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What is Antidiuretic Hormone? - Definition & Function

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  • 0:04 Definition of…
  • 0:31 ADH and Water Reabsorption
  • 1:36 Sodium and Water Reabsorption
  • 2:41 ADH and Increased…
  • 3:22 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb
Although you may have never heard of antidiuretic hormone, it plays an important role in water retention, thirst and blood pressure. This lesson explains the function of the hormone and what can happen if it is not properly regulated.

Definition of Antidiuretic Hormone

Antidiuretic hormone is a small protein secreted by part of the brain called the pituitary gland. Antidiuretic hormone is also known as ADH, or vasopressin. Antidiuretic hormone binds to receptors on cells in the kidney and blood vessels to affect the body. Many species use antidiuretic hormone. It increases the amount of water absorbed by the kidney and increases blood pressure.

ADH and Water Reabsorption

The body uses antidiuretic hormone to retain water and increase blood pressure. As the main organ responsible for water retention in the body, the kidney is directly affected by antidiuretic hormone. The kidney is made of millions of units called nephrons. Nephrons filter blood through a series of tubules, which absorb water, salt and other things needed by the body.

Antidiuretic hormone binds to receptors on the surface of cells in the collecting duct of the nephrons. The hormone causes an increase of water channel proteins, called aquaporins, in the membrane of the cells in the kidney tubules.

Aquaporins are like doors in a nightclub; they let water in and out of the cell. When the doors open, people enter and leave, but because the nightclub is empty, more people go in, rather than out. Similarly, when there is more water inside the nephron than in the blood, when the gates open, water flows in the blood. This causes the body to hold more water and increases the volume of blood, which in turn, increases blood pressure.

Sodium and Water Reabsorption

Antidiuretic hormone also increases sodium reabsorption in a different part of the nephron in the kidney. If sodium is reabsorbed early on in the nephron, the blood concentration of sodium increases. In a process called osmosis, water flows from where there is more water to where there is less water. If there is more water, there must be less solute, the substance dissolved in water. Therefore, water flows from where there is less solute, to where there is more solute. Below, you'll see what happens when you add another type of solute, or glucose, to water.

Water moves from the side with less glucose to the side with more glucose.
osmosis

If sodium is pumped into the blood in the beginning of the nephron, later on, there will be more sodium inside the blood than the nephron. Water flows to where there is more solute, so water will flow into the blood in the later section of the nephron.

Let's go back to our hypothetical nightclub and imagine that, starting at 10pm, all women can get in for free. By midnight, the club is filled with women, at which point, men are admitted for free, and like water, gravitate to the women, or sodium.

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