The document describes the key stages and processes of several human body systems, including: 1) The digestive system breaks down food through a multi-step process of mechanical and chemical digestion involving the mouth, stomach, small intestine and large intestine. 2) The respiratory system inhales oxygen and exhales carbon dioxide through the air passageways of the nose, pharynx, larynx, trachea, bronchi and lungs. 3) The circulatory system transports blood, nutrients, oxygen and waste throughout the body via blood vessels and the heart. 4) Other systems described include the excretory, nervous, locomotor and reproductive systems.