The Human Respiratory System

  The basic function performed by the human respiratory system is remarkably simple—to bring about the exchange of oxygen and carbon dioxide between the blood, the air, and tissues. With each breath, air enters the body through the air passageways and fills the lungs, where gas exchange takes place. The respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and lungs.

The figure at right shows the structures of the respiratory system. Air moves through the nose to a tube at the back of the mouth called the pharynx, or throat. The pharynx serves as a passageway for both air and food. Air moves from the pharynx into the trachea, or windpipe. A flap of tissue called the epiglottis covers the entrance to the trachea when you swallow.

The Respiratory System

Cilia and Mucus The respiratory passageways allow air to pass directly into some of the most delicate tissues in the body. To keep the lung tissue healthy, air entering the respiratory system must be warmed, moistened, and filtered. Large dust particles get trapped by the hairs lining the entrance to the nasal cavity. Some of the cells that line the respiratory system produce a thin layer of mucus. The mucus moistens the air and traps inhaled particles of dust or smoke. Cilia, such as those shown below, sweep the trapped particles and mucus away from the lungs toward the pharynx. The mucus and trapped particles are either swallowed or spit out. These protective measures help keep the lungs clean and open for the important work of gas exchange.

Respiratory Cilia In this cross section of the trachea, the cilia have been colored green.

The Larynx At the top of the trachea is the larynx. The larynx contains two highly elastic folds of tissue known as the vocal cords. When muscles pull the vocal cords together, the air moving between them causes the cords to vibrate and produce sounds. Your ability to speak, shout, and sing comes from these tissues.

The Bronchi From the larynx, air passes through the trachea into two large passageways in the chest cavity called bronchi (singular: bronchus). Each bronchus leads into one of the lungs. Within each lung, the large bronchus subdivides into smaller bronchi, which lead to even smaller passageways called bronchioles. Air moving along this path can be compared to a motorist who takes an exit off an eight-lane highway onto a four-lane highway, makes a turn onto a two-lane road, and ends up on a narrow country lane.

The bronchi and bronchioles are surrounded by smooth muscle that helps to support them and enables the autonomic nervous system to regulate the size of the air passageways. The bronchioles continue to subdivide until they reach a series of dead ends—millions of tiny air sacs called alveoli (singular: alveolus). Alveoli are grouped in little clusters, like bunches of grapes. A delicate network of thin-walled capillaries surrounds each alveolus.