<< Chapter < Page Chapter >> Page >

The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles. The respiratory bronchioles subdivide into several alveolar ducts. Numerous alveoli and alveolar sacs surround the alveolar ducts. The alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles ( [link] ). In the acinar region, the alveolar ducts are attached to the end of each bronchiole. At the end of each duct are approximately 100 alveolar sacs , each containing 20 to 30 alveoli that are 200 to 300 microns in diameter. Gas exchange occurs only in alveoli. Alveoli are made of thin-walled parenchymal cells, typically one-cell thick, that look like tiny bubbles within the sacs. Alveoli are in direct contact with capillaries (one-cell thick) of the circulatory system. Such intimate contact ensures that oxygen will diffuse from alveoli into the blood and be distributed to the cells of the body. In addition, the carbon dioxide that was produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. Because there are so many alveoli (~300 million per lung) within each alveolar sac and so many sacs at the end of each alveolar duct, the lungs have a sponge-like consistency. This organization produces a very large surface area that is available for gas exchange. The surface area of alveoli in the lungs is approximately 75 m 2 . This large surface area, combined with the thin-walled nature of the alveolar parenchymal cells, allows gases to easily diffuse across the cells.

The illustration shows a terminal bronchial tube branching into three alveolar ducts. At the end of each duct is an alveolar sac made up of 20 to 30 alveoli clustered together, like grapes. The airspace in the middle of the alveolar sac, called the atrium, is continuous with the air space inside the alveolus so that air can circulate from the atrium to the alveolus. Capillaries surround each alveolus, and this is where gas exchange occurs. A pulmonary artery (shown in blue) runs along the terminal bronchiole, bringing deoxygenated blood from the heart to the alveoli. A pulmonary vein (shown in red) running along the bronchiole brings oxygenated blood back to the heart. Small, flat mucous glands are associated with the outside of the bronchial tubes.
Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar sacs. Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes. Air flows into the atrium of the alveolar sac, then circulates into alveoli where gas exchange occurs with the capillaries. Mucous glands secrete mucous into the airways, keeping them moist and flexible. (credit: modification of work by Mariana Ruiz Villareal)

Watch the following video to review the respiratory system.

Protective mechanisms

The air that organisms breathe contains particulate matter    such as dust, dirt, viral particles, and bacteria that can damage the lungs or trigger allergic immune responses. The respiratory system contains several protective mechanisms to avoid problems or tissue damage. In the nasal cavity, hairs and mucus trap small particles, viruses, bacteria, dust, and dirt to prevent their entry.

If particulates do make it beyond the nose, or enter through the mouth, the bronchi and bronchioles of the lungs also contain several protective devices. The lungs produce mucus    —a sticky substance made of mucin    , a complex glycoprotein, as well as salts and water—that traps particulates. The bronchi and bronchioles contain cilia, small hair-like projections that line the walls of the bronchi and bronchioles ( [link] ). These cilia beat in unison and move mucus and particles out of the bronchi and bronchioles back up to the throat where it is swallowed and eliminated via the esophagus.

In humans, for example, tar and other substances in cigarette smoke destroy or paralyze the cilia, making the removal of particles more difficult. In addition, smoking causes the lungs to produce more mucus, which the damaged cilia are not able to move. This causes a persistent cough, as the lungs try to rid themselves of particulate matter, and makes smokers more susceptible to respiratory ailments.

In this micrograph, cilia are long, thin, hairlike projections.
The bronchi and bronchioles contain cilia that help move mucus and other particles out of the lungs. (credit: Louisa Howard, modification of work by Dartmouth Electron Microscope Facility)

Section summary

Animal respiratory systems are designed to facilitate gas exchange. In mammals, air is warmed and humidified in the nasal cavity. Air then travels down the pharynx, through the trachea, and into the lungs. In the lungs, air passes through the branching bronchi, reaching the respiratory bronchioles, which house the first site 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 very large. Several protective mechanisms are in place to prevent damage or infection. These include the hair and mucus in the nasal cavity that trap dust, dirt, and other particulate matter before they can enter the system. In the lungs, particles are trapped in a mucus layer and transported via cilia up to the esophageal opening at the top of the trachea to be swallowed.

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

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

[link] B

Got questions? Get instant answers now!

Questions & Answers

I want more mcqs please
Uday Reply
more what?
what is meant by the term decarboxylation
explain the role natural selection in the evolution of organisms
the population produces more than What the environment can support. some organisms adapt well to the environment than others. the less adapted ones disappear leaving the more adapted in the environment.
What are the Advantage of electron microscope over light microscope?
Liticia Reply
an electronic microscope uses electronic so it's more efficient
It helps to view specimen accurately
what are nymph nodes?
Lydia Reply
what imeosis
because the surface tension is tight like a skin
david Reply
what is genetic
Nad Reply
what is écosystème
what is écosystème
what is écosystème
what is écosystème
discuss adaptations of flight in birds
Cassim Reply
what is an enzymes
Joseph Reply
mention two types of metabolic
what is a cell?
is the biological catalyst that speed up chemical reaction in the living thing
an enzymes is a biological catalyst that speed up a chemical reaction in living organisms and it is protein in nature
enzymes are protein that act as biological catalysts
what agglutriation
enzymes are biological catalysts that speed up the chemical reactions in a living organism
what is antecology
agglutination is the process by which rhesus factor of blood react during blood transfusion
Enzymes are biological catalysts, proteins in nature, functioning to increase the rate of chemical reactions and remain unchanged at the end of the reaction.
Be4 transfusions are performed, a major cross-match made by mixing serum frm the recipient wizblood cells frm the donor.If the types don't match eg if the donor z type A,and recipient z type B,the recipient's antibodies attach 2 the donor's red blood cells and form bridges thatcoz the cells2clump.
what is photosynthesis
Conast Reply
photosynthesis is the process by which green plants manufacture their own food from water and carbon dioxide using sunlight as a source of their energy releasing oxygen as a by-product.
what is the function of thé utérus
what are runners
Godfrey Reply
what is an environment
Bwambale Reply
diagrams are not indicated, why?
Simon Reply
diagrams for what?
A vaccination is a treatment which makes the body stronger against an infection. The body fights infections using the immune system, which is made up of millions upon millions of cells including T cells and B cells.
define the term immunity
can simply defined as an exemption from an attack
is the ability of an organism to resist a particular infection or toxin by a specific antibodies or sensitized white blood cells
what is a blood group?
A blood group is any different type that human blood can be separated into for medical purposes.
It is the body ability to resist infection .
difference between cell and cell membrane
Sunday Reply
cell is a basic unit of living body however cell membrane is a protective layer of cell.
very good
which topic would you like to discuss today?
what do you understand by the term vaccine?
guy I need the answer of this question what occurs during photosynthesis
REAL Reply
Photosynthesis is the process that plants use to convert light into food. During this process, plants create carbohydrates starting with only carbon dioxide and water. Sunlight provides the energy that makes photosynthesis possible.
@Stanley,that's how I could as well put it
what are different between genotype and phenotype
Martha Reply
what is light microscope and electronic microscope
what occurs during photosynthesis
what are importance of biology
what is the meaning of computer science
genotype is the genetical constitution of the gene
what is ecology
Adeyemo Reply
what is zoology
zoology branch of biology that study animals
a branch of biology which deals with the study of animals

Get the best Biology course in your pocket!

Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?