- You might be missing an arm
muscle. About 86 percent of us have a
palmaris longus muscle running from
the elbow to the palm. To test whether
you have it, touch your pinkie to your
thumb and tighten your wrist muscles;
a tendon connected to the muscle
will pop out in the center of your
wrist, below your palm. In humans,
it helps wrist flexibility, but there’s no
consequence if you’re born without
one. Scientists aren’t sure why some
people have the muscle and others
don’t, but they think that it probably
helped our primate ancestors grip and
swing through trees.
42. Your heart can pump one million
barrels of blood in a lifetime. Mean-
while, your kidneys filter your entire
supply of blood more than 30 times
every day. The average adult has 4.8 to
6 quarts of blood, and the kidneys fil-
ter about half a cup per minute.
43. Your skeleton contains a lot of
water. The human body is mostly
water (55 and 60 percent for women
and men, respectively). But that fluid
isn’t just in your skin, muscles, and
organs—it’s in your skeleton too. In
fact, water makes up nearly a third of
your bone mass.
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