16 Osmoregulation

  • Created by: lee8444
  • Created on: 12-03-20 13:35

Structure of the mammalian kidney

  • Fibrous capsule - the outer membrane that protects the kidney
  • Cortex - lighter coloured outer region made up of the renal (Bowman's) capsule, convoluted tubules and blood vessels
  • Medulla - darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels
  • Renal pelvis - funnel-shaped cavity that collects urine into the ureter
  • Ureter - a tube that carries urine to the bladder
  • Renal artery - supplies the kidney with blood
  • Renal vein - returns blood back to the heart
  • 1 million nephrons per kidney
1 of 11

Structure of the nephron (1)

  • Bowman's capsule
    • closed-end at the start of the nephron
    • cup-shaped
    • surrounds capillaries (glomerulus)
    • the inner layer of the Bowman's capsule is made up of podocytes (specialised cells)
  • Proximal convoluted tubule
    • a series of loops surrounded by blood capillaries
    • walls are made of epithelial cells which have microvilli
  • Loop of Henle
    • a long, hairpin loop
    • extends from the cortex into the medulla of the kidney and back again
    • surrounded by blood capillaries
  • Distal convoluted tubule
    • series of loops
    • surrounded by blood capillaries
    • walls are made of epithelial cells
2 of 11

Structure of the nephron (2)

  • Collecting duct
    • a tube where multiple distal convoluted tubules empty into
    • lined by epithelial cells
  • Afferent arteriole
    • a small vessel that comes from the renal artery
    • supplies the nephron with blood
    • enters the renal capsule of the nephron where it forms a glomerulus
  • Glomerulus
    • knot of capillaries
    • the fluid is forced out of the blood
    • recombines to form the efferent arteriole
  • Efferent arteriole
    • a vessel that leaves the renal capsule
    • smaller diameter than afferent arteriole
    • causes an increase in blood pressure within the glomerulus
    • branches to form blood capillaries
  • Blood capillaries
    • reabsorb salts, glucose and water before merging back into veins
3 of 11

Stages of osmoregulation

1) Formation of the glomerular filtrate by ultrafiltration

2) Reabsorption of glucose and water by the proximal convoluted tubule

3) Maintainance of a gradient of sodium ions in the medulla by the loop of Henle

4) Reabsorption of water by the distal convoluted tubule and collecting ducts

4 of 11

Formation of glomerular filtrate by ultrafiltratio

  • Blood enters the kidney through the renal artery
  • Renal artery branches into one million arterioles
  • The afferent arteriole divides to form the glomerulus which then forms the efferent arteriole which further subdivides into the capillaries which then form the renal vein
  • The walls of the glomerular capillaries are made up of endothelial cells with pores in them
  • The diameter of the afferent arteriole is larger than the diameter of the efferent arteriole which causes a build-up of hydrostatic pressure
  • This causes water, glucose and mineral ions are squeezed out of the capillary to form the glomerular filtrate
  • Blood cells and large proteins are too big to pass through the across the renal capsule as they are too large
  • The movement of the filtrate out of the glomerulus is resisted by
    • capillary endothelial cells
    • connective tissue
    • epithelial cells of the renal capsule
    • hydrostatic pressure in the renal capsule space
    • low water potential in the glomerulus
5 of 11

Formation of glomerular filtrate by ultrafiltratio

  • Podocytes
    • the inner layer of the renal capsule
    • have spaces in between them
    • allows filtrate to pass in between rather than through
  • All of this results in the hydrostatic pressure of the blood is sufficient enough to overcome the resistance
  • Most of what comes out through ultrafiltration are reabsorbed
6 of 11

