It was formulated by William Henry in The practical description for the law is that the solubility i. In addition, the partial pressure is able to predict the tendency to dissolve simply because the gasses with higher partial pressures have more molecules and will bounce into the solution they can dissolve into more often than gasses with lower partial pressures. Henry's law also applies to the solubility of other substances that aren't gaseous, such as the equilibrium of organic pollutants in water being based on the relative concentration of that pollutant in the media its suspended in.
Where p is the partial pressure of the solute in the gas above the solution, c is the concentration of the solute, the solubility of the substance is k, and the Henry's law constant H , which depends on the solute, the solvent, and the temperature. The solubility captures the tendency of a substance to go towards equilibrium in a solution, which explains why gasses that have the same partial pressure may have different tendencies to dissolve.
Henry's law states that when a gas is in contact with the surface of a liquid, the amount of the gas which will go into solution is proportional to the partial pressure of that gas. The main application of Henry's law in respiratory physiology is to predict how gasses will dissolve in the alveoli and bloodstream during gas exchange.
The amount of oxygen that dissolves into the bloodstream is directly proportional to the partial pressure of oxygen in alveolar air. The partial pressure of oxygen is greater in alveolar air than in deoxygenated blood , so oxygen has a high tendency to dissolve into deoxygenated blood. Conversely the opposite is true for carbon dioxide, which has a greater partial pressure in deoxygenated blood than in the alveolar air, so it will diffuse out of the solution and back into gaseous form.
Recall that the difference in partial pressures between the bloodstream and alveoli the partial pressure gradient are much smaller for carbon dioxide compared to oxygen. Carbon dioxide has much higher solubility in the plasma of blood than oxygen roughly 22 times greater , so more carbon dioxide molecules are able to diffuse across the small pressure gradient of the capillary and alveoli.
So that means that the green partial pressure is going to be half of , which is So this is the partial pressure of the green molecules. I figured it out. And I could actually complicate this a little bit. I could say, well, what if I got rid of those two and replaced them with green molecules? So now the gas is looking different. I've got 6 out of 8 molecules that are green. So what is the new partial pressure looking like? Well, 6 out of 8 means that the percentage is going to be different.
So I've got a new number here and here. This is my new partial pressure. And the reason I actually went through that is because I wanted to show you a way of thinking about partial pressure, which is that if the number of molecules in a group of molecules-- if the proportion goes up-- then really that's another way of saying the partial pressure has gone up. And if you have more molecules, what does that mean exactly?
Well, from this person's standpoint, this person that's watching this surface layer, they're going to see, of course, molecules going every which way. Every once in a while, these green molecules are going to go down and into the liquid. They're going to bounce in different ways, and just by random chance, a couple of these green molecules might end up down here in the surface layer.
So that's something that you would observe. And you'd probably observe it more often if you actually have more green molecules. In other words, having a higher partial pressure will cause more of the molecules to actually switch from the gas part of this cup into the liquid part of the cup. So I don't want to be too redundant, but I want to point out that as the partial pressure rises, we're going to have more molecules, more green molecules, going into the liquid.
So now let me actually ask you to try to focus on this little green molecule, this little fella right here, this guy. Now imagine, he's just entered our world of H2O's, and he's trying to figure out what to do next. And one thing he might do is pop right back out. You'd agree that that's something he could do, right? If he entered the liquid phase, he could also just re-enter the gas phase. He could leave.
And a lot of molecules want to do that. They want to actually get out of the liquid because the liquid is a little stifling. It's kind of crammed in there, a lot of H2O molecules around in this case may not like that.
So it turns out you can actually look up, in a table, this value called K with a little h. And this H with a little h is just a constant. So this is just a constant value that's listed on a table somewhere. And this K sub h actually is going to take into account things like which solute are we talking about. When I say solute, you basically can think of these green molecules.
So which is it? Is it a green molecule or a purple one or a blue one? What exact solute are we talking about? In contrast, when ventilation is insufficient, the partial pressure of oxygen in the alveoli drops. Without the large difference in partial pressure between the alveoli and the blood, oxygen does not diffuse efficiently across the respiratory membrane. The body has mechanisms that counteract this problem.
In cases when ventilation is not sufficient for an alveolus, the body redirects blood flow to alveoli that are receiving sufficient ventilation. This is achieved by constricting the pulmonary arterioles that serves the dysfunctional alveolus, which redirects blood to other alveoli that have sufficient ventilation.
