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Polyuria: Definition, Causes & Symptoms

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  • 0:02 Polyuria = Lots of Urine
  • 0:29 Water, Kidneys & Electrolytes
  • 1:54 Causes
  • 3:52 Symptoms
  • 4:09 Lesson Summary
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Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

There's a saying that 'When you've got to go, you've got to go!' And with polyuria, you've got to go a lot. In this lesson, you'll learn about polyuria and what causes it.

Polyuria = Lots of Urine

Think about your typical day. How many times does it include a stop in the bathroom to urinate? On average, a normally hydrated person's kidneys produce between 0.8 and 2 liters of urine a day. Individuals with polyuria produce over 2.5 liters per day. To understand why kidneys might produce more urine than normal, let's first briefly review how kidneys make urine.

Water, Kidneys & Electrolytes

When blood flows into the kidneys, approximately 20% of the blood's fluid leaves the blood vessels and enters tubules of the nephron, or functional unit of the kidney. This fluid, now called filtrate, contains water, electrolytes (for example, sodium and potassium), nutrients (for example, amino acids and glucose) and waste products (for example, ammonia). The job of the kidneys is to put back, or reabsorb, the right amount of water, sodium, ammonia and so forth from the filtrate to maintain the correct chemical make-up of the blood. If the concentration of any electrolyte is too high or too low in the blood, it can wreak havoc on various systems in the body. What doesn't get reabsorbed from the filtrate is excreted as urine.

The movement of electrolytes, water and waste products in the kidneys is complex and under the control of several hormones. But in general, the movement of water, osmosis, is always in the direction of the more concentrated solution. If there are more solutes in the filtrate, water will remain there. If there are more solutes in the blood, water will be reabsorbed from the filtrate. So, if there is extra sodium in your filtrate because you ate a bunch of salty food, there will be more water in your filtrate, and the amount of urine will be higher. This is why eating a lot of salt without also drinking water can make you dehydrated.

Causes

The main component of urine is water. Even when urine is really yellow and concentrated, it is still over 95% water. Because of this, polyuria is caused by having excess volume of water in the urine. The most common cause of polyuria is drinking lots of water. Marathon runners experience temporary polyuria as they try to hydrate the day before a race. However, life-threatening polyuria can occur when there is something that is keeping water in the filtrate or urine when it should be reabsorbed back into the blood. Since water moves in the direction of the highest concentration of solutes, any condition that causes more solutes to be in the filtrate can cause polyuria. Here are some of the most common medical causes of polyuria:

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