Back To CourseCollege Biology: Help and Review
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Wendy has taught high school Biology and has a master's degree in education.
Many of the internal processes in our body occur without our conscious awareness. Our stomach digests food, our lungs take in oxygen and exhale carbon dioxide, and our kidneys filter our blood each and every day. Our internal organs and systems work automatically, and it is likely that whole days go by when we don't pause to think of our insides at all.
However, one major bodily system may catch our attention more often because we can feel it working in certain parts of our body. Our pulse, rhythmic pulsing of blood through our arteries, is a constant reminder that, at any given moment, blood is coursing through each and every part of us. Our bloodstream is part of our circulatory system, which consists of the heart and blood vessels that carry blood throughout our body. In this lesson, we will learn more about the important vessels that return blood back to the heart after traveling to our body parts. These vessels are called veins.
Within our bodies, there is a massive network of blood vessels with blood racing from our heart to all parts of our body and then back. Imagine an interconnected pipeline with a constant flow of fluid. Blood vessels are the pipeline and blood is the fluid. However, in this example, there is no endpoint. The blood circulates continuously, hence the name circulatory system. There are several different types of blood vessels, with differing size, strength, and composition.
Arteries are the strong, elastic vessels that take oxygenated blood from the heart out to parts of our body to deliver oxygen. Arterial blood is a rich red color, due to the oxygen bonded with hemoglobin on the red blood cells. After the oxygen has been dropped off, the blood is returned back to the heart by the veins. This blood is not only deoxygenated, but it has also picked up carbon dioxide. This gives venous blood a dark red, almost purplish color. We can often see veins just under our skin, where they can tend to have a bluish tint.
Veins serve a critical function within our bodies. When blood has been pumped by the heart to various parts of the body, it must return back to the heart. In a metaphorical sense, veins are the return portion of a round-trip plane ticket. The veins serve the purpose of delivering the blood, now bluish in color, back to the right atrium (chamber) of our heart. In the heart, blood will collect more oxygen and prepare to be pumped back out through arteries. This is a cycle that continues as long as a person is living.
Veins can vary greatly in size. The largest vein in the body is called the vena cava, which is Latin for 'hollow vein.' There are two sections of the vena cava, one below the heart and one above it. The section above the heart is called the superior vena cava, and it returns blood from the head, neck, chest, and upper limbs back to the heart. You can remember this term by associating the word 'superior' with 'above.'
The lower section is called the inferior vena cava, and it returns blood from all other parts of the body back to the heart. Similar to the aforementioned example, 'inferior' can be associated with 'below.' As we get farther away from the heart and toward extremities, veins branch out and get smaller and smaller. The smallest veins are called venules.
Blood vessels are designed to accommodate and move varying volumes and pressures of blood. There is a certain degree of expansion and contraction involved in the process of moving the blood through the body. For that reason, these vessels have features such as muscle, elastic fibers, and proteins like collagen to allow for this type of flexibility. Since veins do not need to withstand as much pressure as arteries, they have much less elasticity and smooth muscle. For this reason, veins do not tend to hold their shape and can appear floppy.
Structurally, the walls of veins are composed of three main layers. These layers are known as tunics, or coats. In veins, the tunics are very thin, especially in comparison to those in arteries. The innermost layer of a vein is called the tunica interna, or intima. The tunica interna itself is made up of three layers. First, there is the endothelium, which is a specialized type of tissue that lines blood vessels, as well as the heart. Next, lies a basement membrane, which provides structural support. Finally, there is an elastic layer called the internal elastic lamina. This layer also borders the lumen, which is the hollow space through which blood flows.
The middle layer making up veins is the tunica media. This is the thickest layer of the three, although in veins it is still very thin. It consists of a small amount of smooth muscle and elastic fibers. In arteries, by contrast, this layer is much thicker and contains a much higher percentage of smooth muscle and elastic fibers.
Finally, the outermost layer of a vein is called the tunica externa. This is the thickest coat of all, and contains collagen and elastic fibers.
Many veins, especially in extremities, have valves on their inner walls. These valves point in the direction of the heart, and they serve a very important purpose. The blood returning back to the heart has a relatively low pressure and could potentially flow backward or pool. The valves inside the veins have the ability to close, ensuring that the blood will only move toward the heart.
Sometimes valves can be defective. If valves are not working properly, blood will not necessarily keep moving forward and tends to pool. When this occurs, veins will swell, and spider veins or varicose veins can appear. This condition is often genetic.
Veins are an important part of our circulatory system. They are responsible for returning deoxygenated blood back to the heart after arteries carry blood out. The vena cava is the largest vein in the body. Veins have much thinner walls than arteries. Because the pressure within them is low, veins have valves inside them to keep blood flowing only one way. This ensures a successful return of blood to the heart, where the cycle begins all over again.
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Back To CourseCollege Biology: Help and Review
24 chapters | 433 lessons
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