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.g., those in humans or pigs), and digesta is processed here in much the same way. 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. many sharks have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of unwanted contents (perhaps developed as a way to reduce exposure to toxins). 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. human digestion process main article: human digestive system upper and lower human gastrointestinal tract the human gastrointestinal tract is around 9 metres (30 feet) long. 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. digestion begins in the mouth with the secretion of saliva and its digestive enzymes. 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 and bicarbonates are 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. mainly proteins are digested in 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. bile helps in emulsification of fats and also activates lipases. 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. gastric secretion at this phase rises to 40% of maximum rate. 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 0.5% hydrochloric acid, which lowers the ph to the desired ph of 1–3. 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. breakdown into nutrients protein digestion protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. the digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. for example, trypsin is secreted by pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin. trypsin then cleaves proteins to smaller polypeptides. fat digestion main article: fatty acid metabolism § dietary sources of fatty acids, their digestion, absorption, transport in the blood and storage digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides. however fats are mainly digested in the small intestine. the presence of fat in the small intestine produces hormones that stimulate the release of pancreatic lipase from the pancreas and bile from the liver which helps in the emulsification of fats for absorption of fatty acids. complete digestion of one molecule of fat (a triglyceride) results a mixture of fatty acids, mono- and di-glycerides, as well as some undigested triglycerides, but no free glycerol molecules. carbohydrate digestion further information: carbohydrate metabolism and carbohydrate catabolism in humans, dietary starches are composed of glucose units arranged in long chains called amylose, a polysaccharide. during digestion, bonds between glucose molecules are broken by salivary and pancreatic amylase, resulting in progressively smaller chains of glucose. this results in simple sugars glucose and maltose (2 glucose molecules) that can be absorbed by the small intestine. lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose and galactose. glucose and galactose can be absorbed by the small intestine. approximately 65 percent of the adult population produce only small amounts of lactase and are unable to eat unfermented milk-based foods. this is commonly known as lactose intolerance. lactose intolerance varies widely by genetic heritage; more than 90 percent of peoples of east asian descent are lactose intolerant, in contrast to about 5 percent of people of northern european descent. sucrase is an enzyme that breaks down the disaccharide sucrose, commonly known as table sugar, cane sugar, or beet sugar. sucrose digestion yields the sugars fructose and glucose which are readily absorbed by the small intestine. dna and rna digestion dna and rna are broken down into mononucleotides by the nucleases deoxyribonuclease and ribonuclease (dnase and rnase) from the pancreas. non-destructive digestion some nutrients are complex molecules (for example vitamin b12) which would be destroyed if they were broken down into their functional groups. to digest vitamin b12 non-destructively, haptocorrin in saliva strongly binds and protects the b12 molecules from stomach acid as they enter the stomach and are cleaved from their protein complexes. after the b12-haptocorrin complexes pass from the stomach via the pylorus to the duodenum, pancreatic proteases cleave haptocorrin from the b12 molecules which rebind to intrinsic factor (if). these b12-if complexes travel to the ileum portion of the small intestine where cubilin receptors enable assimilation and circulation of b12-if complexes in the blood. digestive hormones action of the major digestive hormones there are at least five hormones that aid and regulate the digestive system in mammals. there are variations across the vertebrates, as for instance in birds. arrangements are complex and additional details are regularly discovered. for instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years. gastrin – is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. secretion of gastrin is stimulated by food arriving in stomach. the secretion is inhibited by low ph. secretin – is in the duodenum