Respiratory system mean breath.
~> Respiratory system is about respiration/breathing.

Digestive system mean to eat and make poopoo
~> Digestive system is about the digestion of food.

Circulatory system mean heart and blood.
~> Circulatory system is about the heart and the blood.

Skeletal system mean bone.
~> Skeletal system is about the bones.

Muscular system mean muscle.
~> Muscular system is about the muscles.

Explanations:
1. You need to choose the appropriate words for each ideas you have in mind.
 Example:    means - is all about the

Comment:
Hi, Emma!
I had a good time reading your homework. I was really thankful for your effort on making these sentences but sometimes, we need to choose the most appropriate one for our sentences. Always take note of that but all in all I gotta say this "You are awesome."
-Teacher Joanna ^^