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Pissing the day away

Urine, that yellowish liquid you pee, is formed in the kidneys. It contains urea, which is nitrogen-based waste from metabolism, and water and ions  to control fluid balance in the body. Urine is important to maintaining blood pressure and homeostasis (the state of being stable). 


Urine formation - As you function in your day to day tasks, your kidney is busy filtering your blood through small capillaries (Fancy term: glomerular filtration).  As it does this, it's told by hormones from the pituitary gland whether you need more or less water in your blood. Sodium is co-transported to help maintain pH whenever water is secreted or retained (process called reabsorption). So though your blood is at a steady pH of  7.4, your urine will range from acidic at 4.6 to basic at 8.0. A flux of the blood pH in one direction or the other can cause death - so you see why your urine is so important! Here's a video of urine formation if you want additional details about the kidney function.


Gross fact: the kidneys filter out more than 40 gallons of water a day, luckily it doesn't all end up as urine


Urine color - how dark your urine is an indication of how concentrated it is (concentrated in urea that is). Ideally your urine should be light yellow. Darker yellow indicates dehydration.  Other colorchanges may be due to organ damage, medications, or even a change in diet. 

How it comes out - So the urine is formed in the kidneys by filtering the blood. It then drains through the renal pelvis into the ureters, two tubes leading to the bladder (one from each kidney). The bladder stores urine until it has enough to trigger that urgent feeling that you have to pee. It's made of smooth muscle, so it expands as it fills, but it still only has a certain capacity. Estimates vary, by the general consensus is that it holds less than a liter of fluid.

Urinating is generally a voluntary process, you tell the bladder sphincter to release and urine pours out through the urethra. But overfilling the bladder can put pressure on the seal between the bladder and urethra, causing leakage (aka peeing your pants).

Because of this voluntary/involuntary aspect of urination and the importance of ensuring the bladder doesn't remain filled (it can cause infection), catheters are used to artificially drain the urethra. Hospitals often use these for patients, and it involves a not so pleasant process of inserting a tube into the very small hole at the tip of the penis for men or the urinary meatus for women (i.e. the genitals).

It's ok if you watch the following video without sound

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