Objective 64: Discuss how oxygen and carbon dioxide are transported
When I was scoping out the textbooks website, I came across the MP3 tutor sessions and found this one that was a lot of help. They are very lengthy, I admit but they gave me some very helpful information. What I love about these tutor sessions is that they simplify the topics that they are going over in the session, which helps tremendously. It describes both Dalton's law of partial pressure and Boyle's gas law, and was able to help me understand how the relate to respiratory physiology. Another thing I really like about these tutor sessions is that they always provide multiple examples on how the topics discussed are related to real life. So basically Dalton's law explains how gases behave when they are mixed together and Boyle's law explains how gas pressure and volume are inversley related, or the smaller the space than the greater the pressure. This session was also helpful in finding out how oxygen and carbon dioxide are transported because it gave a great deal of information about the two gases. Being able to listen and read along with the session made the two topics more clear to me and im glad that I was able to come across it.
Pearson welcomes you to MP3 Tutor Sessions for Anatomy and Physiology.
Gas Exchange During Respiration
Section 1: Tough Topics
Take a deep breath. Now hold it, hold it hold it. Okay, let it out. We don't usually think about our breathing under resting conditions. It is something that just happens. Oxygen goes in and carbon dioxide goes out. It seems simple, until something disrupts the process—like holding your breath—at which point we start to appreciate the complexity of the respiratory system. So, what does happen when you breathe in and out? And what happens when you hold your breath?
This tough topics section explores how gas exchange occurs during respiration. Gas exchange associated with the respiratory system involves the exchange of oxygen for carbon dioxide at the lungs and carbon dioxide with oxygen at the tissues. To better organize our discussion, we'll break the discussion into four parts. First, we will discuss the anatomy of the respiratory system. Then we will talk about how pressure affects breathing and gas exchange. The third section will focus on pulmonary ventilation, the process of getting the air in and out of your lungs. Finally, we'll discuss alveolar ventilation, the process of actual gas exchange. So, take a deep breath... and let's begin.
Your respiratory system consists of the nose, nasal cavity, the pharynx, larynx, trachea, bronchi, and lungs. Within the respiratory system are the conducting zone and the respiratory zone. The conducting zone includes everything from the nose to the respiratory bronchioles. It is called the conducting zone because its job is to get the air to the actual site of gas exchange, the alveoli.
The respiratory zone includes the respiratory bronchioles and alveoli. Alveolar ducts lead toalveolar sacs containing the alveoli. The alveolar sacs resemble grape clusters. An alveolus (the singular form of alveoli) is like a single grape in the cluster. The walls of the alveolar sacs are very thin; expanding when air enters and recoiling when it is exhaled. Think of a balloon that you blow up. The balloon expands to a certain volume safely. Then when the air is released, it deflates, recoiling to close to its original size. The alveoli act in much the same way.
The three hundred million alveoli in the lungs comprise a huge surface area for gas exchange. In fact, if they were spread out in a single layer, the alveoli would be the size of a tennis court! The alveoli expand when air enters, using a mechanism called pulmonary ventilation that generally costs us very little energy to perform.
Before we talk about how ventilation works, we need to learn a little bit about the musculature that helps pulmonary ventilation occur and about gas exchange that results from alveolar ventilation. Let's start with the muscles that facilitate breathing. The diaphragm is the primary muscle involved with breathing. It effectively separates your thoracic cavity from your abdominal cavity. Its main function is to increase the volume of the thoracic cavity by contracting downward into the abdomen. The external and internal intercostal muscles increase or decrease the volume by causing expansion or contraction of the ribcage.
That explains how air gets into the lungs. But how does it get into the cells? This is the process of alveolar ventilation. Gases, namely oxygen and carbon dioxide, are exchanged through the respiratory epithelium. This means that the oxygen diffuses out of the lungs into the blood circulation, where it is picked up by the hemoglobin in red blood cells. At the same time, carbon dioxide diffuses from the red blood cells and plasma through the respiratory epithelium where it is exhaled. The gases can diffuse because they are following pressure gradients, which behave similarly to concentration gradients.
