The human trachea, a cylinder about cm long, 2cm in diameter found in front of the esophagus, extends from the larynx into the chest cavity.
It is made of incomplete rings of hyaline cartilage and smooth muscle that divides into the two primary bronchi at the midthorax. The trachea is lined with mucus-producing goblet cells and ciliated epithelia that propel foreign particles trapped in the mucus toward the pharynx. The cartilage provides strength and support to the trachea to keep the passage open. The forced exhalation helps expel mucus when we cough.
Trachea and bronchi structure : The trachea and bronchi are made of incomplete rings of cartilage. Route of inhalation : Air enters the respiratory system through the nasal cavity and pharynx.
It then passes through the trachea and into the bronchi, which bring air into the lungs. The end of the trachea bifurcates to the right and left lungs, which are not identical. The larger right lung has three lobes, while the smaller left lung has two lobes. The muscular diaphragm, which facilitates breathing, is inferior to the lungs, marking the end of the thoracic cavity.
Lung structure : The trachea bifurcates into the right and left bronchi in the lungs. The larger right lung is made of three lobes. To accommodate the heart, the left lung is smaller, having only two lobes.
As air enters the lungs, it is diverted through bronchi beginning with the two primary bronchi. Each bronchus divides into secondary, then into tertiary bronchi, which further divide to create smaller diameter bronchioles that split and spread through the lung. The bronchi are made of cartilage and smooth muscle; at the bronchioles, the cartilage is replaced with elastic fibers.
Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems that control muscle contraction or relaxation, respectively.
In humans, bronchioles with a diameter smaller than 0. Since they lack cartilage, they rely on inhaled air to support their shape. As the passageways decrease in diameter, the relative amount of smooth muscle increases. The terminal bronchioles then subdivide into respiratory bronchioles which subdivide into alveolar ducts.
Numerous alveoli sing. The alveolar ducts are attached to the end of each bronchiole; each duct ends in approximately alveolar sacs. Each sac contains alveoli that are microns in diameter. Alveoli are made of thin-walled, parenchymal cells that are in direct contact with capillaries of the circulatory system. This ensures that oxygen will diffuse from alveoli into the blood and that carbon dioxide 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 relationship of the respiratory and circulatory systems. As there are so many alveoli around 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.
Alveolar structure : 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. Mucus glands secrete mucus into the airways, keeping them moist and flexible.
The air that organisms breathe contains particulate matter such as dust, dirt, viral particles, and bacteria that can damage the lungs. The respiratory system has protective mechanisms to avoid damage. In the nasal cavity, hairs and mucus trap small particles, viruses, bacteria, dust, and dirt to prevent entry. If particulates make it beyond the nose or enter via the mouth, the bronchi and bronchioles contain several protective devices. The lungs produce mucus that traps particulates.
The bronchi and bronchioles contain cilia, small hair-like projections that line the walls of the bronchi and bronchioles.
These cilia move mucus and particles out of the bronchi and bronchioles back up to the throat where it is swallowed and eliminated via the esophagus. Electron microscope image of cilia : The bronchi and bronchioles contain cilia that help move mucus and other particles out of the lungs.
In humans, 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 unable to move. This causes a persistent cough, as the lungs try to rid themselves of particulate matter, making smokers more susceptible to respiratory ailments.
Privacy Policy. Skip to main content. The Respiratory System. Search for:. Systems of Gas Exchange. The Respiratory System and Direct Diffusion Respiratory processes that help organisms exchange O 2 and CO 2 range from simple direct diffusion to complex respiratory systems. Learning Objectives Review an overview of the functions of the respiratory system. Key Takeaways Key Points Respiration ensures that cells, tissues, and major organs of the body receive an adequate supply of oxygen and that the carbon dioxide, a waste product, is efficiently removed; the exchange of oxygen and carbon dioxide occurs via diffusion across cell membranes.
The mechanisms, processes, and structures used for respiration are dictated by the type, size, and complexity of the organism. Direct diffusion of gases through the outer membranes can be used by organisms such as flatworms as a means of respiration due to their small size and simplicity. Key Terms deoxygenated : having removed the oxygen atoms from a molecule diffusion : The passive movement of a solute across a permeable membrane aerobic : living or occurring only in the presence of oxygen.
As the passageways decrease in diameter, the relative amount of smooth muscle increases. 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 Figure In the acinar region, the alveolar ducts are attached to the end of each bronchiole. At the end of each duct are approximately alveolar sacs, each containing 20 to 30 alveoli that are to 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. 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.
Watch the following video to review the respiratory system. 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 Figure 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. 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.
Skip to content Chapter The Respiratory System. Learning Objectives 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. Direct Diffusion. Skin and Gills. Tracheal Systems. Mammalian Systems. Figure Air enters the respiratory system through the nasal cavity and pharynx, and then passes through the trachea and into the bronchi, which bring air into the lungs.
Which of the following statements about the mammalian respiratory system is false? When we breathe in, air travels from the pharynx to the trachea. In aquatic plants, water passes among the tissues and provides the medium for gas exchange. In terrestrial plants, air enters the tissues, and the gases diffuse into the moisture bathing the internal cells. In the leaf of the plant, an abundant supply of carbon dioxide must be present, and oxygen from photosynthesis must be removed.
Gases do not pass through the cuticle of the leaf; they pass through pores called stomata in the cuticle and epidermis. Stomata are abundant on the lower surface of the leaf, and they normally open during the day when the rate of photosynthesis is highest.
Physiological changes in the surrounding guard cells account for the opening and closing of the stomata see Chapter In animals, gas exchange follows the same general pattern as in plants. Oxygen and carbon dioxide move by diffusion across moist membranes. In simple animals, the exchange occurs directly with the environment. But with complex animals, such as mammals, the exchange occurs between the environment and the blood. The blood then carries oxygen to deeply embedded cells and transports carbon dioxide out to where it can be removed from the body.
Earthworms exchange oxygen and carbon dioxide directly through their skin. The oxygen diffuses into tiny blood vessels in the skin surface, where it combines with the red pigment hemoglobin. Carbon dioxide is transported back to the skin by the hemoglobin.
Terrestrial arthropods have a series of openings called spiracles at the body surface. Spiracles open into tiny air tubes called tracheae, which expand into fine branches that extend into all parts of the arthropod body. Fishes use outward extensions of their body surface called gills for gas exchange.
Gills are flaps of tissue richly supplied with blood vessels.
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