You have learned what the major regions of the digestive system are in lab. We will do a little more next week in lab to finish up. On this page are some things that we haven't been able to get to so far... and a little more information about the functions of each region.
Salivary glands are those exocrine glands that secrete the saliva for our mouths. There are three major glands that make up the salivary glands: the parotid, sublingual, and submandibular glands. These glands secrete the saliva, which is a mixture of mucus and a thinner fluid called serous fluid. The serous fluid includes a digestive enzyme, amylase, described more on the digestive enzymes page. These glands secrete their saliva through ducts into the oral cavity.
The structure of the tooth is shown in Figure 17.10, which is also included here. The tooth fits into the alveolar processes of the mandible and maxillae of the skull. It is mainly made up of a material called dentin, which is similar to bone-- but harder! The dentin is surrounded by a thin layer of material called cementum. You may also remember that each tooth forms a non-movable joint with the skull bones called a gomphosis; that is between the cementum and the skull bone, and the ligaments that hold the gomphosis together are called periodontal ligaments.
A tooth has a portion that sticks up over the gums called the crown, and a portion hidden below the gums, called the root. The crown is coated with white enamel, which is a very hard, basically acellular substance. I have noted that it is acellular, so that you understand that if enamel is scraped away, it cannot be replenished since no cells are there to secrete it. Enamel is mainly calcium salts, like lobster and clam shells.
Within the root of the tooth is the root canal, the area that houses the nerves and blood supply for the tooth. This should not be confused with a dental procedure called a root canal-- in the dental procedure, the tooth's root canal is destroyed so that the nerves within the tooth are no longer functional. Any time nerves are destroyed, it is rather painful.
The above is enough of the tooth anatomy. The only other interesting thing about teeth is that we have two sets-- our primary and secondary teeth. Our primary teeth are the ones we have first, and we start getting them around 6 months of age. Our secondary teeth, also called "permanent teeth," come afterward. The secondary teeth develop from our maxillae and mandible. Each moves into position after a primary tooth resorbs its roots.
The esophagus' primary function is to send food from the pharynx to the stomach. It is basically a food transporter. As such, it has to be very good at movement. The esophagus has lots of muscle in it, within its muscular layer. In fact, this muscle is not even all smooth muscle! It is skeletal muscle up toward the pharynx; then down by the stomach it is smooth muscle. You will see how it functions to move the food along on the digestive movements page.
If I have to pick a primary function for the stomach, it would be for food breakdown. Remember from lab that we discussed how food has to get broken down into teeny-enough bits that cells can take in the food by endocytosis. Well, the stomach really works at breaking the food down.
In order to break food down, mechanical and chemical actions work on the food material. The mechanical actions are from the movements of the stomach (and even before this point, from chewing in the mouth). The chemical actions are from the digestive enzymes secreted into the stomach. Once food enters the stomach, these mechanical and chemical effects begin to occur, and we no longer call the material within the stomach "food." Instead, the wet mass of broken up food and digestive chemicals is called chyme.
The chyme in the stomach is at a very acidic pH (you'll see why on the digestive chemicals pages). Also, the digestive enzymes in the stomach are very effective. Why doesn't the acid burn our stomach lining? And why don't the digestive enzymes digest away our stomach lining? The stomach is protected by two things: 1) a thin mucus-coating covers the stomach epithelium; and 2) a viscous, basic (alkaline) solution is secreted by cells in the stomach wall to coat the epithelium. In this way, the cells are protected by mucus and any acid that gets through the mucus hits a basic solution before arriving at the cells. When the acid hits the base, it is neutralized. Nice, huh?
I would like to point out the main function of the small intestines on this page. The small intestines are primarily concerned with absorption! You see, by the time the chyme enters the small intestines, the food particles within it are getting smaller. Certainly, more breaking up of the food particles has to occur, and this will happen in the small intestines, but the main job of the small intestines is to grab up all the small bits of food particles to supply our bodies with nutrients. That is absorption.
By the time the chyme has made it to the large intestines, it is a solution of mainly unabsorbable, useless material; we have already taken up most of the nutrients from it in the small intestines. But now, we have to get rid of the unnecessary material. The chyme is fluid and unnecessary material, so the large intestines have to salvage the fluid and electrolytes within the chyme. This process can be called resorption, and it is what the large intestines do best. By taking all the fluids and useful material back out of the chyme, the large intestines concentrate the useless material into feces for removal from the body.
Digestive System Accessory Organs:
The digestive system can be described as being made up of the alimentary canal plus some additional organs. These non-alimentary, additional ones are the so-called accessory organs of the digestive system, and include the liver, gallbladder, and pancreas.