To understand more about how these pressure gradients work, we will discuss three important laws that describe how gas behaves in confined spaces. These gaslaws are fundamental to understanding how we can take air in, move oxygen into our cells, and expel carbon dioxide. Boyle's Law of Gas Pressure and Volume explains how air is drawn into and expelled from the lungs. Dalton's Law of Partial Pressures describes how pressure gradients can facilitate diffusion for individual gases between your blood and lungs. And Henry's Law explains how gases can move into and out of solution—in the case of our bodies, how oxygen and carbon dioxide can dissolve and diffuse out of the blood. Let's start the discussion with Boyle's Law.
Boyle's Law basically states that gas pressure and volume are inversely related. What this means is that the smaller the space the gas is contained in, the greater the pressure of the gas in that container. Imagine that gas molecules are ping pong balls in a big box. They don't interact very much. Then you put the same amount in a smaller box and a smaller box, they interact more and more. As they interact more, the pressure increases.
Boyle's Law explains how the air gets in and out of your lungs, that is, how it travels through the conducting system and fills the alveolar sacs. It works like this: Gas pressure changes in your thoracic cavity in response to your muscles contracting and your ribcage expanding.
At rest, your diaphragm contracts in a downward movement and the external intercostal muscles pull the ribcage up and out. This causes the volume in the thoracic cavity to increase. But when this happens, the air that did not leave the lungs during the previous exhale completely fills the available space. But since the space is much larger, the air molecules are farther apart, so the air pressure decreases. This drop in pressure prompts you to inhale because the lungs passively expand at the same time as the thoracic cavity does. This is due to the pleural fluid's surface tension which keeps the visceral and parietal pleurae very close together -- literally pulling the lungs outward with the thoracic wall. Air moves into the expanding lungs because air, like any fluid, moves from an area of greater (atmospheric) to lower (intrapulmonary) pressure.
Breathing out is the result of the opposite situation: the pressure in your thoracic cavity is higher than the pressure of the atmosphere around you. Your body wants to equalize the pressure. It does this simply by relaxing the diaphragm and the external intercostal muscles. During forced breathing, for instance, when you are exercising, muscles are recruited for both inhalation and exhalation. They serve the function of not only increasing your respiratory rate, but also the volume in your thoracic cavity in an effort to get rid of oxygen debt.
Let's talk more about how gas exchange occurs between the lungs, blood, and tissues of the body. For this, we have to discuss Dalton's Law of Partial Pressures.
Air is made of a mixture of several gases, including about twenty percent oxygen, eighty percent nitrogen, and minute quantities of carbon dioxide. When these separate gases are mixed together, the total pressure is a result of all the gas pressures combined. When these gases are mixed together the pressure of each is in direct proportion to the percentage the gas takes up in the mixture. So the pressure of the oxygen is twenty and nitrogen is eighty. This is called the partial pressure.
Now, Henry's Law demonstrates that gases mixed together as in air, will dissolve in the liquid based on their partial pressures. Henry's Law deals with partial pressures of gases in two different phases, liquid and gas. So the gases will go in and out of these two phases – BUT – they always follow Dalton's finding on partial pressures. So let's take blood as an example.
You breathe in because the partial pressure in your lungs attached to your thoracic cavity is lower than outside your body. That helps push the air in. Now let's look at what happens at the gas exchange surface, that is, where the oxygen gas gets dissolved in your blood. The blood coming into your lungs has very low oxygen content, which means the partial pressure made up by the oxygen is low. But you just took a breath. So the partial pressure of oxygen in your lungs is much higher. When the oxygen in your alveolar sacs comes in contact with your blood plasma, it dissolves into the plasma until the partial pressures between your lungs and blood plasma are equalized.
The same type of exchange happens with the carbon dioxide – only in reverse. There is more carbon dioxide in your blood plasma than in your lungs, so the partial pressure is higher. Therefore, the carbon dioxide becomes a gas again in order to equalize the partial pressures between the two different phases.
But there are two other factors that will determine how much and how quickly a gas can dissolve into solution. The first is the solubility of the gas. Carbon dioxide is most soluble. Oxygen is only five percent as soluble as carbon dioxide and nitrogen is less than three percent as soluble as carbon dioxide. So at a specific partial pressure, more carbon dioxide dissolves in plasma than oxygen and practically no nitrogen dissolves. That is why having nitrogen make up most of the composition of air is not a big deal. In addition, even though carbon dioxide dissolves more readily, there is so little carbon dioxide in air that you don't have to worry about suffocating. So the only component you get dissolving into your plasma in substantial quantities is oxygen.
