Red Blood Cells

How many red blood cells does the body produce an hour? Hundreds? Thousands? According to Wikipedia, the number is 2.4 million per second! I remember learning at school that red blood cells are made in bone marrow. But I don't remember how they get into the blood stream? Further research required :)

Red blood cells circulate the body every 20 60 seconds. After 100 to 120 days of existence they die (presumably at the same rate of 2.4 million per second) and are disassembled by microphages, who recycle the components. Amazing!

[Edit: updated circulation time: How long does it take for blood to circulate the body? It takes one minute for blood to circulate from the heart, all around the body, and back to the heart again.]

[Edit 2: The average healthy adult has 4.5 to 5.5 litres of blood... this is about 7% of a person's total body weight.]

[Edit 3: Red blood cell (RBC) count: 3.93 to 5.69 million cells per cubic millimeter (million/mm3) ]

Supernovae

Crab Nebula, product of the Supernova 1054

Sometimes stars at the end of their life explode, otherwise known as as a supernova. The average galaxy contains 100 billion stars and would experience a supernova about once a century.

So how many supernovae could you see stargazing on an average evening? Zero? It turns out that there are so many bazillions of galaxies out there that just in a thumbnail portion of the sky you would see 10 supernovae a night. Incredible!

- from War of the Worldviews (Chopra & Mlodinow) p61.