The Circulatory System

The circulatory system consists of the heart, the arteries, the veins, the capillaries, and the blood. These different components work together so that the circulatory system can do the work it must do.

The most important function of the circulatory system is to bring nutrients and oxygen to cells and to carry waste products away from cells. The circulatory system gets oxygen from and returns carbon dioxide to the respiratory system. It gets nutrients and returns waste products to the digestive system.

Before we examine how the circulatory system performs these important functions, let's have a look at the parts of the circulatory system.

Heart

The heart is a muscle about the size of your fist. It is really an incredible machine. It beats more than once per second beginning before you are born and doesn't stop beating until... well... until you are dead!

The heart is divided into four chambers. There are two chambers at the top of the heart and two at the bottom. The chambers at the top of the heart are the atria (the singular form of atria is atrium) and the chambers at the bottom of the heart are the ventricles.

We'll be coming back to the heart in a few minutes, but first let's look at the other components of the circulatory system.

Arteries

Arteries are blood vessels that carry oxygenated blood (blood filled with oxygen) from the heart to all parts of your body. There is one exception to this, and it is called the pulmonary artery. The pulmonary artery is the main artery that connects the heart to the lungs. Unlike other arteries, the pulmonary artery carries deoxygenated blood (blood that has no oxygen left in it). As arteries branch out from your heart, they get smaller and smaller, much the the trunk of a tree becomes large branches, which become smaller branches, which eventually become twigs.

Veins

Veins are blood vessels that carry deoxygenated blood back to the heart. Again, there is one exception. The pulmonary vein carries oxygenated blood from the lungs to the heart. Veins are the blood vessels that you can see. They may look blue to you, but the blood inside of them is not blue. Deoxygenated blood is not bright, bright red the way that oxygenated blood is, but it certainly isn't blue! Some people describe the color as dark red or brown. So why are veins blue? It has to do with the way that light works. When light goes through your skin down toward your veins, certain colors are absorbed. Blue is not absorbed. So only blue light can reflect off of your veins and back to your eyes. Don't worry if you don't understand this. You won't have to explain why veins are blue on the test. You just need to know that blood is red, not blue. You'll learn a lot more about light in second semester, and then you'll understand why veins are blue!

Capillaries

Capillaries are very tiny blood vessels. Remember that the job of the circulatory system is to carry oxygen and nutrients to every cell in your body. That means that there need to be blood vessels that are small enough to reach every cell. Cells are tiny, and so are capillaries. Capillaries have such a small diameter that only a single red blood cell at a time can travel through a capillary. Capillaries are also what connect arteries to veins.

If you could take all of your arteries, veins, and capillaries and string them together, they would be long enough to go around the Earth two times. Now think about your heart again for a moment. Imagine how much work it must do to push blood through that much "pipe"! Are you beginning to understand why you need to avoid foods and behaviors that might harm your heart?

Blood

The circulatory system gets its name because it causes something to circulate, or move around, the body. The thing that circulates is blood. There are four major components to blood. They are:

  • Red blood cells
  • White blood cells
  • Platelets
  • Plasma

Let's take a look at each of these components of blood separately.

Red blood cells

Red blood cells are small, donut shaped structures that are filled with hemoglobin. Hemoglobin is a special protein that acts as a magnet for oxygen. When oxygen comes into contact with hemoglobin, it binds (attaches) to the hemoglobin. The oxygen is held by the hemoglobin until it reaches its destination.

Red blood cells are manufactured in bone marrow. It takes about a week before red blood cells are ready to be released into the bloodstream. Because you need so many red blood cells (they make up about 45% of your blood), you are manufacturing them all the time.

The life of a red blood cell is short and hard. They get pushed out of the heart under high pressure. They have to squeeze through tiny capillaries. And, unlike just about every other type of cell, red blood cells have no nucleus. Because they have no nucleus, they have no DNA. Because they have no DNA, they cannot make proteins. Because they cannot make proteins, they have no way of repairing themselves if they are damaged. Because of this, the average red blood cell only lives for about 4 months.

Red blood cells are shaped a lot like donuts. Instead of a hole in the center, though, they have a depression. You can get a good idea of their shape from the picture below.

White Blood Cells

White blood cells protect you against disease. When they see something in your blood that doesn't belong there, like a bacterium, they surround the bacterium by changing their shape until the bacterium is inside of them. Then they kill it. Just in case you didn't know, bacterium is the singular form of the word bacteria.

White blood cells are made in bone marrow. There are not nearly as many white blood cells as there are red blood cells. In fact, there is only about 1 white blood cell for every 7,000 red blood cells.

In the picture below, you can see two white blood cells among a large number of red blood cells. You'll notice that the white blood cells aren't actually white. In this picture, they are purple. In real life, they aren't white or purple. In real life, white blood cells are transparent. They are purple in this picture because they have been stained with a purple dye. In order to get a clear view of cells, it is often necessary to stain them before putting them under the microscope. The dark purple portion inside of the white blood cell in this picture is the nucleus.

