# Throw astronomical figures into a crusher.

What is the total blood volume of people all over the world?

For this question, you may keep saying that just tell me the amount of blood of a single person multiplied by the total number of people in the world to get the answer. But what if you don’t know how much blood is in your body? At this time, we can consider the scene when donating blood. Ordinary people often donate 200 to 400 milliliters of blood at a time. The ” lost amount” of blood is definitely safe for healthy people. Therefore, let’s roughly think that the amount of blood donated at one time is equivalent to 10% of the total blood volume, so the blood volume in each of us is 2,000 to 4,000 milliliters, multiplied by 7 billion people in the world, and the total blood volume of all people is 1.4 trillion to 2.8 trillion milliliters!

This figure is still difficult to understand ≥ but we can make an analogy. If the blood volume is poured into a pond the size of a large park, the blood depth can reach 10 to 20 meters. Do you feel this time?

If this question makes you feel sick, let’s take a look at a bit more comfortable: what is the total mileage of cars in the world every year? Read the mileage of your own car and the problem will be solved. Suppose your car runs 100,000 kilometers in five years, that is 20,000 kilometers per year. What is the proportion of car owners in the total population in the world? Roughly estimated that 10% of the people in the world belong to car owners, that is 700 million people, and the cars of car owners in the world travel a total of 14 trillion kilometers every year! Not feeling this number? The distance between the nearest star in the solar system and the sun is 40 trillion kilometers. If our spaceship flies at a distance equal to the total mileage of all cars in the world in one year, we will reach another galaxy in 3 years.

These two examples tell us that the most important thing to rely on estimation is to break through the conventional method and solve the problem from another angle, for example, to think about the total blood volume through blood donation and to think about the total mileage by using the odometer data of one’s own car. At the same time, it is necessary to make reasonable guesses based on estimation. For example, the proportion of blood donation and car owners in the total amount is roughly estimated at 10%.

The estimation is not strict, but it can get a data close to the real value in a very short time, thus providing valuable reference for people to think about practical problems.