8.3 Alveoli
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The alveoli are responsible for gas exchange. They are where oxygen is taken into the blood stream from the alveoli, where it is taken to cells in the body, in order for the respiration reaction to take place in cells. Carbon dioxide moves from the blood stream to the alveoli.
As the alveoli are responsible for gas exchange, the composition of air that we inhale and exhale are different. Below is a table below that gives us the percentage volume of inhaled and exhaled air. The concentrations in the table are given in percentage form and not in the form of the number of particles.
As the alveoli are responsible for gas exchange, the composition of air that we inhale and exhale are different. Below is a table below that gives us the percentage volume of inhaled and exhaled air. The concentrations in the table are given in percentage form and not in the form of the number of particles.
From the table, we can see that the concentration oxygen is greater in air that is breathed in compared with air that is breathed out. This is because oxygen is taken into the blood stream in the alveoli, which are in the lungs. Also, we see that the concentration of carbon dioxide is higher in the air that we breath out compared to the air that we breath in. This is because our cells are undertaking respiration reactions and a by-product of respiration is carbon dioxide. The carbon dioxide is taken from the cells that are respiring to the lungs via the blood stream. It is then breathed out in the lungs. Exhaled air tends to be warmer than the air that we breath in. It also contains a lot of water vapour. The amount of water vapour in atmospheric air depends on the weather conditions; if it is humid there is a large quantity of water vapour in the air and when it is not humid, there is a small quantity of water vapour in the air.
The Alveoli
The alveoli in the lungs and they are at the end of the bronchioles. Air travels down the trachea, down one of the bronchi (either left or right bronchi), through the bronchioles and into the alveoli. There are hundreds of million alveoli in the human lungs. There is a diagram of what alveoli look like below.
Blood travels through capillaries very close to the alveoli. There is only two cells gap between the alveoli and the capillary. The two cells are the cells that make up the wall of the alveoli and the cells that make up the capillary wall. Both of these cells allow oxygen and carbon dioxide to pass through them.
The alveoli in the lungs and they are at the end of the bronchioles. Air travels down the trachea, down one of the bronchi (either left or right bronchi), through the bronchioles and into the alveoli. There are hundreds of million alveoli in the human lungs. There is a diagram of what alveoli look like below.
Blood travels through capillaries very close to the alveoli. There is only two cells gap between the alveoli and the capillary. The two cells are the cells that make up the wall of the alveoli and the cells that make up the capillary wall. Both of these cells allow oxygen and carbon dioxide to pass through them.
The lungs are part of the pulmonary circulatory system. This system is where blood is pumped from the heart, through the lungs and back to the heart. The blood in the capillary at the start of the alveoli is deoxygenated. This is because the blood has travelled around the body where it has supplied cells with the oxygen that it requires for respiration. Furthermore, the deoxygenated blood contains a large amount of carbon dioxide, which it has gained from respiring cells (remember that carbon dioxide is a waste product of respiration).
This means that there is a higher concentration of oxygen in the alveoli compared with the concentration of oxygen in the capillaries, which results in the diffusion of oxygen from the alveoli into the capillaries. There is a higher concentration of carbon dioxide in the capillaries compared with the alveoli, which leads to carbon dioxide diffusing from the capillaries to the alveoli. So, oxygen diffuses out of the alveoli into the capillaries and carbon dioxide diffuses into the alveoli from the capillary. This results in blood in the capillaries at the end of the alveoli being oxygenated, which will travel back to the heart, where it will be pumped out on the second circulatory system, which is the systemic system.
This means that there is a higher concentration of oxygen in the alveoli compared with the concentration of oxygen in the capillaries, which results in the diffusion of oxygen from the alveoli into the capillaries. There is a higher concentration of carbon dioxide in the capillaries compared with the alveoli, which leads to carbon dioxide diffusing from the capillaries to the alveoli. So, oxygen diffuses out of the alveoli into the capillaries and carbon dioxide diffuses into the alveoli from the capillary. This results in blood in the capillaries at the end of the alveoli being oxygenated, which will travel back to the heart, where it will be pumped out on the second circulatory system, which is the systemic system.
Earlier I said that there are hundreds of millions of alveoli in the lungs. The reason why there are so many alveoli is because we need to have a very high surface area in the lungs so that diffusion can occur in sufficient quantities. The total surface area of the alveoli is around 60m2. Furthermore, alveoli are designed so that they have a very large surface area.