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Derived copy of Systems of Gas

Exchange*

Shannon McDermott

Based on Systems of Gas Exchange� by

OpenStax

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 4.0�

Abstract

By the end of this section, you will be able to:

• Describe the passage of air from the outside environment to the lungs• Explain how the lungs are protected from particulate matter

The primary function of the respiratory system is to deliver oxygen to the cells of the body's tissues andremove carbon dioxide, a cell waste product. The main structures of the human respiratory system are thenasal cavity, the trachea, and lungs.

All aerobic organisms require oxygen to carry out their metabolic functions. Along the evolutionary tree,di�erent organisms have devised di�erent means of obtaining oxygen from the surrounding atmosphere. Theenvironment in which the animal lives greatly determines how an animal respires. The complexity of therespiratory system is correlated with the size of the organism. As animal size increases, di�usion distancesincrease and the ratio of surface area to volume drops. In unicellular organisms, di�usion across the cellmembrane is su�cient for supplying oxygen to the cell (Figure 1). Di�usion is a slow, passive transportprocess. In order for di�usion to be a feasible means of providing oxygen to the cell, the rate of oxygenuptake must match the rate of di�usion across the membrane. In other words, if the cell were very largeor thick, di�usion would not be able to provide oxygen quickly enough to the inside of the cell. Therefore,dependence on di�usion as a means of obtaining oxygen and removing carbon dioxide remains feasibleonly for small organisms or those with highly-�attened bodies, such as many �atworms (Platyhelminthes).Larger organisms had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passagesaccompanied by complex circulatory systems, to transport oxygen throughout their entire body.

*Version 1.3: May 10, 2016 10:41 am -0500�http://cnx.org/content/m44792/1.10/�http://creativecommons.org/licenses/by/4.0/

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Figure 1: The cell of the unicellular algae Ventricaria ventricosa is one of the largest known, reachingone to �ve centimeters in diameter. Like all single-celled organisms, V. ventricosa exchanges gases acrossthe cell membrane.

1 5.1a Direct Di�usion

For small multicellular organisms, di�usion across the outer membrane is su�cient to meet their oxygenneeds. Gas exchange by direct di�usion across surface membranes is e�cient for organisms less than 1 mmin diameter. In simple organisms, such as cnidarians and �atworms, every cell in the body is close to theexternal environment. Their cells are kept moist and gases di�use quickly via direct di�usion. Flatwormsare small, literally �at worms, which `breathe' through di�usion across the outer membrane (Figure 2). The�at shape of these organisms increases the surface area for di�usion, ensuring that each cell within the bodyis close to the outer membrane surface and has access to oxygen. If the �atworm had a cylindrical body,then the cells in the center would not be able to get oxygen.



Figure 2: This �atworm's process of respiration works by di�usion across the outer membrane. (credit:Stephen Childs)

2 5.1b Skin and Gills

Earthworms and amphibians use their skin (integument) as a respiratory organ. A dense network of capillarieslies just below the skin and facilitates gas exchange between the external environment and the circulatorysystem. The respiratory surface must be kept moist in order for the gases to dissolve and di�use across cellmembranes.

Organisms that live in water need to obtain oxygen from the water. Oxygen dissolves in water but ata lower concentration than in the atmosphere. The atmosphere has roughly 21 percent oxygen. In water,the oxygen concentration is much smaller than that. Fish and many other aquatic organisms have evolvedgills to take up the dissolved oxygen from water (Figure 3). Gills are thin tissue �laments that are highlybranched and folded. When water passes over the gills, the dissolved oxygen in water rapidly di�uses acrossthe gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the otherparts of the body. In animals that contain coelomic �uid instead of blood, oxygen di�uses across the gillsurfaces into the coelomic �uid. Gills are found in mollusks, annelids, and crustaceans.



Figure 3: This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygenfrom water. (credit: "Guitardude012"/Wikimedia Commons)

The folded surfaces of the gills provide a large surface area to ensure that the �sh gets su�cient oxygen.Di�usion is a process in which material travels from regions of high concentration to low concentrationuntil equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulatesthrough the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygenmolecules in gills. As a result, oxygen molecules di�use from water (high concentration) to blood (lowconcentration), as shown in Figure 4. Similarly, carbon dioxide molecules in the blood di�use from the blood(high concentration) to water (low concentration).



Figure 4: As water �ows over the gills, oxygen is transferred to blood via the veins. (credit "�sh":modi�cation of work by Duane Raver, NOAA)

3 5.1c Tracheal Systems

Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct rolein oxygen transport. Insects have a highly specialized type of respiratory system called the tracheal system,which consists of a network of small tubes that carries oxygen to the entire body. The tracheal system is themost direct and e�cient respiratory system in active animals. The tubes in the tracheal system are made ofa polymeric material called chitin.

Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect tothe tubular network, allowing oxygen to pass into the body (Figure 5) and regulating the di�usion of CO2

and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilatethe tracheal system with body movements.



Figure 5: Insects perform respiration via a tracheal system.

4 5.1d Mammalian Systems

In mammals, pulmonary ventilation occurs via inhalation (breathing). During inhalation, air enters thebody through the nasal cavity located just inside the nose (Figure 6). As air passes through the nasalcavity, the air is warmed to body temperature and humidi�ed. The respiratory tract is coated with mucusto seal the tissues from direct contact with air. Mucus is high in water. As air crosses these surfaces ofthe mucous membranes, it picks up water. These processes help equilibrate the air to the body conditions,reducing any damage that cold, dry air can cause. Particulate matter that is �oating in the air is removedin the nasal passages via mucus and cilia. The processes of warming, humidifying, and removing particlesare important protective mechanisms that prevent damage to the trachea and lungs. Thus, inhalation servesseveral purposes in addition to bringing oxygen into the respiratory system.

