Functions of the Digestive System. ▫ ingestion – the oral cavity allows food to enter the digestive tract and have mastication (chewing) occurs, and the resulting. tology of the digestive tract (), the regulation of the digestive system () and the peritoneum (). The anatomy and physiology of each section of the. NOTE: The digestive system consists of the: mouth (oral cavity); pharynx; esophagus; Endoderm gives rise to the epithelial lining of the digestive tract, while.
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Functions of the digestive system. • Digestion-mechanical and chemical breakdown of material. • Motility-movement of material from the oral cavity to the. PDF | On Feb 13, , Dana Bartos and others published Anatomy of the Digestive Tract. PDF |: Digestive system is a demonic & Complex process in human body. The digestive tract starts at the mouth and ends at the anus. It is the.
Teeth are not made of bone, but rather of tissues of varying density and hardness, such as enamel, dentine and cementum. Human teeth have a blood and nerve supply which enables proprioception. This is the ability of sensation when chewing, for example if we were to bite into something too hard for our teeth, such as a chipped plate mixed in food, our teeth send a message to our brain and we realise that it cannot be chewed, so we stop trying.
The shapes, sizes and numbers of types of animals' teeth are related to their diets. For example, herbivores have a number of molars which are used to grind plant matter, which is difficult to digest. Carnivores have canine teeth which are used to kill and tear meat.
Crop A crop , or croup, is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion. In some birds it is an expanded, muscular pouch near the gullet or throat. In adult doves and pigeons, the crop can produce crop milk to feed newly hatched birds. Rough illustration of a ruminant digestive system Abomasum Main article: Abomasum Herbivores have evolved cecums or an abomasum in the case of ruminants. Ruminants have a fore-stomach with four chambers.
These are the rumen , reticulum , omasum , and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material.
Solids clump together to form the cud or bolus. The cud is then regurgitated, chewed slowly to completely mix it with saliva and to break down the particle size.
Fibre, especially cellulose and hemi-cellulose , is primarily broken down into the volatile fatty acids , acetic acid , propionic acid and butyric acid in these chambers the reticulo-rumen by microbes: bacteria , protozoa , and fungi. In the omasum, water and many of the inorganic mineral elements are absorbed into the blood stream. The abomasum is the fourth and final stomach compartment in ruminants. It is a close equivalent of a monogastric stomach e. It serves primarily as a site for acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine.
Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. Microbes produced in the reticulo-rumen are also digested in the small intestine.
A flesh fly "blowing a bubble", possibly to concentrate its food by evaporating water Specialised behaviours Regurgitation has been mentioned above under abomasum and crop, referring to crop milk, a secretion from the lining of the crop of pigeons and doves with which the parents feed their young by regurgitation.
Other animals, such as rabbits and rodents , practise coprophagia behaviours — eating specialised faeces in order to re-digest food, especially in the case of roughage. Capybara, rabbits, hamsters and other related species do not have a complex digestive system as do, for example, ruminants.
Instead they extract more nutrition from grass by giving their food a second pass through the gut. Soft faecal pellets of partially digested food are excreted and generally consumed immediately. They also produce normal droppings, which are not eaten. Young elephants, pandas, koalas, and hippos eat the faeces of their mother, probably to obtain the bacteria required to properly digest vegetation.
When they are born, their intestines do not contain these bacteria they are completely sterile. Without them, they would be unable to get any nutritional value from many plant components. In earthworms An earthworm 's digestive system consists of a mouth , pharynx , esophagus , crop , gizzard , and intestine.
The mouth is surrounded by strong lips, which act like a hand to grab pieces of dead grass, leaves, and weeds, with bits of soil to help chew. The lips break the food down into smaller pieces. In the pharynx, the food is lubricated by mucus secretions for easier passage. The esophagus adds calcium carbonate to neutralize the acids formed by food matter decay.
Temporary storage occurs in the crop where food and calcium carbonate are mixed. The powerful muscles of the gizzard churn and mix the mass of food and dirt. When the churning is complete, the glands in the walls of the gizzard add enzymes to the thick paste, which helps chemically breakdown the organic matter. By peristalsis , the mixture is sent to the intestine where friendly bacteria continue chemical breakdown.
This releases carbohydrates, protein, fat, and various vitamins and minerals for absorption into the body. Overview of vertebrate digestion In most vertebrates , digestion is a multistage process in the digestive system, starting from ingestion of raw materials, most often other organisms. Ingestion usually involves some type of mechanical and chemical processing.
