Jeremy Solomon
imabum14@gmail.com
Friday, March 25, 2011
Shark Blood
Shubin talks about the similarities between the cranial nerves of sharks and ours. He explains that these similarities come from the gill arches in our embryos. Knowing what we know about our similarities with sharks, and our different cardiovascular systems, talk about some ways that environmental pressures and natural selection evolved sharks' methods of cardiovascular regulation into ours. Feel free to focus on one main method, and be sure to explain how each adaptation is a benefit for our environment, and how their adaptations are suited for their environment.
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In some ways the circulatory systems of both sharks and humans are similar, but we can see that each species has adapted its own characteristics to fit its own environment. Compared to the four chambered human heart, with right and left ventricles and atria, sharks possess only a two-chambered heart, with an atrium (also called the auricle) and a ventricle. The heart is an S-shaped tube that is located in the head region of the shark (http://www.enchantedlearning.com/subjects/sharks/anatomy/Heart.shtml). The overall flow of the blood follows this pattern: The blood is pumped by the heart through the afferent branchial arteries to capillaries in the gills to be oxygenated. The blood then flows through efferent branchial arteries (paired dorsal aorta), then through the tissues of the body, and then back to heart in veins (http://www.pc.maricopa.edu/Biology/ppepe/BIO145/lab04_4.html; http://www.enchantedlearning.com/subjects/sharks/anatomy/Heart.shtml).
ReplyDeleteHuman circulation follows a very similar path, with the only difference being the presence of a pulmonary circuit in the lungs as opposed to gills and the complexity of the heart.
The difference between gills and lungs, and the differences between the number of chambers in the heart and the pathways in the circulatory systems of these two species can be explained in the context of evolution. Sharks are organisms that live in the water.
Continued...
ReplyDeleteGas exchange in sharks takes place in the skin, but primarily in the gills. The gill slits close and the pharyngeal chamber expands to suck in water. When the pharynx is filled, the mouth closes and the gill chambers expand and fill with water. Then the gill slits open and the chambers constrict to flush out the water (http://www.saddleback.edu/faculty/steh/bio3bfolder/bio3bgutlab.pdf).
The hearts of fish represent the simplest vertebrate hearts. They are made up of two main chambers. Specifically, the fish has a single-circuit circulation through which blood is driven by a simple two-chambered heart (http://library.thinkquest.org/C003758/Development/heart_evolution.htm).
An important concern of these sharks is thermoregulation, and they have the ability to undergo countercurrent exchange of gases to preserve heat. This ability to undergo countercurrent is an important adaptation to the specific environment in which the sharks live, but the sharks must continually be moving to allow gas exchange to occur (Campbell 707). This connects to the fact that sharks overall. In addition, sharks don’t have high blood pressures and need to move to create the flow of blood (http://www.seaworld.org/animal-info/info-books/sharks-&-rays/anatomy.htm). In that sense, human cardiovascular systems are more advanced in that humans don’t constantly need to be moving in order to respire.
Then again, humans live a very different lifestyle and a completely different environment, not to mention the ponderous amount of evolutionary history in between sharks and humans. However, warm-blooded animals like humans have a high metabolic rate and an efficient double-circuit circulation is a necessity for these species. The hearts of birds and mammals are four-chambered organs, in which blood flowing through pulmonary circulation is completely separated from that of systemic circulation, and vice versa. This allows for no mixing of oxygenated and deoxygenated blood and, as a result, is much more efficient (http://library.thinkquest.org/C003758/Development/heart_evolution.htm). We can look now to the lifestyle changes from sharks to humans to see why these changes in the circulatory system may be present. Turning to the theme of evolution, we find that over time, mutations occur naturally and randomly within the genetic code and present different characteristics in different individuals. From time to time, those new characteristics will give the individual a selective advantage over other individuals, and the individual with the specific characteristic would be more likely to survive and reproduce, continuing on the presence of that specific trait in the next generation. Over time, as the individuals with the selective advantage reproduce and those without the advantage are eliminated, the population sees an overall change. Why might a more efficient cardiovascular system have presented a selective advantage for certain individuals? As Shubin himself points out in the book, the early ancestors of humans were individuals who lived in a hunter-gatherer lifestyle. They needed the cardiac ability to track prey over long distances, and an efficient cardiovascular system allows for a more active lifestyle.
Vickram Pradhan 1/2A
vickram.pradhan@yahoo.com