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 successfully-evolved animal groups on earth. In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles 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. Learning Objectives Describe how circulation differs between fish, amphibians, reptiles, birds, and mammals. Key Points Fish have a single systemic circuit for blood, where the heart pumps the blood to the gills to be re-oxygenated gill circulation , after which the blood flows to the rest of the body and back to the heart.
Other animals, such as amphibians, reptiles, birds, and mammals, have a pulmonary circuit, where blood is pumped from the heart to the lungs and back, and a second, systemic circuit where blood is pumped to the body and back.
Amphibians are unique in that they have a third circuit that brings deoxygenated blood to the skin in order for gas exchange to occur; this is called pulmocutaneous circulation. Simple diffusion allows some water, nutrient, waste, and gas exchange into primitive animals that are only a few cell layers thick; however, bulk flow is the only method by which the entire body of larger more complex organisms is accessed.
The circulatory system is effectively a network of cylindrical vessels: the arteries, veins, and capillaries that emanate from a pump, the heart. In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system, in which the blood is not free in a cavity.
In a closed circulatory system , blood is contained inside blood vessels and circulates unidirectionally from the heart around the systemic circulatory route, then returns to the heart again, as illustrated in Figure a.
As opposed to a closed system, arthropods—including insects, crustaceans, and most mollusks—have an open circulatory system, as illustrated in Figure b. In an open circulatory system , the blood is not enclosed in the blood vessels but is pumped into a cavity called a hemocoel and is called hemolymph because the blood mixes with the interstitial fluid.
As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then reenters the hearts through openings called ostia. This movement allows for gas and nutrient exchange.
An open circulatory system does not use as much energy as a closed system to operate or to maintain; however, there is a trade-off with the amount of blood that can be moved to metabolically active organs and tissues that require high levels of oxygen.
In fact, one reason that insects with wing spans of up to two feet wide 70 cm are not around today is probably because they were outcompeted by the arrival of birds million years ago.
Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to get food faster and possibly to prey on the insects. Circulatory System Variation in Animals The circulatory system varies from simple systems in invertebrates to more complex systems in vertebrates.
The simplest animals, such as the sponges Porifera and rotifers Rotifera , do not need a circulatory system because diffusion allows adequate exchange of water, nutrients, and waste, as well as dissolved gases, as shown in Figure a. Organisms that are more complex but still only have two layers of cells in their body plan, such as jellies Cnidaria and comb jellies Ctenophora also use diffusion through their epidermis and internally through the gastrovascular compartment.
Both their internal and external tissues are bathed in an aqueous environment and exchange fluids by diffusion on both sides, as illustrated in Figure b. Exchange of fluids is assisted by the pulsing of the jellyfish body.
For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body; therefore, more complex circulatory systems evolved. Most arthropods and many mollusks have open circulatory systems. In an open system, an elongated beating heart pushes the hemolymph through the body and muscle contractions help to move fluids.
The larger more complex crustaceans, including lobsters, have developed arterial-like vessels to push blood through their bodies, and the most active mollusks, such as squids, have evolved a closed circulatory system and are able to move rapidly to catch prey.
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 adaptation during evolution and associated differences in anatomy. This is called double circulation. In amphibians, with two atria but only a single ventricle, this results in the mixing of deoxygenated and oxygenated blood, but amphibians also gather oxygen through their moist skin, so this inefficiency is not critical.
Beginning with the reptiles, a septum or wall develops that partly divides the deoxygenated from the oxygenated blood in the ventricle, and this is important because reptiles, with a watertight skin, rely entirely on their lungs for oxygen. Reptiles also have the unique ability to redirect or shunt blood leaving the heart back through the heart without passing through the body circuit, and to shunt deoxygenated body blood back through the body without going to the lungs.
The purpose of this shunt see the purple vessels in the figure below is not entirely understood. Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle figure a. The atrium collects blood that has returned from the body, while the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated; this is called gill circulation.
The blood then continues through the rest of the body before arriving back at the atrium; this is called systemic circulation. The result is a limit in the amount of oxygen that can reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish. Examples of animal circulatory systems : a Fish have the simplest circulatory systems of the vertebrates: blood flows unidirectionally from the two-chambered heart through the gills and then to the rest of the body.
The blood is pumped from a three-chambered heart with two atria and a single ventricle. The heart is three chambered, but the ventricles are partially separated so some mixing of oxygenated and deoxygenated blood occurs, except in crocodilians and birds. In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart pulmonary circulation and the other throughout the rest of the body and its organs, including the brain systemic circulation.
Amphibians have a three-chambered heart that has two atria and one ventricle rather than the two-chambered heart of fish figure b. The two atria receive blood from the two different circuits the lungs and the systems. 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 where gas exchange occurs in the lungs and through the skin. 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 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 successfully-evolved animal groups on earth. In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles 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.
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