【英语时差8,16】骨质疏松-上（在线收听）

Today’s word is: osteoporosis! Yep, osteoporosis. Now, you’ve heard this word before, and you know it has something to do with bones. But can you actually explain what it is? The answer is straight-forward. It comes down to the fact that we human beings need calcium in our blood–it allows us to do all sorts of neat things, from contracting muscles to conducting nerve impulses. Where to get some calcium? Food is a good idea. But what if there isn’t always calcium-rich food to be had, as was probably the case for most of our long evolutionary history? Best to store some for a rainy day. Store it where? The skeleton works! Then, when the blood runs low on calcium, a signaling device called the parathyroid gland can activate a process by which calcium is taken from the skeleton and put back into the blood.