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. In an open circulatory system , the blood is not enclosed in blood vessels but is pumped into an open 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 nutrient exchange, and in some organisms lacking direct gas exchange sites, a basic mechanism to transport gasses beyond the exchange site. Because the gas exchange in many open-circulatory systems tends to be relatively low for metabolically-active organs and tissues, a tradeoff exists between this system and the much more energy-consuming, harder-to-maintain closed system.
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. 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. 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. Exchange of fluids is assisted by the pulsing of the jellyfish body.
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. For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body; therefore, more complex circulatory systems evolved.
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.
The figure below illustrates the basic circulatory systems of some vertebrates: fish, amphibians, reptiles, and mammals. 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.
Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. The atrium collects blood that has returned from the body and 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. In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, which is called pulmonary circulation , and the other throughout the rest of the body and its organs including the brain systemic circulation. In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation.
Amphibians have a three-chambered heart that has two atria and one ventricle rather than the two-chambered heart of fish. 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. However, the ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood. 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.
In mammals and birds, the heart is divided completely into four chambers: two atria and two ventricles. Oxygenated blood is fully separated from deoxygenated blood, which improves the efficiency of double circulation and is probably required for supporting 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. This video gives an overview of the different types of circulatory systems in different types of animals:. Hemoglobin is responsible for distributing oxygen, and to a lesser extent, carbon dioxide, throughout the circulatory systems of humans, vertebrates, and many invertebrates.
The blood is more than the proteins, though. Blood is actually a term used to describe the liquid that moves through the vessels and includes plasma the liquid portion, which contains water, proteins, salts, lipids, and glucose and the cells red and white cells and cell fragments called platelets.
Blood plasma is actually the dominant component of blood and contains the water, proteins, electrolytes, lipids, and glucose. The cells are responsible for carrying the gases red cells and immune response white. The platelets are responsible for blood clotting. Interstitial fluid that surrounds cells is separate from the blood, but in hemolymph, they are combined.
In humans, cellular components make up approximately 45 percent of the blood and the liquid plasma 55 percent. Blood helps maintain homeostasis by stabilizing pH, temperature, osmotic pressure, and by eliminating excess heat.
Blood supports growth by distributing nutrients and hormones, and by removing waste. Blood plays a protective role by transporting clotting factors and platelets to prevent blood loss and transporting the disease-fighting agents or white blood cells to sites of infection. The cells and cellular components of human blood are shown. Red blood cells deliver oxygen to the cells and remove carbon dioxide. Platelets form clots that prevent blood loss after injury.
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.
In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, which is called pulmonary circulation , and the other throughout the rest of the body and its organs including the brain systemic circulation.
In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation. As shown in Figure 3b, amphibians have a three-chambered heart that has two atria and one ventricle rather than the two-chambered heart of fish.
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. Figure 3. The blood is pumped from a three-chambered heart with two atria and a single ventricle. 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 4a.
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 4b.
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.
Figure 4. 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 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. In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system in which the blood is not moving freely in a cavity.
In a closed circulatory system, blood is contained inside blood vessels, circulating unidirectionally in one direction from the heart around the systemic circulatory route, then returning to the heart again. In contrast to a closed system, arthropods including insects, crustaceans, and most mollusks have an open circulatory system.
In an open circulatory system, the blood is not enclosed in the blood vessels, but is pumped into a cavity called a hemocoel. The blood is called hemolymph because it mixes with the interstitial fluid. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity, reentering the heart through openings called ostia singular: ostium.
This movement allows for gas and nutrient exchange.
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