The advantage to this arrangement is that high pressure in the vessels pushes blood to the lungs and body. The mixing is mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit. For this reason, amphibians are often described as having double circulation.
Most reptiles also have a three-chambered heart similar to the amphibian heart that directs blood to the pulmonary and systemic circuits, as shown in Figure c. The ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood.
Some reptiles alligators and crocodiles are the most primitive animals to exhibit a four-chambered heart. Crocodilians have a unique circulatory mechanism where the heart shunts blood from the lungs toward the stomach and other organs during long periods of submergence, for instance, while the animal waits for prey or stays underwater waiting for prey to rot.
One adaptation includes two main arteries that leave the same part of the heart: one takes blood to the lungs and the other provides an alternate route to the stomach and other parts of the body. Two other adaptations include a hole in the heart between the two ventricles, called the foramen of Panizza, which allows blood to move from one side of the heart to the other, and specialized connective tissue that slows the blood flow to the lungs.
Together these adaptations have made crocodiles and alligators one of the most evolutionarily successful animal groups on earth. In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles, as illustrated in Figure d.
The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. The four-chambered heart of birds and mammals evolved independently from a three-chambered heart.
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. Even while acknowledging that "the 4 chamber mammalian heart probably isn't irreducibly complex" as Explore Evolution intimates , they express certainty that "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.
This argument, like its antecedents in the openly creationist literature, is a religious attack on evolution, not a scientifically grounded investigation. Explore Evolution argues that we know nothing unless we know everything.
That attitude is both scientifically and pedagogically inappropriate. The evolutionary process leading to the forms of modern mammalian, reptilian, crocodilian and amphibian hearts remains a subject of research.
Students interested in that ongoing research need a solid understanding of evolution, anatomy, physiology, and developmental biology, but more importantly, they need to understand how scientists know what they know, how scientific inquiry works.
Explore Evolution will teach students none of this. Make a Donation Today. Give a Gift Membership. More Ways to Give. Member Services FAQs. Legacy Society. Science Champions Society. Give a Gift of Stock. Donor-Advised Funds. Employer Matching Gifts. Facebook Fundraisers. Free Memberships for Graduate Students. Teaching Resources. Misconception of the Month.
Coronavirus Resources. Browse articles by topic. Community Outreach Resources. What We're Monitoring. About NCSE. Our History. Our People. The electrical signal from the pacemaker region spreads rapidly across the cardiac muscle cells of the atria via structures called gap junctions, and this ensures that the entire wall of each atrium contracts almost simultaneously. Neurons called Purkinje fibers are also involved in this process in birds but, in general, the mechanisms responsible for the contraction of the atria are similar in most vertebrates.
However, the way in which the electrical signal travels from the atrium to the ventricle differs amongst vertebrates, and the evolution of this pathway has been the focus of considerable attention for many decades Davies, ; Jensen et al. Now, in eLife, Vincent Christoffels of the University of Amsterdam and colleagues — including Bjarke Jensen and Bastiaan Boukens as joint first authors — report new and surprising findings about this phenomenon in alligators Jensen et al.
Back in the 17th century, William Harvey had already noticed that the atria contract before the ventricles in a number of different animals. This meant that the electrical signal generated in the pacemaker region must somehow be slowed down at the 'border' between the atria and the ventricles. In both mammals and birds, a layer of fibrous fatty tissue — which does not conduct electricity — insulates the ventricles from the atria.
The only way that the electrical signal can pass from the atria to the ventricles is via a small structure called the atrioventricular node, which is positioned immediately above the septum that separates the left and right ventricles.
When the electrical signal reaches this node, it activates two bundles of neurons containing His fibers and Purkinje fibers that swiftly relay the impulse and cause the ventricles to contract simultaneously.
However, in extant reptiles, the common ancestor of both birds and mammals, there does not seem to be an insulating layer or an anatomically defined node Davies, Instead, the electrical signal is slowed down by an intricate arrangement of myocardial fibers at the junction between the two atria and the ventricle. In addition, recent studies have been unable to provide any anatomical evidence for a conduction system in the ventricle of reptiles. The electrical signal appears to be conveyed by the internal lining of the heart, which shares molecular signatures with the conduction system found in birds and mammals Jensen et al.
While reptiles rely on their environment to maintain their temperature that is, they are ectothermic , mammals produce their own heat so they are endothermic. The high levels of metabolism needed to produce enough warmth means that the resting and maximal metabolic rates of mammals and birds are about 10 times higher than those of ectothermic animals Bennett and Ruben, The cardiovascular system must keep up with these greater needs by delivering more oxygen to the body.
The four-chambered heart provides an efficient solution by keeping oxygenated and non-oxygenated blood separate. The supply of oxygen to the body can also be improved by increasing how often the heart contracts. This requires cardiac structures that quickly conduct electricity, such as the atrioventricular nodes Burggren et al. Jensen et al. The experiments provided unequivocal evidence of an atrioventricular node in crocodilians.
Among extant reptiles, crocodilians are the closest living sister group to birds. However, despite their four-chambered hearts and an atrioventricular node, all living crocodilians are clearly ectothermic and have low heart rates like other reptiles Hillman and Hedrick, ; Lillywhite et al.
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