Platelets

Platelets are small particles in blood that promote clotting. Clotting is when blood sticks together and gets thicker. If blood did not clot, you would keep bleeding every time you got even a small cut. When blood clots, it "seals" the vein that it is leaking out of.

The large round objects in the picture below are red blood cells. The arrows are pointing to platelets. As you can see, platelets are small and they don't have much of a shape to them.

Plasma

Plasma is the liquid portion of blood. About 55% of the total volume of blood is made up of plasma. Plasma is made mostly of water with a few proteins. Plasma carries nutrients to cells and carries away their waste products.

Now that we've had a look at all of the components contained in the circulatory system, it's time to take a closer look at the heart, because it's the heart that pushes the blood all around the body.

Anatomy of the Heart

There are four chambers in the heart. They are called the right atrium, the right ventricle, the left atrium, and the left ventricle. Both the right and left atrium are located at the top of the heart.

Here is a drawing of the heart. Come back to this drawing to locate the structures whose names and functions and discussed below.

Even though all blood is red, we often use the color red to show oxygenated blood and the color blue to show deoxygenated blood. The illustration shows the direction of blood flow. Since we need to start somewhere, let's start in the right atrium.

Blood enters the right atrium through a large vein called the vena cava. The illustration shows the superior vena cava (which delivers blood from the upper body) and the inferior vena cava (which delivers blood from the lower body). You do not need to remember these names for the test. What is important for you to remember and understand is that the blood that comes into the right atrium is delivered by a large vein and contains the blood that has already traveled around the body. That means that it has very little oxygen in it and a lot of carbon dioxide.

When the right atrium fills with blood, it contracts and pushes the blood through the tricuspid valve and into the right ventricle. The tricuspid valve only allows blood to flow in one direction.

When the right ventricle fills with blood, it contracts and pushes the blood through the pulmonary valve into the pulmonary artery that leads to the lungs. There are two important things to remember from this paragraph. First, you need to remember that there is a valve between the right ventricle and the pulmonary artery. This valve prevents blood from flowing back into the heart from the pulmonary artery, because it only allows the blood to go through in one direction. Second, you need to remember that the pulmonary artery is the only artery in the body that carries deoxygenated blood. Normally, arteries are filled with oxygenated blood.

Blood then leaves the lungs and returns to the left atrium of the heart through the pulmonary vein. The blood flowing through the pulmonary vein is filled with oxygen. Normally, veins carry blood that have no oxygen left in them. The pulmonary vein is the only exception. It is the only vein in the body that carries oxygenated blood.

When the left atrium fills with blood, it contracts and pushes blood through the mitral valve into the left ventricle. The mitral valve allows blood to move in one direction only. Then, the left ventricle contracts, pushing blood through the aortic valve (another one-way valve) into the aorta. The aorta is the largest artery in the body. You can think of the aorta as the trunk of a tree. Many arteries branch from the aorta, just like their are branches that extend from the trunk of a tree. Just as the branches on a tree get smaller and smaller, so do the arteries.

The arteries that extend from the aorta reach every part of the body because they must deliver the oxygen-filled blood to every single cell in the body. In order to reach every cell, the diameter of the arteries must become very, very small. When they are at their smallest, they are called capillaries. Capillaries are tiny enough to fit in between cells. They are so narrow that only a single red blood cell at a time can squeeze through them. One end of each capillary is connected to the arterial system, meaning the arteries that started at the aorta. The other end of each capillary is connected to the venous system, meaning the veins that eventually go back to the right atrium. As the red blood cells move through the capillary, they release their oxygen to the cells they move past. The plasma that carries the red blood cells through the capillary releases the nutrients it is carrying into those same cells. The cells, meanwhile, release their carbon dioxide and other waste products into the plasma Those cells, meanwhile, release their carbon dioxide into the plasma. Now that the blood has released all of its oxygen and nutrients and is filled with waste products, it begins its journey back to the heart through the veins. At the end of its journey, it will enter the heart at the right atrium.

That's it! We began our trip through the circulatory system at the right atrium and we are back where we started from!

So, how do red blood cells pick up oxygen and transfer that oxygen to cells? How does plasma pick up nutrients and transfer the nutrients to cells? How does the plasma pick up carbon dioxide from cells and where does it get rid of the carbon dioxide? Those are the questions that we will answer in the following paragraphs.

Do you remember the story I told in class about someone cutting an onion in the back of the room and how the smell of the onion would eventually fill the whole room? Do you remember the demonstration with the back of cornstarch and water that was suspended in a beaker than contained iodine? The reason that the aroma of the onion filled the room was something called diffusion. The reason that the iodine moved from the beaker into the bag of cornstarch was something called diffusion. Diffusion is the movement of particles from areas of high concentration to areas of low concentration. High concentration means that there is a lot of something. Low concentration means that there is very little or none of something. In the onion example, there was a lot of onion smell in the back of the room where the onion was being chopped up, but none in the front of the room. Because of diffusion, the particles of onion smell moved from the back of the room (the area of high concentration) to the front of the room (the area of low concentration). This movement of particles continued until the concentration of onion smell was equal in all parts of the room. Oxygen moves from the red blood cells in the capillaries to cells using diffusion. Carbon dioxide and other waste products move into the plasma using diffusion. Nutrients move out of the plasma into cells using diffusion.

