MadSci Network: Zoology |
Greetings I don’t know of any animals that do not have hearts , unless you are referring to single celled animals which diffuse nutrients across a cell membrane into its cytoplasm and thus circulation is complete. At any rate the following information should help sort out the different types of circulation. Invertebrates have circulatory systems that range from complex to simple. Some invertebrates, such as earthworms and octopuses, have a closed circulatory system. Other invertebrates have an open circulatory system, in which the blood is only partially confined to the vessels. It fills the hollow spaces of the body as well. Animals with an open circulatory system include insects, spiders, and most shellfish. In many invertebrates, the blood is pumped by contracting vessels or by pumping centers (contracting portions of vessels), or by both. Among insects, for example, the "heart" consists of an internal contracting vessel that extends almost the length of the back. The simplest animals with a true circulatory system include certain Worms have contracting vessels that pump the blood. A group of simpler worms, called ribbon worms or proboscis worms, have a circulatory system with no pumping centers and no contracting vessels. The movements of the animal keep the blood flowing through the body. http://school.discovery.com/homeworkhelp/worldbook/atozscience/c/115905.htm l The distribution of nutrients and gases from the environment is one of the essential tasks of all living organisms. It is a job which takes place at the most basic levels in bacteria and single celled organisms all the way to the intricately complex circulatory systems of mammals and humans. Though the circulatory system begins with simple diffusion and the gastrovascular cavity and seems to end with the four chambered mammalian heart, the development of different hearts in organisms is actually convergent evolution, the frog, worm, and human heart all developed separately and independently of each other. It is the demands of each different type of animal that brings about the very different types of heart in each animal. INVERTBRATES Microorganisms and single-celled animals need only to diffuse nutrients across a cell membrane into its cytoplasm, and thus circulation is complete. However as multicellular animals become larger, with layers of cells stacked on one another, their more interior cells experienced greater difficulty in exchanging materials with the environment by simple diffusion. This problem is solved with the advent of the body cavity seen in such organisms as the jellyfish. It is here that true circulation takes place, the transport of material from one place to another within an organism by passage through an internal fluid. The different types of circulatory systems that then develop are specific to the different phyla of animals. Open and closed circulatory systems then serve organisms whether vessels are needed to transport materials (closed) or merely circulation through the whole body. VERTEBRATES Like invertebrates, vertebrates have also needed different types of systems to suit the many different animals. Any closed circulatory system requires both a system of passageways though which fluid can circulate and a pump to force the fluid through them. The three different types of vertebrate hearts reflect the vastly different needs of each of these animals. Fish The development of gills by fish necessitated a more efficient pump to force blood through the fine capillary network and in fish there is a true chamber-pump heart, beyond the simple closed circulatory system. The fish heartbeat is the peristaltic sequence, starting at the rear and moving to the front. The first of the four chambers to contract is the sinus venosus, then the atrium, then the ventricle, and finally the conus arteriosus. Despite shifts in the relative positions that evolved later heartbeat sequence is maintained and unchanged in all vertebrates. Fish utilize the one-cycle chamber pump, which is ideally suited to its gill respiratory apparatus and represents one of the major evolutionary innovations of the vertebrates. Its greatest advantage is that the blood it delivers to the tissues of the body is fully oxygenated. Thus blood is pumped directly to and through the gills, where it becomes fully aerated; from the gills it flows through a network of arteries to the rest of the body and then returns to the heart through the veins. Thus, in a fish, blood must pass through two capillary beds during each circuit, one in the gills and a second one in another organ. When blood flows through a capillary bed, blood pressure, the hydrostatic pressure that pushes blood through vessels, drops substantially. This means the flow of blood has lost much of the force contributed by the contraction of the heart, so the circulation to the rest of the body is quite slow. However the process is aided by the whole-body movements produced by the fish during swimming. Amphibian The advent of gaseous respiration in the lungs involved a major change in the pattern of circulation. Ultimately this evolutionary change allowed vertebrates to overcome the limitations of the one-chambered heart of fish, which requires aid by swimming. The change consisted of the development of an additional pair of veins. After blood is pumped through a fine network of capillaries in lungs, it is not dispersed to the tissues of the body but is instead returned to the heart through large veins called pulmonary veins for re-pumping. The development of these new veins lead to a great improvement in the performance of the circulatory system, since the blood being pumped to other tissues of the body could be pumped at a much higher pressure than if it were not returned to the heart at this stage. The disadvantage of pulmonary veins is that the aerated blood from the lungs is mixed in the heart with non-oxygenated blood that is constantly being returned to the heart from the rest of the body. Consequently the heart pumps out a mixture of oxygenated and non-oxygenated blood rather than fully oxygenated blood. Mammal and Birds Only a relatively slight alteration is seen in the heart of mammals, birds, and crocodiles. Though the changes occurred separately for these three types of animals, they created the same basic anatomical changes with slight variations. The closure of the ventricular septum (a wall between two cavities) created the double circulatory system toward, the last step in heart evolution. Four-chambered hearts have the advantage re-pumping blood after its passage through the lungs, without mixing oxygenated and non-oxygenated blood. The blood that is pumped by the heart of a mammal or bird into the systemic arterial system if fully oxygenated. The great increase in efficiency that the double circulatory system provides is believed to have been important in the evolution of endothermy in mammals. More efficient circulation is necessary to support the great increase in metabolic rate that is required to generate body heat internally. Also, blood is the carrier of heat within the body, and an efficient circulatory system is required to distribute heat evenly throughout the body. The separation of the blood flow into two circuits also has a second favorable result. Because overall circulatory system is closed, in each full passage through the system, the same volume of blood has to move through the circulation path as through the much more extensive body circulation path. This means that the blood must move through the lungs much faster than through the rest of the body. This more rapid circulation is not accomplished by higher pressure in the pulmonary circuit. Instead the blood vessels in the lung are larger in diameter than those in the rest of the body and offer less resistance to flow. The favorable result is that the rapid flow of blood through the lungs greatly increases the efficiency with which oxygen is captured by the bloodstream. http://www.bhs.berkeley.k12.ca.us/departments/science/anatomy/anatomy98/Hea rts/html/evolution.html Thanks for taking the time to send in a question to the Mad Scientist Network. June Wingert Mad Scientist
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