Types of Circulatory Systems: Open vs. Closed

The circulatory system

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The circulatory system serves to move blood to a site or sites where it can be oxygenated, and where wastes can be disposed. Circulation then serves to bring newly oxygenated blood to the tissues of the body. As oxygen and other chemicals diffuse out of the blood cells and into the fluid surrounding the cells of the body's tissues, waste products diffuse into the blood cells to be carried away. Blood circulates through organs such as the liver and kidneys where wastes are removed and back to the lungs for a fresh dose of oxygen. And then the process repeats itself. This process of circulation is necessary for the continued life of the cells, tissues, and even of the whole organism. Before we talk about the heart, we should give a brief background of the two broad types of circulation found in animals. We will also discuss the progressive complexity of the heart as one moves up the evolutionary ladder.

Many invertebrates do not have a circulatory system at all. Their cells are close enough to their environment for oxygen, other gasses, nutrients, and waste products to simply diffuse out of and into their cells. In animals with multiple layers of cells, especially land animals, this will not work, as their cells are too far from the external environment for simple osmosis and diffusion to function quickly enough in exchanging cellular wastes and needed material with the environment.

Open Circulatory Systems

In higher animals, there are two primary types of circulatory systems: open and closed. Arthropods and mollusks have an open circulatory system. In this type of system, there is neither a true heart nor capillaries as are found in humans. Instead of a heart, there are blood vessels that act as pumps to force the blood along. Instead of capillaries, blood vessels join directly with open sinuses. "Blood," actually a combination of blood and interstitial fluid called 'hemolymph', is forced from the blood vessels into large sinuses, where it actually bathes the internal organs. Other vessels receive blood forced from these sinuses and conduct it back to the pumping vessels. It helps to imagine a bucket with two hoses coming out of it, these hoses connected to a squeeze bulb. As the bulb is squeezed, it forces the water along to the bucket. One hose will be shooting water into the bucket, the other is sucking water out of the bucket. Needless to say, this is a very inefficient system. Insects can get by with this type of system because they have numerous openings in their bodies (spiracles) that allow the "blood" to come into contact with air.

Closed Circulatory Systems

The closed circulatory system of some mollusks and all vertebrates and higher invertebrates is a much more efficient system. Here blood is pumped through a closed system of arteries, veins, and capillaries. Capillaries surround the organs, making sure that all cells have an equal opportunity for nourishment and removal of their waste products. However, even closed circulatory systems differ as we move further up the evolutionary tree.

One of the simplest types of closed circulatory systems is found in annelids such as the earthworm. Earthworms have two main blood vessels—a dorsal and a ventral vessel—which carry blood towards the head or the tail, respectively. Blood is moved along the dorsal vessel by waves of contraction in the wall of the vessel. These contractible waves are called 'peristalsis.' In the anterior region of the worm, there are five pairs of vessels, which we loosely term "hearts," that connect the dorsal and the ventral vessels. These connecting vessels function as rudimentary hearts and force the blood into the ventral vessel. Since the outer covering (the epidermis) of the earthworm is so thin and is constantly moist, there is ample opportunity for exchange of gasses, making this relatively inefficient system possible. There are also special organs in the earthworm for the removal of nitrogenous wastes. Still, blood can flow backward and the system is only slightly more efficient than the open system of insects.

Two-Chambered Heart

As we come to the vertebrates, we begin to find real efficiencies with the closed system. Fish possess one of the simplest types of true hearts. A fish's heart is a two-chambered organ composed of one atrium and one ventricle. The heart has muscular walls and a valve between its chambers. Blood is pumped from the heart to the gills, where it receives oxygen and gets rid of carbon dioxide. Blood then moves on to the organs of the body, where nutrients, gasses, and wastes are exchanged. However, there is no division of the circulation between the respiratory organs and the rest of the body. That is, the blood travels in a circuit which takes blood from the heart to gills to organs and back to the heart to start its circuitous journey again.

Three-Chambered Heart

Frogs have a three-chambered heart, consisting of two atria and a single ventricle. Blood leaving the ventricle passes into a forked aorta, where the blood has an equal opportunity to travel through a circuit of vessels leading to the lungs or a circuit leading to the other organs. Blood returning to the heart from the lungs passes into one atrium, while blood returning from the rest of the body passes into the other. Both atria empty into the single ventricle. While this makes sure that some blood always passes to the lungs and then back to the heart, the mixing of oxygenated and deoxygenated blood in the single ventricle means the organs are not getting blood saturated with oxygen. Still, for a cold-blooded creature like the frog, the system works well.

Four-Chambered Heart

Humans and all other mammals, as well as birds, have a four-chambered heart with two atria and two ventricles. Deoxygenated and oxygenated blood are not mixed. The four chambers ensure efficient and rapid movement of highly oxygenated blood to the organs of the body. This helps in thermal regulation and in rapid, sustained muscle movements.

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Bailey, Regina. "Types of Circulatory Systems: Open vs. Closed." ThoughtCo, Apr. 5, 2023, thoughtco.com/circulatory-system-373576. Bailey, Regina. (2023, April 5). Types of Circulatory Systems: Open vs. Closed. Retrieved from https://www.thoughtco.com/circulatory-system-373576 Bailey, Regina. "Types of Circulatory Systems: Open vs. Closed." ThoughtCo. https://www.thoughtco.com/circulatory-system-373576 (accessed June 10, 2023).