O plasma e as plaquetas estão na bolsa de sangue.

They often are, but not always.

Plasma is a masculine noun in Portuguese, so it takes o:

• o plasma

Portuguese noun gender is not perfectly predictable from the ending. Many words ending in -a are feminine, but some are masculine, especially words of Greek origin or technical/scientific terms.

So this is something you simply learn with the noun:

• o plasma
• a plaqueta

Because plaquetas is:

• feminine
• plural

So the correct definite article is as.

Compare:

• a plaqueta = the platelet
• as plaquetas = the platelets

The article has to match the noun in gender and number.

Because the subject is plural:

• O plasma e as plaquetas = plasma and platelets

When two things are joined by e (and), the subject is normally plural, so the verb must also be plural.

Compare:

• O plasma está... = The plasma is...
• As plaquetas estão... = The platelets are...
• O plasma e as plaquetas estão... = The plasma and the platelets are...

So estão is the 3rd person plural form of estar.

Portuguese usually uses estar for location.

Since the sentence says where something is located, estar is the normal verb:

• estão na bolsa de sangue = are in the blood bag

A very useful rule:

• ser = identity, classification, essential characteristics
• estar = location, condition, state

So:

• O plasma é importante. = Plasma is important.
• O plasma está na bolsa. = The plasma is in the bag.

Na is a contraction of:

• em + a = na

So:

• na bolsa = in the bag

This is very common in Portuguese. Some useful patterns:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

Examples:

• no hospital = in the hospital
• na bolsa = in the bag
• nos exames = in the tests
• nas veias = in the veins

Portuguese often uses noun + de + noun where English might use a noun as an adjective.

So:

• bolsa de sangue = blood bag

Literally, this is something like bag of blood, but in real usage it corresponds naturally to blood bag.

This pattern is extremely common in Portuguese:

• banco de sangue = blood bank
• teste de gravidez = pregnancy test
• dor de cabeça = headache

So de sangue is completely normal here.

Yes, in this context it naturally refers to the bag used for collected blood.

The word bolsa can mean several things depending on context, such as:

• bag
• purse
• pouch
• scholarship

But with de sangue, the medical meaning is clear:

• bolsa de sangue = blood bag

Yes. That is grammatically correct too.

Changing the order of the two nouns does not change the basic meaning:

• O plasma e as plaquetas estão na bolsa de sangue.
• As plaquetas e o plasma estão na bolsa de sangue.

The difference is mostly one of focus or style. The first item may sound slightly more prominent, but both are natural.

You sometimes can, but it changes the feel of the sentence.

With the articles:

• O plasma e as plaquetas estão na bolsa de sangue.

This sounds more like you are referring to the plasma and the platelets as specific components being discussed.

Without the articles:

• Plasma e plaquetas estão na bolsa de sangue.

This can still be understood, but it may sound more like a headline, label, or a more telegraphic style. In normal full sentences, Portuguese often prefers the articles here.

Estão is pronounced roughly like eh-STOWN, with a nasal ending.

A few key points:

• the stress is on the last syllable: -tão
• ão is a very common Portuguese nasal sound
• it does not sound exactly like English ow

A rough English-friendly approximation is:

• es-TOWN
  but with the end nasalized rather than fully pronounced like English town

If you are learning Brazilian Portuguese pronunciation, mastering -ão is very useful because it appears in many common words:

• não
• mão
• situação
• estão

It is common enough in medical, health, and biology contexts, but it is not an everyday word for all conversations.

It means platelets, the blood component.

So a Brazilian Portuguese speaker will understand it, especially in:

• hospital settings
• lab results
• blood donation contexts
• health discussions

It is a normal and correct word, just more specialized than something like sangue.

In Portuguese, verbs agree with the subject in person and number, but not usually in gender.

So the verb changes because the subject is plural, not because one noun is masculine and the other is feminine.

That is why you get:

• estão = plural

The gender appears in articles and adjectives, not in the verb itself:

• o plasma = masculine singular
• as plaquetas = feminine plural
• estão = plural verb form for both together

Yes. Dentro da bolsa de sangue means inside the blood bag, and it is a bit more explicit.

Compare:

• estão na bolsa de sangue = are in the blood bag
• estão dentro da bolsa de sangue = are inside the blood bag

Both are correct. Usually na bolsa already clearly implies location inside it, so dentro de is often unnecessary unless you want extra emphasis.