The human respiratory system consists of the nasal cavity, pharynx, larynx, windpipe, bronchi, bronchioles, and lungs. The nasal cavity warms and filters inhaled air before it reaches the lungs. The windpipe branches into smaller bronchi and bronchioles within the lungs. Oxygen enters the bloodstream and carbon dioxide is released through tiny air sacs in the lungs. The respiratory system works with the circulatory system to supply oxygen throughout the body and remove carbon dioxide.