1. Ch. 14 Part 5
Loop of Henle, Distal Convoluted Tubule, Collecting Duct, Osmoregulation

3. Loop of Henle Review Found in 1/3 of all nephrons in kideny
Begins in Cortex of Kidney and Dips into Medulla of Kidney
Two Parts:
Descending LOH
Ascending LOH
Function:
To create a very high concentration of Na+ and Cl- ions in tissue fluid of medulla, enabling water to be reabsorbed from fluid in collecting duct as it flows through medulla
Enables concentrated urine production
Ensures water is conserved in body and not released in urine
Prevents dehydration

4. Loop of Henle Reabsorption
Ascending Loop of Henle
Loop of Henle Reabsorption
Descending Loop of Henle
Impermeable to water
Permeable to Na+ and Cl- ion
Permeable to Urea
Cells ACTIVELY TRANSPORT Na+ and Cl- ions OUT of Filtrate in LOH and INTO tissue fluid in medulla
Decreases water potential in tissue fluid
Increases water potential of filtrate fluid inside of ascending ALOH
High concentrations of Na+ and Cl- build up near bottom of LOH (between DLOH and ALOH)
As filtrate moves UP ALOH, filtrate becomes less concentrated with Na+ and Cl- (it has been losing them the whole way up)
Still removed b/c higher portions of ALOH are found in the medulla that has less concentrated tissue fluid (near top of medulla, close to cortex)
Concentration of Na+ and Cl- inside ALOH is never drastically different from concertation of ion sin tissue fluid
Permeable to Water
Permeable to Na+ and Cl- ions
Filtrate flows down DLOH
Water moves OUT of LOH into tissue fluid via OSMOSIS
Na+ and Cl- in medulla tissue fluid diffuse INTO DLOH (down their conc.grad.)
Bottom of LOH
Filtrate contains LESS water and LOTS of Na+ and Cl- ions (lower water potential)
Longer loops = more concentrated filtrate becomes in LOH

5. Counter-Current Multiplier
Mechanism used in the Loop of Henle
Created by have the DLOH and ALOH run side by side
Fluid flows down in DLOH
Current goes down
Fluid flows up in ALOH
Current goes up
Enables maximum concentration of solutes to build up inside and outside the tube at the bottom of the LOH

7. Urea in Loop of Henle and Collecting Duct
Ascending LOH and Collecting Ducts PERMEABLE to UREA
Urea has diffused into tissue fluids
High concentrations on UREA in Medulla tissue fluid
Collecting Duct dips into medulla
Some Urea diffuses back out of collecting duct and INTO medulla tissue fluid (increasing solutes, lowering water potential)
Water flows OUT of collecting duct by OSMOSIS
Water is reabsorbed into tissue fluid
Happens until water potential of medulla tissue fluid is equal to water potential of urine(may be greater than water potential in blood)
Controlled by ADH
Anti-diuretic hormone (more on this later)
Some urea remains in collecting duct and is excreted in urine

9. Reabsorption in the Distal Convoluted Tubule (DCT) and Collecting Duct
First part of DCT
Functions same as ALOH
Na+ and Cl- actively pumped out
Second Part of DCT
Functions same as collecting duct
Na+ ions are actively pumped from filtrate fluid IN tubule TO tissue fluid INTO peritubular capillaries
K+ ions ACTIVELY TRANSPORTED INTO tubule
Rate of ions pumping in and out varies
Regulates concentration of ions in blood

11. Animal Adaptations
Mammals can produce highly concentrated urine to conserve water
Increase thickness of medulla of kidneys = more concentrated urine (more water reabsorbed by tissues)
Deep LOH with ascending LOH that have lots of Na+-K+ pumps
Cytoplasm has lots of mitochondria (lots of ATP to pum Na+)
Human concertation of urine = 4x that of our blood plasma
Desert rodents concertation of urine = 20 x that of their blood plasma

12. Osmosregulation Control of water potential of body fluids
Important part of homeostasis
Involves:
Hypothalamus
Osmoreceptors sensory neurons
Monitor water potential of blood
Posterior pituitary gland
Kidney

13. ADH Anti-diuretic hormone
“Diuresis” = production of dilute urine (release of water)
ADH stops urine from being dilute by stimulating reabsorption of water
Peptide hormone made of 9 amino acids
Hypothalamus detects changes in water potential of blood
Using OSMORECPTOR sensory neurons
Detect decrease in water potential and send nerve impulse to stimulate Posterior Pituitary Gland
Secreted by posterior pituitary gland
Target cells: cells of the collecting duct of nephron in kidney
Function of ADH: REDUCE water loss in urine by making kidney reabsorb as much water as possible
Stops dilute urine from being made
Creates MORE aquaporins in Collecting duct so more water can leave collecting duct, go into tissue fluid and be reabsorbed by peritubular capillaries

14. Decrease in Water Potential in Blood
Decrease in water potential below SET POINT is detected by osmoreceptors in hypothalamus
Nerve impulses sent along neurons that end at posterior pituitary gland
Impulses stimulate posterior pituitary gland to release ADH into blood capillaries and travel all over body

17. Effects of ADH on Kidney
Collecting ducts allow reabsorption of water (OUT of nephron and into medulla/tissuecapillaries)
ADH molecules BIND to receptor proteins on cell surface membranes of collecting duct cells
Makes collecting duct more permeable to water by INCREASING number of aquaporins
Aquaporins -water permeable channels in cell surface membrane of collecting duct cells
Binding of ADH to receptor proteins on cell membrane activate ENZYMES inside of cell
Activates series of enzyme controlled reactions that end in final product: ACTIVE PHOSPHORYLASE ENZYME
PHOSPHORYLASE causes vesicles in cytoplasm of cell (vesicles contain aquaporins in their membrane) to begin moving towards cell membrane (review Golgi body modifications to proteins)
Vesicles fuse with cell membrane, thus increasing the number of aquaporins
Water can now freely move through the membrane DOWN water potential gradient, into concentrated tissue fluid peritubular capillaries  blood plasma

19. Fluid flows through collecting duct
Fluid in collecting duct = HIGH water potential
Tissue fluid in medulla = LOW water potential
Water leaves collecting duct cells
Fluid in collecting duct becomes more concentrated
Increase of water in blood
This is due to reabsorption of water due to release of ADH
Volume of fluid flowing into bladder will be less = more concentrated urine (less dilute)

22. Effects of Lots of Water in Body
Water potential in blood INCREASES
Osmoreceptors in hypothalamus detect increase
Osmoreceptors are no longer stimulated
Neurons in posterior pituitary gland no longer stimulated to release ADH
No ADH being secreted
Aquaporins in cell membrane of collecting duct cells move OUT of membrane and become vesicles in the cytoplasm again
Takes minutes for this to happen
Not immediate
Collecting duct becomes impermeable to water
Lots of water flowing in fluid in collecting duct = increased volume into bladder = DILUTE urine
Ensures maintenance of water potential

23. Response to decrease in ADH
Collecting duct cells do NOT respond immediately to decrease in ADH secretion by posterior pituitary gland
ADH already in blood needs to be broken down
Half of ADH is blood is destroyed every minutes
Eventually ADH stops arriving at collecting duct cells
No attachment to receptor proteins
Aquaporins removed and become vesicles in cytoplasm again
10-15 minutes

24. Alcohol and ADH Alcohol affects the pituitary gland
Reduces secretion of ADH
No ADH attaching to receptors on collecting duct cells = no aquaporins = impermeable to water = water stays in collecting duct with urine (NO water reabsorption as filtrate moves through collecting duct)
Dilute urine
Dehydration