Wednesday, March 25, 2009
Heart Size and Metabolism
On page 24, Carroll says that icefish have "larger hearts and blood volumes than those of their red-blooded relatives." If icefish are ectotherms and therefore, by definition, have a lesser need for a large and complex heart than endotherms do due to their comparative lack of metabolic activity, what is the selective advantage of having such a large heart?
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Icefish, as their name implies, live in one of the most extreme environments in our planet. Water, in general, has less O2 per unit volume than air and the concentration of O2 decreases as depth increases. Plainly, the deeper the ice fish go, the less O2 present for them to take in to sustain life. So, it is no only important that the icefish develop the most efficient way to distribute whatever oxygen present in their atmosphere to their surrounding body tissues. This is more easily done with a larger heart. The icefish heart is characterized by a spongy myocardium with a highly developed arterial coronary system. This larger, “compact” structure allows for a greater intake of o2 from the water in their extremely cold and pressurized environment. Without this enlarged heart, the ectotherm would be unable to survive in such an extreme environment because it would be unable to sustain enough oxygen from the ocean’s depths.
ReplyDeleteEndotherms, such as humans have huge and complex heart. Humans need to supply blood through various organs. On the other hand, Ectotherms do not have large hearts because of their lack of strenuous activities. However Icefish are exceptions among the ectothermes: they have large hearts, large blood volumes and very high cardiac output.
ReplyDeleteAs Mellisa had pointed out, water has less O2 per unit volume than air. Therefore, Icefish have to develop some way to develop efficient way to send O2 throughout the body. In addition, Icefish live in very cold water which makes them harder to survive and send the blood around. It is true that O2 is more dissolved in the cold water but still, it may be too cold for Icefish to have functional heart. This is why Icefish have large heart, large blood volumes and very high cardiac output. Icefish is using large heart not only to send the O2 through whole body efficiently but also large hearts are more durable in the cold water. For example, when humans enter cold water, heart rate slows down and eventually we die. However Icefish can survie in the cold water because of their large heart which may stay functional.
Another interesting fact about Icefish is that Myoglobins, which are located in heart muscles or any kind of muscle, which stores O2 for strenuous activities are lacking in Icefish. Therefore, Icefish is just using large amount of O2 dissovled in the cold water and they are not storing in the muscles they are just simply pumping it through the body. This is why ICefish have large hearts, blood volumes, and very high cardiac output. Without this selective advantage, they will perish…
I’m going to have to disagree with Sammy on more than one occasion here. Just because Ice Fish are ectotherms that does not mean they have a lack of strenuous activities. In fact ectotherms generally refer to organisms that require environmental influences to stabilize body temperature. So Varad was right when he said that ice fish have “lack of metabolic activity.” Ice fish depend on their environment to regulate the body temperature in their body, but they live in such a stable environment. The Arctic Ocean maintains a 0° C all year around and since the ice fish are ectotherms who have adapted to this insane climate with the development of “antifreeze” proteins and large hearts and capillaries. So technically they are not excluded from “strenuous” activity, they are wildlife organisms nonetheless and still have to fight and prey in order to survive and reproduce.
ReplyDeleteIt is true that O2 decrease in depth but O2 is much more soluble in colder water. The Ice fish adapted a scales skin which allowed for direct diffusion of oxygen directly into the ice fish. So technically they don’t have to have large hearts to pump the much diluted blood they have. “For example, when humans enter cold water, heart rate slows down and eventually we die. However Ice fish can survive in the cold water because of their large heart which may stay functional.”First of all just because the Ice fish have a large heart does not mean that they can survive in extreme temperatures and cause their heart rate to stop slowing down like humans. Human’s heart beat slows down because of reflex that human’s experience. The Bradycardia is the first response to submersion into cold water. This causes the heart rate of humans to slow down to 110 -25% slower because this lessens the need for blood stream oxygen and leaves more for the organs. This is not because we have a smaller heart, but rather because our deeply embedded genetic instincts which tell us to conserve oxygen. The Ice fish on the other doesn’t have to worry about this because their skin allows oxygen to diffuse from the water into their organs. Then humans die because of the remaining reflexes that it experiences like peripheral vasoconstriction which is when the capillaries start closing off due to sphincters. This then starts cutting off circulation to the toes, fingers, slowing closing off “useless” body parts in order to protect the heart and the brain, eventually the oxygen runs out in the body and the human dies. These receptors are located within the nasal cavity and other areas of the face supplied by the cranial nerve V which relay information to the brain and then innervate cranial nerve X which controls the autonomic nervous system.
Both Melissa and Sammy said that a nice and large healthy heart will allow the fish to send oxygen more efficiently through the body. They seemed to miss that the Ice fish don’t have hemoglobin. The oxygen carrying cell in the blood which is assembly of 4 globular protein subunits each with an IRON ion to hold on to the oxygen. Without this cell they generally don’t need to have such an efficient heart to transfer the oxygen because the oxygen mainly isn’t in the blood. Carroll even says that large gills allow more oxygen to pass and so does a scale less skin. I think that’s how they get their oxygen efficiently.
I think their large heart does however contribute to a greater cardiac output. A larger heart would have a greater volume of blood pumped through each time having a stronger hydrostatic force pushing the near freezing nutrients through the enlarged capillaries easier and more efficiently. In such a cold environment the blood constantly has a danger in freezing to keeping it thin and fast moving makes it difficult to freeze. Fluid flows out of a capillary at the upstream end near an arteriole due to blood pressure and hydrostatic pressure. The greater the pressure the greater the amount of nutrients pushed out into organs in need which allows for the Ice fish to continue to survive and reproduce which is key here. Natural selection at its best.
http://en.wikipedia.org/wiki/Mammalian_diving_reflex
http://en.wikipedia.org/wiki/Arctic_Ocean
http://occawlonline.pearsoned.com/bookbind/pubbooks/campbell6e_awl/medialib/assets/e-book/htm/campbell6e.htm
http://en.wikipedia.org/wiki/Hemoglobin
http://en.wikipedia.org/wiki/Cardiac_output
http://en.wikipedia.org/wiki/Ectotherm