Excretion

• Excretion

  The removal of waste substances of metabolic reactions, toxic materials and substances in excess requirements
• Organs for excretion in humans

  • Kidneys
  • Lungs
  • Liver
• Carbon dioxide

  Must be excreted as it can dissolve in water to become acidic, reducing the pH of cells and enzyme activity
• Too much carbon dioxide is toxic
• Urea is also toxic in higher concentrations, so it must be excreted
• Kidney

  Located in the back of the abdomen, has two important functions: regulating water content of blood and excreting toxic waste products of metabolism (urea)
• Nephrons


  • Millions of structures in the kidney, also called kidney tubules or renal tubules
  • Start in the cortex of the kidney and loop down into the medulla and back up the cortex
• Reabsorption of glucose


  1. After the glomerular filtrate enters the Bowman's capsule, glucose is the first substance to be reabsorbed in the proximal (first) convoluted tubule by active transport

  2. The nephron is adapted for this by having a lot of mitochondria to provide energy for active transport of glucose

  3. Reabsorption of glucose can't happen anywhere else in the nephron as the proximal convoluted tubule is the only place where the gates that facilitate active transport are found
• People with diabetes

  Can't control their blood glucose levels, meaning not all glucose is filtered out of blood and can be reabsorbed, so it is excreted in urine
• Ultrafiltration

  1. Smaller molecules in blood are forced out of the capillaries and into Bowman's capsule to form the filtrate (mix of water, salts, glucose, urea)

  2. This process increases pressure on the blood as the capillaries get narrower in the glomerulus
• Reabsorption of water and salts

  1. As the filtrate drips through the loop of Henle, necessary salts are reabsorbed back into blood by diffusion

  2. As salts are reabsorbed, water follows by osmosis

  3. Water is also reabsorbed from the collecting duct in different amounts depending on the body's needs
• Urea dissolves in blood and is taken to the kidney for excretion, and small amounts are also excreted in sweat
• High urea levels

  Causes cell death, reduced response to insulin leading to diabetes, and deposits inside blood vessels
• Deamination

  Excess amino acids absorbed in the blood that aren't needed to make proteins are broken down, with the part containing carbon turned into glycogen and stored, and the part with nitrogen converted into ammonia and then urea