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Figure 6: Air enters the respiratory system through the nasal cavity and pharynx, and then passesthrough the trachea and into the bronchi, which bring air into the lungs. (credit: modi�cation of workby NCI)

Which of the following statements about the mammalian respiratory system is false?

a.When we breathe in, air travels from the pharynx to the trachea.b.The bronchioles branch into bronchi.c.Alveolar ducts connect to alveolar sacs.d.Gas exchange between the lung and blood takes place in the alveolus.

From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box), as it makesits way to the trachea (Figure 6). The main function of the trachea is to funnel the inhaled air to thelungs and the exhaled air back out of the body. The human trachea is a cylinder about 10 to 12 cm longand 2 cm in diameter that sits in front of the esophagus and extends from the larynx into the chest cavitywhere it divides into the two primary bronchi at the midthorax. It is made of incomplete rings of hyalinecartilage and smooth muscle (Figure 7). The trachea is lined with mucus-producing goblet cells and ciliatedepithelia. The cilia propel foreign particles trapped in the mucus toward the pharynx. The cartilage providesstrength and support to the trachea to keep the passage open. The smooth muscle can contract, decreasingthe trachea's diameter, which causes expired air to rush upwards from the lungs at a great force. The forcedexhalation helps expel mucus when we cough. Smooth muscle can contract or relax, depending on stimulifrom the external environment or the body's nervous system.



Figure 7: The trachea and bronchi are made of incomplete rings of cartilage. (credit: modi�cation ofwork by Gray's Anatomy)



4.1 Lungs: Bronchi and Alveoli

The end of the trachea bifurcates (divides) to the right and left lungs. The lungs are not identical. Theright lung is larger and contains three lobes, whereas the smaller left lung contains two lobes (Figure 8). Themuscular diaphragm, which facilitates breathing, is inferior to (below) the lungs and marks the end of thethoracic cavity.

Figure 8: The trachea bifurcates into the right and left bronchi in the lungs. The right lung is made ofthree lobes and is larger. To accommodate the heart, the left lung is smaller and has only two lobes.

In the lungs, air is diverted into smaller and smaller passages, or bronchi. Air enters the lungs throughthe two primary (main) bronchi (singular: bronchus). Each bronchus divides into secondary bronchi,then into tertiary bronchi, which in turn divide, creating smaller and smaller diameter bronchioles as theysplit and spread through the lung.

Alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles (Figure 9). Gas exchangeoccurs only in alveoli. Alveoli are in direct contact with capillaries (one-cell thick) of the circulatory system.Such intimate contact ensures that oxygen will di�use from alveoli into the blood and be distributed to thecells of the body. In addition, the carbon dioxide that was produced by cells as a waste product will di�usefrom the blood into alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizesthe structural and functional relationship of the respiratory and circulatory systems. Because there are somany alveoli (∼300 million per lung) within each alveolar sac and so many sacs at the end of each alveolarduct, the lungs have a sponge-like consistency. This organization produces a very large surface area thatis available for gas exchange. The surface area of alveoli in the lungs is approximately 75 m2. This large



surface area, combined with the thin-walled nature of the alveolar cells, allows gases to easily di�use acrossthe cells.

Figure 9: Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolarsacs. Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes.Air �ows into the atrium of the alveolar sac, then circulates into alveoli where gas exchange occurs withthe capillaries. Mucous glands secrete mucous into the airways, keeping them moist and �exible. (credit:modi�cation of work by Mariana Ruiz Villareal)



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Watch the following video1 to review the respiratory system.

5 Section Summary

Animal respiratory systems are designed to facilitate gas exchange. In mammals, air is warmed and humid-i�ed in the nasal cavity. Air then travels down the pharynx, through the trachea, and into the lungs. In thelungs, air passes through the branching bronchi, reaching the respiratory bronchioles, which house the �rstsite of gas exchange. The respiratory bronchioles open into the alveolar ducts, alveolar sacs, and alveoli.Because there are so many alveoli and alveolar sacs in the lung, the surface area for gas exchange is verylarge. Several protective mechanisms are in place to prevent damage or infection. These include the hairand mucus in the nasal cavity that trap dust, dirt, and other particulate matter before they can enter thesystem. In the lungs, particles are trapped in a mucus layer and transported via cilia up to the esophagealopening at the top of the trachea to be swallowed.

1http://openstaxcollege.org/l/lungs_pulmonary



Glossary

De�nition 9: alveolus(plural: alveoli) (also, air sac) terminal region of the lung where gas exchange occurs

De�nition 9: bronchus(plural: bronchi) smaller branch of cartilaginous tissue that stems o� of the trachea; air is funneledthrough the bronchi to the region where gas exchange occurs in alveoli

De�nition 9: bronchioleairway that extends from the main tertiary bronchi to the alveolar sac

De�nition 9: diaphragmdomed-shaped skeletal muscle located under lungs that separates the thoracic cavity from theabdominal cavity

De�nition 9: larynxvoice box, a short passageway connecting the pharynx and the trachea

De�nition 9: mucussticky protein-containing �uid secretion in the lung that traps particulate matter to be expelledfrom the body

De�nition 9: nasal cavityopening of the respiratory system to the outside environment

De�nition 9: pharynxthroat; a tube that starts in the internal nares and runs partway down the neck, where it opensinto the esophagus and the larynx

De�nition 9: primary bronchus(also, main bronchus) region of the airway within the lung that attaches to the trachea and bifurcatesto each lung where it branches into secondary bronchi

De�nition 9: tracheacartilaginous tube that transports air from the larynx to the primary bronchi


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