Diffusion & Surface Area

We have learned that several body systems work together in order to provide all of your cells with the nutrients they needs, provide them with the oxygen they need, and carry away and dispose of the waste products they create.

The Role of the Circulatory System

In some ways this process is simple. Nutrients, oxygen, and waste products are moved from place to place via your circulatory system. Your heart pumps blood to every cell in your body through a series of arteries, veins, and capillaries. Blood is used to carry nutrients and oxygen to cells and to carry waste products away from cells.

If you look at a typical illustration of the circulatory system, you will notice that some blood vessels are colored red and others are colored blue. The blood vessels that are colored red are arteries and the blood vessels that are colored blue are veins. It is unfortunate that blue is used, because this might be one of the reasons that some people believe that the blood in veins is blue. The blood in veins is red. It is not the same bright red as the blood in arteries, but it is still red.

Most people believe that arteries carry oxygenated blood (blood that contains oxygen) and veins carry deoxygenated blood (blood that has no oxygen). This is not always true. The artery between the heart and the lungs, called the pulmonary artery, carries deoxygenated blood, and the vein between the lungs and the heart, called the pulmonary vein, carries oxygenated blood. The best way to remember the difference between arteries and veins is that arteries are always carrying blood away from the heart and veins are always carrying blood back to the heart.

Blood must make a round trip from and to the heart. That means that there must be a physical connection between the system of arteries and the system of veins. Because blood needs to reach every cell in your body, some of the blood vessels that carry blood must be very tiny -- microscopic, in fact. These tiniest of blood vessels are called capillaries. It is in the capillaries that the connection between the system of arteries and the system of veins is made.

Capillaries are incredibly small. The average capillary is just 3-4 µm (micrometer.) A micrometer is one millionth of a meter. If you look at a meter stick or ruler that includes millimeters, then you can see how small a millimeter is. Now imaging dividing the space between the millimeter markings 1,000 times. That is how large a micrometer is. A typical red blood cell has a diameter of 6 µm. So in order to squeeze through a capillary, a red blood cell has to change its shape to fit.

You might think that you need to have a lot of blood vessels in order to reach every cell in your body. You're right! If you were to take all of the arteries, veins, and capillaries found in the average adult human and you could place them all end to end in a single line, you would have 60,000 miles of blood vessels. That's enough to go completely around the Earth twice and still have 10,000 miles of blood vessels left over!

The Role of the Respiratory System

Now that we have a way to transport blood to every cell in the body, how do we get oxygen to our cells? With our lungs! It is in the lungs that oxygen is transferred to blood so that it can be carried to our cells. And, it is in the lungs that carbon dioxide carried away from our cells is "dropped off" by blood.

The Role of the Digestive System

Our cells rely on the foods we eat to get the nutrients they need. Obviously, a cell can't eat a carrot. Instead, we eat the carrot. The nutrients that are in the carrot need to be removed, separated, broken down before they can be delivered to cells. This process is called digestion and, not surprisingly, happens in the digestive system. Some digestion happens in your mouth, some happens in your stomach, and some happens in your intestines, but it is in the small intestine that the nutrients from the carrot are transferred to blood.

The Role of the Excretory System

As you might expect, blood picks up a lot of different waste products as it travels around the body. It needs a place to dispose of these waste products so they can be removed from the body. Luckily, we have specialized organs that are designed for doing this. Some waste products are removed from blood by the kidneys. Some are removed from blood by the liver. Eventually, these waste products reach the bladder (where they are disposed of through urination) or the large intestine (where they are disposed of through defecation.) Unfortunately, there are some substances that are not excreted and stay in the body. If you look at the label of a can of tuna fish, you might see a warning advising pregnant women to limit how much tuna they eat. This is because tuna may contain mercury. Mercury is an example of a toxic substance that is often stored in the body instead of being excreted.

It's All About Diffusion

The actual process by which substances are transferred into and out of the blood is called diffusion. Diffusion is defined as the movement of particles from areas of high concentration to areas of low concentration. You see real life examples of diffusion all the time.

Diffusion can even occur through a barrier. In class, I used the example of a helium balloon that loses helium overnight. Even if the balloon is tightly sealed, the helium is able to diffuse into the air surrounding the balloon. Many membranes, like the rubber membrane of a balloon, actually have tiny holes in them that are large enough for certain substances to leak through.

It's a good thing that diffusion can happen even through a barrier, because cells are surrounded by a cell membrane. The cell membrane prevents the cytoplasm and organelles inside the cell from leaking out, but it allows oxygen, nutrients, and waste products to pass through the cell membrane. These substances must also be able to move through the walls of capillaries. In the small intestine, they must be able to pass through the walls of the villi, and in the lungs they must be able to pass through the walls of the alveoli.

