysis fluid are separated by a semi-permeable membrane, allowing exchange of substances based on concentration gradients

  3. Dialysis fluid is continually refreshed to maintain concentration gradients

  
• Dialysis may take 3-4 hours and needs to be done several times a week to prevent damage from toxic substance buildup