In order to speak about internal organs, you need the appropriate vocabulary.
Stomach - the organ inside your body where food goes after it has been eaten and where it starts to be digested;

Lung - one of the two organs inside your chest that are used for breathing;

Brain - the organ inside your head that controls your thoughts, feelings, and movements;
Tissue - the material that animals and plants are made of;

Spine, backbone, spinal column - the long structure of bones down the centre of your back, which supports your body;

Kidney - one of the two organs in your body which remove waste from the blood and produce urine;

Heart - the organ inside your chest that sends blood around your body;

Skeleton - the structure made of all the bones in the body of a person or animal;

Skull - the part of your head that is made of bone and which protects your brain.
Some people can have problems with their lungs because of smoking.

She will have an operation on the heart.

Fast food destroys your stomach.