Diffusion occurs naturally and does not require any energy. This is important, because it means that cells can obtain many of the substances they need without using any energy.

Where Diffusion Happens - Oxygen & Carbon Dioxide

The place where oxygen diffuses into blood for delivery to cells, and where carbon dioxide produces by cells diffuses out of blood, is in the lungs. When you inhale, air containing oxygen is pulled into your lungs. It begins in the trachea, which then branches into to bronchi. Each bronchus descends into a lung, where it eventually branches into smaller and smaller airways. As these airways get smaller, they become bronchioles. Eventually, each bronchiole terminates (ends) in an alveolus.

Alveoli are tiny air sacs that fill with oxygen-filled air each time you inhale. Each alveolus is surrounded by capillaries. The walls of each alveolus are just a single cell thick. When the oxygen concentration in the alveolus is higher than the oxygen concentration in the capillaries, oxygen diffuses through the wall of the alveolus, through the wall of the capillary, and forms a chemical bond with the hemoglobin inside a red blood cell. At the same time, there is blood in the capillary that is filled with carbon dioxide. Because the concentration of carbon dioxide in the blood is higher than the concentration of carbon dioxide in the alveolus, the carbon dioxide diffuses through the capillary wall and into the alveolus. Then, when you exhale, the air containing the carbon dioxide is pushed out of the alveoli.

Where Diffusion Happens - Nutrients

The place where nutrients diffuse into blood for delivery to cells is in the small intestine. The digestion of food that began in your mouth comes to an end in the small intestine. By this time, most of the nutrients that were present in the food have been removed and broken down and are ready to be used by cells.

The inside of the small intestine is covered with microscopic, finger-like projections called villi. Each villus contains capillaries. As nutrients wash over the villus, they diffuse into the blood by passing through the wall of the villus, through the wall of the capillary, and into the blood where they are carried away to cells. The villi are designed to allow for very close contact between the nutrients and the capillaries. They also help to ensure that the concentration of nutrients is very high, making the process of diffusion faster and easier. Finally, the wall of the villus is just a single cell thick, enhancing the ability of nutrients to diffuse through the wall and into the blood.

Increasing Surface Area

The transfer of oxygen, nutrients, and waste products can only take place at a microscopic level, where the capillaries are. That means that only very small amounts of these substances can be moved at one time. Given the size of our bodies, how can we possibly get enough oxygen or nutrients to stay alive?

The transfer of nutrients and oxygen can only happen when the nutrients and/or oxygen come into close contact with a capillary. Let's use the small intestine as an example. The small intestine is about twenty feet long and about an inch in diameter. Lets assume that the entire interior of the small intestine is covered in capillaries. What would be the total area that could be covered with capillaries?

We can calculate the surface area of the small intestine by multiplying the length by the circumference. The calculation looks like this:

Length = 20 feet x 12 inches/foot = 240 inches

Circumference = πd = 3.14 x 1 = 3.14 inches

Surface area = length x circumference = 240 inches x 3.14 inches = 754 inches² = 5.25 feet²

Even if we could completely cover 5.25 feet² with capillaries, it would not be enough to absorb the nutrients we need to stay alive. The only way to increase the absorption of nutrients is to increase the surface area of the small intestine. And, in fact, the actual surface area of the small intestine is larger than 5.25 feet². Much larger. The surface area of the small intestine is 250 m² or 2,700 feet².

This huge increase in surface area, without increasing the size of the small intestine, is made possible by villi. Villi are the tiny finger-like projections that cover the surface of the inside of the small intestine. You can see how this increase in surface area works by doing a little demonstration with your own hand. Using a piece of string, trace around the outside of your hand starting and ending at the wrist. Then, measure how much string you used. I used about 18 inches. Now, do it again, but this time run the string up and down the space between your fingers, as if they were villi. How much string did you use that time? I used about 40 inches.

Just as the villi increase the surface area in the small intestine, the alveoli do it in the lungs. Your lungs are small enough so that two of them fit inside your chest. But, thanks to the alveoli, the surface area of your lungs available for the exchange of oxygen and carbon dioxide is far greater than what you would expect given the size of your lungs. Your lungs have a surface area of 250m². If that number looks familiar to you, it's because you've seen it before. You've seen it here. You saw it just a few paragraphs above, because the lungs have roughly the same surface area as the small intestine.

What You Need for the Test

1. All of the information contained here in this section of the Online Textbook.
2. The notes you took from my lecture in class.
3. The information contained in the Powerpoint from class.
4. The information contained in the video we watched in class.
5. The math skills to calculate surface area and volume of cubes and spheres if you are given the formulae.
6. You can use the practice test in Schoology to identify areas and topics that you need additional study in.

Here is a link to the Powerpoint.

Here is a link to the video.