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All living things must take in nutrients and expel waste products. In the case of animal cells, this means that they must take in carbohydrates and oxygen, and release carbon dioxide.
In single-celled organisms that live in water, this can be accomplished by transport across the cell wall.
But what about multicellular organisms where not all cells come in contact with the outside environment? And what about animals that live on land and can't just absorb nutrients from their surrounding environment? These animals must find a way to distribute nutrients to every cell in their body, and the bigger and more complex the organism, the more difficult this task becomes. So it shouldn't be too surprising that many different types of distribution systems are found in various animals, depending on their size, complexity and the environment they live in.
Some of the simplest multicellular organisms are cnidarians. These are animals which belong to the phylum cnidaria, and include jellyfish, corals, anemones and hydra. Cnidarians are only two cell layers thick and have a sac-like body plan consisting of a body wall surrounding a central gastrovascular cavity, which is a structure found in some animals that serves as the main site of both digestion and distribution of substances throughout the body.
For example, let's take a look at a very simple cnidarian, the hydra. Hydras are very small aquatic animals which capture prey with their tentacles and then deposit it into their gastrovascular cavity.
Food is digested right there in the cavity and the nutrients are distributed throughout the animal. Notice that the gastrovascular cavity extends throughout the entire body, including the tentacles so that every cell of the hydra has direct contact with either the cavity or the water surrounding the animal.
Oxygen is absorbed by the epithelial layer of cells which is this outer layer of cells that are directly exposed to the environment. Nutrients from digested prey are absorbed by the inner layer of cells exposed to the gastrovascular cavity, so oxygen and nutrients never have to travel across more than two cell layers.
Gastrovascular cavities work well for aquatic animals with two cell layers, but cannot support animals that have many layers of cells, especially those that don't live in the water. For these more complex animals, the digestive and circulatory systems became separated and specialized to perform the more complicated processes needed to support many cell layers and life out of water.
There are two main types of circulatory systems: open circulatory systems and closed circulatory systems. Open circulatory systems are systems where internal organs and body tissues are surrounded by circulatory fluid. This fluid is called hemolymph, and it's pretty much like blood, except that it doesn't contain oxygen transporting cells.
Let's take a closer look at an example of an open circulatory system.
In an open circulatory system, the hemolymph surrounds all internal organs and fills pretty much all of the internal spaces between organs, muscles and other body tissues. These hemolymph-filled spaces are called sinuses . Hemolymph moves around between organs and tissues as body movements of the animal changes the size and shape of the sinuses. In addition, animals with open circulatory systems, like our grasshopper here, usually have some sort of main vessel, like this one here, with simple tubular hearts that contract and partially direct circulation of the hemolymph through the sinuses, like this.
Open circulatory systems solve the problem of how to transport nutrients through organisms with more than two cell layers. But transport in open systems is most effective over only short distances and occurs slowly. So how do large animals transport nutrients long distances? The answer to this question can be found in most modern homes: internal plumbing!
Think of it this way, before the widespread use of plumbing, human settlements were limited in size to areas within walking distance of a water source, whether it was a central well, a stream or a river. However, once humans started using plumbing to quickly and efficiently direct water long distances, cities could grow larger and suburbs began appearing around the cities. As long as there was plumbing to provide a source of fresh water, there was really no limit to how big a city or metropolitan area could be.
The same principles hold true for circulation in animals. A closed circulatory system is a circulatory system where blood is contained within vessels. The purpose of these blood vessels is really not much different from the purpose of water pipes in a city, which is to transport a necessary commodity long distances and distribute it to many locations.
And just as a city water system requires pumps to move water through the pipes, closed circulatory systems require a heart to pump blood through the vessels. Because the blood is restricted to well-defined vessels and pumped directionally through the system by one or more hearts, nutrients and waste products can be transported long distances in a very short amount of time.
A closed circulatory system also ensures that blood can be directed to every cell in an organism by providing blood vessels that reach every cell of its body. This is also similar to a city water system which directs water to every house and apartment of the city that has a water pipe running into it.
As if speed, efficiency and distance weren't enough, closed circulatory systems have one final advantage over open circulatory systems and that is control. Because the blood is contained in vessels, the flow can be adjusted in several ways.
First, the flow rate through the entire system can be adjusted up or down by adjusting the heart rate. The faster the heart pumps, the more blood is pumped through the system.
Second, flow can be also be increased to a localized area by dilation of blood vessels. Dilation of blood vessels occurs when a blood vessel increases in diameter to allow a larger volume of blood to flow through. So, here we have our blood vessel and the blood is flowing through. When the blood vessel dilates, the vessel expands, and the amount of blood flowing through increases.
Likewise, flow can be decreased to a localized area by constriction of blood vessels. Constriction of blood vessels occurs when a blood vessel decreases in diameter to allow a smaller volume of blood to flow through. When the vessel constricts, the vessel gets smaller, and the amount of blood flowing through decreases.
Flow can also be completely shut down in some smaller vessels when blood flow is not necessary or if there is an advantage to stopping blood flow. For example, when parts of the digestive tract aren't in use, a subset of vessels that are used to absorb nutrients from digested food will be completely closed off and shut down since there is really no purpose to absorb nutrients that aren't there.
To review, all animal cells need access to oxygen and carbohydrates, and need to expel carbon dioxide. Some very simple multicellular organisms have a gastrovascular cavity, which is a structure found in some animals that serves as the main site of both digestion and distribution of substances throughout the body.
More complex organisms with more than two cell layers need more complex circulatory systems to get nutrients to all cells of the organisms. Open circulatory systems are systems where internal organs and body tissues are surrounded by circulatory fluid. This fluid is called hemolymph and it is the circulatory fluid in open circulatory systems.
Large animals, need a faster circulatory system that can transport nutrients and oxygen longer distances. These animals have a closed circulatory system which is a circulatory system where blood is contained within vessels.
Animals with closed circulatory systems can change the rate of flow in the entire system by altering their heart rate, or they can change the rate of flow in localized areas by changing the size of specific blood vessels. Blood vessels can dilate, or increase in diameter to allow a larger volume of blood to flow through, or blood vessels can constrict and decrease in diameter, which allows a smaller volume of blood to flow through. Flow can even be completely shut down in some smaller blood vessels when it is advantageous for the organism.
Stay tuned, because next we'll be taking a closer look at the human circulatory system.
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Back To CourseCLEP Biology: Study Guide & Test Prep
24 chapters | 224 lessons