Skin Reflex
Blushing
When you are embarrassed, blood vessels in the face open wide and the skin turns red. You cannot stop it. Darwin called it the most human of all expressions, and no other animal seems to do it.
Everyone
Heart and blood
Heart rate, blood pressure, blood flow, and fainting.
10 oddities. 4 have this as their primary region.
Body Rare ability
CCR5-delta32
About 1% of people of Northern European descent carry two copies of a broken gene that makes them almost immune to the most common form of HIV. A bone marrow transplant from one such donor produced the first cure of HIV.
About 1% of Northern Europeans (two copies)
Organs Reflex
Cold diuresis
Being cold makes you need to urinate. The body pulls blood away from the skin into the core, the heart reads this as too much blood, and the kidneys are told to dump water.
Everyone
Heart Reflex
Diving reflex
Cold water on the face makes the heart slow down, sends blood to the brain, and holds the breath. Every mammal has it. You can trigger it with a bowl of cold water.
Everyone
Organs Rare ability
Extraordinary breath-holding
Most adults can hold their breath for 30 to 90 seconds. Trained freedivers manage over 11 minutes, and over 24 minutes after breathing pure oxygen. One group of sea nomads has evolved larger spleens for diving.
Trained divers only
Heart Trick
Heart rate control
Slowing or speeding up your own heartbeat on purpose, using breath, attention, or training. Most people can shift it a little within minutes. A few trained people shift it a lot.
Most people, a little
Nerves Involuntary
Precordial catch syndrome
A sudden, sharp, stabbing pain near the heart that lasts seconds to a few minutes and gets worse when you breathe in. It is the most common cause of chest pain in children and teenagers, and it is harmless.
Most chest pain in ages 6–20
Ears Sound
Pulsatile tinnitus
A whooshing or thumping in one ear that keeps time with your pulse. It is the sound of blood flowing near the ear. It is usually harmless, but it always deserves a check-up.
About 3–5% of people with tinnitus
Heart Involuntary
Seeing stars when standing up
You stand up fast, your vision goes grey or sparkly, and your head feels light for a few seconds. Blood has briefly pooled in your legs. It passes on its own.
Almost everyone, at some time
Heart Reflex
Vasovagal syncope
The most common reason people faint. A trigger like blood, pain, heat, or standing too long makes the vagus nerve slow the heart and drop blood pressure. The brain briefly runs short of blood and you go down.
About 4 in 10 people faint at least once