Reabsorption of glucose and water by the proximal

  • 85% of the filtrate is reabsorbed here
  • Small molecules are removed, some such as urea are waste, but most are useful
  • The proximal convoluted tubules are adapted to reabsorb substances into the blood by having epithelial cells that have:
    • microvilli to maximise surface area
    • infoldings at their base to further increase the surface area
    • a high density of mitochondria to produce ATP for active transport
  • The process
    • sodium ions are actively transported out of the cells lining the PCT into blood capillaries which carry them away - this lowers the sodium concentration
    • sodium ions now diffuse down the concentration gradient from the lumen of the PCT into the epithelial cells lining the PCT by facilitated diffusion
    • These carrier proteins carry glucose and amino acids along with the sodium ions (co-transport)
    • The molecules have been co-transported into the cells of the PCT then diffuse into the blood
  • About 180dm3 of water enters the nephrons every day but only 1dm3 leaves as urine every day
7 of 11

Loop of Henle function & DCT

  • The loop of Henle is responsible for reabsorbing water from the collecting duct by concentrating the urine so that it has a lower water potential than the blood
  • The concentration of the urine produced is directly related to the length of the loop of Henle
    • the descending limb is narrow with thin walls that are very permeable to water
    • the ascending limb is wider with thick walls that are impermeable to water
  • Loop of Henle acts as a counter-current multiplier
  • The water that passes out of the collecting duct by osmosis happens through aquaporins
  • ADH alters the number of aquaporins in the collecting duct which change the permeability to water
  • Overall, the filtrate that passes through has a lower water potential than blood
  • Distal convoluted tubule
    • the cells that line the DCT have microvilli and many mitochondria which allows for reabsorption by active transport
    • The main role of the DCT is to make final adjustments and to ensure the proper pH of the blood is maintained by selecting specific ions
8 of 11

Loop of Henle process

  • 1) Sodium ions are actively transported out of the ascending limb of the loop of Henle using ATP provided by the many mitochondria in the cells
  • 2) This creates a lower water potential in the medulla in between the two limbs (interstitial region)
  • 3) The walls of the descending limb are permeable to water and so water passes out by osmosis. The water enters the blood capillaries by osmosis and is carried away
  • 4) The filtrate progressively loses water as it moves down the descending limb and reaches its lowest water potential at the tip of the hairpin
  • 5) Sodium ions are either diffused or actively transported out of the ascending limb and so the filtrate has a progressively higher water potential
  • 6) In the interstitial space, there is a gradient of water potential with the highest water potential in the cortex and a lower water potential the further into the medulla
  • 7) The collecting duct is permeable to water so as the filtrate moves down it, water passes out by osmosis into the blood vessels that occupy this space
  • 8) As water passes out of the filtrate the water potential is lowered throughout the counter-current multiplier system which ensures there is always a potential gradient drawing water out of the tubule
9 of 11

If water potential falls

  • Cells called osmoreceptors in the hypothalamus of the brain detects the fall in water potential
  • When the water potential of the blood is low, water is lost by the osmoreceptor cells by osmosis
  • Due to this, the cell shrinks that makes the cell produce a hormone called antidiuretic hormone (ADH)
  • ADH goes to the pituitary gland where it is secreted into the capillaries
  • ADH in the blood goes to the kidneys where it increases the permeability to water of the cell-surface membrane of the cells that make up the walls of the DCT and the collecting duct
  • Specific protein receptors on the CSM of these cells bind to ADH molecules leading to the activation of phosphorylase within the cell
  • The activation of phosphorylase causes vesicles within the cell to move to and fuse with the CSM
  • These vesicles contain numerous water channel proteins (aquaporins) so when they fuse with the membrane, the permeability to water increases
  • More water passes out of the collecting duct and re-enters the blood
  • Overall, this doesn't increase the water potential of the blood but prevents it from going lower
  • Osmoreceptors also send nerve impulses to the thirst centre of the brain to encourage the individual to drink more water
10 of 11

If water potential rises

  • Can be due to
    • large volumes of water being consumed
    • salts used in metabolism aren't replaced by the diet
  • Response:
    • osmoreceptors in the hypothalamus detect the rise in water potential and so it tells the pituitary gland to reduce the amount of ADH it is releasing
    • less ADH causes the membrane to be less permeable to water
    • less water is reabsorbed
    • the urine produced is more dilute
  • All of this is negative feedback
11 of 11


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Organisms respond to changes in their environment resources »