The Respiratory System :

Unity and Diversity

Created and Edited By Ryan Shea, Josh Agranat, and Nick Simmons-Stern

The respiratory system is responsible for gas exchange in animals - that is, the respiratory system transfers oxygen and carbon dioxide from the air to the circulatory system, and vice versa, across the respiratory surface. Different organisms utilize different methods of respiration. Some animals, like sponges, don’t have particular organs for gas exchange and therefore take gases from the water that surrounds them. On the other hand, vertebrates use lungs internally located while fish and arthropods have gills.

The Human Respiratory System

The respiratory system in humans, like most mammals, is centered around a set of lungs used for gas exchange. In humans, the lungs are connected to the oral and nasal cavities via the trachea to allow for transfer of oxygen to and carbon dioxide from the bronchial tree (figure 1.2) of the lungs. The bronchial tree consists of bronchi and thousands of smaller alveoli that branch off and are surrounded by capillary beds wherein gas is exchanged accross the capillary membrane into the lungs - the process by which oxygen enters the blood stream and carbon dioxide leaves. The structure of the lung meets this function of gas exchange in that there are thousands of alvioli in a single lung, thus allowing for a maximized suface are across which molecules can diffuse. That there are thousands of alvioli and millions of capillaries across which the essential process of gas exchange can occur allows humans to extract the most oxygen from each "breath" of air.

When one or both lungs become damaged, their respiratory capacity is greatly diminished. Diseases such as lung cancer, pneumonia, and bronchitis are all common ailments of the lung that can be potentially deadly to humans. In pneumonia, which can be caused by viruses, bacteria, infection, fungi, or even parasites, fluid builds up in and around the lungs (a pleural effusion), which, combined with a general swelling of the lung diminshes its alveolar capacity for gas exchange. That is, in cases of pneumonia, a human cannot extract as much oxygen from the air. In extreme cases, the restriction of lung capacity
The Respiratory System
The Respiratory System

The Respiratory System

(Figure 1) The Human Respiratory System (Figure 1.2) A computer-generated image of the human respiratory system.

The Fetal Pig Respiratory System
A pig's respiratory system is nearly identical to that of a human. Like a human, a pig inhales air through oral and nasal cavities ( nasopharynx and pharynx ), and transfers oxygen to the bronchi and alvioli of the lung through the larynx and trachea (figure 4). Gas exchange occurs across the membranes of capillaries in the capillary beds of the alvioli in the various lobes of the lung and again, like humans, the maximized surface area of these bronchioli and avlioli allows for highly efficient and rapid extraction of oxygen from the air.

Bellow are various photographs taken during the disection of a pig fetus depicting the various organs and pathways involved in respiratio:


Figure 2. Fetal Pig Chest - here pictured prior to organ extraction. Simlar to humans, the lobes of the pig lung envelope the heart, caudal to the trachea and oral cavity and superior to the diaphragm and abdominal cavity.


Figure 3. Fetal Pig Lungs - here pictured dorsal side up, after extraction from the pig fetus.


Figure 4. The passageway to the lungs. At left, the trachea and larynx are depicted attached to the inner dorsal lining of the respiratory cavity. This pathway connects the oral and nasal cavities to the lungs as pictured at right, where the lungs have been extracted and the trachea cut to reveal a cross section of the tracheal wall and the enclosed cavity that serves as the pathway for gas transfer to and from the lungs. Also visible are the bronchi leading from this cavity to the bronchial tree of the lung lobes and the pulminary artery and vein that carry oxygen-poor and oxygen rich blood to and from the lungs, respectively.

(Figure 4.2) A closer view of the trachea, larynx, and epiglotis. Here, the lower jaw is fully removed and the upper trachea and larynx are detached from the dorsal lining of the upper respiratory cavity. The epiglotis, pictured at right, controls the opening into the trachea and discriminates air from the bolus (chewed up food to be sent down the esophogus for digestion). Below, as held open by the forceps, is the opening to this esophogus.


Figure 5. The diaphragm. This smooth muscle creates negative pressure during inspiration and positive pressure during exhalation, allowing air to enter and exit the lungs, respectively. This muscle is controlled involuntarily and is moved by a central tendon. The entire mechanism is controlled by the phrenic nerve.

The Grasshopper Respiratory System
The respiratory system in the grasshopper mainly constists of numerous tracheae, which are connected to the respiratory medium at the outside of the insect through spiracles, which are holes in the exoskeleton of the insect (Figure 7). The tracheae are reinforced with chitin, which prevent them from collapsing under the atomespheric pressure. This respiratory system is not intimately connected to the circulatory system, and does not transport the oxygen and carbon dioxide using the open circulatory system that is present in the grasshopper.

This system for gas exchange highlights the diversity of living things. Insects, unlike mammals, do not have lungs or any defined and isolated organ for gas exchange. Rather, they continuously take in and let out air from their trachael network, which can exchange gasses throughout. Similar to the mammal lungs, however, the structure of this system of respiration meets its function by maximizing the surface area of the respiratory surface.

external image TrachealSystem150.gif
external image TrachealSystem150.gif
(Figure 6) Diagram of the Tracheal System of the grasshopper.


(Figure 7) Photograph of the ventral side of the grasshopper. Note the spriracles - openings into the trachael networks of the animal.

Fish Respiratory System
Unlike grasshoppers (insects), frogs (amphibians), pigs, or humans (mammals), fish extract oxygen from the water (two parts hydrogen, one oxygen) through gills. The process by which water is forced through the mouth and across the gill filaments (which, again, maximize the surface area... unity in diversity...) allows for the extraction of oxygen into the circulatory system. The opercullum, which regulates the pressure of the this water and thus the ability of a fish to extract oxygen, plays a crucial role in this process.

(Figure 8) A right lateral anterior view of a fish, before disection.

(Figure 9) A left lateral view of a crayfish, shell removed. Here, the striated gills are clearly visible (and with magnification, the individual gill filliments).

Frog Respiratory System
Frogs, and amphibians in general, have a unique form of respiration that differs greatly from their mamallian counterparts. For one, their skin acts as a respiratory surface. In frogs, gas exchange is able to occur directly accross the epithelial tissue - oxygen enters the blood stream and carbon dioxide enters the air. The glossy, "wet" nature (structure) of a frog's skin highlights this function as a respiratory surface, as all respiratory surfaces need a mucus layer to facilitate diffusion accross cell membranes.

The frog's form of inspiration and exhalation is also unique. Instead of using a diaphragm (figure 1, 5) to draw air into the lungs, frogs first draw air into the oral cavity from the nasal opening by retracting the throat. Once the mouth is full of air, the throat then contracts and forces air into the lungs. After gas exchange occurs across the lung's respiratory surface (similar here to the mammallian system of bronchi), the throat again draws air from the lungs into the mouth and then forces it out the nostrils. (Click here for an animated demonstration).


As is evident by the vast diversity of the various animal respiratory systems showcased in this WooKISPACE, the animal kingdom has evolved some very unique ways of extracting oxygen from the air for use in cellular respiration. Where humans and pigs make use a diaphragm to draw air in, frogs utilize their mouth and skin, and fish their opercullum. Where some have distinct pathways for air to flow, others such as grasshoppers have an open network of tubes. Ultimately, however, these adaptations all serve the same purpose: the sustanance of life on earth.