How do the Lungs relate to the Respiratory System?
All animals must use a respiratory system (whether with lungs or gills) in order to receive oxygen and dispose of carbon dioxide. In mammals, the respiratory system includes the lungs, airways, and muscles which aid in air movement into and out of the body. The lungs provide to be the most important organ for the respiratory system.

The pathway of air...
Continuing on the subject of mammals, respiration begins with air entering the body through nasal cavities. As the air enters, it is humidified and the internal body temperature warms the air. Dust is caught in the mucus surrounding the nasal cavity and the trachea. The air then travels to the lungs, which undergoes gas exchange.

The entire process of respiration has a lot to do with the circulatory system. The heart pumps blood from the right ventricle to the pulmonary arteries, where it then travels to the pulmonary blood vessels. These vessels are connected to the branches within the lungs (the aveoli). At this site of gas exchange, the oxygen from the air is carried via Hemoglobin through the blood. After the gas exchange is complete, the blood returns to the left side of the heart through the pulmonary veins. The blood returning to the heart through the veins is oxygen rich because of the hemoglobin carrying the oxygen from the inhaled air.
Here is a simple picture to introduce you to the main idea:
Simultaneously, as the Oxygen leaves the alveoli, the Carbon Dioxide is taken in by the alveoli (to be exhaled)
Simultaneously, as the Oxygen leaves the alveoli, the Carbon Dioxide is taken in by the alveoli (to be exhaled)

How does Structure meet Function?
The necessary exchange of gases (which transports oxygen from the environment into the blood and removes carbon dioxide from the blood into the environment) is achieved by the "mosaic" of air-filled sacs (specialized cells) called alveoli. Alveoli are particular to mammalian lungs, and are shaped like small hollow cavities. This structure enables them to literally hold the oxygen-rich air. Because of their thin walls, alveoli are able to easily exhange gas with the bloodstream (i.e. with the surrounding blood vessels).

external image Alveoli_diagram.pngAs seen from this picture, the alveoli have pulmonary veins surrounding their exterior. The blue in this picture represents the arteries going from the heart to the alveoli branches of the lungs. The red represents the pulmonary veins (as depicted in picture) which are oxygen-rich and head back to the heart.

The constant exchange of oxygen and carbon dioxide is made possible within certain organisms by simple diffusion. This can occur when the animal's cells are in close proximity to the environment, so that oxygen is able to be diffused by passive exchange.
However, adaptations were needed in order for multicellular organisms to function. There are two major adaptations which allow for this majority of organisms to function:
1) A circulatory system which allowed for the transportation of gases to and from the tissues of the body and..
2) A centrally organized respiratory system which could obtain oxygen from the environment and quickly distribute these molecules to the circulatory system, so it is then distributed throughout the entire body.
These two adaptations are represented within mammalian lungs. A human is a perfect example, giving that the lungs are centralized internally, and are connected to the trachea, mouth, and nose. This allows for the lungs to work from the inside of the body yet obtain oxygen from the outside.

What does the shape of the lungs have to do with gas exchange?
The pathway of air in air breathing vertebrates consists of entering through either the nose or the mouth. A series of airways (pathways) ensures the air to be directed to the lungs. This consists of the pharynx, the larynx, the trachea, the bronchi and bronchioles, and the alveolus. The lungs of mammals are made up of a lattice of alveoli, which enables it to contain great surface area. Thus, gas exchange is able to quickly occur. This is important because the bloodstream must constantly be obtaining oxygen and releasing carbon dioxide, or else the entire body would deteriorate. Rapidly distributing the oxygen is obtainable because of the expansive surface area of the lungs.
external image lungs4.gif
Seen on the left is a depiction of the lungs. The bronchioles break apart like branches into the little cavities called alveoli. The "lattice" of alveoli is much like leaves on a tree.
The trachea, coming down from the mouth, breaks apart into two branches, which continuously break apart into smaller and smaller branches (like on a tree). The vast amount of branches all contain alveoli on the tips, a lot like leaves attached to the ends of trees. This enables all the capillaries and blood vessels attached to the alveoli to gather the oxygen from the branches of the lungs and bring it to the bloodstream.

The two lungs of the human body seem similar but actually are not identical. The right lung consists of three lobes while the left consists of two. The right lung's lobes are the superior, middle, and inferior. The left lung contains the superior and the inferior. The significance of the left lung being much smaller is because of the heart. The left lung contains an indent (a "cardiac notch") which is a concave impression which is molded to shape the heart which the left lung covers.
The lungs are coverd in a moist coating which enables the gas exchange to occur. However, this moist coating is susceptible to bacteria growth, which leads to many diseases.

