The Role of ADH (DP IB Biology: HL)

• Living organisms can maintain a safe balance of water and solutes in their bodies by osmoregulation
  • Osmoregulation is an example of homeostasis
• The kidneys play an important role in osmoregulation by altering the amount of water reabsorbed from the glomerular filtrate into the blood
• The amount of water reabsorbed by the kidneys can be regulated by changing the permeability of the walls of the distal convoluted tubule and collecting duct to water
• The permeability of these parts of the nephron is regulated by a hormone called antidiuretic hormone, or ADH
• ADH is released from the posterior section of the pituitary gland in the brain, which is regulated by a region of the brain called the hypothalamus
  • The hypothalamus monitors the composition of the blood as it flows past osmoreceptor cells in the brain, as well as receiving signals from receptors elsewhere in the body

Blood water content is monitored by osmoreceptor cells in the hypothalamus, which then regulates the release of ADH from the posterior pituitary gland into the blood.

Low blood water content

• Blood water content might drop as a result of reduced water intake, sweating, or diarrhoea
  • Low blood water content can also be referred to as high blood solute concentration, or high blood osmolarity
  • If blood water content gets too low it can lead to dehydration
• A reduction of blood water content is detected by the hypothalamus in the brain
• The hypothalamus causes the pituitary gland to secrete ADH into the blood
  • The target cells of ADH are in the distal convoluted tubule and collecting duct in the kidneys
• ADH increases the permeability of the walls of the distal convoluted tubule and collecting duct in the kidneys to water
  • The permeability of the walls of the distal convoluted tubule and collecting duct are increased by increasing the number of channel proteins called aquaporins in the cell surface membranes of the cells lining the nephron lumen
• More water is reabsorbed into the blood via the distal convoluted tubule and collecting duct
  • The activity of the loop of Henle generates a concentration gradient across the medulla, meaning that as the collecting duct descends into the medulla the osmolarity of the tissues of the medulla increases; this means that water is reabsorbed by osmosis all the way down the length of the collecting duct
• The reabsorption of water leaves a concentrated filtrate that passes through the collecting duct and into the renal pelvis
  • This remaining filtrate is the urine; from the renal pelvis it passes along the ureter to the bladder
• The blood water content increases and a small quantity of concentrated urine is produced

High blood water content

• Blood water content might increase due to increased water intake or loss of salts during sweating
  • High blood water content can also be referred to as low blood solute concentration, or low blood osmolarity
  • If blood water content gets too high it can lead to overhydration
• High blood water content is detected by the hypothalamus
• The hypothalamus no longer stimulates the pituitary gland to release ADH and ADH levels in the blood drop
• The distal convoluted tubule and collecting duct walls become less permeable to water
  • Fewer aquaporins are present
• Less water is reabsorbed from these regions of the nephron into the blood, and the water instead passes down the collecting duct into the renal pelvis along with the rest of the filtrate
• Blood water content decreases and a large quantity of dilute urine is produced

Osmoregulation is an example of homeostasis; the amount of water reabsorbed by the kidneys into the blood is regulated