What are Veins? - Functions & Explanation

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  • 0:00 A Look at Veins
  • 1:00 Circulatory System Overview
  • 2:05 Functions of Veins
  • 3:35 Structure of Veins
  • 5:25 Valves in Veins
  • 6:10 Lesson Summary
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Lesson Transcript
Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Veins are the blood vessels that carry deoxygenated blood from parts of our body back to the heart. Learn more about how they function and quiz yourself at the end.

A Look at Veins

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.

Overview of the Circulatory System

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.

Function of Veins

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.

Structure of Veins

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.

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