In the figure below, glycerol is shown to have three hydrogen atoms removed, and three fatty acid monomers each lose an oxgyen and hydrogen. This allows three bonds to be made, and produces a polymer in this case, a triglyceride , and three water molecules. A polymer is made from two or more monomers. In chemical digestion, the process of dehydration synthesis is reversed.
Three water molecules are used to separate the glycerol from the fatty acids, thereby producing monomers the body can absorb and utilize. Similar processes occur with proteins, carbohydrates, and nucleic acids. Therefore, the ultimate goal of chemical digestion is to utilize enzymes to hydrolyze nutrient polymers, creating monomers that are readily absorbed by the body. Questions: 1 What monomers will be produced by the chemical digestion of proteins?
Although the small intestine is the workhorse of the system, where the majority of chemical digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process. Figure 2. All digestive organs play integral roles in the life-sustaining process of digestion. As is the case with all body systems, the digestive system does not work in isolation; it functions cooperatively with the other systems of the body.
Consider for example, the interrelationship between the digestive and cardiovascular systems. Arteries supply the digestive organs with oxygen and processed nutrients, and veins drain the digestive tract. These intestinal veins, constituting the hepatic portal system, are unique; they do not return blood directly to the heart. Rather, this blood is diverted to the liver where its nutrients are off-loaded for processing before blood completes its circuit back to the heart.
At the same time, the digestive system provides nutrients to the heart muscle and vascular tissue to support their functioning. The interrelationship of the digestive and endocrine systems is also critical. Hormones secreted by several endocrine glands, as well as endocrine cells of the pancreas, the stomach, and the small intestine, contribute to the control of digestion and nutrient metabolism.
In turn, the digestive system provides the nutrients to fuel endocrine function. Table 1 gives a quick glimpse at how these other systems contribute to the functioning of the digestive system. The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body.
Accessory digestive organs, despite their name, are critical to the function of the digestive system. The main function of the organs of the alimentary canal is to nourish the body. This tube begins at the mouth and terminates at the anus.
Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. All alimentary canal organs are part of the tube of the alimentary canal, and therefore have food physically traveling through them. Both the beginning of the alimentary canal the mouth and the end of the canal the anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body.
However, that is not the only outer surface. The earthworm is basically a tube. At one end of the tube, its mouth consumes soil.
The soil passes through the tubular body. Its digestive system breaks down and absorbs nutrients from the soil. The remaining soil exits from the anus at the far end of the tubular body. After all, they are now coursing through its blood. However, did the undigested soil that exits from the anus ever actually enter the tubular body of the worm? No, the undigested soil merely passed through the tubular body. It never actually entered the body. While obviously more complex than an earthworm, the human body retains the basic tubular structure.
If you accidentally swallowed a seed while eating watermelon, inevitably the watermelon seed would pass through your digestive system, eventually being expelled in the feces.
This watermelon seed never entered your body. It merely passed through the alimentary canal. Therefore, while discussing digestion, it is important to always remember that the surface of the alimentary canal is considered an outside surface, and substances within the canal are outside of the body.
Each accessory organ aids in the breakdown of food, yet does not have food pass through it. These digestive organs do not make up part of the alimentary canal tube. Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue.
Together, these are called accessory organs because they sprout from the lining cells of the developing gut mucosa and augment its function; indeed, you could not live without their vital contributions, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts.
Throughout its length, the alimentary tract is composed of the same four tissue layers; the details of their structural arrangements vary to fit their specific functions. Starting from the lumen and moving outwards, these layers are the mucosa, submucosa, muscularis, and serosa, which is continuous with the mesentery. Figure 3. The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa.
The rates of chemical digestion processes involving enzymes are dependent on the pH of the surrounding fluids. The scale covers the range 1—14, with pH 7 being neutral, pH 1—6 acidic and pH 8—14 basic.
The enzyme amylase, which is present in saliva, has its maximum activity at pH 7. The activity of amylase changes as the pH changes — a higher enzyme activity will result in a faster rate of reaction. Enzymes are globular proteins with a characteristic shape.
This shape can alter with changing pH, the result being a decrease in activity. Add to collection. Surface area Chewing breaks the large into the small. Surface area increases as particle size decreases for a given bite-sized piece of food:. This may entail sending a message that activates the glands that secrete digestive juices into the lumen, or it may mean the stimulation of muscles within the alimentary canal, thereby activating peristalsis and segmentation that move food along the intestinal tract.
The walls of the entire alimentary canal are embedded with nerve plexuses that interact with the central nervous system and other nerve plexuses—either within the same digestive organ or in different ones. These interactions prompt several types of reflexes. Extrinsic nerve plexuses orchestrate long reflexes, which involve the central and autonomic nervous systems and work in response to stimuli from outside the digestive system.
Short reflexes, on the other hand, are orchestrated by intrinsic nerve plexuses within the alimentary canal wall. These two plexuses and their connections were introduced earlier as the enteric nervous system. Short reflexes regulate activities in one area of the digestive tract and may coordinate local peristaltic movements and stimulate digestive secretions. For example, the sight, smell, and taste of food initiate long reflexes that begin with a sensory neuron delivering a signal to the medulla oblongata.
The response to the signal is to stimulate cells in the stomach to begin secreting digestive juices in preparation for incoming food. In contrast, food that distends the stomach initiates short reflexes that cause cells in the stomach wall to increase their secretion of digestive juices.
A variety of hormones are involved in the digestive process. The main digestive hormone of the stomach is gastrin, which is secreted in response to the presence of food. Gastrin stimulates the secretion of gastric acid by the parietal cells of the stomach mucosa. Other GI hormones are produced and act upon the gut and its accessory organs. Hormones produced by the duodenum include secretin, which stimulates a watery secretion of bicarbonate by the pancreas; cholecystokinin CCK , which stimulates the secretion of pancreatic enzymes and bile from the liver and release of bile from the gallbladder; and gastric inhibitory peptide, which inhibits gastric secretion and slows gastric emptying and motility.
These GI hormones are secreted by specialized epithelial cells, called endocrinocytes, located in the mucosal epithelium of the stomach and small intestine. These hormones then enter the bloodstream, through which they can reach their target organs. The digestive system ingests and digests food, absorbs released nutrients, and excretes food components that are indigestible. The six activities involved in this process are ingestion, motility, mechanical digestion, chemical digestion, absorption, and defecation.
These processes are regulated by neural and hormonal mechanisms. Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Module 2: The Digestive System. Search for:. Digestive System Processes and Regulation Learning Objectives By the end of this section, you will be able to: List in order the organs of the alimentary canal.
Describe the types of movements that occur in the digestive system. Distinguish between physical and chemical digestion. Describe the neural and hormonal regulation of digestion. Visit this site for an overview of digestion of food in different regions of the digestive tract. Note the route of non-fat nutrients from the small intestine to their release as nutrients to the body.
Critical Thinking Questions Offer a theory to explain why segmentation occurs and peristalsis slows in the small intestine.
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