Lesson Explainer: Blood Vessels Biology

In this explainer, we will learn how to describe the structures and functions of the major blood vessels in the human circulatory system.

If you laid them out end to end, there would be over 100‎ ‎000 km of blood vessels throughout the human body. They vary from as large as around 3 cm to as small as 5 μm (0.0005 cm) in diameter. The three types of blood vessels, veins, arteries, and capillaries, are the major structures of the circulatory system along with the heart, as shown in Figure 1. All 4–6 litres of blood in an adult human body is contained within this vast, closed network of branched and connected blood vessels, which carry materials to, from, and throughout every tissue and structure of the body.

Figure 1: A diagram showing the heart and the major blood vessels of the circulatory system.

Most blood vessels are made of several layers of different types of tissues. They are generally composed of an inner layer (endothelium) of epithelial (or lining) tissue, a middle layer of smooth muscle tissue, and an outer layer of connective tissue. The smooth muscle tissue can change the diameter of the blood vessels to adjust the pressure of the blood within them or the speed of the flow of blood through them. The space that the blood flows through is called the lumen. The general structure of a blood vessel is shown in Figure 2.

Figure 2: A diagram showing the general structure of a blood vessel. The inner space is called the lumen. The blood vessel walls are typically composed of three layers.

The three types of blood vessels, arteries, veins, and capillaries, have adaptations specific to their different functions.

Arteries are blood vessels that carry blood away from the heart. The heart is a muscular organ responsible for pumping blood throughout the circulatory system. The blood vessels that are leading directly away from the heart carry blood under especially high pressure and are adapted for that purpose. Arteries have thick, muscular walls and relatively small lumens. They also possess a membrane of elastic fibers, that helps them to stretch and withstand the pressure of the constant pumping by the heart without being damaged. Unlike veins, arteries do not possess valves.

Another feature of arteries is that they tend to be located deep within the body, farther from the surface of the skin. Since arteries carry blood under high pressure, damage to a major artery can lead to extremely rapid blood loss. For this reason, arteries tend to be located deeper within our bodies to decrease the risk of injury, blood loss, and death.

Key Term: Artery

An artery is a blood vessel that carries blood away from the heart.

Example 1: Relating the Structure and Function of Arteries

Why do arteries have thick, muscular walls containing elastic fibers?

  1. To prevent oxygen from being lost from the blood
  2. To prevent the backflow of blood
  3. To help them carry blood at low pressure
  4. To allow the transport of substances to other cells
  5. To help them carry blood at high pressure

Answer

Arteries are one of the three major types of blood vessels in the human body. Arteries carry out the function of carrying blood away from the heart toward the tissues of the body. Because arteries carry blood directly from the heart, they carry the blood under the highest pressure within the circulatory system. The circulatory system is a closed system, meaning that all of the blood in the body is contained within it. The blood is pushed through the blood vessels by the pumping action of the heart. The blood vessels that carry the blood directly away from the heart will be under the most pressure. The thick muscles of the walls of the arteries give them strength. The elastic fibers in some arteries allow them to stretch without being damaged by the pumping action of the heart.

Arteries have thick, muscular walls containing elastic fibers to help them carry blood at high pressure. The correct answer is option E.

Veins are blood vessels that carry blood toward the heart. The blood in the veins is under considerably less pressure than that in the arteries. The veins have thinner walls with less smooth muscle and relatively larger lumens. They are less elastic than arteries, as they contain less elastic fiber.

Veins also have valves within them. Valves are special structures that prevent blood from flowing backward. Since the blood in veins is traveling to the heart from the various tissues of the body, it is not subjected to the high pressure of blood pumping from the heart. For this reason, there is less pressure to keep it moving in the right direction. There are also valves within the heart itself that work to keep all the blood flowing in the right direction.

Another feature of most veins is that they tend to be located closer to the surface of the skin when compared with arteries, which are generally located deeper within the body. Since the blood within the veins is under considerably lower pressure than the blood within the arteries, damage to a vein would not cause severe bleeding like damage to an artery.

This difference between arteries and veins is outlined in Table 1 and in Figure 3.

Table 1: A table contrasting the structures and functions of arteries and veins.
ArteriesVeins
Carry blood away from the heartCarry blood toward the heart
Thicker wallsThinner walls
Smaller lumenLarger lumen
Higher blood pressureLower blood pressure
No valvesValves
Figure 3: A diagram illustrating the differences in structure between veins and arteries.

Key Term: Vein

A vein is a blood vessel that carries blood toward the heart.

Key Term: Valve

A valve is a structure that only allows fluid to pass through in one direction, preventing it from flowing backward.

Capillaries are microscopic blood vessels that form vast networks throughout the tissues of the body. More than 80% of the total length of blood vessels in the human body consists of capillaries. Laid out end to end, that it about 80‎ ‎000 km of capillaries!

