The Cirulatory System: Unity and Diversity

In any given animal, there must be a method for which nutrients such as energy and oxygen enter individual cells. The process of osmosis and diffusion will work over a period of short distances, but what about delivering nutrients throughout the entire body of an organism? This is where the circulatory system comes into play. The circulatory system, in all animals, acts as a pathway, in which nutrients that couldn't otherwise be delivered to cells through diffusion and osmosis, are transported to the cells in an organism. In a way, the circulatory system is like a highway, and the nutrients are the cars traveling on the highway. There are many different circulatory system adaptations that different organisms possess. One of these adaptations is a heart, which acts as a muscular pump in a closed circulatory system to keep the nutrients flowing throughout the body. In an open circulatory system, hemolymph, which is similar to blood, is pumped through a gastrovascular cavity called a hemocoel, which acts as the highway for the nutrients to be delivered. Regardless of the organism, all circulatory systems have circulatory fluid, a set of tubes for the fluid to move, and a muscular pump.

The heart is the muscular organ, located beneath the breast bone, which is responsible for pumping blood through all of the vessels in the body in order to deliver oxygen and other nutrients to cells all throughout the body. The heart is covered by the pericardium, which isolates the heart from the surrounding tissues, and is a sac of membrane that completely encloses the heart. The heart receives deoxygenated blood through the superior and inferior vena cavas in the right atrium. The blood then flows down to the right ventricle, which pumps the blood through the pulmonary artery and to the lungs, where it can absorb oxygen. The oxygenated blood returns through the pulmonary vein and enters the left atrium. Next, the blood flows down to the left ventricle, the chamber with the most muscle, where it is pumped through the aorta and then continues to the rest of the body. A very important characteristic of the heart is the valves it has at the openings of the chambers which allows it to control the flow of blood in the right direction. Because the pig is a mammal, its heart has the exact same structure as that of a human.

This cross section of the heart shows the atrium and ventricle which is surrounded by a massive amount of tissue. This picture is a good example of how one side of the heart is more muscular than the other. As it can be seen, the left side of the heart (the viewer's right) is much larger than the other side. This is because this is the side that pumps blood to the rest of the body. This part of the heart must be strong in order to maintain blood pressure throughout the body. The heart is one of the strongest muscles in the body, and this picture helps demonstrate how large and powerful the heart muscle really is.

This view of the fetal pig's heart shows the hole where blood would be pumped through the aorta to the rest of the body.

Grasshoppers have an open circulatory system, meaning that the blood and interstitial fluid are the same. This fluid, called hemolymph, is pumped by a long tube located on the dorsal side of the organism, called the heart, through the vessels and into sinuses of the insect. Hemolymph is moved through this open circulatory system by a system of body, not heart, muscle contractions, that move the hemolymph through out the sinuses. After the hemolymph transports its nutrients, the fluid returns to the heart through ostia. Although the heart complex in a grasshopper is simple, it still incorporates valves to control the hemolymph during contractions. Unlike the pig, the heart plays no role in transporting oxygen to the cells in the organism.


A mussel, like the grasshopper, has an open circulatory system.Many of the circulatory system adaptations that the mussel exhibits are the same as that of the grasshopper.Both organisms have a hemolymph-filled cavity called a hemocoel, which serves as the cavity through which hemolymph is pumped to different cells of the body.
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Open Circulatory Sysetm Vs. Closed Circulatory System |

Closed circulatory system Vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. The closed circulatory system developed as organisms became more complex and had higher metabolic rates. In these organisms, the circulatory system need more powerful hearts as well as a more organized method of nutrient transfer to supply the basic necessities of an organism with a high metabolic rate. The fish is the first organism to make a circulatory adaption. It has a two chambered heart producing a single circuit of blood flow. Blood is first pumped to the gills to pick up oxygen and release carbon dioxide, and then continues to the capillaries in other parts of the body as the vessels get smaller. Amphibians have made adaptions to make their circulatory systems much more efficient. They have a three-chambered heart, unlike the human's four-chambered. The ventricle pumps the blood into two circulations pulmocutaneous and systemic. Pulmocutaneous circulation leads the blood to the capillaries that surround gas-exchange organs so that the blood can pick up oxygen and release carbon dioxide. The oxygenated blood returns to the heart to be pumped through the rest of the body, systemic circulation. The double circulation of blood allows the blood to be pumped throughout the body quickly and efficiently. The adaption of a four-chambered heart for mammals is one step up from this, because in the three-chambered heart some of the non-oxygenated blood mixes with the oxygenated blood. Endotherms developed these four-chambered hearts because they require much more energy. The independent systemic and pulmonary circulations allow for the most efficient circulation of blood.
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|| Open circulatory system

The open circulatory system is common to molluscs and arthropods. Open circulatory systems (evolved in crustaceans, insects, mollusks and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood. In an open circulatory system, there is no distinction between blood and interstitial fluid – this is one substance called hemolymph. Furthermore, it is important to note that the hemocoel (cavity in organisms that have open circulatory systems) serves to bathe cells directly in oxygen and nutrients.
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Disadvantages of Having a closed vs. open circulatory system

- In a closed circulatory system, blood is used to transfer oxygen to tissues and remove carbon dioxide through tissues. In an open circulatory system such as that of the grasshopper, the tracheal system is responsible for this, putting much less stress on the circulatory system.


Blood, the central liquid of the closed circulatory system, has many functions in the body of many organisms. Most of these functions involve regulating the internal environment of an organism, homeostasis. It carries oxygen and other nutrients to the cells of the organism. It also carries waste away from the cells, like carbon dioxide and lactic acid. Blood also serves to regulate the pH, water, electrolytes, blood clotting and temperature of the organism it is a part of. It also protects the body from harmful organisms such as bacteria through white blood cells and antibodies. The structure of blood cells fit these functions, but on a much smaller level. The chemical makeup of blood cells and their proteins allow them to perform their functions. The protein hemoglobin in red blood cells allows it to carry oxygen to other cells from the lungs and also to carry hydrogens back to the lungs from the cells. The donut shaped structure of red blood cells is necessary for them to do these and other functions in the body. The structures of other parts of the circulatory system are also necessary for blood to do its part. The many arteries, veins, and capillaries of the body bring the blood to the many cells.

A scanning electron microscope picture of different blood cells (red, white, and platelet) photos/whatis/img/4.jpg

In an open circulatory system, hemolymph is the main transporter of nutrients. Hemolymph "bathes" the cells directly with the nutrients it carries. The disadvantage of hemolymph is that there is no distinction between hemolymph and interstitial fluid, so more than just nutrients can be "bathed" on the cell, increasing the chances that unwanted substances such as toxins enter the cell. Hemolymph is generally considered the insect equivalent of blood.