Further protein digestion takes place in the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by the pyloric sphincter.
When digesting protein and some fats, the stomach lining must be protected from getting digested by pepsin. There are two points to consider when describing how the stomach lining is protected. First, as previously mentioned, the enzyme pepsin is synthesized in the inactive form. This protects the chief cells, because pepsinogen does not have the same enzyme functionality of pepsin. Second, the stomach has a thick mucus lining that protects the underlying tissue from the action of the digestive juices.
When this mucus lining is ruptured, ulcers can form in the stomach. Ulcers are open wounds in or on an organ caused by bacteria Helicobacter pylori when the mucus lining is ruptured and fails to reform.
Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi.
The apical surface of each villus has many microscopic projections called microvilli. These structures, illustrated in Figure 4, are lined with epithelial cells on the luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the blood stream on the other side.
The villi and microvilli, with their many folds, increase the surface area of the intestine and increase absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver.
There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic substances, including drugs, alcohol, and some pathogens. Figure 4. Villi are folds on the small intestine lining that increase the surface area to facilitate the absorption of nutrients.
The human small intestine is over 6m long and is divided into three parts: the duodenum, the jejunum, and the ileum. The duodenum is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum. In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer.
Pancreatic juices also contain several digestive enzymes. Digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells of the intestinal wall itself, enter the duodenum. Bile is produced in the liver and stored and concentrated in the gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize starches, disaccharides, proteins, and fats.
These digestive juices break down the food particles in the chyme into glucose, triglycerides, and amino acids. Some chemical digestion of food takes place in the duodenum. Absorption of fatty acids also takes place in the duodenum. The second part of the small intestine is called the jejunum , shown in Figure 3. Here, hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. The bulk of chemical digestion and nutrient absorption occurs in the jejunum.
The ileum , also illustrated in Figure 3 is the last part of the small intestine and here the bile salts and vitamins are absorbed into blood stream. The undigested food is sent to the colon from the ileum via peristaltic movements of the muscle. The ileum ends and the large intestine begins at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity. Figure 5. The large intestine reabsorbs water from undigested food and stores waste material until it is eliminated.
The large intestine , illustrated in Figure 5, reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter.
The colon can be divided into four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material. Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet. The rectum is the terminal end of the large intestine, as shown in Figure 5. The primary role of the rectum is to store the feces until defecation.
The feces are propelled using peristaltic movements during elimination. The five major organs that secrete digestive juices are the salivary glands, stomach, pancreas, liver and small intestine. Each of these organs synthesizes its mixture of digestive juices that breaks down food into smaller pieces that can be absorbed into the body. The main salivary glands are found in the cheeks, under the tongue and around the jaw.
They secrete about 1 quart of saliva each day. Amylase, also called ptyalin, is an enzyme in saliva that breaks down starches or complex carbohydrates -- such as bread, rice and potatoes.
Lysozyme is another salivary enzyme, which helps to keep the mouth free from germs. Saliva also contains mucus, which coats the food and enables each bite to travel smoothly through the digestive tract. The stomach, an important organ for digestion, produces gastric juice which is comprised of hydrochloric acid, water and enzymes.
Hydrochloric acid works with the main gastric enzyme called pepsin to aid the digestion of protein-rich foods like eggs, meat and tofu. The production of acid is increased by a hormone known as gastrin, which is made by specific cells lining the stomach. The stomach also produces gastric lipase, which assists in digesting fats. Once the chemical reaction within this lock and key arrangement has been completed, the products are released and the enzyme is free to attract another substrate molecule.
The rate of reaction for such a process is thousands of substrate molecules per minute. If a solution of sugar is left in a sealed container, it breaks down into glucose and fructose extremely slowly. In the presence of a small amount of the enzyme sucrase, the rate of breakdown is millions of times faster.
Sometimes, chemical substances other than substrates can bind with the active sites of enzymes, blocking their normal function. For example, water-soluble compounds of arsenic and mercury are extremely poisonous because they can permanently bind to some enzyme systems, markedly reducing their efficiency. Depending on the dose, the end result could be death. Another unique property is that they are extracellular enzymes that mix with food as it passes through the gut.
The majority of other enzymes function within the cytoplasm of the cell. The chemical digestion of food is dependent on a whole range of hydrolase enzymes produced by the cells lining the gut as well as associated organs such as the pancreas. These can then be readily and rapidly absorbed through the gut wall and into the bloodstream for transport to the liver and from there to other parts of the body.
The main enzyme-producing structures of the human digestive system are the salivary glands, stomach, pancreas, liver and small intestine. The following pathway summarises how starch present in a food like bread is broken down chemically into glucose, which can then be absorbed through the intestinal wall and into the bloodstream for transport to the liver and from there to other parts of the body.
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