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Respiratory System for AP Biology

By — McGraw-Hill Professional
Updated on Apr 25, 2014

Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology

We are going to head down to the lungs now. Please stay close because it will get a little loud in these windy tunnels. Air comes into the body through the mouth and the nose. We are currently in the nasal passages, and along with the air that came into the nose, we are being warmed and moistened in the nasal cavity before we head down toward the pharynx region, where the air and food passages cross. We will come back to this area again later on in the tour when we take the road that food uses to get from the mouth to the stomach. During inhalation, the air goes through a structure called the glottis into the larynx (human voicebox). From there the air moves into the trachea, which contains rings of cartilage that help it maintain its shape. The trachea is the tunnel that leads the air into the thoracic cavity. If you look outside your windows, you will notice some tiny arms waving at us as we go by. They are the cilia, which beat in rhythmical waves to carry foreign particles (like our tour bus) and mucus away from the lungs.

We are now at a fork in the road. Here the trachea divides into two separate tunnels: the two bronchi, which are also held open by cartilage rings, one going to the left lung, and one going to the right lung. The bronchus divides into smaller branches, which divide into even smaller branches, which divide into tunnels called bronchioles. These bronchioles branch repeatedly until they conclude as tiny air pockets containing alveoli.

In Figure 15.2, notice how thin the walls of the alveoli are. They are usually a single cell in thickness, are covered by a thin film of water, and are surrounded by a dense bed of capillaries. You might have questioned earlier exactly where the exchange of O2 and CO2 actually occurs—this is the place. The alveoli are considered to be the primary functional unit of the lung. Oxygen enters the alveolus the same way we just did—it dissolves in the water lining of the wall and diffuses across the cells into the bloodstream. At the same time, the CO2, which is carried by the blood, primarily in the form of bicarbonate (HCO3), passes out of the blood in a similar manner. The O2 moves easily into the bloodstream because it is moving down its concentration gradient. Once there, it travels with the blood to the rest of the body.

Respiratory System

Before we move on to the digestive system, we should discuss the mechanism by which breathing actually occurs. The rib cage and the diaphragm play important roles in the breathing process. Inhalation causes the volume of the thoracic cavity to increase. As a result, the air pressure in the chest falls below that of the atmosphere, and air flows into the body. This is accompanied by a contraction of the ribcage muscles and the diaphragm, allowing for the increase in thoracic volume. After the air exchange occurs, the muscles relax, causing the diaphragm to move up against the lungs, reducing the thoracic volume. This causes the pressure in the lungs to exceed that of the atmosphere—driving the air containing CO2 out of the body.

Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology

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