Why do our bodies ache (joints, muscles, etc.) when we are sick?

One of the main reasons that our body aches when we are sick, like with a cold, is that the body's immune system is producing plenty of antibodies in addition to the effects of all those viruses replicating in our cells killing them and leaving the area 'raw' and exposed.

These antibodies also promote the release of histamine which typically dilates (widens) blood vessels near an infection, this allows for more of the body's defences to get at the infection. There are histamine receptors in blood vessels that cause them to dilate.

As these chemicals are released into the blood stream they can end up in the muscles or other body parts. Various body systems can have receptors to histamine that can then trigger a pain receptor.

In addition to histamine there are biochemicals called cytokines that are released when the body has an immune response that are also known to trigger a  biochemical pathway that can affect pain receptors.

Histamine and cytokines releases can change the perception of pain receptors in the body making them more sensitive to pain factors.

There are other factors that come into play also such as biochemicals called interleukins that relate to fever conditions and temperature increases, all of which can affect pain receptors in different ways, for example heat receptors.

The overall perception of pains and aches over the whole body can vary from person to person and there may be other combinations of psychological,  physiological or even nutritional factors that may influence this.

What is the difference between Savings account and Checking account?

A checking account:

* usually accrues no interest
* can used to pay bills via checks, debit card transactions, and ATM withdrawals
* usually has no limit on the number of transactions that can be done

A savings account:

* accrues interest (though not much these days)
* typically limits the kind and number of transactions that can be done with it

So basically a checking account is where money for your day to day payments flows through, while a savings account is where you keep your savings.

In the US, when you put money into a checking account, the bank has to keep 10% (unless it is small bank, then the number is smaller) in reserve, and it can lend out the rest to people seeking loans (as long as it meets other conditions not relevant here).  If you put money into a savings account, the bank can lend out all of it without keeping any in reserve (as long as it meets other conditions not relevant here).

So, the bank can lend out more of the savings deposits than the checking deposits, which makes savings deposits worth more to them.

Because the regulators impose these differences in regulation between the two accounts, they don't want banks to create something which is called a savings account but acts like a checking account.  So they have regulation D, which forces banks to limit savings account, such as "6 transactions per month" limit.

Why is the liver one of the only organs that grows back when most of it is removed?

The best explanation right now seems to come down to two main factors.

1) Liver cells (called hepatocytes) have a relatively high rate of division in response to acute injury, which allows them to replace liver cells that have been damaged or are missing. This is similar to skin cells (fibroblasts and keratinocytes), but unlike, for example, heart cells (cardiomyocytes) which have an exceptionally low rate of division.

2) A special type of liver regeneration helper cells called "hybrid hepatocytes" were recently discovered by a group at UCSD, which appear to facilitate regeneration and reduce the risk of tumor formation (which can be an unfortunate side-effect of attempts at regenerative therapies).

Related reading:
Newly Discovered Cells Regenerate Liver Tissue Without Forming Tumors

Why do calories matter for losing weight but the weight of the food doesn't?

Actually the weight of the food does matter, but not in the way that you think.

Food is composed of a bunch of different types of chemicals. Most has a fair bit of water in it which we urinate or sweat out after a while. Then there are proteins (building blocks that the body uses to grow or repair itself), carbohydrates (sugars and starches that the body burns to provide energy), fats (same) and fibre (which is the part that the body can't digest and passes through).

Really only three of those ingredient types - carbohydrates, fats and proteins - are stored by the body for future emergency use and contribute to weight gain. They all become body fat or muscle, and they all contribute to the calorie count of food.

But food that weighs a lot but has really low amounts of those three types, like say celery, has almost all of its weight locked into water and fibre. It actually takes energy to go through the process of digesting food, so a celery stick or two actually helps with weight loss by both filling you up so you don't eat as much other stuff, and consuming energy to process through your body.

What are the different blood types, and why can't we mix them?

Blood types are determined by the presence, or absence, of certain markers on the surface of red blood cells, called antigens. An antigen is a molecule that is of sufficient size and complexity that the body can recognize it as "not self" and mount an immune response to destroy it as a potential invader. There are actually many antigens that can define various blood types but the most important come in two groups; 1) A, B, O, and 2) Rh.

