How the Kidneys Regulate Water Balance

How the Kidneys Regulate Water Balance
Coming up next: How the Kidneys Regulate Acid Base Balance

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  • 0:56 Osmoregulation
  • 2:03 Creating Concentrated Urine
  • 3:22 Diluting the Urine
  • 4:55 Diabetes Insipidus
  • 6:07 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
Water balance is essential to our health and survival. This lesson explores how the kidneys regulate water balance with special cells known as osmoreceptors and a process called osmoregulation. You'll also learn how this process relates to a rare form of diabetes known as diabetes insipidus.

Renal Maintenance of Water Balance

Your kidneys have three main purposes that influence the rest of your body. These three roles include the regulation of water balance, acid-base balance, and the excretion of waste.

Before proceeding to the information in this lesson, I highly recommend that you be familiar with the following concepts: glomerular filtration, GFR, renal tubules, tubuloglomerular feedback, the renin-angiotensin-aldosterone system and its negative feedback hormone and loops. We have lessons on each of these topics, and it's important that you understand them before moving on to one of the most important big-picture roles that your kidneys are involved in: the regulation of water balance in your body and how it is clinically relevant.

Diagram of a nephron, including the glomerulus, in the kidney
Glomerulus Nephron


Under normal conditions, your body is constantly guarding against the loss of precious water. While water by itself is a non-energy-producing nutrient in our body, it is absolutely critical to our survival. No water, no life.

In fact, it's so critical that over 99% of water filtered by the glomerulus every day is reabsorbed back into circulation. When dehydrated, your kidneys create super-concentrated urine, sometimes more than six times the concentration of blood!

On the flip side, if you drink too much and therefore have fluid overload, your kidneys can alter their role and cause diuresis, or the increased release of urine, in order to get rid of excess fluid.

The processes I just described - the active regulation of fluid and electrolyte movement in and out of a cell or body - is called osmoregulation and is chiefly accomplished on a systemic (or whole-body) scale by the kidneys.

Creating Concentrated Urine

One of the ways by which this is accomplished is through a countercurrent exchange system between a structure called the vasa recta and the loop of Henle. This system helps to concentrate urine in the renal medullary interstitium, the tissue surrounding the loop of Henle, in order to help move water out of the renal tubule and back into the bloodstream. By the time the filtrate enters the distal convoluted tubule and the collecting duct, the remaining filtrate is hypotonic, or dilute.

Structures involved in urine concentration
Kidney Structures

This seems counterintuitive, as I just said that this mechanism forces water out of the renal tubule and back into the blood. You'd think, logically, that less water in the renal tubule means there's more solute per amount of water, causing a higher concentration of fluid in the distal segments of the renal tubule.

However, keep in mind that the loops of Henle also reabsorb a lot of solutes as well, not just water. This, coupled with the fact that the distal tubule reabsorbs even more solutes, causes the fluid passing out of the loop of Henle and into the distal convoluted tubule to actually become hypotonic, or dilute.

Diluting the Urine

So, to dilute or not dilute? That is the question. Shakespeare may not have known this, but your body has the answer: it's called ADH. ADH is a hormone released to further concentrate the dilute filtrate passing out of the distal loop of Henle and moving through the distal segments of the distal tubule and the collecting duct.

If osmoreceptors, which are cells that monitor the osmolality (or solute concentration) in blood, sense an increase in the osmolality of your blood, ADH will be released in order to conserve more water in the body so that the water can dilute the blood back down to a normal concentration.

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