The Respiratory System Study Guide

Updated on Sep 22, 2011


Cellular respiration is a part of metabolism and is a set of chemical reactions. In this lesson on the respiratory system, we will examine the physical process of exchanging gases between our bodies and the atmosphere – in other words, breathing. It is important for organisms to get rid of waste gases and to absorb helpful gases necessary for the chemical processes of metabolism.

When Is Respiration Not Breathing?

The word respiration is used in two different but related ways in biology. In one sense, respiration means the act of bringing air into the lungs and expelling waste gases. We call this form of respiration breathing. However, the chemical reactions of metabolism partly contain a set of reactions that use enzymes and oxygen to burn food molecules and release the energy of their bonds. This process is more specifically known as cellular aerobic respiration (or anaerobic when no oxygen is present). Sometimes, the two types of respiration are differentiated by being internal (cellular respiration) and external (the act of breathing). Here, we are talking about the process of breathing, which has the important function of allowing for the exchange of gases.

Who Respires?

All organisms, even single-celled ones, must perform cellular respiration (either aerobically or anaerobically) during metabolism to obtain necessary energy to support life functions. Because single-celled animals are small, they have no need of a respiratory system to help them breathe. They can simply exchange gases across their cell membranes. However, multicellular animals are too large to rely only upon diffusion for gas exchange. They must also have specialized tissues to perform this function.

The Respiratory System

The vertebrate respiratory system can essentially be divided into two parts, the part that conducts air and the gas exchange portion. The series of conducting tubes brings air into the gas exchange portion. Breathing involves inhalation (the taking in of air) and exhalation (the releasing of air).

Conduction of Air

Organisms breathe through their nose or mouth, and air enters the upper part of the throat or the pharynx. The next point of note is the larynx or what is called the Adam's apple on the inside of the throat. The larynx is also known as the voice box because the cartilage here changes shape to help form the sounds of speech. Inhaled air next passes into the trachea, which is a tube in the center of the chest. Within the chest, the trachea branches into two bronchi. Each bronchus leads to a lung. Once inside the lung, the bronchus branches repeatedly into small tubes called bronchioles. Bronchiole tubes then lead to a microscopic sac-like structure called an alveolus (plural is alveoli). The alveoli are the site of the second function of the respiratory system: gas exchange.

Gas Exchange

The lung provides a moist and warm surface for gas exchange. Each lung has over two million alveoli, which provide a huge surface area for gas exchange, equal to about 800 square feet. In contrast, the skin surface area of your body is only ten square feet. All of that is packed within each of your lungs, which really only have a relatively small volume if it weren't for all of the alveoli. So again we see the power of branching and folding to increase surface area in relation to volume.

The alveolus is a small sac surrounded by blood vessels. The walls of the sac and the walls of the surrounding blood vessels are very thin, allowing for diffusion of gases in either direction. The hemoglobin of the red blood cells has a greater affinity for oxygen than it does for carbon dioxide. Thus, when the blood stream interfaces with the outside air in the alveolus, oxygen diffuses into the blood and carbon dioxide diffuses into the sac of the alveolus, to be exhaled with the next breath. Blood entering the lungs is low in oxygen and high in carbon dioxide because of cellular respiration (metabolism), whereas air entering the lungs from the atmosphere is high in oxygen and low in carbon dioxide. Thus, the alveoli contain a high concentration of oxygen and a low carbon dioxide concentration. However, the blood in the blood vessels lining the alveoli has exactly the opposite condition, so the two gases flow in opposite directions. This is the gas exchange of respiration. The blood in the blood vessels lining the alveoli has a high oxygen concentration and flows out of the lungs to go to the rest of the body.

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