Video Transcript
What gas is transported out of the bloodstream and into the lungs to be exhaled?
This question asks us about the lungs, which are soft, spongy organs located in the chest cavity and protected by the ribs. They function within the respiratory system to exchange gases between the blood and the external atmosphere. Let’s review some key facts about their structure and role so that we can come up with the correct answer.
The structure of the lungs is often likened to an upside-down tree. On this diagram, you can see the main tube going towards the lungs. This is the trachea and is like the trunk of a tree, wide and strong. It is supported by rings of cartilage to keep it open. The trachea then splits into the two bronchi, main branches of the tree, which further divide into bronchioles, smaller tree branches, and finish in the small air sacs called alveoli, like the leaves of a tree. It is when the air has got to the alveoli that gaseous exchange can occur. This is because the walls of the alveoli are only one cell thick. And they are made of flattened cells, keeping the diffusion distance short.
The alveoli are surrounded by a dense capillary network. And the total surface area of the alveoli is about 70 meters squared. So there is a large surface area for exchange. Oxygen in the air is in higher concentration than in the blood. So it diffuses from the alveoli into the blood of the capillaries. The oxygen then enters the red blood cells, which are specialized to carry the oxygen around the body to where it is needed. All of our body cells require oxygen, but what for?
Oxygen is needed for aerobic respiration, a type of cellular respiration. Aerobic respiration is when carbon-containing compounds, usually glucose, are broken down in the presence of oxygen to release energy in the form of ATP. The cells then use this energy for a variety of functions, such as muscle contraction, production of new cellular components, driving metabolism, and active transport.
As you can see from this equation, the waste gas of respiration is carbon dioxide. This diffuses from the cells down its concentration gradient into the blood. The blood then carries the carbon dioxide in the red blood cells and dissolved in the plasma up to the lungs. Here, it diffuses out of the blood down its concentration gradient into the alveoli. It is then exhaled out of the body, breathed out, so we can then bring more fresh air back into the alveoli when we inhale, breathe in. Then the process starts all over again.
Now we’ve looked over the information you need to know about gaseous exchange in the lungs, we can return to our question, which asks “What gas is transported out of the bloodstream and into the lungs to be exhaled?” We now know that the correct answer is carbon dioxide.
