After the bolus of food passes from the pharynx, it enters into the esophagus, which I've mentioned before. The esophagus is another long, muscular tube. This tube is about 25 cm. long and it joins the laryngopharynx to the stomach. You could see the stomach below this sac here.
I've just brought in the muscle layer because I just want to remove it to show you where the esophagus passes through into the abdomen. I'll just gradually remove the muscle layer. Actually, the skeleton as well. I don’t know why I put that there. You can see that the esophagus passes through the diaphragm, this kind of dome-shaped muscle which helps in respiration. It passes through the esophageal hiatus and the diaphragm to enter into the abdomen.
I'll just remove that. You can see that the esophagus joins the stomach. You can see that sac-like organ sitting here. This is the stomach.
I'm just going to remove some of these accessory organs of digestion. To the right of the stomach and a little bit anteriorly, we've got the liver. I'm just going to remove that. We've got this little sac sitting under the liver called the gallbladder. I'll remove that as well.
And to the left of the liver, the stomach, you've got the spleen, which isn't an organ of the digestive system. It's an organ of the lymphoid system. I'll just get rid of that as well.
You can see the esophagus joins the stomach. The region where it joins is called the cardiac region of the stomach because I guess this is the part of the stomach that lies closest to the heart. I've just added the vessels in here and the cardiovascular system and you can see that it sits just above the stomach.
Something to point out is that at this junction between the esophagus and the stomach, you have a sphincter. This is called the lower esophageal sphincter or the cardiac sphincter because it lies in the cardiac region.
A sphincter is essentially a circular muscle in the body which keeps a passage closed. A sphincter relaxes to open and it contracts to close.
You've got the lower esophageal sphincter or cardiac sphincter at the junction between the esophagus and the stomach. The sphincter is formed by circular muscle within the walls of the esophagus. And you've also got an upper esophageal sphincter at the other end of the esophagus.
The muscular wall of the esophagus contracts in waves, which is called peristalsis. This propels the bolus of food from the laryngopharynx down the esophagus into the stomach.
Next, we've got the stomach which I'm sure you're well away of and I've pointed out a few time already. The stomach is a muscular sac and it sits in the upper left abdomen. You can see its position here relative to the surface of the body.
It also lies under the diaphragm. If I just bring in the muscles, you can see the diaphragm sitting over the stomach.
And if you remember, we've got the liver sitting to the right of the stomach. You can see that the liver has these two parts. The left part of the liver called the left lobe lies in front of the stomach. And then to the left of the stomach, you've got the spleen, which isn't a digestive organ. It's part of the lymphoid system.
This end, the proximal end of the stomach is called the cardiac region. And then at the distal end, you've got the pyloric region. And you've got another sphincter at the end of the stomach called the pyloric sphincter.
The sphincters help to keep the contents of the stomach contained. The upper sphincter, the cardiac sphincter or lower esophageal sphincter prevents contents of the stomach refluxing back up into the esophagus. You've got things like acid and other enzymes and things in the stomach that you don’t want coming back up there into the esophagus. And if you were to stand in your head or flip upside-down, you don't want all the contents of your digestive system to come exploding out of your mouth. This is the purpose of sphincters, to keep things contained.
The function of the stomach is to mechanically digest the food by churning it. The muscular walls of the stomach churn up the food and mix it with digestive enzymes. And you've also got some cells in the walls of the stomach producing acid which kills bacteria. In this way, it serves as an infective barrier.
The partially digested food of the stomach through this chemical acidic and mechanical digestion passes into the duodenum, which is the next part we're coming onto, this part here of the digestive system. This partially digested food is called chyme.
That’s the stomach and esophagus.