At the same time, the pulmonary arterioles that serve alveoli receiving sufficient ventilation vasodilate, which brings in greater blood flow. Factors such as carbon dioxide, oxygen, and pH levels can all serve as stimuli for adjusting blood flow in the capillary networks associated with the alveoli.
Ventilation is regulated by the diameter of the airways, whereas perfusion is regulated by the diameter of the blood vessels. The diameter of the bronchioles is sensitive to the partial pressure of carbon dioxide in the alveoli. A greater partial pressure of carbon dioxide in the alveoli causes the bronchioles to increase their diameter as will a decreased level of oxygen in the blood supply, allowing carbon dioxide to be exhaled from the body at a greater rate.
As mentioned above, a greater partial pressure of oxygen in the alveoli causes the pulmonary arterioles to dilate, increasing blood flow. Gas exchange occurs at two sites in the body: in the lungs, where oxygen is picked up and carbon dioxide is released at the respiratory membrane, and at the tissues, where oxygen is released and carbon dioxide is picked up.
External respiration is the exchange of gases with the external environment, and occurs in the alveoli of the lungs. Internal respiration is the exchange of gases with the internal environment, and occurs in the tissues. The actual exchange of gases occurs due to simple diffusion. Energy is not required to move oxygen or carbon dioxide across membranes. Instead, these gases follow pressure gradients that allow them to diffuse.
Cellular respiration see the review of cellular respiration below in the tissues maintains the pressure gradients for internal respiration. The anatomy of the lung maximizes the diffusion of gases: The respiratory membrane is highly permeable to gases; the respiratory and blood capillary membranes are very thin; and there is a large surface area throughout the lungs.
The pulmonary artery carries deoxygenated blood into the lungs from the heart, where it branches and eventually becomes the capillary network composed of pulmonary capillaries. These pulmonary capillaries, along with the alveolar walls and a shared basement membrane, create the respiratory membrane.
As the blood is pumped through this capillary network, gas exchange occurs. Although a small amount of the oxygen is able to dissolve directly into plasma from the alveoli, most of the oxygen is picked up by erythrocytes red blood cells and binds to a protein called hemoglobin , a process described later in this chapter.
Oxygenated hemoglobin is bright red, causing the overall appearance of bright red oxygenated blood, which returns to the heart through the pulmonary veins. Carbon dioxide is released in the opposite direction of oxygen, from the blood to the alveoli. Some of the carbon dioxide is returned on hemoglobin, but can also be dissolved in plasma or is present as a converted form, also explained in greater detail later in this chapter. External respiration occurs as a function of partial pressure differences in oxygen and carbon dioxide between the alveoli and the blood in the pulmonary capillaries.
Figure 2. External respiration — Oxygen diffuses across the respiratory membrane from the alveolus to the capillary, whereas carbon dioxide diffuses out of the capillary into the alveolus. Although the solubility of oxygen in blood is not high, there is a drastic difference in the partial pressure of oxygen in the alveoli versus in the blood of the pulmonary capillaries. This difference is about 64 mm Hg: The partial pressure of oxygen in the alveoli is about mm Hg, whereas its partial pressure in the blood of the capillary is about 40 mm Hg.
This large difference in partial pressure creates a very strong pressure gradient that causes oxygen to rapidly cross the respiratory membrane from the alveoli into the blood. The partial pressure of carbon dioxide is also different between the alveolar air and the blood of the capillary.
However, the partial pressure difference is less than that of oxygen, about 5 mm Hg. The partial pressure of carbon dioxide in the blood of the capillary is about 45 mm Hg, whereas its partial pressure in the alveoli is about 40 mm Hg.
However, the solubility of carbon dioxide is much greater than that of oxygen—by a factor of about 20—in both blood and alveolar fluids. As a result, the relative concentrations of oxygen and carbon dioxide that diffuse across the respiratory membrane are similar.
Internal respiration is gas exchange that occurs at the level of body tissues Figure 3. Similar to external respiration, internal respiration also occurs as simple diffusion due to a partial pressure gradient. However, the partial pressure gradients are opposite of those present at the respiratory membrane. The partial pressure of oxygen in tissues is low, about 40 mm Hg, because oxygen is continuously used for cellular respiration.
In contrast, the partial pressure of oxygen in the blood is about mm Hg.
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