Regulation of kidney function

Why the kidney needs a control

You drink two glasses of water, and your urine becomes pale. You sweat on a hot day, and your urine is dark and small in volume. The kidney changes its work to match the body. This control of the water and the salts of the body fluids is called osmoregulation.

The kidney function is controlled by the hypothalamus, the juxtaglomerular apparatus and, to a certain extent, the heart.

ADH and the hypothalamus

Special cells called osmoreceptors in the body notice the changes in the volume of the blood, the volume of the body fluid and the concentration of the ions.

1. An excessive loss of fluid, or a rise in ionic concentration, activates the osmoreceptors.
2. They stimulate the hypothalamus, which makes the antidiuretic hormone (ADH), also called vasopressin.
3. The neurohypophysis, the back part of the pituitary, releases ADH into the blood.
4. ADH helps the reabsorption of water from the later parts of the tubule. So less water is lost in the urine. This is the meaning of antidiuretic: it prevents diuresis, the passing of a large volume of urine.

An increase in the volume of the body fluid switches the osmoreceptors off and stops the release of ADH. This completes the feedback.

ADH also makes the blood vessels narrower, which raises the blood pressure. A higher blood pressure raises the blood flow in the glomerulus, and so it raises the GFR.

Diabetes insipidus. A person who does not make enough ADH passes a very large volume of dilute urine, because the water is not reabsorbed from the collecting duct. The person loses much water and is very thirsty. This is diabetes insipidus. It has nothing to do with sugar. The name diabetes mellitus is for the disease in which the urine has glucose.

The juxtaglomerular apparatus

The juxtaglomerular apparatus (JGA) is a special sensitive region. It is formed by cells of the distal convoluted tubule and of the afferent arteriole, at the place where they touch each other.

A fall in the glomerular blood flow, the glomerular blood pressure or the GFR activates the JG cells to release renin. The steps follow.

1. Renin changes angiotensinogen, a protein of the blood, into angiotensin I, and then into angiotensin II.
2. Angiotensin II is a powerful vasoconstrictor. It narrows the blood vessels, and so it raises the glomerular blood pressure and the GFR.
3. Angiotensin II also makes the adrenal cortex release aldosterone.
4. Aldosterone causes the reabsorption of Na$^+$ and water from the distal parts of the tubule. This raises the blood pressure and the GFR.

This complex mechanism is the renin-angiotensin mechanism.

ANF, the check from the heart

An increase in the flow of blood to the atria of the heart makes the heart release the atrial natriuretic factor (ANF). ANF causes vasodilation, the widening of the blood vessels, and so it lowers the blood pressure. The ANF mechanism acts as a check on the renin-angiotensin mechanism.

• ADH: triggered by loss of fluid or a rise in ionic concentration. It makes the later part of the tubule take back more water, so there is less urine.
• Renin-angiotensin: triggered by a fall in glomerular blood flow, pressure or GFR. It narrows the vessels and releases aldosterone, so the blood pressure and the GFR rise.
• ANF: triggered by more blood flowing to the atria. It widens the vessels, so the blood pressure falls.

A model of ADH and the volume of urine

A person must remove about 600 mOsmol of solute in a day. The simulation uses a model with values from physiology books, not from NCERT. With much ADH, the urine can be as concentrated as 1200 mOsmol/L. With no ADH, it can be as dilute as 50 mOsmol/L. The volume is the solute divided by the osmolarity.

You should find that with no ADH the volume is about 12 L a day, which is a picture of diabetes insipidus. With the largest ADH effect it is only 0.5 L a day, the least that the kidney can pass.

Try it in Python

The program uses the model to find the osmolarity and the volume of the urine for a chosen ADH effect.

Practise with new numbers

Common mistakes

• Saying that ADH is made by the pituitary. It is made by the hypothalamus. The neurohypophysis releases it.
• Saying that ADH increases the urine. It reduces the volume of urine.
• Mixing up diabetes insipidus and diabetes mellitus. In insipidus, ADH is short and the urine is dilute. In mellitus, the urine has glucose.
• Saying that renin is a hormone that raises the blood pressure by itself. Renin is an enzyme. Angiotensin II raises the pressure.
• Saying that ANF acts like renin. ANF lowers the blood pressure and checks the renin-angiotensin mechanism.

Remember these

• Control: hypothalamus (ADH), JGA (renin) and the heart (ANF).
• ADH: made in the hypothalamus, released by the neurohypophysis, helps the reabsorption of water in the later tubule, prevents diuresis. A lack of ADH causes diabetes insipidus.
• Renin: from JG cells. Angiotensinogen to angiotensin I to angiotensin II. Angiotensin II: vasoconstriction, aldosterone, which gives Na$^+$ and water reabsorption.
• ANF: from the atria, vasodilation, lowers the blood pressure.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which parts of the body control the working of the kidney?

2. Describe what happens when a person loses much water in sweat.

3. What is the order of the renin-angiotensin mechanism?

4. What is the work of the ANF?

5. What is diabetes insipidus?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 16, Excretory Products and Their Elimination.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapters on urine concentration and dilution, and on the regulation of extracellular fluid.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapters 25 and 26.