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Fluid Balance and Blood Pressure

Fluid Balance and Blood Pressure

Author: Amanda Soderlind

Identify the role the kidneys play in maintaining homeostasis in the body.

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Source: Video and Images Created by Amanda Soderlind

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Welcome to this lesson today on fluid balance and blood pressure.

Today we'll be discussing how the kidneys help to regulate fluid balance and blood pressure.

The kidneys can concentrate urine, and by concentrating urine, it allows for the regulation of that blood volume, and therefore blood pressure. Blood volume actually has an effect on blood pressure. The more volume of blood there is, the higher the blood pressure will be. If the blood volume drops, then blood pressure will drop, as well. So kidneys help to concentrate urine, or help to have urine be more dilute, depending on the body's needs-- depending on what the blood volume and blood pressure is. So the kidneys have that ability to concentrate urine, if necessary, to help regulate blood volume and blood pressure.

Hormones can help to adjust the amount of water that is in urine, to help maintain this volume of cellular fluids. Some hormones that play a role in this are ADH, which stands for antidiuretic hormone. SO ADH, antidiuretic hormone, is released when less water is taken in than is lost. So basically what this hormone does is, it helps the kidneys conserve water, and it makes the urine more concentrated. So this hormone, ADH acts in the distal tubules and connecting ducts of a nephron. It is helping urine to become more concentrated to help conserve water. When ADH levels are low, then you'll have dilute urine. When ADH levels are higher, urine will be more concentrated. So by concentrating urine, it's allowing the body to regulate and adjust blood volume and pressure.

Our next hormone is aldosterone. Aldosterone is a hormone that basically, what it does is it causes urine to become concentrated. So basically, it mostly targets the collecting ducts, and cells in these distal tubules and collecting ducts will reabsorb more sodium at a faster rate, which then indirectly causes the body to reabsorb more water, as well, because the water will follow the salts. It's a hormone that helps to concentrate urine, as well. Similar to ADH, but it acts in a different part of the nephron.

A diuretic. A diuretic is a substance that reduces reabsorption of sodium. So basically what it does is, it promotes loss of water in urine. So the body is losing more water if you're taking something that's considered a diuretic. Caffeine is a good example of a diuretic. You may notice that when you drink a lot of caffeine, you urinate more often. Your body is losing more water through urine, because caffeine is a diuretic.

Renin is an enzyme released when the blood volume is reduced. If the blood volume drops or is lower, blood pressure is also going to drop, and this enzyme renin will become released in the body. And basically, it stimulates the secretion of this hormone, here. And what this hormone does, as we said, concentrates urine, increases the amount of water that's reabsorbed, and helps to increase that blood volume again, and therefore help to bring the blood volume and blood pressure back to a normal level.

We're going to take a look at this sheet here, and I'm going to give you just a little example of how this works. So ADH is actually an example of a negative feedback loop. If you remember, a negative feedback loop, basically what that means is that a stimulus is detected by the body, and then reversed. We going to take a look at how ADH is an example of a negative feedback. Let's say that your body detects that water is lost. So if our body loses water, what that's going to end up doing is decreasing our blood volume. And it also will increase the sodium levels in our blood. Actually, what it does is, it increases the concentration of sodium in our blood if we're losing water. So what's going to happen then is that ADH, or antidiuretic hormone, be released.

And as that antidiuretic hormone is released, the kidneys will absorb more water. So more water is being reabsorbed back in the kidneys-- or by the kidneys-- and what that's going to do is increase blood volume.

So the ADH is causing water to be reabsorbed back into the body, and as that water is reabsorbed into those extra cellular fluids of the body, it's going to increase that blood volume. And as that blood volume increases, it's also going to help, therefore, to increase the blood pressure, as well. And the release of ADH will be stopped.

So it's a negative feedback loop because our stimulus was that our body lost water, our blood volume was decreased, and the sodium concentrations were increased. This is our stimulus, and then this is our response, here. So we detected this change, and then something happened. The ADH being released reversed that change in order to maintain homeostasis. So it's a negative feedback

This lesson has been an overview on fluid balance and blood pressure.

Terms to Know

Nicknamed the “salt-retaining hormone," aldosterone stimulates the kidneys to retain sodium while simultaneously excreting potassium in the urine.

Antidiuretic Hormone (ADH)

A hormone secreted from the posterior pituitary gland that stimulates the kidneys to retain water. The receptors for ADH are located on the distal convoluted tubules and collecting ducts. High levels of ADH cause a person’s urine output to decrease and its color to become darker and more concentrated.


A medication or substance that increases a person’s urine output.


An enzyme found in blood that plays an important role in increasing blood pressure during hemorrhaging and diarrhea.