BIG IDEAS: <Inhalation> Carbon Dioxide builds up in the blood, making it acidic. This triggers a part of the brain stem (the medulla oblongata) to send a message to the diaphragm. In response, the diaphragm (which at rest it curved upward) contracts, flattens out, and drops down. At the same time, the muscles between the ribs (called intercostal muscles) contract, lifting the ribcage up and out. These two actions increase the volume inside the chest cavity, which decreases the pressure. The outside air rushes into the lungs, through the air passages, to equalize the pressure difference.

<Exhalation> The diaphragm relaxes and returns to its normal, upward position. The intercostal muscles relax and the ribcage drops back down. This decreases the volume inside the chest cavity, which increases the pressure. The air rushes back out of the lung and air passages.

Alveoli are small, sac-like structures at the ends of bronchioles. These are the site of gas exchange in the respiratory system. After inhalation, the air sacs have a high concentration of oxygen which diffuses into the capillaries surrounding them. At the same time, carbon dioxide (a waste product from cellular respiration) diffuses from the capillaries into the alveoli. This carbon dioxide is then exhaled.

External respiration is the process of breathing, exchanging air between your body and the surrounding air. Internal respiration is the exchange of blood gases (oxygen and carbon dioxide) between the lungs and the blood. Cellular respiration is using the inspired oxygen to break the nutrients from digestion down into useable energy (ATP) - carbon dioxide is created as a waste product.

Build a Lung Activity: As a class, we made a model of a group of alveoli. Small balloons represented alveoli, bright red and dark red yarn represented oxygen rich and poor capillaries around the alveoli, and a straw taped on the balloon represented a bronchiole.Each person made one alveolus and we put them together in a group.

Build a Chest Cavity Activity: Each lab group made a model of the chest cavity. One 2-liter bottle represented the ribcage (chest cavity), a straw represented the trachea and air passages, a large balloon represented a lung, and a plastic bag represented a diaphragm.

Lung Capacity Lab: Each lab group took a milk jug, a sink full of water, a rubber hose, and a drinking straw. The milk jug was filled with water and turned upside down in the water-filled sink. Each person breathed a full breath through the straw and hose into the milk jug, displacing water with expelled air. The jug was then set on the counter and filled back up with water, measuring how much volume was displaced with expelled air. The process was done for a normal breath as well. Text from the lab also explained tidal volume, vital capacity, dead space, residual volume, and total lung volume.

Alveoli are air sacs at the ends on bronchioles in the lungs. Each one is surrounded by some oxygen rich capillaries and some oxygen poor capillaries. They are found in grapelike clusters.

When you pull down on the diaphragm, the lung inflates. When you push up on the diaphragm, the lung deflates. If you plug the trachea, you can still pull down on the diaphragm, but the lung doesn't inflate. Then, if you unplug the trachea while the diaphragm is still held down, air rushes in and fills the lung.

A full breath (vital capacity) is the maximum amount of air you can expel from your lungs after a deep breath in. A normal breath (tidal volume) is the amount of air expelled from your lungs during one normal respiration. Residual volume is the amount of air left in the lungs that cannot be expelled. Dead space is the air left in your air passages. Total lung capacity is the total volume of air in your lungs.

Alveoli are the sites of gas exchange in the lungs. Oxygen diffuses from the alveoli into the capillary blood and carbon dioxide diffuses from the capillary blood into the air sacs.

When the diaphragm contracts, it drops down and increases the volume inside the chest cavity. The air rushes into the lungs to fill the space (equalize the pressure). When the diaphragm relaxes, it moves back up, decreasing the volume and the air rushes back out. The volume change is what causes the air to rush in or out. It's not "sucked" in by the movement of the diaphragm.

During the breathing process, we bring air into and out of our lungs and airways. There are different terms that describe the volume of air exchanged during normal breaths, deep breaths, and that left in the lungs and airways.