Dear believing brother, in an ayah in Quran, Allah mentions blood saying:

“Say: "I find not in the Message received by me by inspiration any (meat) forbidden to be eaten by one who wishes to eat it, unless it be dead meat, or blood poured forth, or the flesh of swine - for it is an abomination”

[Surat Al Anaam, verse 145]

 What catches your attention is that the prohibited blood is only the spilled one, whereas the blood running in the living being’s veins is pure, but the minute it is spilled it turns into a prohibited blood according to the above ayah. What is the scientific explanation for that?
 Scientists said: In blood there is a percentage of urea and uric acid, and as we all know the uric acid is a very toxic substance.
 There are 50 grams of this acid in one liter of urine; on the other hand the kidneys throw out of the body 0.4 grams of this substance in each liter of blood, while part of it is thrown out of the body through sweat perspiration.
 Also the toxic gas called carbon dioxide in the blood is thrown out of the body through the respiratory system, while the spleen takes care of harvesting all the dead cells in the blood in order to purify it.
 So the function of the lungs, kidneys, sweat glands, and spleen is to purify the body’s running blood from all the toxic substances and gazes.

 So the running blood in the human being is pure, and the minute it is spilled it keeps the urea in it with all, not only the toxic acids but the very toxic acids, and in that case the blood turns to a cause of damaging the cells.
 Basing on that fact we are not allowed to eat animals killed in other ways than slaughtering and forcing the blood out of its body like in hitting the animal or eating a dead and head butted one, because in these cases the blood stays still in the body of the animal, which means keeping all the toxic substances in it causing harm to the cells of the person who eats it.
 This is the reason behind prohibiting the spilled blood, and this is an amazing truth because Allah the Almighty mentioned the facts in his book before we know the reasons, explanations, and analysis of these facts.
 One more thing, the Prophet PBUH said:

‘Surely there is in the body a small piece of flesh; if it is good, the whole body is good, and if it is corrupted, the whole body is corrupted, and that is surely the heart’.

[Related by Al-Bukhari through Nu’man Bin Basheer]

 Scientists now are saying that the heart’s disorders increase the rate of urea in the blood in that case the cells will be destroyed, so this small piece of flesh mentioned about 15 hundred years ago in the above Hadith is an evidence that this is a revelation from Allah and that the Prophet didn’t say out of his own desire, it was rather a revelation sent down to him.
 Liver and spleen are the first organs to generate blood in human, and are considered as the factory that manufactures blood red cells, and when the main factories to generate blood red cells are ready in the bone marrow, the liver and spleen turn into a storage area for blood, the Prophet PBUH said:

((Two kinds of dead animals and two kinds of blood are lawful to you. The two kinds of animals are fish and whale, and the two kinds of blood are liver and spleen.))

[ Narrated in Sahih Muslim and Bukhari]

 Did the Prophet perform an anatomy on human organs before in his life? How did he know that the liver and spleen are not but blood factories and storage places to it? But these two organs don’t have spilled blood and that is why the prophet allowed us to eat them, he said:

((Two kinds of dead animals and two kinds of blood are lawful to you. The two kinds of animals are fish and whale, and the two kinds of blood are liver and spleen.))

[ Narrated in Sahih Muslim and Bukhari]

 Dear believing brother, these are few facts confirming that the Quran is Allah’s words and that human can’t create any thing like Quran even if they all met together for this purpose because there are eternal facts in it and because no falsehood can approach it in front or behind it.