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Humans, like all other mammals, have a closed circulatory system. You may remember that in a closed system blood is contained within vessels, and that the main components of a closed circulatory system are the heart, blood vessels and blood. In this lesson, we're going to take a look at the human vascular system, which is the network of blood vessels in the human body.
Now, we already know that blood vessels make transport of blood over significant distances possible, and that they transport blood to every cell of the body, but just how extensive is the human vascular system? If we laid all of the blood vessels in the average adult human body end-to-end, how far do you think they'd reach? Across the San Francisco Bay? Nope, too short! What about the width of California, from San Francisco to Lake Tahoe? Still too short! How about all the way across the United States, from San Francisco to New York? Would you believe that's still too short? What about all the way around the world? Seem ridiculous? Maybe, but only because it's still too short! The fact is, if laid end to end, all of the blood vessels in the average human body would reach all the way around the Earth two and a half times! Hard to believe, isn't it? But it's true; the human body has so many cells that, to reach every single one, it contains about 62,000 miles of blood vessels, which is roughly 2.5 times the circumference of the Earth!
There are three main types of blood vessels in the human body: arteries, veins and capillaries.
Each type of blood vessel serves a different purpose and has specific features to help it achieve that purpose. Let's start with arteries. Arteries are blood vessels that carry blood from the heart to other tissues of the body. Because arteries are the first vessels that the heart pumps blood through, they experience the highest blood pressures, so they have thick, muscular walls to withstand the high pressures and maintain a very round shape. Artery walls are very elastic, which allows them to expand and accommodate the spike in blood volume that occurs with every heartbeat. After expanding, the arteries quickly contract back to their original size, and in the process help to push the blood along its way. When people feel their pulse, they are actually feeling the expansion of an artery that occurs after each heartbeat.
In contrast, veins are blood vessels that carry blood from various tissues to the heart. Because blood pressure is much lower in veins, they have a much thinner layer of muscle surrounding them and therefore do not maintain a rounded shape nearly as well as arteries. Also, because blood pressure is so low in veins, they have one-way valves that ensure unidirectional flow toward the heart. This is a cross-section through a tissue sample of the human gall bladder. In it, you can see several arteries and veins of various sizes. You can see some very large arteries here, here and here, and some much smaller arteries here and here.
You can also see two small veins here, between the two smaller arteries, and some larger veins here and here. Notice that the main difference between the arteries and veins is that the arteries have much thicker walls, with a thick layer of muscle, and that the veins have much thinner walls, with such a thin layer of muscle that it's often hard to see them in tissue samples.
The smallest blood vessels in the human body are called capillaries. Capillaries are microscopic blood vessels with very thin walls that allow the transfer of nutrients and gases between the blood and tissues. The smallest capillaries are just big enough for blood cells to pass through one at a time. They weave in and out through every tissue in the human body to ensure that every cell has access to the blood supply. Capillaries are specifically adapted to facilitate transfer of oxygen, carbon dioxide, carbohydrates and other nutrients between the blood and tissues. The capillary walls are only one cell thick, and composed of endothelial cells, which are very thin, flattened cells that line the inner wall of all blood vessels. Because capillary walls are so thin, nutrients and waste products can cross them almost instantly. Even so, the flow rate in capillaries is slowed down to allow time for red blood cells to release their oxygen into the blood.
Now that we know the three basic types of blood vessels and what they do, let's follow the path that the blood takes through the human vascular system. In this diagram, oxygen-poor blood is represented by the blue, and oxygen-rich blood is represented by the red.
Starting in the right side of the heart, the blood is pumped into the pulmonary arteries, which are the arteries that bring oxygen-poor blood to the lungs. This oxygen-poor blood then enters into a dense network of capillaries that blankets the entire respiratory surface of the lung. It is here where the blood releases carbon dioxide into the lungs and absorbs oxygen. The now oxygen-rich blood enters the pulmonary veins, which are the veins that bring oxygenated blood from the lungs back to the heart.
From the pulmonary veins, blood enters the left side of the heart and is pumped into the aorta, which is the main artery that leads directly out of the left side of the heart. The aorta is the largest blood vessel in the human body, and branches off into many smaller arteries that bring oxygenated blood to the various tissues of the body. The blood then enters capillaries that distribute the blood to every cell of the body, oxygen is released into the tissues and carbon dioxide is picked up by the blood. The blood is then collected back into veins, which bring the now oxygen-poor blood back to the right side of the heart, and the cycle is ready to repeat itself.
Now you may think that an adult's vascular system is fully mature and doesn't change at all, but that's not exactly true. People's bodies change even after they stop growing taller. Muscles can grow bigger with exercise, or layers of fat can get thicker if people don't exercise. In either case, there is an expansion of tissue which will require a blood supply in order to survive, so the vascular system must expand too. But how does the vascular system know when and where to expand? The answer is it lets the cells tell it where and when it needs to expand.