We have mentioned that the circulatory system is a closed system. This means that, except in the case of illness or injury, blood itself does not leave the network of the blood vessels. Instead, the contents of the blood can be exchanged, with some substances leaving and some being absorbed, through the thin walls of the capillaries. This is why the capillaries are so important. They are the place where materials get into and out of the bloodstream.

Key Term: Capillary

A capillary is a small blood vessel that connects arteries to veins and forms networks around body tissues to carry out the exchange of gases and other materials.

The capillaries have very small lumens, only wide enough for red blood cells to pass through in a single file, one behind another. Capillaries have an average diameter of 7–10 micrometres—that is between 0.007 and 0.01 millimetres! Capillaries do not have smooth muscles associated with them. Instead, they are surrounded by just a single layer of epithelial tissue, as shown in Figure 4. These thin capillary walls are permeable and allow some materials to pass into and out of the bloodstream. This includes nutrients, wastes, gasses like oxygen and carbon dioxide, fluids, ions, and hormones. The blood flow through the capillary beds is controlled by structure called “the precapillary sphincters,” since the capillaries do not have a muscle layer of their own.

Figure 4: A diagram showing the structure of a capillary. Capillaries are the site of the exchange of materials between the blood and the other tissues of the body.

Capillaries exist in networks called capillary beds. Blood enters the capillary bed from a small artery (arteriole) and leaves through a small vein (venule), as shown in Figure 5.

Figure 5: A diagram showing the structure of a capillary bed. A capillary bed is a network of capillaries that receives blood from a small artery and empties it into a small vein. Capillary beds are found throughout the tissues of the body.

Example 2: Identifying the Type of Blood Vessel Based on Its Function

Which of the major blood vessels carries blood back into the heart?

  1. Veins
  2. Capillaries
  3. Arteries

Answer

There are three major types of blood vessels in the human circulatory system, and they each serve a different function. The three types of blood vessels are veins, capillaries, and arteries. The circulatory system is a closed system. This means that the blood never leaves the blood vessels and the heart. However, one of the main functions of the blood is to transport materials. The materials are able to enter and leave the blood through the capillaries, which have especially thin walls and are extremely numerous. Arteries carry blood to the capillaries, which means that they transport blood away from the heart. Veins have the opposite function, and they carry blood from the capillaries to the heart.

The blood vessels that carry blood back into the heart are veins. Therefore, the correct answer is option A.

Example 3: Relating the Structure and the Function of Capillaries

Why is it important for capillaries to have thin, permeable walls?

  1. To allow them to carry blood at a high pressure
  2. To increase the rate of blood flow
  3. To allow substances to diffuse into and out of cells
  4. To prevent the backflow of blood in the vessels
  5. To provide capillaries with the flexibility to move around cells

Answer

Capillaries are the site of the exchange of materials between the blood and the other tissues of the body. They are known for having permeable walls, which means that materials can easily pass through the walls of capillaries. Larger blood vessels like veins, which carry blood toward the heart, and arteries, which carry blood away from the heart, do not have permeable walls. For this reason, the function of blood, to carry materials to where they are needed, could not be completed without the microscopic capillaries that are present in almost every part of the body. Capillaries are extremely small blood vessels that exist in networks, called capillary beds, that receive blood from a small artery and empty it into a small vein. In the capillary beds, materials can diffuse or be transported into and out of the bloodstream. Unlike veins and arteries, capillaries have walls that are only one layer of thin epithelial cells in thickness. This is what allows materials to move into and out of the blood in the capillary beds.

So, this means that the correct answer is option C, to allow substances to diffuse into and out of cells.

Now that we understand the structures of the different blood vessels, let’s relate this information to the functions of blood vessels within the larger circulatory system.

A flow chart illustrating the flow of blood through the circulatory system is provided in Figure 6 below.

Figure 6: A diagram illustrating the flow of blood through the pulmonary circuit and the systemic circuit of the respiratory system.

Figure 6 shows that blood flows from the heart to the lungs and back again. This is called the pulmonary circuit. Pulmonary is a word that means “related to the lungs.” The Arab physician Ibn al-Nafis in the 13th century was the first known person to suggest that blood could not travel from one side of the heart to the other and instead flows through the pulmonary circuit to do so.

After its return to the heart from the lungs, the blood flows from the heart to the tissues of the body, and back again. This is called the systemic circuit since it circulates materials through the various organ systems of the body. The English physician William Harvey was the first known person to detail the systemic circulatory system in the 1620s, writing a clear account of how the heart pumped blood around the body in a complete circuit! Interestingly, he also discovered that the valves in the veins control the flow of blood when he tightly tied a ligature around a person’s arm and observed small “bumps” that would only allow the blood to be pushed in one direction.

Let’s imagine the path of an individual red blood cell leaving the heart for the pulmonary circuit. The cell will be pumped into a pulmonary artery and then through smaller and smaller branching arteries until it reaches the capillary beds within the lungs, as shown in Figure 7. In the capillaries in the lungs, oxygen will diffuse into the bloodstream through the capillaries’ thin walls. The red blood cell will pick up this oxygen and then be pumped back through larger and larger connecting veins until it reaches the pulmonary veins and reenters the heart. We refer to this blood, which is now rich in oxygen, as “oxygenated.”

