Anatomical Comparison of Respiratory Organs


            As organisms evolve, they manifest anatomical differences that allow them to adapt to their environment and therefore increase their probability of survival - Anatomical Comparison of Respiratory Organs introduction. Therefore, although numerous similarities exist among species, they still exhibit characteristic features that define them as a single unit of animal classification. Throughout the geologic timescale, many animals and plants have already emerged and disappeared. But in comparing their different organs and organ systems, we could see how a single celled creature is related to the most complicated and highest form of organism, humans.

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An Anatomical Comparison of Respiratory Organs

            There are two common mechanisms of respiration, external and internal. External respiration is a process of oxygen intake and carbon dioxide expulsion from and to the environment. This mechanism utilizes respiratory membranes that are frequently found in a particular organ, collectively considered as the respiratory system. Internal respiration, on the other hand, is a more advanced form of gaseous exchanged between the organism and its environment. It is the process of oxygen and carbon dioxide exchange between the capillary blood and tissue fluids. It is highly essential to expel carbon dioxide for it is a toxic gas that causes the inhibition of numerous cellular activities. Therefore, this reinforces the vitality of the entire process of the respiratory system (Kent, 1992, p. 415).

            Except in early embryos, external respiration is accomplished using membranes that need direct contact to the environment. These membranes need to be kept vascularized, moistened, while the epithelium should be kept thin. The primary organs employing external respiration include the external and internal gills, oropharyngeal mucosa, swim bladders or lungs, and skin of adult vertebrates. Other less common devices for respiration include bushy or filamentous outgrowths of the pectoral fins as that of male Lepidosiren, and the posterior trunk region and thigh as that of African hairy frog (Kent, 1992, p. 415).

            Generally, modern amphibians use their skin for respiring in both water and air environments. Although the skin is not responsible for most of an organism’s oxygen uptake, this organ does the majority of carbon dioxide expulsion. Cutaneous respiration is an integral mechanism in the gaseous exchange of amphibians. However, their respiration is not limited through the skin, as amphibians also possess lungs. Relatively, these structures are simple sacs that follow the shape of the pleuroperitoneal cavity. Amphibian lungs vary greatly among different species, as it can be rudimentary in caecilians, and even absent in salamanders. In urodeles, it functions as a hydrostatic orgs, while in Necturus it only obtains two percent of the needed oxygen. But the unique feature of amphibian lungs that separate it from other taxonomic groups is that it does not employ vacuum pump. Instead, a pressure pump is used in order to force in the air from the oropharyngeal cavity while lung elasticity pushes the air out (Kent, 1992, 416).

            On the other hand, avian and mammalian lungs use a different form of respiration, internal respiration. Lungs of birds are morphologically unique. The air inhaled by the organism enters the lungs through the trachea via the primary bronchus. The air continues nonstop through the mesobronchus and then it enters the air sacs prior its return to the lungs. These air sacs are characteristic of avian lungs, but are not limited to birds alone. Their morphology resembles an accordion bellow that fills alternately due to compression. When they empty, a steady and uninterrupted airflow travels across the lung respiratory epithelia (Kent, 1992, p. 438).

            These air sacs in birds are open-ended duct systems that consequently bring the total replacement of air within the lungs in each cycle of the bellows. But its more important function is designed specifically for organisms in flight. These anatomical features dissipate the heat muscles produce while flying. Because of the poor vascular supply in air sacs, they become the direct recipient of muscle heat instead of the blood stream. While a bird is at reset, its chief defensive mechanism against overheating is automatic physiological response of increased rate of breathing. Another importance of air sacs is its extension into the bones causes the conservation of energy while a bird is in flight.  This anatomical feature decreases the entire body density of a bird, which allows its increased buoyancy in air. As the air sacs become warmer, therefore filled with light air, the bird’s body becomes more buoyant (Kent, 1992, p. 440).

            In mammals, the lungs are different from those in birds. Mammalian lungs involve an airway passage system that resembles a branching deciduous tree. When air enters the system, it is arborized and proceeds to the gradually decreasing diameter of passageway until it reaches the alveoli. The alveoli have an epithelial lining that is specialized for gaseous exchange. Here, oxygen enters the lungs and carbon dioxide exits the respiratory system using the reverse path (Kent, 1992, 440).

            Mammalian lungs do not involve air sacs, as they are commonly terrestrial animals that do not require special mechanisms for flight adaptation. Usually, mammals possess asymmetrically lobed lungs, with few exceptions. Several aquatic and terrestrial mammals, such as whales, sirenians, elephants, and horses, do not have lobes. While other mammals, such as monotremes and rats, are only lobed at the right portion (Kent, 1992, p. 440).

            Basically, lung ventilation is accomplished using the diaphragm, which is a muscular dome-shaped suction pump in the mammalian respiratory system. Its muscular contraction causes it to flatten, therefore decreasing the subatmospheric pressure within the pleural cavity containing the lungs. Consequently, normal atmospheric pressure is pushed into the lungs by the nares and respiratory tract in order to fill the vacuum inside. This is otherwise known as inhalation and is an active process. The diaphragm is analogous to a suction pump as it creates a vacuum inside the lungs causing the air to be driven into the duct system (Kent, 1992, 442).

            On the other hand, exhalation is a passive phenomenon that mammals use in expelling a carbon dioxide laden air. This is fundamentally caused by the several factors including diaphragm relaxation, upward pressure exertion by the abdominal viscera, resilience of the abdominal wall, return of ribs to resting position and relaxation of inter and supracostal muscles, and lung elasticity.      All contribute to the process of squeezing the air out of the lungs. It is possible that during forceful expiration, an individual will experience panting, coughing or roaring (Kent, 1992, p. 442).

            Animals have evolved with different mechanisms for breathing. Some employ more primitive means such as cutaneous respiration, while others have more advanced structures such as the mammalian lungs. Other organisms, such as the birds, have specialized and uniquely designed respiratory systems that enable them to fly. But the main function of all these organs is to obtain the oxygen from the environment, utilize it for synthesizing energy to enable our metabolic processes, and expel the carbon dioxide to the atmosphere. It is a constant cycle that maintains the balance of use and reuse of vital resources (Kent, 1992, 415).


Kent, G. C. (1992) Comparative Anatomy of the Vertebrates. Missouri: Mosby – Year Book, Inc.


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