Short answer: Both sentences are understandable, but “The lungs fill the blood with oxygen” sounds more natural and clear. A better, clearer way to say it is: “The lungs oxygenate the blood.”
Step-by-step explanation:
- About the word “the”: When we say “the blood”, we mean the specific blood in a person’s body — the blood we already know about. When we say just “blood”, it can sound more general. For the sentence about lungs, we want the specific blood in your body, so “the blood” is better.
- Which one sounds natural? Saying “The lungs fill the blood with oxygen” is the natural way to say it. Saying “The lungs fill blood with oxygen” is not wrong, but it sounds less usual.
- A clearer biology phrase: Scientists often say “the lungs oxygenate the blood.” That means the lungs add oxygen to the blood so your body’s cells can use it.
- Small biology note (easy): The lungs don’t pour oxygen into blood like pouring water into a glass. Instead, tiny air sacs in the lungs (called alveoli) let oxygen pass into the blood and let carbon dioxide pass out into the air you breathe out.
Examples to compare:
- "I drank the water." (a specific water you and the listener know about)
- "I drank water." (some water, general)
- "The lungs fill the blood with oxygen." (best choice for the sentence you gave)
- "The lungs oxygenate the blood." (even better and more scientific)
Tiny quiz:
- Which sentence is more natural: "The lungs fill the blood with oxygen" or "The lungs fill blood with oxygen"? — Answer: "the blood" is more natural.
- What is a better scientific phrase? — Answer: "The lungs oxygenate the blood."
Tip: Use normal capitalization: "The lungs..." instead of writing LUNGS in all caps, which is used only for emphasis.