The independent evolution of the same or a similar biological trait is referred to as convergent evolution. In most animals, the circulatory system is used to transport blood through the body. Some primitive animals use diffusion for the exchange of water, nutrients, and gases.
However, complex organisms use the circulatory system to carry gases, nutrients, and waste through the body. Circulatory systems may be open mixed with the interstitial fluid or closed separated from the interstitial fluid. Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptions during evolution and associated differences in anatomy.
Fish have a two-chambered heart with unidirectional circulation. Amphibians have a three-chambered heart, which has some mixing of the blood, and they have double circulation. Most non-avian reptiles have a three-chambered heart, but have little mixing of the blood; they have double circulation. Mammals and birds have a four-chambered heart with no mixing of the blood and double circulation. Some animals use diffusion instead of a circulatory system. Examples include:.
A closed circulatory system is a closed-loop system, in which blood is not free in a cavity. Blood is separate from the bodily interstitial fluid and contained within blood vessels. In this type of system, blood circulates unidirectionally from the heart around the systemic circulatory route, and then returns to the heart.
Systemic circulation flows through the systems of the body. The blood flows away from the heart to the brain, liver, kidneys, stomach, and other organs, the limbs, and the muscles of the body; it then returns to the heart. Skip to content The Circulatory System. Learning Objectives By the end of this section, you will be able to do the following: Describe an open and closed circulatory system Describe interstitial fluid and hemolymph Compare and contrast the organization and evolution of the vertebrate circulatory system.
Circulatory System Architecture The circulatory system is effectively a network of cylindrical vessels: the arteries, veins, and capillaries that emanate from a pump, the heart. In a closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Most vertebrates and some invertebrates, like this annelid earthworm, have a closed circulatory system.
In b open circulatory systems, a fluid called hemolymph is pumped through a blood vessel that empties into the body cavity. Hemolymph returns to the blood vessel through openings called ostia. Arthropods like this bee and most mollusks have open circulatory systems. Simple animals consisting of a single cell layer such as the a sponge or only a few cell layers such as the b jellyfish do not have a circulatory system.
Instead, gases, nutrients, and wastes are exchanged by diffusion. The blood is pumped from a three-chambered heart with two atria and a single ventricle. Thus, both the loop and the new atrium are necessary for a 3 chambered heart to function.
And though the loop interior circuit sounds simple, it really must be a complex tube network with valves in the right places to keep fluid flowing properly. Single circulation hearts pump blood directly through the gas exchange organ and out to the body figure 1a. And if this new loop doesn't connect with the gas exchange organ, then the new loop is functionless and useless. The easiest way to make this transition happen is probably to have the vein leaving the gas exchange organ feedback to become an artery feeding back into the heart.
Of course this means the new vein-artery simultaneously needs the proper valves so it can function like an actual part of the heart itself. Probably of most importance is the fact that there is now no vein leaving the heart and pumping blood back out to the body. In other words, to create this interior circuit of double circulation, a new vein must be created and blood flow out of the heart completely rewired to the rest of the body.
Even if gas exchange organ could be bypassed making it a less complex, though functionless and not selectively advantageous circuit , the main problem with going from single circulation to double circulation is that somehow this new circuit has to wire itself to the lungs.
Either way, the vein leaving the heart must somehow also become an artery fed back into the heart through a new functional atrium and then a new vein must be created so that blood leaving the heart still gets out to the rest of the body through the circulatory network. Finally, the heart muscle has to adapt to all of these changes, especially such that beating can occur to pump through the new atrium and associated fluid pressure changes. In other words 4 primary changes are needed to go from single to double circulation: The duplication of the atrium such that fluid transport through new atrium is functional A conversion of the vein leaving the heart into an artery at the other end such that it is fed back into the heart.
A complete rewiring of how blood finally leaves the heart and goes out to the rest of the body the creation of a new vein and rewiring. Modification of the heart muscle to beat properly and accommodate the additional atrium and fluid pressure changes associated with the rewiring.
If any of these steps are missing, double circulation won't work. And this says nothing about the many valves and other smaller veins and arteries associated with double circulation which characterize true hearts as well changes needed in the pumping mechanism of the heart muscle to accommodate a completely new atrium and fluid-pressure balance. The transition from 2 to 3 chambers requires a change from single to double circulation which involves at least 4 major simultaneous changes including the complete rewiring of how blood leaves the heart to the rest of the body.
Many more minor simultaneous changes associated with mechanics of proper fluid transport would also be necessary. It not possible for double circulation system to evolve from a single circulation heart system in a Darwinian step-by-step manner because too many changes are necessary, making the 3 chambered heart unevolvable from a 2 chambered heart. Getting a 4 chambered heart: With respect to hearts with "double circulation", the 4 chamber mammalian heart probably isn't irreducibly complex.
Going from 3 to 4 chambers really doesn't look all that complicated of course they're still very different and this is totally oversimplified, but I'm only talking about basic organ design. Basically, the single ventricle in the 3-chambered heart is split into 2 chambers in the 4 chambered heart, making 2 ventricles instead of one.
A "double circulation" heart could work with 3 chambers--and it does in reptiles and amphibians. The human heart has 2 atrium-ventricle pairs, which beat in succession something like pistons in a car.
Only one ventricle is really needed to pump the blood. But just like an 8 cylinder engine put out a lot more horsepower than a 4 cylinder, so does a 4 chamber 2 pairs heart have a much more power to supply the body with energy and oxygen-rich blood than a 3 chamber 1 ventricle heart.
There is a huge advantage to having a 4 chamber heart. Taking away the additional capillary complexities associated with suddenly having a second ventricle, one might be able to argue that the 3 to 4 chambered heart transition isn't difficult to imagine, relatively speaking, as all one has to do is note the strong advantage of separating oxygenated and de-oxygenated blood and then divide the ventricle in half.
Of course the key word here is "imagine". Irreducible complexity is a real phenomena, and it can be analyzed, and so in some cases it might not exist, and in the case of the 4-chambered heart case it probably doesn't.
Though the 4 chambered heart may not be irreducibly complex with respect to "double circulation", it might still be the result of intelligent design and not evolution, and irreducible complexity doesn't have to exist in all instances for it to exist in some. Overall, regardless of chambers, the heart has had much design put into it.
These animals also have separate circuits of blood vessels for oxygenating blood and delivering it to the body. Deoxygenated blood returning from the body empties into the right atrium. From there, blood is conducted to the ventricle and is then pumped to the lungs.
After picking up oxygen and getting rid of carbon dioxide in the lungs , blood returns to the heart and empties into the left atrium. The blood then enters the ventricle a second time and is pumped out to the body. The second trip through the heart keeps blood pressure strong and blood flow rapid as blood is pumped to the tissues , helping the blood deliver oxygen more efficiently.
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