Кровотечение остановилось.

Кровотечение is in the nominative singular, because it is the subject of the sentence: it is the thing that stopped.

A good literal way to think about the structure is:

[The bleeding] stopped.

So Russian treats кровотечение as the subject, just as English treats the bleeding as the subject.

Because Russian past-tense verbs agree with the subject in gender and number.

Кровотечение is a neuter singular noun, so the past tense uses -ло:

• masculine: остановился
• feminine: остановилась
• neuter: остановилось
• plural: остановились

So:

• кровотечение остановилось
• рана затянулась
• поезд остановился

The ending matches the noun.

A major clue is the ending -ие. Nouns ending in -ие are typically neuter in Russian.

So:

• здание — neuter
• состояние — neuter
• кровотечение — neuter

That is why the verb is остановилось, not остановился or остановилась.

The verb here is остановиться — to stop, to come to a stop, to cease.

The non-reflexive verb is остановить, which usually means to stop something.

Compare:

• Врач остановил кровотечение. — The doctor stopped the bleeding.
• Кровотечение остановилось. — The bleeding stopped.

So the reflexive form is used because the sentence describes the event as ceasing, not as something explicitly stopped by an outside agent.

In остановилось, you can think of it as:

• остановило = stopped
• -сь = reflexive marker

Остановилось is perfective.

That means the sentence presents the action as a completed result: the bleeding has come to an end.

This is exactly why perfective is natural here. The focus is not on the process, but on the outcome.

Compare:

• Кровотечение останавливалось. — The bleeding was stopping / used to stop / was being brought under control.
  This is imperfective and would need more context.
• Кровотечение остановилось. — The bleeding stopped.
  This is perfective and sounds like a complete event.

Not in this exact sentence.

Остановить is transitive: it needs an object.

So you can say:

• Врач остановил кровотечение. — The doctor stopped the bleeding.

But if кровотечение itself is the subject, then the natural verb is остановиться:

• Кровотечение остановилось.

So the choice depends on sentence structure:

• someone stopped it → остановить
• it stopped → остановиться

Not really, and it usually sounds less natural.

Russian normally uses кровотечение for bleeding as a medical event or condition.
Кровь means blood, the substance itself.

So:

• Кровотечение остановилось. — natural; the bleeding stopped.
• Кровь остановилась. — sounds odd in many contexts, because it suggests the blood itself literally stopped moving.

In more everyday speech, people might say:

• Кровь перестала идти. — The blood stopped coming out.

But the sentence you have is the most standard and concise way to say The bleeding stopped.

It is fairly neutral, but it does sound somewhat medical or clinical because of the noun кровотечение.

You might hear it:

• in medical contexts
• in first-aid situations
• in news reports
• in written descriptions

In casual speech, people may choose something less technical, such as:

• Кровь перестала идти.

But Кровотечение остановилось is completely normal and natural.

Yes. Russian word order is flexible.

The neutral order is:

• Кровотечение остановилось.

You could also say:

• Остановилось кровотечение.

That version is possible, but it sounds more marked. It may emphasize the result or fit a certain narrative style.

For a learner, Кровотечение остановилось is the safest and most neutral order.

The stress is:

• кровотечЕние
• остановИлось

So the sentence is:

кровотечЕние остановИлось

A rough pronunciation guide:

• кровотечЕние ≈ kruh-vuh-tye-CHEH-nee-ye
• остановИлось ≈ uh-stuh-nuh-VEE-ləsʲ

A few helpful notes:

• Unstressed о is usually reduced in pronunciation.
• The -сь at the end is soft.
• The stressed syllables are the clearest and strongest parts of the words.

Then you would switch to the transitive verb:

• Врач остановил кровотечение. — The doctor stopped the bleeding.
• Ей быстро остановили кровотечение. — They quickly stopped her bleeding.

You can also use a passive-style construction:

• Кровотечение было остановлено. — The bleeding was stopped.

So:

• Кровотечение остановилось. = the bleeding stopped
• Кровотечение было остановлено. = the bleeding was stopped
• Врач остановил кровотечение. = the doctor stopped the bleeding

These are related, but not identical in meaning or focus.