1.
Interestingly, hormones regulate the most important reabsorptions in the distal

2.
One important hormone is Aldosterone, which is secreted by the adrenal cortex. It is secreted when the blood pressure gets too low (as part of the renin-angiotensin-aldosterone system).

3.
Aldosterone will activate sodium pumps so that more sodium (and water) is reabsorbed back into the blood in exchange for potassium, which is excreted in the urine.

4.
If, on the other hand, the aldosterone level is decreased, then less sodium is reabsorbed leading to an increase in sodium excretion and a decrease in potassium excretion in the urine.

5.
The second hormone is the Atrial Natriuretic Peptide (=ANP). This hormone is released by the cardiac atria and increases when the blood pressure gets too high!

6.
ANP works mainly in the collecting ducts and inhibit sodium reabsorption, which will also reduce water reabsorption, and thereby decrease the blood pressure.

7.
In fact, ANP works in the opposite direction to ADH!

8.
Another important hormone is the parathyroid hormone (=PTH), secreted by the parathyroid glands (at the back of the thyroid gland in your neck), which reabsorbs calcium and secretes phosphate in the urine.

1.
This is an interesting ‘complex’ of special cells that are located at the beginning of the distal tubule and close to the glomerulus (‘juxta’ means ‘next’ to the glomerulus).

2.
It consists of three group of cells:
a) the macula densa
b) the juxtaglomerular cells
c) the extraglomerular mesangial cells

3.
The complex is responsible for two mechanisms for regulating the function of the nephron!
a) intra-renal (tubulo-glomerular feedback)
b) extra-renal (renin-angiotensin-aldosterone).

4.
The macula densa consists of specific cells located in the wall of the distal convoluted tube at the point where the distal tubule comes ‘close’ to the glomerulus and the two afferent and efferent arterioles (under the microscope, these cells look dark or dense, hence the name ‘densa’)

5.
The cells in the macula densa detect the amount of salt (=NaCl) in the pre-urine flowing in the distal convoluted tubule.

6.
If the salt concentration becomes too high, then the macula cells will make the adjacent afferent arteriole constrict.

7.
This will decrease the blood flow to the glomerulus, decrease the amount of fluid secreted in Bowman’s capsule and therefore decrease the filtration rate (GFR). This will lower the salt concentration in the pre-urine.

1.
The extra-renal functions of the Juxtaglomerular complex deals with hormones that are secreted from these cells and regulate several functions in the body.

2.
The most important hormones are:
a) Renin
b) Erythropoietin

3.
When the blood pressure in the afferent arteriole is too low, the hormone renin will be secreted by the juxtaglomerular cells. This renin is actually the start of the renin-angiotensin-aldosterone system (see: B.6.3. Long Term Regulation, panel A)

4.
This renin will, in the circulating blood, convert the existing angiotensinogen into Angiotensin I. This angiotensin I is then converted into angiotensin II when the blood passes through the lungs!

5.
This angiotensin II will constrict the arterioles in the body, thereby increasing the arterial blood pressure, which will lead, amongst others, to an increase in blood flow to the kidneys!

6.
But the angiotensin II will also stimulate the cortex of the adrenal glands to secrete aldosterone. This aldosterone, in turn, will induce the kidney to reabsorb more salt, and therefore more water, which will increase the blood pressure. It is actually an extra-renal long term regulation of the blood pressure in the body!

7.
Finally, what about the extraglomerular mesangial cells??

8.
Spoiler alert! We don’t know the full details! Sorry!!

9.
Some people think that the extraglomerular mesangial cells have something to do with the secretion of erythropoietin and/or renin.

10.
We have already seen the effects of renin by the juxtaglomerular cells (previous text). Erythropoietin is a hormone that stimulates the production of erythrocytes in the bone marrow. (link).

11.
Personally, I don’t think there is a big difference between the functions of the juxtaglomerular cells and that of the extraglomerular mesangial cells.

12.
They are both located in the same area, close to each other, and support each other in detecting the signals and producing the required hormones when necessary.

1.
As I said in my introduction to this page, the tubular system not only reabsorbs water and stuff but also secretes (other) stuff!

2.
This is necessary to get rid of solutes that may be harmful to the body and are usually the waste products of other (metabolic) systems.

3.
Examples are urea (waste product of the nitrogenous protein metabolism) and uric acid (waste of the nucleic acid metabolism).

4.
But also excessive potassium ions and hydrogen ions (to regulate the blood pH) can be secreted if necessary (see acid-base system).

5.
Several drugs are also actively secreted such as penicillin and Phenobarbital for example.