The human Digestive System and layers
The trunk of mammals is divided into a thorax, which is bordered by ribs, and an abdomen. Most of the digestive system is within the abdomen, but the head, neck, and thorax contain the superior end of the digestive system
The digestive system includes the alimentary canal or gastrointestinal tract, a tubular, muscle-lined passageway that extends from the mouth to the anus. The muscles in the walls of the passage way are responsible for peristalsis. You will view the regions along this tube that are specialized for various activities of the digestive system.
Path of Air
Since the paths of air and food are closely associated, let’s also trace the path of air through the head and neck. When the mouth is closed, air enters the body through the external nares or nostrils.
The external nares lead to a chamber dorsal to the hard palate, called the nasal cavity. Air then passes from the nasal cavity to the nasopharynx, which is a chamber dorsal to the soft palate. Only mammals have a hard palate, which separates the breathing and eating passageways.
This allows mammals to chew food and breathe at the same time. Only mammals truly chew their food and have teeth specialized for chewing. After air enters the glottis, it proceeds through the larynx and trachea to the lungs.
Path of Food
The head and neck contain the mouth, oral cavity, and pharynx. Food enters the oral cavity through the mouth, which is bounded by the lips. The roof of the oral cavity is formed by a bony palate, the hard palate. At the posterior end of the hard palate the bone ends and the roof of the mouth becomes soft, i.e. the soft palate. (The difference in texture is not notable on the models, but bone is modeled in the hard palate. ) The oral cavity is that part of the digestive tract from the lips to the end of the hard palate.
Digestion begins in the oral cavity, as digestive enzymes such as amylases secreted in saliva, are mixed with the food during chewing. Amylase catalyzes the breakdown of amylose, or starch, into simpler carbohydrates.
The tongue pushes food from the oral cavity into the region ventral to the soft palate, which is called the oropharynx.
Projecting into the oropharynx is a flap, the epiglottis, which partially surrounds an opening at its base, called the glottis. The glottis eventually leads to the lungs. Note the two openings in the pharynx. One goes to the trachea, which is always open due to its rings of cartilage, and the other opening is to the esophagus.
When food enters the oropharynx, a reflex makes the epiglottis fold back, closing the glottis and allowing food to slide into the esophagus. Once in the esophagus, peristaltic contractions of the esophagus push the food to the stomach. No digestion occurs in the esophagus. It is just a passageway that connects the pharynx to the stomach.
The abdomen is the area inferior to the ribs that contains most of the digestive, reproductive, and excretory organs, collectively known as the viscera. The cavity in the abdomen that contains the viscera is the peritoneal cavity, which together with the thoracic cavity constitutes the coelom. The diaphragm, a muscle that aids in inhalation, forms a partition between the thoracic and abdominal cavities.
Pressed against the inferior side of the diaphragm on the right side of the abdominal cavity is the dark brown liver. Its color is due to its rich blood supply.
The liver has a variety of functions, including the production of bile, which aids in the emulsification of fats. Bile is stored temporarily in the gall bladder, a greenish sac embedded in the posterior face of the liver.
Find the spleen, which is a fist-shaped, brown organ on the left side of the abdominal cavity that superficially resembles the liver in color, due to a rich supply of blood.
The spleen functions in the storage, destruction, and production of red blood cells. The spleen is not part of the digestive system, but is mentioned here because students always ask about it.
Inferior to the diaphragm on the left side of the abdominal cavity is the sac-like stomach. Locate the cardiac end of the stomach near the diaphragm where the esophagus enters the stomach. It is named the „cardiac end because it is the end closest to the heart.
There is a weak muscle here called the cardiac sphincter that keeps food in the stomach from splashing on to the esophagus. The stomach walls secrete HCl as well as pepsin, an enzyme that catalyzes the breakdown of some proteins.
At the posterior end of the stomach is a constriction where the stomach joins the anterior end of the small intestine. This constriction between the stomach and small intestine is the pyloric sphincter, which is a circular muscle (a sphincter) that controls the movement of acidic chyme from the stomach to the small intestine. It allows just a little bit of stomach contents to move into the small intestine at a time.
As the stomachs peristaltic waves push food from the cardiac end to the pyloric end, the pyloric sphincter will open and let a bit of partially digested food into the small intestine.
The anterior end of the small intestine is the duodenum– so named becasue it is about 12 inches long in the average human. (Latin for twelve is „duocecim.) The small intestine on the model is not loose as it is in real life. In a living person it is actually a tube about 3 meters long.