To do well on your test, you will need to focus on oxygen and carbon dioxide. In order to have a really good understanding of the process, you need to know a little bit about how the lungs work.

Inside your lungs are millions of tiny air sacs (bags) called alveoli. Alveoli is the plural form of the word alveolus. So if you are talking about just one air sac, you would refer to it as an alveolus. If you are talking about more than one, you would refer to them as alveoli. The picture below shows what alveoli look like.

First, you should notice that the alveoli are attached to tubes. These tubes are very, very small in diameter. They are what connect the alveoli to your nose and mouth. In other words, these tubes let air get from your nose and mouth to your alveoli. The second thing you should notice is that alveoli are in a cluster, almost like a bunch of grapes. This is a little bit easier to see if you look at the cluster of alveoli at the bottom. Here, the artist has cut off a chunk so that you can see inside. Finally, you should notice that the alveoli are covered with capillaries. The red capillaries are filled with oxygenated blood (they are arterial capillaries) and the blue capillaries are filled with deoxygenated blood (the are venous capillaries).

Now, let's look at a second picture that focuses on the movement of blood, oxygen, and carbon dioxide.

In this illustration, you are looking at the blood that is contained in a single capillary on the surface of a single alveolus. The artist has drawn the red blood cells that are flowing through the capillary. You can see that the red blood cells slowly change color from blue to red as they move through the capillary. Please remember that these colors are not real! Red blood cells never turn blue. The artist is just sticking with the standard way of depicting blood: blue for deoxygenated and red for oxygenated. As the red blood cells move past the alveolus, oxygen that is inside the alveolus diffuses through the wall of the alveolus, through the wall of the capillary, and into the red blood cells. At the same time, carbon dioxide that is in the plasma in the capillary diffuses through the capillary wall, through the wall of the alveolus, and into the alveolus. When you exhale (breathe out), that carbon dioxide comes out of your mouth and nose. When you inhale, oxygen from the air you inhale fills the alveolus. If there is more oxygen inside the alveolus than there is in the blood in the capillary, then it will diffuse from the alveolus (the area of high concentration) to the blood (the area of low concentration). If there is more carbon dioxide in the blood than there is in the alveolus, then it will diffuse from the plasma (the area of high concentration) to the alveolus (the area of low concentration). This process is occurring all the time, and it is occurring simultaneously. That means that at the same time that oxygen is diffusing from the alveoli into the blood, carbon dioxide is diffusing from the blood into the alveoli.

You already know that blood reaches every cell in the body thanks to your capillaries. When you blood reaches a cell, the oxygen that is held by the red blood cells diffuses into the cell. At the same time, the nutrients that are in the plasma diffuse into the cell. While this is happening, carbon dioxide and other waste products that are contained inside of the cell diffuse into the blood.

What You Need to Know for the Test

For your test, you will need to write an explanation of how the circulatory system works. First, you will need to write a paragraph in which you describe the components found in blood and what each component does. Then, you will need to write a paragraph in which you briefly describe the anatomy of the heart. Finally, you will need to describe the flow of blood through the circulatory system. You will do this by starting in any chamber of the heart and following the flow of blood everywhere it goes until you end up back in the same chamber you started in. You will need to explain what is happening along the way. For example, if red blood cells are picking up oxygen, you will need to explain where that is happening and how it is happening. If the plasma is getting rid of carbon dioxide, you will have to explain where that is happening and how it is happening. You will need to explain how nutrients move from blood into cells, and how waste products move from cells into blood. You will not need to explain where the waste products are taken (except for carbon dioxide) or where the nutrients come from, because you haven't learned that yet. These are the words and terms that you will need to include and use properly in your essay:

  • red blood cells
  • white blood cells
  • platelets
  • plasma
  • oxygen
  • carbon dioxide
  • nutrients
  • waste products
  • bacteria
  • pulmonary artery
  • pulmonary vein
  • aorta
  • arteries
  • veins
  • capillaries
  • right atrium
  • left atrium
  • right ventricle
  • left ventricle
  • mitral valve
  • tricuspid valve
  • pulmonary valve
  • aortic valve
  • lungs
  • diffusion
  • high concentration
  • low concentration

You should have no problem with the test if you know the information contained in the PowerPoint, can remember the important parts of the Bill Nye video, and have an understanding of the topics covered in this section of the Online Textbook. I do not expect your answer to be nearly as detailed as the information I have presented here. You answer only needs to include enough detail so that I can tell that you know the parts of the circulatory system, can explain the different components in blood, and can describe how diffusion allows the circulatory system to deliver oxygen and nutrients to cells while removing waste products.

Click here to view the Bill Nye Circulatory System video that we watched in class.

Click on the link below to view the PowerPoint from class.