Although two of these organs have other functions besides digestion, when we talk about their role specifically in digestion, you will see that these organs act as exocrine glands. Exocrine glands are ones that secrete their material into ducts. Therefore, after the description of each organ, I have also added a section just to describe their ducts. Their ducts lead into the alimentary canal, so that their secretions directly affect the chyme.
The cells of the liver, the hepatic cells, are arranged in rows like spokes in a wheel. The entire wheel is called a hepatic lobule. The spaces between the spokes are the hepatic sinusoids. The hepatic sinusoids are compartments full of blood, allowing for nutrients in blood to flood into the liver. You see, the liver is a metabolic machine. It has many roles in building and breaking down nutrients, as well as in detoxifying our blood. To do this, its cells need lots of exposure to blood. The hepatic sinusoids allow for that. We'll get into the metabolic functions of the liver in the next unit.
For the digestive system, the important thing that the liver does is to make bile. The hepatic cells make it. Bile contains:
The hepatic cells make the bile and secrete it into tiny, tiny ducts, called bile canaliculi. Bile is NOT secreted into the hepatic sinusoids-- that's for blood. Bile goes into the canaliculi, and then these canaliculi fuse to make larger ducts called bile ducts. The bile ducts come together to make hepatic ducts. Each lobe of the liver has a hepatic duct that comes together to make the common hepatic duct.
Remember, these accessory organs act as exocrine glands for digestion. Exocrine glands always secrete into ducts, never into blood.
The gallbladder isn't truly an exocrine gland. You see, to be an exocrine gland, you have to make something for secretion and then secrete it into a duct. But the gallbladder doesn't really make anything-- it just secretes!
The liver makes bile and then sends it out into the common hepatic duct. But the common hepatic duct doesn't hold much bile in it, and the liver keeps making bile. So, the bile backs up, and it can enter the gallbladder through the cystic duct. Once bile finds itself in the gallbladder, it stays there until the gallbladder receives a signal (from cholecystokinin, as you'll see later) to release it into the small intestine. While inside the gallbladder, the bile gets concentrated. You see, the gallbladder does work on the bile, reabsorbing a lot of the water the liver secreted when making the bile. When the signal comes to release the bile, it all flows out of the gallbladder, through the cystic duct, and on into the common bile duct (as described below).
Please note that often people experience problems with their gallbladders. You see, sometimes the gallbladder gets too active at resorption, and the bile crystallizes... this is what forms gallstones. These chunks of material, the gallstones, can get stuck in the cystic or common bile ducts and cause pain. This problem is usually fixed by removing the gallbladder so that it doesn't continue to occur. Note that the gallbladder is only a storage area, so we can do just fine without it!
The pancreas makes most of the digestive enzymes used in the small intestines, as you will see on the digestive secretions pages. The pancreas is extremely important for digestion.
The pancreas makes pancreatic juice, and that flows through the pancreatic duct (via the hepatopancreatic duct) into the small intestines. The enzymes that are in this pancreatic juice are described on the digestive secretions pages. But here I'll mention a bit about the pancreatic structure.
You see, the pancreas is both an exocrine gland as well as an endocrine gland. You learned about its endocrine functions when we discussed the endocrine system (it makes glucagon and insulin), and for this function it secreted its hormones into blood. But it also secretes other chemicals and fluid into a duct. These are what make up exocrine secretions for the pancreatic juice. For exocrine secretion, cells called pancreatic acinar cells are arranged around ducts and secrete into them. These small ducts join together to form the pancreatic duct. Meanwhile, other cells within structures called Islets of Langerhans (you are not responsible for this term) are the ones that have the endocrine function. Your book shows all of this back in the endocrine chapter on page 507 in Figures 13.32 and 13.33.
Accessory Organ Ducts!
I threw this in, because these three organs all send their secretions to a common duct that enters the small intestine at the proximal aspect of the duodenum. The duct is called the hepatopancreatic ampulla, since it is a short, bulbous-shaped duct. The liver and gallbladder secretions run in the common bile duct, while the pancreatic secretions run in the pancreatic duct. These two separate ducts, the common bile duct and the pancreatic duct, join together to form the hepatopancreatic ampulla. The only thing left is to explain the common bile duct... you see, both the liver and the gallbladder secrete into the common bile duct, but through their own smaller duct connections to this common bile duct. The liver secretes into the common hepatic duct, while the gallbladder secretes into the cystic duct. So, when the common hepatic and cystic ducts merge, they create the common bile duct.
I had hoped to get to all of these ducts in lab, but we just didn't have the time.
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