A. Remember all that has happened in all the previous sections of the nephron??
1.
In the glomerulus, blood was highly filtered so that only the blood cells and the major proteins stayed in the blood, all the rest (a lot of water and salts) were filtered into the glomerulus; the pre-urine.
2.
In the proximal convoluted tubule, the vast majority of water (80%) and most if not all of the small proteins, salts, glucose etc. were reabsorbed back into the blood.
3.
Then the pre-urine flowed into the loop of Henle where its osmolality increases from 300 to 1200 mOsm (in the descending loop) and decreased back to ±200 mOsm (in the ascending loop). This created a huge osmotic gradient in the medulla of the kidney.
4.
Then in the distal convoluted tubule, we saw several (hormonal) mechanisms that are crucial, for the whole body, in stabilizing the salt concentration, the blood pressure, etc. etc.
5.
This brings us to the final chapter of the function of the kidneys: the collecting ducts!
1.
As you may remember from the structure of the nephron, the pre-urine, after flowing through the distal convoluted tubule, flows into the collecting ducts.
2.
These ducts do exactly what their name implies; they collect the pre-urine from several neighbouring nephrons.
3.
But, in addition to that, they are selectively permeable to water and this selectivity is controlled by a hormone called ADH (= Anti-Diuretic Hormone).
4.
Look at the word; this hormone works Against (= inhibits) Diuresis (= peeing, voiding, losing urine), Hormone.
5.
So, if there is a lot of ADH, the cells in the collecting ducts become highly permeable to water, to keep the water in the kidney and the body.
6.
Another (old) name for ADH is Vasopressin.
7.
So, if there is little or no ADH, then water will not be reabsorbed, the urine remains diluted, and more water will be voided.
8.
But, in the presence of ADH, because the medullary interstitium becomes more hypertonic as you go down the collecting duct, water will go out of the pre-urine and the pre-urine becomes more and more concentrated.
9.
In conclusion, ADH, in the collecting ducts, with the help of the osmotic gradient in the medulla (that was created by the loop of Henle, remember?) determines the amount of water that the kidney excretes.
1.
Ah, but then, who controls the release of ADH? The brain! More specifically, a region at the base of the brain called the hypothalamus.
2.
In that region in the brain, the blood vessels contain osmo-receptors. They measure the osmolality of the blood that flows through these vessels.
3.
If the osmolality is too high (= too concentrated), then nerve signals are send to the posterior pituitary gland.
4.
The posterior pituitary is a gland that produces several hormones under command from specific regions in the brain.
5.
If the osmolality in the blood is too high, then the hypothalamus signals the posterior pituitary gland to release ADH into the blood.
6.
This ADH goes to the kidney and makes the collecting ducts cells more permeable for water, so that more water is sucked back into the interstitium, and back to the blood, while the urine gets more concentrated.
7.
I should add that the hypothalamus will also make you more thirsty so that you drink more water which will help to decrease the blood osmolality.
8.
On the other hand, if the osmolality of blood has decreased (= too much water), then the ADH release is reduced.
9.
This will reduce the permeability of the collecting ducts to water and more water is lost, resulting in diluted urine.
1.
This diagram summarizes the function of the loop of Henle and the collecting ducts.
2.
In the loop of Henle, the osmolality of the pre-urine increase and decreases during the course of the loop (thereby creating the medullary osmolality gradient).
3.
In the collecting ducts, as determined by the presence or absence of ADH, water is more (or less) reabsorbed from the urine, which thereby produces diluted or concentrated urine.
1.
But suppose that you don’t have ADH or that this hormone does not work?
2.
Then the kidneys will not be able to concentrate the urine and you will lose too much water.
3.
This disease is called Diabetes Insipidus (not be confused with Diabetes Mellitus; which is the ‘real’ diabetes disease).
4.
There are actually two types of Diabetes Insipidus:
a) Caused by a problem in the pituitary gland (trauma etc.): Central Diabetes Insipidus b) Problem with the ADH-receptor in the collecting ducts: Nephrogenic Diabetes Insipidus.
5.
The word “diabetes” comes from the old Greek, meaning “working like a syphon” which really means losing a lot of water!
6.
The word “insipidus” is from Latin and means “tasteless” (the urine has no taste!). This, in contrast to diabetes mellitus, where the urine contains lots of glucose (taste sweet!).
7.
The symptoms of diabetes insipidus are:
a) Produce large amounts of urine
b) Increased thirst (logical!)
c) Dehydration
d) Seizures (!)
8.
In the case of Central Diabetes Insipidus, one can give a medicine (desmopressin) that works like ADH. In the case of nephrogenic insipidus, it becomes more difficult but one can try to treat the complications.
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Location of the collecting duct after the nephron:
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The function of ADH on the Collecting Duct:
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The location of the posterior pituitary gland in the hypothalamus:
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The osmolarity curve along the Opp of Henle and the Collecting Duct:
F.3.4. Distal Convoluted Tubule