in the previous video in the previous video we have talked about the artery and arterials so in this particular video we are going towards the next blood vessels which are known as the capillaries the capillaries as you can see there are the smallest and thinner's blood vessels to allow the exchange of substances before we go into capillaries in detail in the previous video I did mention to you that arterials have a very important function where they have to make the blood flow slower before it enters the capillary and why does it need to make the blood flow slower this is where we are going to be talking about the reason so talking about the capillary I'm drawing out a three-dimensional structure of a capillary as you can see there are the capillary has very thin walls how thin are the walls uh the wall is so thin that we just call it a one cell thick endothelium that is how you describe the wall of the capillary right and it has an extremely narrow Lumen how never is it so to understand how Neville the Lumen is we have to look at its longitudinal view in two Dimensions so I'm drawing I'll be capillary in a longitudinal View and it has the one cell thick endothelium wall and as you can see that the Lumen is so Neville that the diameter of the Lumen is only 7 micrometers to give you a bit of context as to how wide seven micrometers is seven micrometers is just basically the diameter of a red blood cell so a wet blood cell can only move in a single line as they are traveling through the capillary that's how small the capillary Lumen is another extra thing that we have to know is the wall of the capillaries are not directly attached to each other they will have tiny gaps between between the capillary endothelium wall the reason for the tiny gaps will be important when we are talking about tissue fluid in another video but for now what we have to know is the when we look at the longitudinal view of the capillary you can see the one Celtic endothelium wall the Lumen is seven micrometers it's so small that our red blood cells can only move one at a time in a single line and the capillary walls have very tiny minuscule gaps so drawing out the capillary in a cross section you can also appreciate the one Celtic endothelium wall you can see the Lumen is seven micrometers and a beam like structure over there and that being like structure or that weird looking the red color structure is just the Bic concave cross section of the red blood cell as a reminder that only one red blood cell can pass at a time when it's moving through the capillary because that's how small the Lumen is and I'm just going to label the tiny gaps over there so that you can see that too now the function of the capillary is to allow exchange between blood and body cells and I'll be looking let's just talk about an example of a body cell like a neuron the neuron is a cell in our nervous system that constantly needs oxygen because oxygen allows it to do aerobic respiration so this cell needs to get oxygen from the blood and within the blood there is a cell called the wet blood cell that carries the oxygen so I'm drawing out the blood vessel as you can see arterial and you can also see the capillary what's the difference between the arterial and capillary if you notice the arterials have very thick walls and the capillary has very thin walls of one cell pic endothelium so the and I'm just I'm blowing out a red blood cell over there and inside the red blood cell it's carrying oxygen now as a reminder the arterials mean function is to slow the blood down before it goes into the capillary why does it have to do so because imagine if it did not slow the blood down and the red blood cell just washed through the capillaries very quickly and if it travels through the capillaries extremely quickly the there will be no time for the red blood cell to give its oxygen to the neuron so the neuron will not obtain the oxygen from the blood so the function of the arterial is it has to slow the blood down because by slowing the blood down the red blood cells will move slowly within the capillaries in order to allow time for exchange to happen in the capillary so as the red blood cell is moving very slowly it enters the capillary then and the one cell thick wall of the capillary minimizes the diffusion distance so that diffusion can easily happen oxygen can diffuse from the red blood cell into the neuron so that's a good thing and the narrow Lumen is also important because it ensures the red blood cell is as near to the body cell as possible now students will be like I don't understand that part because they were like can you elaborate further on the narrow Lumen okay to understand the never Lumen concept let's look at three situations I'm drawing out three blood vessels and inside the blood vessels you can see that we say red blood cell inside there and there are transporting oxygen and there is a body cell outside the blood vessel now if we look at this first blood vessel can the oxygen from the red blood cell diffuse into the body cell the answer is no it can't because even though the blood cell is near the body cell okay the wall is too thick so that's not a good thing now let's look at the second blood vessel can this oxygen diffuse into the body cell the answer is also no because the wall is thin yes that's a good thing but the red blood cell is too far away because the Lumen is so wide that the red blood cell is not near the body cell so the third example of this blood vessel is the perfect example and in this case oxygen is able to diffuse into the body cell because not only is the wall thin but the natural Lumen forces the red blood cell to be as near to the body cell as possible so the thin wall and the narrow Lumen minimizes the diffusion distance so that exchange can happen between the body cells and the blood so in this case here the red blood cell will provide the oxygen to the body cells and the body cells will provide carbon dioxide back to the blood that is why the blood going to the body cells is oxygenated which I've highlighted in pink and the blood going away from the body cells becomes deoxygenated which I've highlighted in blue so this is what we have to know about the capillaries in detail the the capillaries have a one cell thick wall to minimize distance of diffusion and the narrow Lumen ensures that the red blood cell is as near to the body cell as possible so that diffusion can easily happen