There are four levels of structural organization in the human body: cells, tissues, organs, and systems. The cardiovascular, respiratory, digestive, skeletal, nervous, muscular and endocrine systems were highlighted. Key aspects of cardiovascular anatomy and function like the heart chambers and blood flow were described. Respiratory system components like the lungs and gas exchange process were outlined. Muscle contraction occurs via the sliding filament model in the sarcomere. Flexibility is influenced by factors like age, gender, joint structure and past injury through soft tissues like fascia, muscles and tendons.