1. The Kidney

2. Water Reabsorption
After selective reabsorption in the proximal convoluted tubule, the loop of Henle creates a low (very negative) water potential in the medulla to ensure more water is reabsorbed from the collecting duct

3. Loop of Henle
Consists of a descending limb into the medulla and an ascending limb back out to the cortex
Allows salts (sodium and chloride ions) to be transferred from the ascending limb to the descending limb
The overall effect is to increase the concentration of salts in the tubule fluid so they diffuse out from the ascending limb into the surrounding medulla tissue giving a low (very negative) water potential

4. Water Potential
As the fluid in the tubule descends into the medulla down the descending loop, the water potential becomes lower (more negative)
This is due to :
loss of water by osmosis to the surrounding tissue fluid
diffusion of the sodium and chloride ions into the tubule from surrounding tissue fluid
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5. Water Potential
As the fluid in the tubule ascends back up towards the cortex, the water potential becomes higher (less negative)
This is due to :
sodium and chloride ions diffusing out of the tubule into the tissue fluid at the base
higher up the tubule, sodium and chloride ions are actively transported out into the tissue fluid
wall of the ascending limb is impermeable to water so it cannot leave the tubule
SO…the fluid loses salts but not water as it moves up the ascending limb
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6. This arrangement is known as the hairpin counter-current multiplier system.
It increases the efficiency of salt transfer from the ascending limb to the descending limb
This causes a build up of salt in the surrounding tissue fluid
‘Student Speak’ Water moves out of the descending limb, making the fluid in the tubule very salty. Salt then diffuses out of the base of the ascending limb as it is at high concentrations (very negative water potential), and then is transported out using active transport at the top of the ascending limb
The removal of ions from the ascending limb makes the urine very dilute and water can then be reabsorbed by the body from the distal tubules and collecting ducts
Water Potential
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7. The Collecting Duct
From the top of the ascending limb, the tubule fluid passes through the distal convoluted tubule where active transport adjusts the concentration of various salts
The fluid has a high water potential (contains a lot of water) and flows into the collecting duct
The collecting duct carries fluid into the medulla which contains a lot of salts (low/very negative water potential)
As the fluid passes through, water moves by osmosis, from the tubule fluid into the surrounding tissue
It then enters the blood capillaries by osmosis and is carried away
The amount of water absorbed depends on the permeability of the walls of the collecting duct
By the time the urine reaches the pelvis it has a low (very negative) water potential and the concentration of urea and salts is high

8. Osmoregulation
Osmoregulation is the control of water and salt levels in the body
Water is gained from food, drink and metabolism
Water is lost in urine, sweat, water vapour in exhaled air and faeces

9. The Collecting Duct and ADH
The walls of the collecting duct can be made more or less permeable according to the needs of the body
The walls of the collecting duct respond to levels of antidiuretic hormone (ADH) in the blood
Cells in the wall have membrane bound receptors for ADH
The ADH binds to these receptors and causes a chain of enzyme controlled reactions inside the cell
The end result is to insert vesicles containing water permeable channels (aquaporins) into the cell surface membrane
This makes the walls more permeable to water
More ADH in the blood means more aquaporins are inserted allowing more water to be reabsorbed, and less, more concentrated urine with a lower (more negative) water potential

10. Less ADH Less ADH in the blood means that less water is reabsorbed
The cell surface membrane folds inwards to create new vesicles that remove the aquaporins from the membrane
The wall is less permeable and more water passes out in urine with a higher (less negative) water potential

11. Adjusting the Concentration of ADH
Osmoreceptors in the Hypothalalmus monitor the blood’s water potential
When the water potential is low, these cells lose water by osmosis and shrink, stimulating Neurosecretory cells (specialised nerve cells)
Neurosecretory cells produce ADH in their cell body, which flows down the axon to the terminal bulb in the posterior pituitary gland where it is stored until needed

12. Adjusting the Concentration of ADH
When the Neurosecretory cells are stimulated, action potentials are sent down the axons, causing a release of ADH
ADH enters blood capillaries running through the posterior pituitary gland and is transported round the body and acts on the wall of the collecting duct
When water potential rises (less negative) less ADH is released
ADH is slowly broken down (it has a half life of 20 minutes)