Did you know that the average human lung has a respiratory surface area that is roughly the same size as half of a tennis court? Believe it or not, that’s how much surface area an active, healthy human needs to ensure that the body gets plenty of oxygen.
Why Doesn’t the Skin Absorb Oxygen?
Let’s talk about the human respiratory system and why we need it. I mean, we’re all surrounded by the air, which is about 20% oxygen, so why don’t we just absorb it through our skin? The problem is, if we just absorbed oxygen through our skin, it might reach the first two, maybe three layers of cells, before it was all used up, and it would never make it all the way to our brain, or muscles, or most other places in our body. We just don’t have enough surface area to supply enough oxygen and we need an efficient way to get it into the bloodstream so that it can be distributed throughout the body.
Parts of the Respiratory System
So, let’s start at the beginning. A person breathes air in through their nostrils or their mouth. The mouth and nasal airways come together at a junction called the pharynx, which is located at the back of the mouth, and continues down the throat until it reaches the larynx. The larynx is more commonly known as the voice box, and some people call it the Adam’s apple.
The air then enters the trachea, which is the main airway that runs from the larynx down to the lungs. The trachea is known to most people as the windpipe. Just as the air reaches the lungs, the trachea splits into two smaller tubes called bronchi.Each bronchus enters one side of the lungs and then splits repeatedly into ever smaller tubes called bronchioles.
And finally, at the ends of the smallest bronchioles are clusters of spherical structures that look a lot like bunches of grapes. These grape-like structures are called alveoli and are basically tiny air sacs with very thin walls that serve as the main site of gas exchange.
The average human lung has about 500 million alveoli with a combined inner surface area that is about the same size as half of a full-size tennis court. That’s 40 times the surface area of a person’s skin!So how is this even possible? The organization of the alveoli in clusters creates a honeycomb-like internal structure where the alveoli walls create a massive amount of surface area for the air to come in contact with.In combination with the sheer numbers of the alveoli, they solve our first problem by increasing the surface area available to absorb oxygen through.But how do the lungs take oxygen out of the air and get it into the bloodstream? Well, surrounding the alveoli is a very dense network of capillaries. You may remember that capillaries are very small, flattened blood vessels with very thin walls that allow gases and nutrients to be easily transferred between the blood and surrounding tissues.
How Blood Becomes Oxygenated
The blood collects oxygen from the alveoli as it passes over the entire surface area, created by the alveolar walls. However, the oxygen can’t just magically jump from the air across the alveoli and into the blood; it has to cross each barrier one at a time.First, the oxygen has to be dissolved, so there is a thin film of water coating the inside surface of the alveoli. Because the volume of water is so small, it becomes saturated with oxygen almost instantly and the oxygen is then free to diffuse across the cell membrane. You may recall that dissolved molecules diffuse quickly in the direction of a lower concentration, which in this case is the oxygen-depleted blood in the surrounding capillaries, so oxygen diffuses from the saturated water coating the alveoli, across the alveolar wall, through the capillary wall and into the oxygen-depleted blood.Once in the blood, you may also remember that oxygen enters red blood cells and then binds to hemoglobin, taking it out of solution and keeping the dissolved oxygen level low in the blood. This ensures that the oxygen continues to flow from the alveoli and into the blood.
As the oxygen is diffusing into the blood, carbon dioxide, which is the primary waste product of animal cells, is being released. Because there is so little carbon dioxide in the air, compared to the blood, diffusion of carbon dioxide occurs in the opposite direction to that of oxygen. Carbon dioxide diffuses out of the capillaries, into the alveoli, into the thin layer of water, and then quickly exits into the air. The end result is that oxygen is taken out of the air and enters the bloodstream, while carbon dioxide is removed from the blood and released into the air.
Problems of Liquid in Alveoli
It is important that only a very thin layer of water coats the alveoli. If the alveoli begin to fill up with water, three problems occur.
First, the large volume of water would require much more oxygen in order to reach saturation of the larger volume. Saturating the large volume takes time and slows oxygen transfer.Second, the oxygen and carbon dioxide would have further to travel across the water to reach the alveolar wall.
This further slows the transfer of gases to and from the blood.Third, the surface area would be reduced as the surface area of the alveoli becomes covered by a thick layer of water. The surface area of the water has now become the effective surface area of the lung. For these reasons, excess water in the lungs creates a barrier to oxygen uptake and can cause drowning.
To review, humans need a large surface area for gas exchange and need to easily transfer gases between the blood and the air.
Specialized structures in the lungs called alveoli dramatically increase the surface area of the lung and the dense capillary network around the alveoli creates a very short route between the bloodstream and the air. A very thin film of water coating the alveoli allows oxygen and carbon dioxide to instantly transition between dissolved and gaseous forms, thereby facilitating the uptake of oxygen and the release of carbon dioxide.
When you are done, you should be able to:
- Identify the parts of the respiratory system
- Describe the alveoli and their role in transferring oxygen to the bloodstream
- Explain how blood becomes oxygenated
- Discuss what would happen if the alveoli filled up with water