The Urinary System – Selective Reabsorption

Last week, we learned about the general filtration process that occurs in our kidneys. This week, we’ll learn just how our kidneys modify the filtrate to produce urine.

The filtrate that is in the renal tubule consists of water as well as other small molecules, like sugars and urea. Some of these molecules, like sugars, can return to the bloodstream in a ‘process’ known as selective reabsorption. It’s called ‘selective’ reabsorption because the bloodstream is picky as to what it absorbs from the tubule. The molecules that leave the tubule enter tiny blood vessels next to the tubule, which are called peritubular capillaries. The molecules can then be carried through the rest of the circulatory system, to provide our body cells with nutrients (if they’re sugars).

Parts of the Kidney

Selective reabsorption mainly occurs in the proximal tubule, which is the beginning of the tubule. The proximal tubule is just after the Bowman’s capsule. Whenever a molecule leaves, it is accompanies by water, which means a lot of water is reabsorbed by the bloodstream in this process.

By returning the molecules to the bloodstream, the remaining filtrate’s composition changes. As water leaves, the concentration of particles in the tubule increases.

Hormones can affect what is reabsorbed in the distal tubule. These hormones are the anitidiuretic hormone (ADH), which is also known as vasopressin, and aldosterone. They’ll be discussed next week when we talk about Concentration and Dilution!

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Pins and Needles aka Parasthesia

Nowadays, it seems like my legs get a lot more sleep than I do. “#firstworldproblems, #universitywoes”. But, really, why do our limbs fall asleep and why do they tingle us so uncomfortably when they do?

Pins and Needles

The sleeping of our limbs, also termed ‘parasthesia’, is the result of our nerves acting abnormally due to an increased pressure on them for a prolonged period of time. Our nerves essentially act as little messengers between our limbs and our brain. So this prolonged pressure on the nerves results in the loss of communication between the limbs and our brain.

There is also a prolonged pressure placed on our blood vessels, which results in our nerves not receiving enough oxygen or nutrients.

So, in response to this pressure, nerves, much like how we respond to pressure, can react in to different ways: They can either become unresponsive and wait until the pressure has been removed, or they will essentially begin to spaz out and rapidly send signals in hopes of sending them in the right direction.

Now, the latter causes a problem because we have a lot of different nerves feeding our brains with a lot of different information: some inform us about temperature, some others about pressure on our skin and so forth. So when the nerves start spastically sending signals, the brain is unable to fully interpret what is happening and gets a mix of signals about warmth and numb sensations as well as conflicting signals about being cold and tingling sensations. This is why our sleeping limbs are described as having pins and needles. The mix of signals results in a mix of sensations, including an odd duality of numbness and tingling.

How to wake up your limbs

Many people try to hit their limbs that have fallen asleep. That doesn’t really do anything, unfortunately. The best we can do is to change our positions to remove the pressure and wait for the blood flow and nerve responses to return to normal.

And that’s all there is to know about pins and needles. Ironically, my foot has now fallen asleep so it’s time to practice what I preach.

 

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