 You must have seen your mom preparing hung curd or we also call it the greek yogurt She puts some curd in a thin muslin cloth and then hangs it right so that all the way can drip out all the liquid drips out and Finally we get a thick creamy consistency of the curd now Can you tell me why does your mom uses this thin muslin cloth and not any other fabric like the fabric in your uniform or denim? Well, I know it's a silly question, but still I want you to answer this The first point would be unlike other fabrics muslin cloth is very thin and because of that the liquid can easily pass through it, right? Okay, so does that mean we can use anything which is very thin? For example the clean film which is used in the kitchen to wrap food the plastic sheets Can we use those? Probably not right because nothing can pass through a plastic sheet without cutting and tearing it, right? So we need something which is permeable So the second reason for using this thin muslin cloth is because it is permeable, right? Now with this whole hung curd and muslin cloth story the point I want to drive home is that if you want to allow Something to pass through a surface Then the best surface would be the one that is very thin and permeable to whatever needs to pass through it, right? Now I want you to relate this to the membranes in the biological world Membranes and plants and animals which we often call the biological exchange membranes Exchange membranes because these membranes are the one through which some kind of exchange happens either of gases nutrients some kind of fluid something goes in or goes out of these membranes, okay? That's why we call it the exchange membranes and if something has to pass through these membranes It has to be thin and it has to be permeable. We will all agree to that now, right? So here we have the example of root hair through which the water and minerals are sucked into a plant and Then we can talk about the digestive system where the small intestine has very thin lining through which the nutrients are absorbed into our blood vessels and Then we can talk about the lungs the alveolus the wall of the alveolus through which exchange of gases takes place So in this video, let's have a closer look at this alveolar membrane Let's see how many layers or how many barriers oxygen and carbon dioxide needs to cross every time we breathe and Here we have the magnified view of the alveolus now as you can see It is made of just one layer of cells. These are special cells called the squamous cells now Squamous comes from a Latin word, which means scale. Let me write fish scale here Okay, and it is called so because these are very flat just like the scales of a fish and The membrane which is formed out of these cells are called the squamous epithelium Okay, now this squamous epithelium is so thin and flat that it cannot hold its structure alone It needs something to sit on and for that the structural support is provided by This membrane around the squamous cells, which is called the basement membrane now Remember that basement membrane is not cellular. It is not made of any cells Okay, it is just the extra cellular matrix providing just the right support for the alveolus and With these squamous cell layer and the basement membrane they together form a very thin layer just perfect for the air to pass through Okay, now when the gas enters the alveolus, let's just say oxygen entered the alveolus Okay, and it passed through this layer the layer of squamous cells and basement membrane Its next destination is our blood, right? So the best position for our blood capillaries will be to sit very closely to this alveolar wall, right? You must have seen images like this where you can clearly see how dense Network of blood capillaries surrounds the alveolus, right? So let's make our blood capillary here There you go and again for the gas to get through these blood capillary walls The wall has to be very thin. We discussed that right? It has to be thin and it has to be permeable and for that reason the wall or the membrane of the blood capillaries are made of the same kind of cells squamous cells But the membrane in this case is not called a squamous epithelium, but a squamous endothelium Now let's try to understand why there is this difference in the name. Okay, even though they are made of squamous cells So thelium simply means a layer of body's tissues So the body tissue which lines the pathways of your body which is open to the external environment Like the respiratory system or the digestive system. It is called the epithelium Okay, but the pathways in your body which is completely closed like your circulatory system We have a closed circulatory system, right? So those closed pathways are lined by endothelium Okay, now the reason I'm emphasizing on these two terms is because they are often asked in multiple choice questions in exam and it often often Confuses students. Okay, so always remember that endothelium means inside. It will always line something which is closed Which is inside. Okay Now do you remember that we discussed that this squamous cells of the alveoli they can't sit alone They need a basement membrane for their structural support Well, the squamous cells of the blood capillaries also need the same They will also require basement membranes to support their structure Okay, with this we got all the layers that stands on the way of our respiratory gases Okay, so the gas to pass from our alveolar cavity into our blood It has to pass through number one squamous epithelium of the alveoli Second is the basement membrane now We won't consider this basement membrane of the blood capillary and the alveoli as separate membranes because these are made of the same Substances, okay, so basement membrane is also called basement substances Okay, next to the basement membrane we have these squamous endothelium of the blood capillaries and We call these three together as the diffusion membrane through which the gases can diffuse Now the thickness of all these three together is Somewhere around 400 micrometer or we can just say that it is less than one millimeter Okay, so this is the thickness So this was all about diffusion membrane, which is the site of cashier's exchange in the alveolus of our lungs