[SOUND] Welcome back. This is the milk ejection module. This is the first of a series of videos that we're going to explore this physiological process called milk ejection. So let's try to put this in context. We've gone through different stages of mammary development, development during pregnancy, lactogenesis, the initiation of lactation, lactation, the maintenance of lactation. We talked a lot about one of the key things there being milk removal. And milk ejection is actually part of this process of milk removal. So let's start to examine this a bit more. But first let's review. We used this in the lactation module where we kind of got the concepts of systemic factors, hormones. And in particular, I think we focused mostly on prolactin as being one of those key systemic factors that's causing the mammary gland to refill after milk is removed. When the animal is suckled, when the milk is removed from the milking machine, whatever the case may be, a very strong, positive impact there. So it's basically telling the gland to refill and again, prolactin being a key there. At the same time we have removal of local factors, local feedback inhibitor of lactation, FIL. It's kind of an abbreviation. There's a number of factors involved in that. These have been removed because they're actually out in the milk and when the milk's removed that's removing those. And over time, as we go through time, this balance starts to shift where we get here to the kind of the other end when the gland is filled up. These systemic factors have declined in their impact. And we have a lot of the accumulation of the feedback inhibitive lactations of heavy negative or inhibitory components of this. Milk the animal, the milk is removed. Boom, we go right back over. Milk ejection is really part of this component of it. This is part of the process of physically how the milk is removed. And I want to remind you of several characteristics of the anatomy of the mammary gland. First of all, well, we'll start down here. These white areas here represent the cisterns. So we have a teat cistern in the teat, right above that. Another example over here would be the gland cistern. And then the large cisternal ducts. Those kind of tissues do not secrete milk. They do not produce milk. And so they're just kind of there and they're kind of milk collecting areas. We'll expand on that later in some of the other videos, the fact that they're not just collecting milk. Milk in those areas can be removed without milk ejection. On the other hand, 80% or more of the tissue up here, where the alveoli or the small ducts are that is producing milk, these milk ejections come out because it's held up there by capillary forces, or capillary action. So it has to have milk ejection for that milk to come out of the gland. So now we're going to go and look a bit more carefully at this idea of capillary forces and capillary action and see what that means. The milk that's in the alveoli and the udder and the alveoli in the small ducts is held in there by capillary action. So we want to talk a little closer look at capillary action. But let's work our way down to this. So take some fairly wide bore tubes here. So this is a ten milliliter pipette, and I've cut the ends of this pipette off. And we have a beaker of milk here. If we put that in there, it doesn't come up very far. So there's not a lot of capillary action going on here. And of course, the reason for that is because the diameter of this is too broad. We have a 1 milliliter pipette that I've cut the ends off of, and that's this guy. And so this is about, this broader one was about 7 millimeters in inside diameter. This is about 2, 2.5 milimeters inside diameter. We put that down in our milk now. We've got a little bit of capillary action. There's little bit of drawing up there, and certainly nothing stays in there. It comes right out when I pull it up. So again not very much in the way of capillary action going on there. If we take a real capillary tube, on the other hand, and this guy is only about a little less than one millimeter in diameter, about .9 millimeters. We put that in there, and we watch this. The fluid's going up, it's going up by almost a centimeter there above the surface of the milk. So what's happening? Why is this happening? What we find is that if you look at the diameters, say, a large diameter versus a very, very small diameter of a tube, we can kind of draw a curve like this. So as it gets smaller and smaller in diameter, you get a higher and higher water height, or water being drawn in. Again, this one is about a little less than a millimeter but those alveoli and small ducts, the diameter of those is almost one-tenth or less of that. So about 100 microns, which about 0.1 millimeters or smaller. And so you can see that, that fluid's going to be drawn up as it's secreted in the alveoli and the small ducts. The only way to get that out is to squeeze it out. So let's take a look at that. So let's change our perspective a little bit. So we need to think of this plastic pipette as a capillary that is very, very small bore. And remember that the alveoli and the end of the ducts are basically blind sacs. So if we draw some milk up in that, again, we're kind of cheating here, because of the way this thing's tapered and not very much milk is going to come out on its own. But the only way you can get that milk out is to squeeze it out. And that's what's happening with the myoepithelial cells that are lining those small ducts and the alveoli, squeezing the milk out of the lumen of the alveoli and the small ducts and down through the gland. So let's go back and look a little bit more carefully, with a little bit more detail, at this milk ejection process. [SOUND]
