El médico dice que mi corazón está bien.

In Spanish, a singular countable noun usually needs an article unless it is being used in a very specific pattern.

Here, el médico means the doctor: a specific doctor, probably the one treating or examining the speaker.

• El médico = the doctor
• Un médico = a doctor

Saying just médico by itself would usually sound incomplete in this sentence.

Dice is the third person singular present tense of decir, which means to say or to tell.

So:

• yo digo = I say
• tú dices = you say
• él / ella / usted dice = he / she / you say

In El médico dice..., the subject is el médico, so Spanish uses dice.

Que means that in this kind of sentence.

So:

• El médico dice que... = The doctor says that...

In Spanish, que is very commonly kept after verbs like decir, pensar, creer, and saber when introducing a full clause.

English often drops that:

• The doctor says my heart is fine.

Spanish normally keeps que:

• El médico dice que mi corazón está bien.

Mi corazón means my heart.

The possessive adjective mi agrees with the thing possessed, not with the possessor. Since corazón is singular, you use mi:

• mi corazón = my heart
• mis pulmones = my lungs

A common point of confusion is that Spanish often uses the definite article with body parts, but that usually happens in other structures, especially with reflexive verbs:

• Me duele el corazón = My heart hurts / The heart hurts me

In this sentence, the speaker is simply identifying whose heart is being discussed, so mi corazón is perfectly natural.

Because Spanish uses estar for states or conditions, and bien works with estar to mean well, fine, or okay.

• está bien = is fine / is okay / is well

Using ser here would be wrong:

• es bien is not natural Spanish in this meaning

This is a very common pattern:

• Estoy bien = I’m fine
• Todo está bien = Everything is fine
• Mi corazón está bien = My heart is fine

Here bien means something like fine, okay, or in good condition.

With estar, bien often describes a state:

• Estoy bien = I’m fine
• La comida está bien = The food is fine
• Mi corazón está bien = My heart is fine

In a medical sentence like this, it suggests that the heart is functioning normally or that there is no problem.

The accent marks show which syllable is stressed.

• médico → stress on mé
• está → stress on the last syllable, tá
• corazón → stress on the last syllable, zón

These accents are important because they help with pronunciation and sometimes distinguish words:

• esta = this
• está = is

So the accent in está is not optional; it changes the word.

In standard Peninsular Spanish, it is approximately:

el MÉ-di-co DI-the ke mi ko-ra-THÓN es-TÁ byen

A few useful points:

• médico → stress on the first syllable
• dice → the c before e is pronounced like the th in thin in most of Spain
• corazón → the z is also pronounced like th in most of Spain
• está → stress on the last syllable
• bien is one syllable

So in much of Spain:

• dice sounds like DEE-the
• corazón sounds like ko-ra-THON

In this sentence, it is most naturally understood literally: the physical heart.

That is because the context is medical:

• El médico dice... = The doctor says...

Outside medical contexts, corazón can also be figurative, meaning heart in the emotional sense, just like in English.

But here the doctor clearly makes the literal meaning the most likely one.

Normally, no. In standard Spanish, que is expected after decir when a full clause follows.

So:

• El médico dice que mi corazón está bien. = correct

Leaving out que would sound wrong or very non-standard in this sentence.

This is one of those places where Spanish is less flexible than English.

Yes. Both are possible, but they are not always identical in tone.

• el médico = the physician / the doctor
• el doctor = the doctor

In everyday Spanish, médico is very common for a medical professional. Doctor is also common, but médico can sound a bit more precise in the sense of medical doctor.

So:

• El médico dice que mi corazón está bien.
• El doctor dice que mi corazón está bien.

Both are natural.