The duodenum begins at the pylorus of the stomach, extends to the right side of the abdomen, then loops back to the left half of the body, passing close to the stomach and spleen.
Accessory organs – these are not part of the alimentary canal, but are important organs for digestion.
Inferior to the stomach and posterior to the small intestine is the pancreas. The pancreas is a long, irregularly-shaped gland, with superficial resemblance to cottage cheese. Some of its products, like the hormones insulin and glucagon, are dumped into the blood as they are needed. (Thus, the pancreas is an endocrine gland.) The pancreas also produces enzymes that catalyze protein digestion. These enzymes which are released as needed into the duodenum through a duct. Thus, the pancreas is also an exocrine gland.
Bile produced in the liver also empties into the duodenum. Ducts from the liver and gall bladder join to form the common bile duct which enters the anterior side of the duodenum right next to the pylorus. Bile emulsifies fats, and the digestion of fats does not begin until they reach the small intestine.
The small intestine loops back and forth, and fills much of the abdominal cavity. The small intestine is held in place by fan-like folds of connective tissue (mesentery) that contain many blood vessels. Why would so many blood vessels be attach to the small intestine? (Think about the major reason we have a digestive system)
Find where the small intestine is attached to the large intestine and you will find 3-way junction with the colon (a.k.a. large intestine). A dead end tube or sac, the caecum, will be on one side of the junction. The colon is on the other side (the small intestine is, of course, the third). In humans there is a relatively short caecum off of which an even thinner extension is found This dead end tube, about as big around as a pencil and a couple of inches long.
The colon has three regions, each named for its orientation. Within the ascending colon material moves upward. Within the transverse colon material moves from right to left, and within the descending colon, material moves inferiorly toward the rectum.
The distal portion of the colon extends into the true pelvis, which is the cylinder surrounded by bone in the center of the pelvis. The colon ends at the sigmoid colon and then the rectum. The external opening of the rectum is the anus. (Sigmoid means “similar to sigma” or “similar to the letter S” as that part of the colon makes a few zigzags on its way to the rectum.)
Layers of the Digestive system
The GI tract from the esophagus to the anal canal is comprised of 4 layers or tunics. Each layer performs specific functions in the digestive process.
These layers are :
1. Mucosa- the innermost layer lines the lumen of the GI tract. It is both absorptive and secretory in function. It contains lymph nodes as well as goblet cells which secrete mucous. There is also a thin layer of smooth muscle in this tuned.
2. Submucosa- this is the second layer, much thicker than the mucosa. It is primarily vascular and nerve containing. Absorbed molecules pass through the mucosa to enter blood or lymph vessels here. The submucosa contain glands and a nerve plexus (Meissner’s plexus) which provides autonomic innervation to the muscle layer in the mucosa.
3. Tunica muscularis- This is the primary smooth muscle layer of the GI tract which is responsible for peristalsis. It has an inner circle and an outer longitudinal layer of muscle. Contraction of this layer causes the movement of food as well as helping to pulverize and churn the food with digestive enzymes. There is a large nerve plexus (Aurebach’s plexis ) located between the 2 muscle layers. It provides both sympathetic and parasympathetic innervation.
4. Serosa- is the outermost layer of the GI tract wall. It is binding an protective in function.
Defense mechanisms of the GI tract
The GI tract has several mechanisms for protecting against harmful materials ingested. The lining of the stomach produces concentrated hydrochloric acid which can kill some organisms. The mucous lining of the GI tract serves as a protective layer as well producing copious amounts of mucous that may dilute harmful substances. The most helpful mechanism is vomiting which is a reflexive response to toxins and irritants. Diarrhea can also help remove harmful substances as well.
Innervation of the GI tract
The GI tract is innervated by the sympathetic and parasympathetic divisions of the nervous system. The vagus nerves are the source of parasympathetic activities in the esophagus, stomach, pancreas, gall bladder, small intestine and part of the large intestine.
The lower portion of the large intestine receives parasympathetic innervation from spinal nerves of the sacral region. Stimulation of the parasympathetic nerves increases peristalsis and GI tract secretions.
Sympathetic nerve fibers pass through the submucosal and myeteric plexuses that innervate the GI tract. The effect of sympathetic nerves is in opposition to parasympathetic nerves. Sympathetic impulses inhibit peristalsis, reduce secretions and constrict muscle sphincters along the GI tract.