The smooth muscles of the esophagus undergo a series of wave like movements called peristalsis that push the food toward the stomach, as illustrated in Figure 9. The peristalsis wave is unidirectional—it moves food from the mouth to the stomach, and reverse movement is not possible. The peristaltic movement of the esophagus is an involuntary reflex; it takes place in response to the act of swallowing.
A ring-like muscle called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach.
When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Many animals have a true sphincter; however, in humans, there is no true sphincter, but the esophagus remains closed when there is no swallowing action.
A large part of digestion occurs in the stomach, shown in Figure The stomach is a saclike organ that secretes gastric digestive juices. The pH in the stomach is between 1. This highly acidic environment is required for the chemical breakdown of food and the extraction of nutrients.
When empty, the stomach is a rather small organ; however, it can expand to up to 20 times its resting size when filled with food. This characteristic is particularly useful for animals that need to eat when food is available. Figure The human stomach has an extremely acidic environment where most of the protein gets digested. The stomach is also the major site for protein digestion in animals other than ruminants.
Protein digestion is mediated by an enzyme called pepsin in the stomach chamber. Pepsin is secreted by the chief cells in the stomach in an inactive form called pepsinogen. Pepsin breaks peptide bonds and cleaves proteins into smaller polypeptides; it also helps activate more pepsinogen, starting a positive feedback mechanism that generates more pepsin.
Another cell type—parietal cells—secrete hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices.
Hydrochloric acid helps to convert the inactive pepsinogen to pepsin. The highly acidic environment also kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles mixes the stomach contents about every 20 minutes. The partially digested food and gastric juice mixture is called chyme.
Chyme passes from the stomach to the small intestine. 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 11, 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.
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 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 10 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.
The large intestine reabsorbs water from undigested food and stores waste material until it is eliminated. The large intestine , illustrated in Figure 12, 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 The primary role of the rectum is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary and the outer sphincter is voluntary.
The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs are organs that add secretions enzymes that catabolize food into nutrients. Accessory organs include salivary glands, the liver, the pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by hormones in response to the food consumed. The liver is the largest internal organ in humans and it plays a very important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fatty components of the food in the duodenum.
The liver also processes the vitamins and fats and synthesizes many plasma proteins. Because carnivorous animals come in many different sizes, the total length of their digestive tracts do vary. Generally speaking however, the overall length is considered to be rather short averaging only about six times that of their total body length, much shorter than omnivores or herbivores.
The Oral Cavity Carnivores have a wide mouth opening in relation to their head size. This provides obvious advantages in developing the forces used in seizing, killing and dismembering prey. In all mammalian carnivores, the jaw joint is a simple hinge joint lying in the same plane as the teeth.
The primary muscle used for operating the jaw in carnivores is the temporalis muscle. This muscle is so massive in carnivores that it accounts for most of the bulk of the sides of the head. This is because the muscles masseter and pterygoids that attach there are of minor importance in these animals.
The lower jaw of carnivores cannot move forward, and has very limited side-to-side motion. When the jaw of a carnivore closes, the blade-shaped cheek molars slide past each other to give a slicing motion that is very effective for shearing meat off bone. The teeth of a carnivore are discretely spaced so as not to trap stringy debris. The incisors are short, pointed and prong-like and are used for grasping and shredding. The canines are greatly elongated and dagger-like for stabbing, tearing and killing prey.
The molars carnassial are flattened and triangular with jagged edges such that they function like serrated-edged blades. Because of the hinge-type joint, when a carnivore closes its jaw, the cheek teeth come together in a back-to-front fashion giving a smooth cutting motion like the blades on a pair of shears.
When eating, a carnivore gorges itself rapidly and does not chew its food. Since proteolytic protein-digesting enzymes cannot be liberated in the mouth due to the danger of auto digestion damaging the oral cavity , carnivores do not need to mix their food with saliva; they simply bite off huge chunks of meat and swallow them whole.
The Stomach Carnivores have a simple single-chambered stomach. Since these animals average a kill only about once a week, a large stomach volume is advantageous because it allows the animals to quickly gorge themselves when eating, taking in as much meat as possible at one time which can then be digested later while resting.
Additionally, the ability of the carnivore stomach to secrete hydrochloric acid is exceptional. Carnivores are able to keep their gastric ph. This is necessary to facilitate protein breakdown and to kill the abundant dangerous bacteria often found in decaying flesh foods.
Dogs hold chewed food in their stomachs for 4 to 8 hours after ingestion. Only a little food at a time is released into the intestine, which it passes through quickly. This gives any bacteria that may live through the repeated acid baths little time to colonize and produce gastrointestinal distress. The Small Intestine The small intestine, approximately twenty feet in length in a dog, is vitally important. Without it, no digestion could take place and the animal could not survive. Because meat is relatively easily digested, the small intestine where absorption of food molecules takes place is relatively short — about three to five or six times the body length.
It is in the small intestine where food is digested and ultimately enters the bloodstream. In addition to digesting and absorbing feedstuffs, the intestine is critical for water and electrolyte balance, endocrine regulation of digestion and metabolism, and immunity. The intestines of carnivorous fish have evolved for processing a highly digestible, nutrient dense diet that is high in protein and low in carbohydrate. Correspondingly, abilities to digest protein are well developed, but carbohydrate digestion is low compared to omnivorous and herbivorous fish.
Furthermore, the evolutionary stable diet is associated with a lack or reduced abilities to adaptively modulate digestive functions to match changes in diet composition.
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