There are two ways to pronounce the word "blood": the first way to pronounce "xuè" is used for the pronunciation of written language; When reading xiě (white), it is used as the pronunciation of spoken language. However, it is sometimes difficult to distinguish written language from spoken language. If we distinguish according to the usage of the word "blood", it will be easier to master.
"Blood" is used in compound words and idioms, which belongs to written usage and should be pronounced as xuè. Such as blood, blood vessels, sweat, blood, bloody case, blood, bloodline, blood color, blood debt, blood transfusion, anemia, painstaking efforts, congestion, blood, blood bath, platelets, vampires, mixed blood, cerebral hemorrhage, blood feud, blood feud.
"Blood" is used alone or in a phrase (phrase), which belongs to oral usage and should be pronounced as xiě. For example, vomiting blood, blood lessons, blood price, red as blood (used alone above), blood pus, blood clot, blood spot, chicken blood, pig blood, selling blood, donating blood, blood test and drawing blood (blood).
It is worth mentioning that the boundary between Chinese compound words and phrases has not been completely solved by grammarians. But under normal circumstances, they can still be distinguished, that is, to see whether their conceptual differences and internal integration are close. The concept of compound words is simple and fixed, not the simple addition of morpheme meanings, but the internal combination is relatively close. However, the concept of phrase expression lacks specificity and is the sum of the meanings of several words, and its internal combination is relatively loose. For example, the word "bleeding" in "Blood Sacrifice" and "Blood Event" refers to "sacrificing life or injury", so it is a compound word, so it should be pronounced as xùe (in the dictionary); The word "bleeding" of "excessive bleeding" does not mean "sacrificing life or injury" but "bleeding", so it is a phrase and should be pronounced xiě. The internal structure of the phrase is loose, and some other words can be inserted, such as "chicken blood-chicken blood", "blood donation-blood donation" and "blood drawing-blood drawing-blood drawing". Only a few verb-object compound words can be inserted with other words, such as "blood transfusion", which can be said to be "blood transfusion once" and "blood transfusion of 300CC", but it is still a compound word because it is a medical term, not a simple addition of morpheme meanings, nor does it mean "blood transfusion".
Of course there are exceptions. That is, some colloquial words such as "hematemesis, bloodletting, hemoptysis, fainting, blood exchange, hematochezia, hemorrhage (dialect), bloody, bloody" and the idiom "hit the nail on the head". Perhaps the idiom "hit the nail on the head" comes from the spoken language of the masses. Remember the special case, as long as you master its usage, you will master its pronunciation.