בכורות נה, א
אמר רב כהנא כרותא דדמא כבדא
Rav Kahana also states: The source of blood is the liver.
The liver has long been considered a very special organ (as indeed it is). The Babylonians used the organ for divination, as is mentioned in our own Book of Ezekiel (21:26):
כִּי־עָמַד מֶלֶךְ־בָּבֶל אֶל־אֵם הַדֶּרֶךְ בְּרֹאשׁ שְׁנֵי הַדְּרָכִים לִקְסָם־קָסֶם קִלְקַל בַּחִצִּים שָׁאַל בַּתְּרָפִים רָאָה בַּכָּבֵד׃
For the king of Babylon has stood at the fork of the road, where two roads branch off, to perform divination: He has shaken arrows, consulted teraphim, and inspected the liver.
The Greek physician Galen (129-c.207 CE) believed that the liver stored the four humors of the body, and, just like Rav Kahana, he claimed that it produced blood. But both were mistaken
Where your blood actually comes from
Blood is not made in the liver (unless you are a fetus) but in the bone marrow, which is the spongy middle bit of bones. And your bone marrow works really hard. Each day it makes 200,000,000,000 red blood cells, 150,000,000,000 platelet cells, and 20,000,000,000 lymphocytes. It all begins with an all powerful stem cell, which then can turn into (among other things) a red blood cell that carries oxygen and has life of only 120 days), a mast cell which is involved in inflammation and allergies, a neutrophil (also known as a “white blood cell”) which fights infections, or a macrophage, which is a microscopic vacuum cleaner and whose job get rid of all the cellular bits and pieces that don’t belong.
But in The Fetus, the Liver does make blood
In the growing embryo, blood is first made in the yolk sac, but at about 6 weeks gestation the fetal liver takes over. The liver happily churns out all the cells of the blood line, until, at about 5 months gestation, the bone marrow takes over. So neither Rav Kahana nor Galen were totally mistaken when he declared that the source of blood is the liver.