16 Osmoregulation

• 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

• 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

• 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

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

• 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

• 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

• 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

• 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

• 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

• 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

• 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