The Urinary System – Concentration and Dilution

Last time, we talked about how the urinary system allows for our bodies to selectively reabsorbs things that were filtered out of our circulatory system. One of the molecules that can be reabsorbed is water, which is an important molecule for several reasons. In terms of the urinary system, water is important for the concentration and dilution of our urine.

The concentration and the dilution of urine is regulated by 2 hormones: Anti-diuretic hormone and Aldosterone.

The anti-diuretic hormone is released if you’re dehydrated, and literally translates to “against the passing of urine”. In situations where you are dehydrated, your blood volume will be low due to the lack of a sufficient amount of water, resulting in a lower blood pressure. To counteract this, the anti-diuretic acts to reduce the amount of water lost by the body and minimizes how much urine you make and release. This will result in a more concentrated urine that has a less water than usual and is a deeper yellow due to the higher concentration of urea.

Aldosterone is also released when you’re dehydrated. This hormone is responsible for the increase in thirst while also helping your body retain water by increasing the amount of sodium in your body. The more solute there is in your body, the more likely water is to stay with the solute (rules of osmosis, hurray!).

These hormones exert their effects primarily on the distal tubule of the nephron (after the loop of Henle), so that is where the water is reabsorbed!

So when these hormones are released, your urine will be more concentrated because water is retained by your body! Amazing, right?

Next week: We will finally get to talking about the effect of Alcohol on Urine!

Is there something you’d like to learn about? Tell me about it here and I may just do a post about it 🙂

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Lactose Intolerance and Diarrhea

I know, I know. Oy with the poo already! But I swear, this is the last one (for now). Since I’ve talked about diarrhea due to gastrointestinal infections, I should probably explain why some people get diarrhea as a symptom to lactose intolerance.

Difference between diarrhea caused by GI infections and Lactose Intolerance

Last week, we learned that our bodies don’t absorb the water we ingest when there is a GI infection, which means we don’t gain any water in our systems. In contrast to diarrhea as a result of an infection, water is actually taken from the body and enters the intestinal tract; that means we actually lose water we already had. But why?

Diarrhea as a symptom of Lactose Intolerance

When you have lactose intolerance, you have a decreased efficiency in the digestion of lactose. While there are bacteria in your intestinal tract that are capable of digesting the lactose, they may not be able to digest all of the lactose you’ve consumed. This means that there is a concentration of lactose in your intestines that has not been taken up by your body. This is where the water loss comes in.

Remember that pesky biology lesson about Osmosis? Well, it’s actually applicable here (thank you, high school!). So, you have a high concentration of sugar (lactose) in your intestines and a lower concentration in the external environment of the intestine. Water likes to do this thing where it moves from areas of low concentrations of solutes, like sugars, to higher concentrations of solutes. Basically, water is a gold-digger and the more solutes an area has, the richer it is. So the water flows into your intestinal tract until the concentration of the sugars in your intestine is roughly the same as the concentration of the sugars outside of the intestine (this is called an equilibrium). This increased volume of water in your intestines causes a watery stool (aka diarrhea!).

Always remember that it’s important to stay hydrated anytime you have diarrhea.

Lesson here is: don’t ingest crazy amounts of lactose, especially if you’re lactose intolerant… unless you’re taking lactase pills. If you’re taking lactase pills, you can eat all the poutine that your heart allows! (That reminds me, happy belated Canada day!)