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The familiar sounds of your heart are made by the heart's valves as they open and close.
Hyperthyroidism leads to an increased rate of metabolism and affects about 1% of women but only 0.1% of men. For most people, this increased metabolic rate causes the thyroid gland to become enlarged (known as a goiter).
It is believed that the Incas used anesthesia. Evidence supports the theory that shamans chewed cocoa leaves and drilled holes into the heads of patients (letting evil spirits escape), spitting into the wounds they made. The mixture of cocaine, saliva, and resin numbed the site enough to allow hours of drilling.
Blood is approximately twice as thick as water because of the cells and other components found in it.
All adults should have their cholesterol levels checked once every 5 years. During 2009–2010, 69.4% of Americans age 20 and older reported having their cholesterol checked within the last five years.

![People have seen patterns in the stars since ancient times.[5] This 1690 depiction of the constellat](https://biology-forums.com/gallery/47/medium_12359_26_01_13_3_44_40.png)