食道

after the food is swallowed it leaves the mouth and then goes next to the esophagus so let's focus on that and just to make sure we all know kind of how far the esophagus ghosts so I'm gonna draw in some lines right here to show where it starts up here and then it ends right about there and kind of where I've drawn the lines as well there are sphincters that's it to make sure food only flows in one way up here we have the upper esophageal sphincter esophageal sphincter and so just to reiterate a sphincter is just a circular localization of muscle so just a bunch of muscle that sits in a ring right here that makes sure that it's closed unless we tell that muscle to relax so we can pass food along and so when I say that we're going to tell the muscle to relax that implies then that this muscle here is under our control so it's composed primarily of skeletal muscle skeletal muscle so it's under voluntary control and down here I'll draw a squiggly line we have the lower esophageal sphincter so this is a little tricky because we call it a sphincter you know it smells like a sphincter looks like a sphincter but it doesn't actually work like a sphincter so it's sort of I guess a scientific joke if you'd like to call it that because we don't have a ring of muscle that sits here that opens and closes whenever our body deems it's an appropriate time to what do we have instead well we actually have a sheet that kind of sits right here there's a sheet of muscle that lines the connection between the thoracic cavity and the abdominal cavity so you can imagine that you have a thoracic cavity above things that would sit here would be your lungs in your heart and then below and I'll draw that right here you have an abdominal cavity so the abdominal cavity that's going to have most of the GI tract and this muscle that we have that sits here that's going to be the diaphragm you've probably heard of this this is voluntary skeletal muscle that contracts to pulled downwards to help us breathe when it moves downward the lungs inflate with air so the diaphragm sits here and it actually makes a ring around this lower esophageal sphincter and the diaphragm is actually the lower esophageal sphincter because it just sits there holding the esophagus in place and that's why over time we can have what's called a hiatal hernia the esophagus can move upward and downward through this lower esophageal sphincter and will have gastric acid from the stomach reflux upwards and give us heartburn or gastroesophageal reflux disease GERD so that's why we have heartburn but rodents or horses dumped rodents and horses actually have their own true sphincter that sit here so instead they have an actual dedicated ring of muscle rather than a sheet now the other thing about the esophagus which is pretty interesting is that it works as a passageway for fluid it doesn't do much the only process that it can kind of claim to have any stake in is what's referred to as peristalsis peristalsis and this is going to be an important term for the discussion of the rest of the GI tract peristalsis is just the wave-like propulsion of food wave-like propulsion I'll just write that for now because you'll have the contraction of the esophagus at one point up here like so and by doing that it relaxes down here it relaxes here and so when it relaxes here and contracts up here that means that if you have a bolus of food like a little circle that's right there it's more inclined to move this way directly ahead and that's peristalsis and the way we control that is kind of in three parts along the esophagus because the esophagus is not just skeletal muscle all the way are not even just smooth muscle its lit up in the Third's so I'll draw three parts of the esophagus here so the first part that we have is actually just skeletal muscle so it's under our voluntary control skeletal muscle and so that's one third I'll say the top one-third of the esophagus in the middle right here we actually have a mix we have skeletal under our control plus smooth muscle and they kind of work together so it's sort of in our control but also not really so that's going to be the middle one third of the esophagus the middle one-third and kind of as the progression goes finally when we get to the end right here this last 1/3 of the esophagus is going to be smooth muscle so that's our last one third and so that's how the esophagus is split up into three parts skeletal at the top one-third skeletal and smooth for the middle one-third and then smooth entirely for the last one-third that's how our esophagus works as well