The other factor that affects how efficiently gas dissolves into a liquid is temperature. The warmer the liquid, the lower the gas solubility will be. For this example, think of soda – which is only carbonated sugar-water. Carbon dioxide is forced into solution because high pressures are used. As long as the cap stays on, it stays carbonated. If you take off the cap two things happen. First, some of the excess carbon dioxide immediately escapes because you relieved some of the pressure by removing the cap. Second, at room temperature, a soda goes flat--that is, all the excess carbon dioxide has escaped and now you have flavored water without the fizz. As the temperature increased, the carbon dioxide became less soluble, and left the solution.
Applying this to humans, we see that as body temperature rises, oxygen exchange is less efficient because the solubility of oxygen in the heated plasma decreases. That is why many athletes don't perform as well in hot weather. They prefer cooler temperatures because their oxygen exchange is better.
Okay, before we move on to discussing gas exchange between the lungs, blood, and tissues, let's summarize what we have learned about gas laws. Boyle's Law tells us that the decrease of pressure in your lungs causes you to inhale because the internal pressure is less than that outside your body. After you inhale, the pressure in the confined space of your lungs is greater than outside and as a result, you exhale. Dalton's Law of Partial Pressures explains how gases behave when they are mixed together. The pressure in a confined space is the total of all the gas pressures combined. Henry's Law explains how gases can dissolve in a solution. For us, that is air dissolving in our blood plasma. This directly relates to the partial pressure of the gas in the air and in the blood. If the partial pressure is higher in the lungs, the gases will dissolve into the plasma. If the pressure is lower in the lungs, the dissolved gases will come out of solution and become a gas again.
Now let's talk about the mechanisms involved in Alveolar Ventilation. These are the actual events that are occurring between our lungs, blood, and tissues to make gas exchange possible.
Keep in mind that gas exchange occurs by simple diffusion along pressure gradients. So no energy is expended and the gas goes from high to low pressures. The space between your alveoli and the blood—called the respiratory membrane—is extremely small, only half a micrometer wide, facilitating diffusion. The respiratory membrane consists of the alveoli cell membranes, the fused basal lamina of endothelial and alveolar epithelial cells, and the endothelial cells that make up the lining of the capillaries. This space is only half a micrometer wide. So the gas molecules have a very small distance to go. They are small enough to slip right through the cell membranes.
Now when you inhale, the amount and partial pressure of oxygen is greater in the alveolar sacs. This pressure gradient causes the oxygen to traverse the respiratory membrane and enter the blood plasma, where it quickly binds to hemoglobin. Oxygen has such a high attraction for the iron in hemoglobin that under normal conditions, less than five percent of the oxygen is freely circulating. The rest is bound to hemoglobin.
The red blood cells carry oxygen to all parts of the body, where the partial pressure of oxygen is lower in the tissues. This causes the oxygen to again follow the pressure gradient, dissociating from the iron and flowing into the tissues. That is how oxygen gets delivered to the tissues.
The process of expelling carbon dioxide works in reverse, so we will start at the tissues. There, cellular metabolism generates carbon dioxide as a by-product. As a result, the partial pressure of carbon dioxide is greater in the tissues than in the plasma. Therefore, as oxygen is released, carbon dioxide is picked up by the red blood cell in exchange and transported to the lungs. It isn't released anywhere along the way because the pressure of carbon dioxide is higher in tissues than in the red blood cells. Carbon dioxide is released in the lungs because the partial pressure of carbon dioxide in the alveoli is very low.
To summarize, partial pressure and gas solubility are key factors in efficient gas exchange. The partial pressure difference for oxygen between the lungs and tissues is twelve to fifteen times greater than the partial pressure differences for carbon dioxide. But due to the higher solubility of carbon dioxide, the same amounts are exchanged. In addition, the distance across the respiratory epithelium is small enough to allow simple diffusion to occur rapidly, facilitating effective gas exchange.
So, to return to the question we asked at the beginning of this section, what happens when you hold your breath? Your body has central chemoreceptors that are more sensitive to increases in carbon dioxide than decreases in oxygen. And during normal breath-holding it's the increase in carbon dioxide, not the oxygen decrease, that stimulates taking a breath.
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