Kidneys

• Ultrification of blood:

  • the high hydrostatic pressure in each glomerulus, from contraction of the left ventricle, forces water and small molecules from the capillaries into the Bowman's capsule. Forming glomerular filtrate
  • Filtration occurs through the fenestrations in the basement membrane of the endothelial cells lining the capillary, and then through the podocytes of the Bowman's capsule
• Selective reabsorption in PCT:

  • many of the ions, small molecules and water are reabsorbed from the filtrate back into the capillaries surrounding the PCT
  • glucose molecules are reabsorbed by co-transport
  • water molecules are reabsorbed by osmosis as the glucose lowers the water potential, first in the epithelial cell, and then in the blood
  • diabetes results in a high concentration of glucose in the glomerulus filtrate because all the carrier proteins are fully saturated.
• Water reabsorption:

  • The ascending limb actively transports Na+ ions and Cl- ions from the filtrate into the medulla
  • Water moves out of the filtrate from the permeable descending limb, into the medulla and then into the blood by osmosis, down its water potential gradient
  • Na+ and Cl- diffuse back into the filtrate in the descending limb
• The loop of Henle as a counter current multiplier:

  • because very ions are pumped out of the ascending limb, deep in the medulla, the water potential is very low in the surrounding fluid.
  • moving through the medulla to the cortex, the water potential increases as there are fewer ions pumped out.
  • water in the collecting duct leaves by osmosis, down its water potential gradient along its whole length due to the decrease in water potential as the ascending limb goes deeper in the medulla
  • water enters the blood in the capillaries by osmosis
  • the longer the loop of Henle the more water is reabsorbed
• Diuresis:

  • excess water loss in the urine is known as diuresis
  • an antidote to diuresis is called an antidiuretic
  • mammals produce a hormone called antidiuretic hormone (ADH) that reduces water loss in the urine.
  • ADH is produced in the hypothalamus then secreted to and stored in the posterior pituitary gland.
  • When the water potential of the blood decreases (becomes more negative), osmoreceptors in the hypothalamus detect this and send impulses to the posterior pituitary stimulating it to release ADH
• Hypothalamus

  • contains osmoreceptors
  • produces ADH, which it secretes into the posterior pituitary body
• Posterior Pituitary Body

  • releases ADH into the blood when simulated to do so by osmoreceptors in the hypothamalus
• ADH

  • stimulates reabsorption of water by collecting ducts and distal convoluted tubules. It binds to receptors on their walls and causes more aquaporins to be incorporated into their membranes.
  • more water can then move out of the filtrate and into the blood, by osmosis, down the water potential gradient.
  • A smaller volume of more concentrated urine is produced.