Let's look at ABO first. A and B are antigens that humans may or may not carry on their red blood cells, but the important thing about A and B is that they are also abundant in our environment, in microorganisms, pollen, and in animal dander. We are constantly exposed to the antigens and as a result, we develop antibodies that will attack and destroy red blood cells that are unlike our own.

We get one gene from each of our parents to determine our ABO type. The gene for A codes for the A antigen, the gene for B codes for the B antigen, there really is no gene for O, it's more of a blood banker's placeholder, it doesn't code for anything at all. If you inherit two A genes you'll be type A, but you'll still type as A if you only get one gene for A and one for O.

Whichever way you get there, if you are type A you will carry in your bloodstream antibodies that will fiercely attack red blood cells displaying the B antigen. It goes the same way for people of type B. One copy of the B gene will make you type B if your other one is also a B or if your other is for O. Either way you'll carry antibodies that will attack any cells showing the A antigen. Of course, you could get genes for both A and B and your type comes out AB and you don't make antibodies against either antigen. People who are type O make both Anti-A and Anti-B antibodies.

Once they got the ABO grouping sorted out transfusions got a lot safer but still, people occasionally died in obvious transfusion reactions. Something else was going on. For the safety of the humans involved the search for the mystery antigen causing these reactions was done in Rhesus monkeys. People to whom the Rhesus monkey's blood reacted to were dubbed Rhesus Positive, which got shortened to Rh+. People to whom the monkeys did not react are of course Rh-. Modern blood bankers refer to the Rh antigen as D.

The D antigen is uniquely human, it is not abundant in our environment, the only way for a person to become sensitized to, and develop antibodies to attack the D antigen is to actually be exposed to Rh+ red cells when they themselves are Rh-. The most common way for this to occur is not via transfusions it often occurs in childbirth. When a Rh- woman gives birth to a Rh+ child there is a risk the fetal red blood cells that make get into her bloodstream will be recognized as "not self" and an immune response will soon destroy all of the Rh+ cells, and any such cells her body may encounter in the future.

An ABO mismatch transfusion can be quickly fatal. An Rh- Rh+ mismatch is an entirely different matter. Of course, a Rh- woman of childbearing age should never be given Rh+ cells for the sake of the safety of future pregnancies, but in a life threatening emergency situation there is no reason a man of Rh- blood type couldn't safely be transfused with Rh+ cells in sufficient quantity to reverse a trend towards death. This would be a once in a lifetime deal. Over several weeks all of the recipient's transfused Rh+ cells would be destroyed for being "not self" and an enduring immune response would remain.

Why do lines appear in the picture when we photograph a computer screen?

Screens do not generate an entire image at once. Rather, each row of pixels is rendered one at a time until the entire image is rendered. Imagine the screen as filling in a color by numbers from left to right one line at a time.

This happens a number of times per second based on your framerate. A 59.99 hertz screen does this nearly 60 times per second. If your screen has 4000 pixels, it does one pixel every .015 seconds.

Similarly, most digital cameras use what's known as a rolling shutter. They don't capture the whole image at once, but instead one line at a time. This is similar to how a photocopier works.

When the rolling shutter is out of time with the framerate of a screen, it catches different cycles of rendering in the same still.

Imagine if you could instantly swap out a series of photos while a photocopier tried to scan them. You'd end up with a similar effect.

What would happen when water is put into an unbreakable container and then frozen?

There is no rule saying the container has to break. All freezing water does is increase the pressure inside the container. Higher pressure may cause a different type of ice crystal to form.

So the answer is that the water still turns into ice; however, if it genuinely cannot break the bonds of the container it is trapped inside, it turns into a very different kind of ice than we're used to seeing.

We currently know of 15 different solid phases of water, aka ice, with each type being distinct due to differing density and internal structure. The form you're likely most familiar with is Hexagonal Ice which is what happens when water freezes normally under regular conditions. If you keep lowering the temperature of Hexagonal ice, it eventually becomes Cubic Ice; tweak the temperature and pressure further and you can create more kinds — Ice II, Ice III all the way up to Ice XV.
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