External Respiration

External/Alveolar Respiration

Students often forget that the term “external respirationThe exchange of gases between the lungs and the blood in the pulmonary capillaries.” specifically refers to the gas exchange that is happening in the lungs.  Some textbooks might call this alveolar gas exchange.  No matter what you call it, we are exchanging gases across that respiratory membrane.  Gases in the air are going into the blood.  Of course, what is in your alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood. is not yet in your body, and so this gas exchange is external.  When your red blood cellsThe basic structural and functional units of life. drop off that oxygen to your tissues, that is called internal respirationThe exchange of gases between the blood and body tissues..  Atmospheric air contains almost all nitrogen and oxygen. There are also some other gases like carbon dioxide at smaller partial pressures.  The air in alveoli is almost all oxygen and carbon dioxide.  If you were to stand here with your mouthThe opening of the digestive tract where food enters and mastication begins. open, gases would diffuse down their gradients. They would move into your tissues and out of them.   The rate would not be fast enough to keep you alive, but the process is happening.

Making Oxyhemoglobin

Recall the section A cut or slice of the body or an organ for study. on blood that hemoglobinThe oxygen-carrying protein in red blood cells that gives blood its red color. is contained in red blood cells. Oxygen moleculesGroups of atoms bonded together. must travel through the respiratory membrane into the plasmaThe liquid component of blood.. Then, they must move from the plasma through the erythrocytes(RBCs): Red blood cells responsible for oxygen transport. cell membrane and into the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. of the erythrocyte. This allows oxygen to bind with the iron in hemoglobin. Here in external or alveolar respirationThe process of gas exchange, including ventilation, external and internal respiration. we are taking deoxyhemoglobinHemoglobin that has released its oxygen and is carrying carbon dioxide or hydrogen ions. and turning it into oxy hemoglobin. From the previous section on blood, we know that deoxyhemoglobin has a very dark reddish, almost purplish color. Oxyhemoglobin, on the other hand, has a bright ruby red color. These colors are created from the bonding of the iron in the hemoglobin molecule with the oxygen. Just as metal rusts when exposed to oxygen, the iron in hemoglobin does the same thing in changing color.

Respiratory Membrane

The respiratory membrane is the air blood barrier. This is where items in air must diffuse through membranes to enter into the plasma of blood and vice versa. The respiratory membrane is the fusion of the type 1 alveolar cells and the endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces f of the capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. near it. The fusion of these two membranes gives them one basement membrane through which gas molecules must diffuse. By fusing these membranes the distance through which the molecules must diffuse becomes less. This reduction is just one adaptation in the respiratory membrane. These adaptations encourage the passive diffusionPassive movement of molecules from areas of high to low concentration. of gases. The point here is not to use active transport and consume ATPThe energy currency of cells used for muscle contraction.. Let’s look at this picture a little bit more closely.

On the right side of the diagram is a blue capillary. In this case, it is on the arterial side of the bed. Here in the pulmonary circuitThe circulation of blood between the heart and the lungs, where blood is oxygenated. arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr bring deoxygenated blood to the alveoli. And veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge carry oxygenated blood away from the alveoli and back to the left atrium of the heart. This distribution of oxygenated and deoxygenated blood in the pulmonary circuit is exactly the opposite. It is the reverse of how it is in the systemic circuitThe part of the circulatory system that carries oxygenated blood from the heart to the body and retu. This also means that these red blood cells are visible in the capillaries. They are flowing in this diagram from right to left.

This alveolus is lined with simple squamous cells. These are the type 1 pneumocytes or alveolar cells. They are responsible for the diffusion of gases. This blue cell here that looks like it has hair is a type 2 great alveolar cell that secrets surfactantA substance secreted by Type II pneumocytes that reduces alveolar surface tension.. We can see a very faint highlight of blue. It separates these type 1 pneumocytes or alveolar cells from the open air gravity of the alveolus. This is the surfactant covering. It breaks up the alveolar surface tensionThe force exerted by the liquid lining the alveoli, which tends to collapse them; reduced by surfact. This tension is the tendency for this membrane to close. This happens due to the surface tension cohesion of waterThe universal solvent essential for life. molecules. We have an alveolar macrophage here patrolling the area. It is looking for any pathogens. We can also see the start of the columnar slash cuboidal ciliated cells of the respiratory bronchiole. Absent from this picture are the many elastic fibers that are running over this alveolus. These elastic fibres are very important in the collapse of this alveolus during exhalation.

Factors Affecting Diffusion

If you have ever opened a soda bottle then you have experienced something called Henry’s law. You are tasty little soda has carbon dioxide bubbles that have been pushed into your soda under pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange.. When you twist off the top of your soda bottle a whole bunch of that carbon dioxide comes rushing out. This is because there is a lot of carbon dioxide in your bottle. It is a high concentration. There is relatively less carbon dioxide in the air, which is a low concentration. Therefore carbon dioxide is simply just diffusing down its gradient. If you leave your soda bottle open for a very long time, all of the carbon dioxide will diffuse out. It will escape from your bubbly soda and join the rest of the carbon dioxide molecules in the atmosphere. The same principle applies here at the alveolus. Oxygen diffuses into the liquid of the plasma. Meanwhile, carbon dioxide diffuses out of the liquid of the plasma.

As mentioned before, diffusion is a passive transport process. This is what takes place here in external or alveolar respiration. For my entire body to be sufficiently oxygenated, this respiratory membrane must meet a few qualifications. The gas exchange needs to be efficient.

#1: the distance that the gases have to travel must be small. This shows that the basement membrane of the type 1 alveolar simple squamous cells is fused. It is fused with the simple squamous endothelium of the blood vessel.

#2: There must be enough alveoli that exchange gases during external respiration. It is essential to oxygenate enough capillaries. This serves the needs of my peripheral tissues. This means that the lung has to have a lot of surface area per volume. As I mentioned once before many textbooks say that the lung has enough surface area to cover a tennis court. And my response to that is always yeah. Who did that? Who took a human lung and spread it all out to cover a tennis court?

#3: lucky for us carbon dioxide and oxygen are both nonpolar lipid soluble gases. as such they are capable of diffusing through the cell membranes of the simple squamous cells of the respiratory membrane. No transport channel is required. These gases are able to just push through cell membranes in between the phospholipids of the phospholipid bilayer.

#4 If you were to stand here with your mouth open, oxygen would actually diffuse into your blood. Carbon dioxide would actually diffuse out of it. Diffusion alone is not enough for you to live. However, this means that gradients are established. These gradients allow these gases to flow in these directions. Think of this respiratory membrane. For oxygen to diffuse into your blood, it must first enter one of the type 1 pneumocytes. It then moves into the simple squamous lining the blood vessel. From there, it diffuses into the plasma and finally into the red blood cell. This gradient is very complicated. It doesn’t just have two points that create a high and low concentration. It has many points between that create that gradient.

#5 this is a concept called ventilation perfusion coupling. You do not inflate all of your alveoli with every breath. However, the alveoli that you do inflate require blood flow in the surrounding capillaries. This is necessary for gas exchange. There is no use sending blood to alveoli that do not inflate during inhalation.

All of these concepts will be explored in more detail throughout the respiratory Physiology mini lectures . @1