Muscular System: It comprises of muscles that permit movement of the body, maintains posture and circulates blood throughout the body. So, MUSCLES are attached to bones and help them MOVE.
Circulatory System: Is system that permits blood to circulate and transport nutrients, oxygen, carbon dioxide, hormones, and blood cells to and from the cells in the body to provide nourishment to the body. So, BONE MARROW inside the bones produce the cells inside your BLOOD.
Nervous System: It is a complex collection of nerves and specialized cells known as neurons that transmit signals between different parts of the body. So, the BRAIN tells the skeletal system when and where to move. Bones protect the ORGANS.
Digestive System: It consists of the central gastrointestinal (GI) tract and its associated accessory organs that break down food into smaller components so that nutrients can be absorbed and assimilated. So, CALCIUM help to grow and repair bones. SKELETAL SYSTEM protect the organs.