Here's how it works: when a cell or group of cells is not getting enough oxygen, it produces several growth factors, including vascular endothelial growth factor, or VEGF for short. These growth factors are signaling molecules that spread out from the cells that aren't receiving enough oxygen until they reach an existing blood vessel. When they do reach an existing blood vessel, these signaling molecules cause existing endothelial cells to migrate out of their current vessel, multiply and form a new blood vessel.
This new blood vessel branches off from the original vessel, out towards the cell that didn't receive enough oxygen. This entire process is called angiogenesis, and it is defined as the process by which new blood vessels are formed by extension of existing blood vessels. Angiogenesis plays a key role in the development of vasculature in response to any type of new tissue growth, whether it is due to wound repair, expansion of normal tissues or the growth of a tumor.
Although humans have a closed circulatory system, it is not completely watertight. The thin walls of the capillaries are a little bit leaky, and lose about one percent of the fluid that passes through them, along with some of the associated blood proteins. This may not seem like a lot, but over the course of a day, this adds up to about three total liters of fluid! If you consider that the average human has about five liters of blood in their body, then clearly the amount of fluid lost is significant and must be returned to the bloodstream.
This is accomplished by the lymphatic system, which is a system of vessels that returns fluid to the blood. The lymphatic system is often overlooked, but it's an essential part of the vascular system nonetheless. Mixed among the capillary beds is a network of tiny lymphatic capillaries, which collect fluid from the surrounding tissue. Once the fluid and any proteins it contains are collected into the lymphatic capillaries it is called lymph, or the fluid contained within the lymphatic system. The lymphatic capillaries converge into larger lymphatic vessels, which some people simply call lymphatics.
Since the lymphatics are not a part of the closed circulatory system, they cannot rely on the heart to propel fluid through the system. Instead, the lymphatic system relies on skeletal muscle and rhythmic contractions of the lymphatic vessel walls to squeeze the lymphatics and propel the lymph through the system. To ensure that lymph flows in the correct direction within the system, lymphatic vessels have one-way valves similar to those found in veins which prevent backflow. The lymphatics direct lymph toward the shoulders, converging into larger vessels along the way. They finally drain their lymph into the circulatory system in two locations near the shoulders.
Scattered throughout the lymphatic system are numerous enlargements of the lymphatics called lymph nodes . These are small bean-shaped organs that filter lymph and are packed with immune cells that monitor the lymph for any sign of infectious agents like bacteria and viruses, but we'll talk more about the immune system in another lesson.
So, to review, the human vascular system is the very extensive network of blood vessels that supplies blood to every cell of the body. The three main types of blood vessels in the vascular system are arteries (which are blood vessels that carry blood from the heart to other tissues of the body), veins (which are blood vessels that carry blood from various tissues to the heart) and capillaries (which are microscopic blood vessels with very thin walls that allow the transfer of nutrients and gases between the blood and tissues).
The blood flows through the vascular system in the following path: starting in the right side of the heart, the blood is pumped into the pulmonary arteries, which are the arteries that bring oxygen-poor blood to the lungs. This oxygen-poor blood then enters into the capillaries of the lung, where the blood releases carbon dioxide and absorbs oxygen. The now oxygen-rich blood enters the pulmonary veins, which are the veins that bring oxygen-rich blood from the lungs back to the heart. From the pulmonary veins, blood enters the left side of the heart and is pumped into the aorta, which is the main artery that leads directly out of the left side of the heart. The aorta then branches off into many smaller arteries that bring oxygenated blood to the various tissues of the body. The blood then flows into veins, which bring the now oxygen-poor blood back to the right side of the heart, and the cycle is ready to repeat itself.
Endothelial cells are very thin, flattened cells that line the inner wall of all blood vessels. When cells don't receive oxygen, they release VEGF and other growth factors that signal the need for new blood vessels. When the growth factors reach an existing blood vessel, angiogenesis occurs and a new blood vessel sprouts from the old one.
Although humans have a closed circulatory system, the capillaries are a little bit leaky and would lose way too much fluid if it wasn't recycled back into the circulatory system by the lymphatic system, which is a system of vessels that returns fluid to the blood. The fluid in the lymphatic system is called lymph, and it is filtered through several lymph nodes, which are small bean-shaped organs that filter lymph. Lymph nodes also contain immune cells that monitor the lymph for any sign of infectious agents like bacteria and viruses before it is returned to the circulatory system.
Well, that pretty much covers the vascular system.
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Back To CourseCLEP Biology: Study Guide & Test Prep
24 chapters | 224 lessons