Lesson Video: The Human Respiratory System Biology

In this video, we will learn how to describe the structure of the lungs, and explain how they are adapted for efficient gas exchange.

09:33

Video Transcript

In this video, we will learn to describe the structure of the lungs. We’ll also learn to explain how they’re adapted for the mechanics of breathing and for efficient gas exchange. So, let’s take a deep breath in together and get started.

The lungs are two soft, spongy structures located within the chest to either side of the heart. The lungs are very delicate, and they can’t move on their own. They’re protected and supported by the ribs, the intercostal muscles, which are the muscles between the ribs, and the diaphragm, a dome of muscles stretched against the bottom surface of your lungs, just above your liver and stomach.

Also important are the passageways that connect our lungs to the outside atmosphere. This includes the pharynx, which is the common passage in your body for food and for air, the trachea or windpipe, which is the rigid tube you can feel in your neck. The trachea branches off into a left and a right bronchus, and each bronchi branches further into smaller and smaller bronchioles. These passageways are lined with mucus, which traps fine particles that might damage or infect the lungs.

Some other organs related to the respiratory system include the epiglottis, which is a flap of tissue that closes the trachea when you swallow. This prevents your food from entering your lungs. We also have the larynx or voice box, which vibrates, allowing you to speak. And nestled inside of the bones of your skull, we have several sinuses, which help to warm and moisten the air we breathe before it enters our lungs.

Our lungs are the primary organs of our respiratory system, which is the organ system responsible for the exchange of gases between our bodies and the air around us. In this video, we’ll be referring to the mix of gases that you find filling the space around you as well as inside of your lungs, as air. Even though we’ll be talking mostly about oxygen and carbon dioxide, it’s important to note here that air is composed of more than 75 percent nitrogen gas. It contains about 20 percent oxygen and just a tiny amount of carbon dioxide. And these concentrations play a role in how gases get into and out of our bloodstream, which we’ll look at more closely later.

The lungs are the primary organ of gas exchange in the human body. This means that there how we get oxygen in and carbon dioxide out of our bloodstream. Recall that the cells of your body need oxygen in order to carry out cellular respiration, which generates the cellular energy that we need to power all of our life functions. We take that oxygen in through our lungs when we inhale. One of the byproducts of cellular respiration is excess carbon dioxide, which is removed from our bodies when we exhale.

But we’ve already stated that the lungs can’t move on their own. So, how do we inhale and exhale at all? Well, that is where the diaphragm and the intercostal muscles come into play. Let’s start by recalling that fluids, such as gases, flow from areas of high pressure to areas of low pressure. When we inhale, our diaphragm contracts pulling it downwards. And our intercostal muscles also contract, pulling the ribs upwards and outwards. This causes the space within our lungs to expand, lowering the pressure, and air from our surroundings flows in.

When we exhale, our diaphragm relaxes, stretching upwards again. The intercostal muscles relax and our ribs move downwards and inwards. The volume or space in our lungs become smaller, which increases the pressure, and this pushes air out of our bodies. This action happens reflexively, without conscious thought. But if you start to think about it, you can control your breathing voluntarily. Your breathing rate or the number of breaths you take per minute also adjust automatically based on your level of activity. When you’re more active and your tissues need more oxygen, the number of breaths you take per minute will increase. And when you’re relaxing and using less oxygen, your breathing rate will naturally decrease again.

Well, we’ve learned how the lungs are able to move air into and out of our bodies. But how does this lead to gas exchange with our bloodstream? To answer that question, we’ll need to take a closer look at our lung tissue. So, we’ll clear away most of this diagram. As we described a little bit earlier, within the lungs, there are many branched vessels called bronchioles, and they carry air in and out of the lung tissue. At the end of these branched vessels are systems of thin-walled sacs that look something like a pile of soap bubbles or a bunch of tiny grapes. These are called alveoli, and they’re the site of gas exchange.

Here, we have a detailed diagram of one alveolus surrounded by one capillary. We’ve enlarged all the parts so that we can see how this gas exchange works, and it’s more simple than you may think. You may recall that molecules tend to move from areas of high concentration to areas of low concentration in a process known as diffusion. When blood reaches the lungs through the pulmonary arteries, the oxygen concentration is very low because the oxygen molecules have already been delivered to the tissues of the body that need them to generate cellular energy. Relatedly, the concentration of carbon dioxide is relatively high. And I’m gonna go ahead and represent this information graphically so we can visualize it even more clearly.

And we’ve learned that the air we breathe contains quite a bit of oxygen and contains very, very little carbon dioxide. So, the oxygen from the air will diffuse into our blood, where it’s carried by the red blood cells out of the lungs by the pulmonary veins and pumped by the heart to the tissues of the body. At the same time, the carbon dioxide from our blood diffuses into the air inside of the lungs. And the air with this excess carbon dioxide is removed from your body when you exhale.

Here, at the alveoli, the deoxygenated blood becomes oxygenated again, and the excess carbon dioxide is removed from our bloodstream. There are some special adaptations of the alveoli that allow this gas exchange of oxygen and carbon dioxide to occur continuously and efficiently. The first is that the walls of the alveoli, like the walls of the capillary, are just one thin squamous epithelial cell thick. This decreases the distance that gases need to diffuse across, which allows diffusion to happen more quickly. The second is that the surfaces of the alveoli are moist. Gases can only diffuse across cell membranes when they’re dissolved in liquid.

The last adaptation we’ll discuss is the surprisingly massive surface area within our lungs. The shape of the alveoli allows a large surface for gas exchange within the limited volume of our lung tissue. It’s sort of the same as how we can fit 100 folded paper tissues into a small box. If we took all of those tissues out of the box, unfolded them, and then spread them out, they would cover a pretty large surface area.

In the same way, if you were to spread out the surface of all the alveoli within our lungs, they would cover over 800 square feet or 75 square meters, which is half the size of a tennis court or the floor plan of a one- bedroom apartment. All of the alveoli are almost completely surrounded by capillary beds. And this large surface area means that we’re able to exchange a large amount of carbon dioxide and oxygen between our blood and the air with each breath.

Now that we’ve covered the basic structure of the lungs, the mechanics of breathing, and the adaptations that facilitate gas exchange, let’s take a moment to review what we’ve learned. In this video, we learned about the general structure of the lung. We learned about the alveoli, their structural adaptations, and their function. We also learned about the mechanics of gas exchange, how we inhale and exhale due to the relative pressure in our lungs as well as how we exchange carbon dioxide and oxygen with the air due to the relative concentration in our blood.

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