Digestion is separated into four steps: Ingestion : placing food into the mouth entry of food in the digestive system , Mechanical and chemical breakdown: mastication and the mixing of the resulting bolus with water, acids , bile and enzymes in the stomach and intestine to break down complex molecules into simple structures, Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis , active transport , and diffusion , and Egestion Excretion : Removal of undigested materials from the digestive tract through defecation.
Underlying the process is muscle movement throughout the system through swallowing and peristalsis. Each step in digestion requires energy, and thus imposes an "overhead charge" on the energy made available from absorbed substances.
Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc.
The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter by gut bacteria and for resorption of water from digests before excretion. In mammals , preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach.
Mechanical and chemical digestion begin in the mouth where food is chewed , and mixed with saliva to begin enzymatic processing of starches.
The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. Absorption occurs in the stomach and gastrointestinal tract , and the process finishes with defecation. Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours.
Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis. Gastric juice contains hydrochloric acid and pepsin which would damage the walls of the stomach and mucus is secreted for protection.
In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach.
The partially digested food enters the duodenum as a thick semi-liquid chyme. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile , pancreatic juice and intestinal juice.
The intestinal walls are lined with villi , and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine.
In the large intestine the passage of food is slower to enable fermentation by the gut flora to take place. Here water is absorbed and waste material stored as feces to be removed by defecation via the anal canal and anus.
Neural and biochemical control mechanisms Different phases of digestion take place including: the cephalic phase , gastric phase, and intestinal phase. The cephalic phase occurs at the sight, thought and smell of food, which stimulate the cerebral cortex. Taste and smell stimuli are sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal secretes acid and G cell secretes gastrin activity via D cell secretion of somatostatin.
The gastric phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes.
This activates the release of acetylcholine , which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin , which in turn stimulates parietal cells to secrete gastric acid.
Gastric acid is about 0. Acid release is also triggered by acetylcholine and histamine. The intestinal phase has two parts, the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes. Absorption of nutrients occurs in the jejunum and ileum, the second and third regions of the small intestine.
The lining of the small intestine is shaped into microscopic folds villi , which increase the surface area available for absorption. Although an age-related reduction in villus height has been shown, the impact on nutrient uptake does not seem to be clinically significant Drozdowski and Thomson, Populations of certain bacteria that reside in the small intestine have been shown to increase as we age, leading to bloating, pain and decreased absorption of nutrients such as calcium, folic acid and iron.
This can have a negative effect on health. In addition, PPIs have been shown to provoke bacterial overgrowth in the small intestine, which may exacerbate NSAID-induced small intestinal injury and foster the development of systemic conditions, including inflammatory bowel disease, diabetes and autoimmune diseases Fujimori, The large intestine As already mentioned, oesophageal peristalsis slows with age, but research has recently shown that small intestinal transit time does not seem to be affected Fischer and Fadda, This causes a delay in colonic transit of waste, leading to constipation Wiskur and Greenwood-Van Meerveld, Peristalsis is also affected by the age-related atrophy of the mucosa and muscle layers of the colon.
The walls of the colon sag, prompting the formation of pouches diverticuli. Straining to eliminate faeces may put additional pressure on weakened blood vessel walls, giving rise to haemorrhoids. The rate of cell division declines in the digestive epithelium, which cannot repair and replace itself as well as it needs to. There is also a drastic age-associated rise in the incidence of several gut pathologies including cancer of the colon — in fact, age is the key risk factor for colorectal cancer.
Recent studies indicate that ageing induces changes in the DNA of epithelial intestinal cells, particularly in the colon; this process — known as DNA methylation — is believed to play a significant part in the development of colorectal cancers Masoro and Austad, The ageing process mimics the intestinal microbe profile that accompanies inflammatory bowel diseases and obesity Neish, The commensal microorganisms inhabiting the lumen of the colon are prevented from entering surrounding tissues by a single layer of epithelial cells that form an impermeable mucosal barrier.
As the barrier function of the mucosal immune system is impaired, the incidence of GI pathogen infections is higher — and is a major cause of morbidity and mortality in older people Mabbott et al, The accessory organs With age, the pancreas, which generates four major digestive enzymes, decreases in weight and some of its tissue undergoes fibrosis. Its exocrine function is impaired and the secretion of chymotrypsin and pancreatic lipase reduced Laugier et al, , adversely affecting the ability of the small intestine to digest food.
The liver undertakes more than functions for the body; as it shrinks with age and blood flow to it decreases, its functional capacity also decreases Drozdowski and Thomson,