Definition: Oxygenated Blood

Oxygenated blood refers to blood that has a high concentration of oxygen.

Figure 7: A diagram showing blood flowing through a capillary in the lung. The blood becomes oxygenated by absorbing oxygen from the air we breathe, which diffuses through the thin walls of the capillaries.

Next, the same blood cell, now oxygenated, will begin its systemic circuit. It will be pumped through the aorta, which is the body’s largest artery. It will pass through smaller and smaller branching arteries until it reaches the capillary beds of whichever part of the body it encounters first. Here, the oxygen within the red blood cell will diffuse out, through the thin walls of the capillaries, and into the tissues of the body where the cells can use it to carry out cellular respiration. Since the oxygen has been removed from the cell, it is now called “deoxygenated.” The deoxygenated blood cell will be pumped through larger and larger connecting veins, past several valves along the way, until it reaches the vena cava. The venae cavae are the largest veins in the human body, and they return the deoxygenated blood back to the heart.

Key Term: Deoxygenated Blood

Deoxygenated blood refers to blood that has a low concentration of oxygen.

Example 4: Describing the Function of the Lungs in the Circulatory System

What happens to the blood when it is taken to the lungs?

  1. It becomes oxygenated.
  2. It becomes deoxygenated.
  3. It absorbs glucose.
  4. It releases glucose.

Answer

Blood in the human circulatory system is pumped by each side of the heart on two separate circuits. The pulmonary circuit carries blood from the heart, to the lungs, and back to the heart. The systemic circuit carries blood from the heart, to the body, and back to the heart.

The primary function of our blood is to carry materials from place to place. One of the most important of these materials is oxygen. Oxygen is used by the cells of our body in a process called cellular respiration. Cellular respiration generates the cellular energy that keeps our cells alive and functioning. In order to carry out cellular respiration, the cells of our bodies need a constant supply of oxygen, and that oxygen is transported to those cells by the blood in the systemic circuit of the circulatory system. Since the blood in the systemic circuit carries oxygen that it drops off to be used by cells, the blood is said to become “deoxygenated.” In the pulmonary circuit, the deoxygenated blood absorbs oxygen that we breathe in from the air. The blood will then carry this oxygen to the rest of the body on its next trip through the circulatory system.

This means that when blood is taken to the lungs, it becomes oxygenated. Therefore, option A is the correct answer.

As you can see, most arteries carry oxygenated blood to the tissues of the body, but the pulmonary artery will carry deoxygenated blood from the heart to the lungs in the pulmonary circuit. In the same way, most veins carry deoxygenated blood from the tissues of the body to the heart, but the pulmonary vein will carry oxygenated blood from the lungs to the heart. While it may be true that arteries generally carry oxygenated blood, and veins generally carry deoxygenated blood, the exact opposite is true in the pulmonary circuit of the circulatory system.

It takes a single red blood cell, on average, just one minute to complete both the systemic and the pulmonary circuits of the circulatory system. And, since the average red blood cell lives for about 115 days, it will likely make this journey over 165‎ ‎000 times, becoming oxygenated and deoxygenated and traveling from the heart, to the lungs, to the heart, to the body, and back again.

Figure 8: Blood pressure decreases as distance from the heart increases. The speed of blood flow is lowest in the capillaries; this allows an efficient exchange of materials.

We have mentioned the pressure and the speed of blood within the circulatory system a couple of times already. You may be aware that fluids move from areas of high pressure to areas of low pressure. The heart contracts to push blood through the circulatory system. Therefore, blood pressure is highest in the blood vessels leading away from the heart. We can see in the graph in Figure 8 that the blue line for blood pressure is very high in the arteries and quite low in the veins.

We have also learned that even though capillaries are the smallest, they are by far the most numerous blood vessels in the circulatory system. The orange line showing the total area in the graph reflects this information. We can also notice that the velocity, or speed, of blood flow is the lowest in the capillaries. Since the capillaries are where materials are exchanged between the blood and the other tissues of the body, it makes sense that the blood would move the slowest in these areas to allow this important exchange to take place.

The circulatory system consists of the heart, veins, arteries, and capillaries. The structures of the different types of blood vessels are adapted to their function within the circulatory system.

Let’s review what we have learned about blood vessels in this explainer.

Key Points

  • The major types of blood vessels are capillaries, veins, and arteries.
  • Each type of blood vessel is adapted to its function within the circulatory system.
  • Arteries are large, muscular blood vessels with thick walls that often contain elastic fibers to help them carry blood at high pressure away from the heart.
  • Veins are blood vessels with large lumens that carry blood toward the heart and will contain valves to encourage the one-way flow of blood at low pressure.
  • Capillaries are networks of tiny blood vessels with walls only one cell thick that allow the exchange of materials between the blood and the other tissues of the body.

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