in chapter 24 we're going to cover the digestive system now when we're eating food we cannot absorb food in its whole form our bodies need to break down each of these food items into the smaller subunits or the monomers by reactions in chapter two we learned that reactions that break substances or molecules into their smaller subunits are is called catabolism and this is what we need to do in order to absorb those monomers so our cells can use these particular molecules conversely when we're growing or repairing our bodies we're going to take those smaller monomers from these food substances and build larger molecules and eventually tissue we call these building reactions anabolism now the digestive tract is going to be also called the gastrointestinal tract or the elementary canal and this is essentially one long muscular tube that begins with the mouth our oral cavity continues throughout our body until the food exits our body at what we would call the anus along the way the food is going to be broken down and absorbed and there will be accessory organs such as our teeth and glands gallbladder our pancreas that are going to assist with this process this diagram or illustration shows all of the components of the digestive system that we're going to cover both the main organs as well as the accessory organs so the processes that will be included as we're looking through the digestive system is going to be ingestion of our food beginning with our mouth we'll see how digestion involves not just mechanical processes but also chemical processes the mechanical is going to involve breaking down the food into smaller parts as well as moving it along the digestive tract and the chemical digestion is going to be using various chemical substances in our body to help break those food substances down by catabolic reactions secretion is a very big component of activities in the organs of the digestive system most of the food we and our monomers will see are going to be absorbed in the small intestine area lastly defecation is the term that we used to our rid our bodies of the components of waste products or substances in the food that we don't eat or absorb and so these are called the feces most of the digestive system can be found in the peritoneal cavity remember the peritoneum is the serous membrane that is lining this peritoneal cavity it is going to include two layers because the serous membranes are double membranes the visceral peritoneum is the portion of that double membrane that is directly on the organ so we see it covering the the liver and the stomach whereas the parietal peritoneal membrane is the membrane that is going to line the body wall such as the abdominal body wall now the peritoneal cavity is filled with fluid if we have a some type of dysfunction in our body in which the fluid is building up it can result in what we call ascites and we see here the buildup of the fluid in the peritoneal cavity and let me switch to a pointer here i'll put it on the laser we looked at the mesentery in our fetal pigs and that is the connective tissue that we see holding the components of the intestines within the peritoneal cavity together this we will see is going to have blood vessels within it as well as lymphatic tissue and nerves we may be able to see mostly some small artery arterioles and capillaries in this area now we also can look at mesenteries that are called the omentum this we did not see in the fetal pig the lesser omentum is going to be present in the lesser curvature of the stomach uh you may have noticed the falsiform ligament uh extending through the liver when we were looking at the pig liver there is another omentum that is referred to as the greater omentum and this is one structure that we did not see in the fetal pig but we would have seen if we had continued with adult cat dissections when opening up the abdominal pelvic area there is a large apron like structure that extends below the stomach it's attached to the greater curvature of the stomach and it is consisting of connective tissue and adipose tissue there are more mesenteries present and identified within the abdominal pelvic area remember learning about retroperitoneal in 168 this is referring to organs that are actually found behind the peritoneal cavity so they're still in the abdominal pelvic area they're but they're not within the peritoneal cavity this would be kidneys or the pancreas this is an illustration showing organs that are present within the peritoneal cavity for instance the liver and the stomach and the other organs that are retroperitoneal such as the pancreas and i don't see do they have the kidneys i don't see the kidneys shown on here as being retroperitoneal also and the duodenum also the digestive tract is going to be comprised of four layers and these layers are pretty consistent throughout the digestive system even though in each different area there's going to be a different organization of these layers and that is what you would expect because structure determines function and the different areas of our digestive system are going to have different functions the first or most superficial layer of the digestive system that we will look at is called the mucosa and we touched on this a little bit in 168 this layer is going to be responsible for secreting the mucus as we need to help protect the epithelial lining the mucous membrane has the layer of the epithelial tissue deep to this is a lamina propia so connective tissue and also a muscular layer so a muscular mu must muscular muscularis mucosae that's a mouthful let's look at the epithelial layer that we see in the mucosa as we progress through the digestive system it's going to vary so in the esophagus where there's quite a bit of abrasive activity with food traveling down it's going to be the stratified squamous epithelial tissue as we go into the further organs of the digestive system such as the stomach and then here we are in the small intestine we and even into the large intestine you notice that the cells are going to now be the simple columnar epithelial tissue that's going to uh be involved in secretions and then we also if we zoomed in we would be able to see quite a few of the mucus secreting cells in goblet cells another type of cell that is present within the epithelial tissue are called enteroendocrine cells when you hear the term entero the prefix that's going to tell you that it's going to be found in the gut so mostly associated with intestinal area endocrine means that it's going to be releasing substances into the tissue to the fluids in the body such as blood or into interstitial fluid we will look at the function of these type of cells later on deep to the epithelial layer is the lamina propria this is going to be connective tissue and it will be that loose areolar connective tissue that we find so often in our bodies this is going to be where we find the blood vessels and nerves and lymphatic tissue um as you would expect because the epithelial lining is going to need the nourishment but it's not vascularized now unique in the digestive system is we find smooth muscle throughout and even in this mucosal layer is some smooth smooth muscle that muscularis mucosa that was giving me a tongue twister earlier this is just another view of the muscularis mucosa and it is showing that there's two layers as we so often see with smooth muscle that there's more than one layer and they the cells these smooth muscle cells will be oriented in different directions to allow for movement in different directions deep to the mucosa is the submucosa this is irregular connective tissue or dense irregular connective tissue