The open cavity within a lung provides the lung to work within the alveolar system of the lungs. The oxygen and carbon dioxide molecules passively exchange by means of diffusion from the gaseous environment to the blood. Because of the lungs' ability to simultaneously remove carbon dioxide and take in oxygen, the system is at a constant balance. This connects to the cirulatory system as well because it maintains a balance between the acids and bases in the blood (by removing CO2 and other waste products from the bloodstream).

What happens when it becomes off balance?
When homeostasis is incomplete and the acid-base balance is not maintained, many respiratory diseases and conditions could occur. One such condition is Respiratory Acidosis which occurs when the acidity within the blood is abnormally increased. This comes to be due to alveolar hypoventilation, causing a lot of carbon dioxide to be produced quickly. This elevation arterial carbon dioxide is known as PaCO2, and decreases the blood's pH levels. An interesting website which could be checked out is here .
This website is a bit scientific, but gives a much more detailed description of this chronic condition.
Another condition is Respiratory Alkalosis which occurs when there is a decrease in carbon dioxide (contrasting with the previously mentioned condition). Due to the decreased plasma carbon dioxide, a decrease of hydrogen ions and bicarbonate concentrations occurs as well. Both of these conditions can occur in either an acute or a chronic form.

Diseases Continued:
Asthma: Asthma occurs when there is too much mucus on the inside of the lungs. The airway becomes constricted and the area usually inflames. Environmental stimulants can cause these reactions, such as allergic reactions to moist air, dry air, or cold air, exercise, or stress. Coughing, wheezing, and panicking often occur during an asthma attack.
Emphysema: This disease occurs from the loss of elacticity of the lung tissue (particularly around the alveoli and therefore causing destruction of the capillaries around the alveoli). It is due to exposure to toxins or long time exposure to tobacco. During exhalation, the small airways collapse, causing the airflow to be impeded and oftentimes "trapping" the air within the lung. This relates to the shortness of breath and hyperventilation often felt due to this disease.
Pneumonia: pneumonia acts as an inflammatory disease, prompted from infection from bacteria or a virus. Heavy breathing, a cough, and chest pain are the usual symptoms of this illness. The lungs are sometimes filled with liquid, called "consolidation."

Diversity and Unity between different organisms
Mammalian Lungs:
A fetal pig, like the one we dissected, contains two lungs just like a human. Because it is a mammal, the pig also contains a trachea and a series of bronchioles and alveoli within their lungs.
The picture below shows the lungs from a different view, seeing the bronchioles in the bottom side of the lungs.

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Invertebrate Lungs:
Invertebrates do contain so-called "lungs" which aid in respiration but are not at all related to the mammalian lungs. A grasshopper, for example, uses spiracles for air to pass through. The spiracles then spread to tracheal tubes, which transports the gases to all the cells within the body. A grasshopper does not contain hemoglobin, so the blood does not carry the oxygen supply to the cells as a mammal's circulatory system does. Insects like the grasshopper have a closed circulatory system which means the circulatory system and the respiratory system are not connected.

Here is a picture of the series of tracheal tubes which lead to all cells of the grasshopper's body.
Here is a picture of the series of tracheal tubes which lead to all cells of the grasshopper's body.

Amphibian Lungs:
In frogs and other amphibians, the lungs are balloon-shaped structures, with the gas exchange occurring only in the outer surface of the lung. Many amphibians are able to diffuse their own oxygen through their moist skin while living in wet environments. Frogs, in particular breathe by filling its mouth with air, closing the mouth, closing the openings to the nostrils, then opening the glottis to the lungs, thus forcing the air into the lungs.
As exemplified in the picture below, the frog's glottis enables it to force air into the lungs:
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This picture shows the lungs behind the liver. In comparison to the human body, the frog's lungs are hidden behind the massive liver. They are the two small sacs on either side of the liver.
This picture shows the lungs behind the liver. In comparison to the human body, the frog's lungs are hidden behind the massive liver. They are the two small sacs on either side of the liver.

Avian Lungs:
Instead of containing alveoli within the lungs, birds contain tiny "para-bronchi" where air flows through into air apillaries and then undergoes a countercurrent exchange. The carbon dioxide and the oxygen are diffused with cross-flowing into the capillaries.
Avian lungs consist of two sacs, one near the front and one towards the back. When a bird inhales, the air rushes back towards the rear sac and while rushing by, oxygenates the blood in the para-bronchi. Upon exhalation, the air then exists the caudal (rear) sac moves over the parabronchi because it is still oxygenated. The air in the cranial sac is exhaled. This complex system enables the bird to always have the airflow travel from posterior to anterior. This helps in extracting a greater concentration of oxygen from inhaled air, because it flows in one direction. Oxygen is able to be diffused into the bloodstream during both inhalation and exhalation ( This also enables birds to fly at high altitudes and still extract oxygen from the air, even if there is not a high concentration of it.
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