11. Controlling blood water potential✅

The kidneys regulate the water potential of the blood

  • water is essential to keep the body functioning, so the amount of water in the blood and the water potential must be kept constant

  • Mammals excrete urea and other waste products in solution. This means water is lost during excretion. Water is also lost in sweat

  • The kidneys regulate the water potential of the blood and urine, this is called osmoregulation


Water potential is too low

  • The body is dehydrated

  • More water is reabsorbed by osmosis into the blood from the tubules of the nephrons

  • This means the urine is more concentrated, so less water is lost during excretion


Water potential is too high

  • The body is hydrated

  • So less water is reabsorbed by osmosis into the blood from the tubules of the nephrons

  • This means the urine is more dilute, so more urine is lost during excretion


Location

  • water is reabsorbed into the blood along almost all of the nephron

  • But regulation of water potential mainly takes place in the loop of Henle, DCT and collecting duct

  • The volume of water reabsorbed by the DCT and collecting duct is controlled by hormones


Maintaining a sodium ion gradient - Loop of Henle

— loop of Henle is located in the medulla/ inner layer of the kidneys. It’s made of two ‘limbs’, the descending limb and the ascending limb. The limbs control the movement of sodium ions so that water can be reabsorbed by the blood —



  • Near the top of the ascending limb, Na+ ions are pumped out into the medulla using active transport.

  • The ascending limb is impermeable to water, so the water stays inside the tubule

  • This creates a low water potential in the medulla because there’s a high conc of ions



  • because there’s a lower water potential in the medulla than in the descending limb, water moves out of the descending limb (which is permeable to water) into the medulla by osmosis

  • This makes the filtrate more concentrated (the ions can’t diffuse out as the descending limb isn’t permeable to them)

  • The water in the medulla is reabsorbed into the blood through the capillary network



  • near the bottom of the ascending limb Na+ ions diffuse into the medulla

  • Further lowering the water potential in the medulla

  • The ascending limb is impermeable to water so it stays in the tubule



  • water moves out of the distal convoluted tubule (DCT) by osmosis and is reabsorbed into the blood



  • the first 3 stages massively increase the ion conc in the medulla which lowers the water potential

  • This causes water to move out of the collecting duct by osmosis

  • The water in the medulla is reabsorbed into the blood through the capillary network

— the volume of water reabsorbed into the capillaries is controlled by changing the permeability of the DCT and the collecting duct —



Water reabsorption

  • the water potential of the blood is monitored by cells called osmoreceptors in a part of the brain called the hypothalamus

  • When the water potential decreases, water will move out of the osmoreceptors cells by osmosis, causing cells to decrease in volume

  • This sends a signal to other cells in the hypothalamus, which sends a signal to the posterior pituitary gland

  • This causes the posterior pituitary gland to release a hormone called antidiuretic hormone (ADH) into the blood


ADH

  • makes the walls of the DCT and collecting duct more permeable to water

  • This means more water is reabsorbed from these tubules into the medulla and into the blood by osmosis

  • A small amount of concentrated urine is produced which means less water is lost from the body


How does ADH change the water content of the blood

Water content is too low - dehydrated

  • the water content of the blood drops, so its water potential drops

  • This is detected by osmoreceptors in the hypothalamus

  • The posterior pituitary gland is stimulated to release more ADH in the blood

  • This means that the DCT and collecting duct become more permeable, so more water is reabsorbed into the blood by osmosis

  • A small amount of highly concentrated urine is produced and less water is lost


Water content is too high - hydrated

  • water content of the blood rises, so its water potential rises

  • This is detected by osmoreceptors in the hypothalamus

  • The posterior pituitary gland releases less ADH into the blood

  • This means that the DCT and collecting duct become less permeable, so less water is reabsorbed into the blood by osmosis

  • A large amount of dilute urine is produced and more urine is lost