and the vasculature that we saw in the mucosa is ex and the lymphatics and nerves they're extending into this submucosa and we should be aware that our digestive system even though it can function independently is going to be impacted by the autonomic nervous system just a good case of nerves and how it impacts your your gut feelings should help you understand that we see in the submucosa a neural plexus and so this is going to include parasympathetic as well as sympathetic fibers of course the digestive system is primarily under parasympathetic control now deep to the submucosa we find the muscular layer and this is going to be comprised of smooth muscle cells and once again there are two layers and these layers of the smooth muscle cells are going to be oriented in different directions so when they correct contract it's going to result in a different type of movement and we need this muscular movement in the digestive system because it is going to be moving the food through our system now the digestive system can function often independently of the central nervous system so we refer to this as the enteric nervous system and we're learning about this quite a bit how extensive it is and involved it is and once again here we see that it is primarily under parasympathetic control now the neural tissue that we find in muscular layer we refer to as the myenteric plexus the under control the autonomic nervous system the uh last layer that we look at the outermost is going to be the serosa and this is going to be a membrane sometimes this is referred to as the adventitia and that is generally when the serosa is attaching it to outer structure such as the body wall food moves through the digestive tract and there are two types of movement that we will identify these smooth muscle cells in the digestive tract can function similarly to the autorhythmic activity of the cardiac cells in that they can function independently and they have this rhythmic type of contraction activity now the mechanism is not the same as we looked at the pacemaker cells the type of movement is going to be referred to as either peristalsis or segmentation peristalsis is the type of movement that is going to be waves of the muscle contractions and this is going to propel the bolus of food through the digestive tract whereas segmentation is random contractions that are turning up the food but keeping it more in place so peristalsis what we see as this food bolus is moving is the circular muscles will contract behind the food bolus so it is moved forward and then the longitudinal muscles will contract and that is going to be the coordination of those two muscular contractions those two layers of the muscles is going to result in movement of that food bolus forward through the digestive system whereas segmentation is going to be random contractions in different locations in order to break up this food bolus now how we regulate the activities of the digestive system are going to depend on three different mechanisms they'll respond to local factors neural factors and hormonal activity as well locally this is going to be when the organs of the digestive system respond to chemical components such as ph which we would recognize as hydrogen ion concentration or perhaps volume of food in the system so let's say we'll look at when the stomach is is full and distended what happens this the organs of the digestive system will respond to these type of stimuli other chemical constituents that we've touched on little bits previously prostaglandins histamine can also cause an effect the neural right mechanisms are going to be with reflexes and they can either be of the type of short reflexes such as locally functioning or long reflexes the short reflexes are independent of the central nervous system they're going to be very localized within that digestive system responding to let's say stretch of a particular area resulting in contractions whereas long reflexes are going to include the central nervous system so these are much more complex mechanisms and we'll see these when we're looking at peristalsis that occurs with large volumes of food substances and the hormone mechanism is going to involve the secretions from those entering endocrine cells and this is going to involve several different types of hormones we're going to look at it quite a few as we go through this chapter and as true with hormones compared to neural impact it's going to be more widespread and of generally a longer duration we'll touch just a bit on the oral cavity sorry dental people um you will have to glean most of your information as you enter into your programs we'll just uh briefly touch on some of these areas our oral cavity which we recognize as our mouth of course is going to have the sensations of food the taste the texture but this is also an area where mechanical digestion is beginning because we're beginning to chew our food and break it down into smaller components so we can swallow it saliva is also going to be released and help with this process but in addition we see that digestion is actually starting in the oral cavity there is a mucosal lining in this oral cavity once again stratified squamous epithelial tissue because there's a lot of abrasion from the foods that we eat the oral vestibule we looked at in our pig so that's you know that area in your what we would recognize as our cheek between our cheek and our teeth so if you're swishing mouthwash around that's going to be in your oral vestibule and the gingiva are our gums this shows some of the structures the only structure i'm going to point out here is the uvula our tongue we looked at that in the pig and most of us are familiar with our tongue this is going to be an organ that's involved in the mechanical digestion it helps move food around in our mouth and helps us to swallow as well as taste and texture that structure that that ligament that's hanging holding the tongue down to the floor of our mouth is called the frenulum and we saw that in the pig uh there is um a lipase uh enzyme also that is present from the salivary gland deep to the tongue our teeth are also involved in mechanical digestion the root of the tooth is the portion of the tooth that is uh within the the gums or the gingiva so it's within the socket and there is a that periodontal ligament that's going to hold the tooth in and that creates that joint we identified back in chapter 9 the gomphosis the part of the tooth that's exposed that we see we call that the crown and the surface is coated with the substance recognized as enamel and i believe an animal is even more uh hard and rigid than the bones in our body extremely durable we do have two sets of teeth those first teeth that we recognize as our baby teeth are called the deciduous teeth and they're replaced with the permanent teeth and there are more of the permanent teeth and this picture here i find it very interesting because you can see the adult teeth behind the baby teeth we have three uh salivary gland pairs in our oral cavity area the parotid glands the submandibular glands and the sublingual glands each of these glands has a secretion that's a bit unique and here is an illustration showing those glands the submandibular gland you may if you uh go a little bit above your jugular veins in your pig you are likely to see it it's pretty clear it looks like a lima bean the parotid is large but it's also very thin and flat and it's very it is very hard to find because it blends right in with this masseter and we don't dissect deep enough to find the sublingual gland in our pigs and that takes us to where we picked up in lecture so i will stop here