Digestion in the context of Swallowing

The nematodes (/ˈnɛmətoʊdz/ NEM-ə-tohdz or NEEM-; Ancient Greek: Νηματώδη; Latin: Nematoda), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (helminths) are the cause of soil-transmitted helminthiases.

They are classified along with arthropods, tardigrades and other moulting animals in the clade Ecdysozoa. Unlike the flatworms, nematodes have a tubular digestive system, with openings at both ends. Like tardigrades, they have a reduced number of Hox genes, but their sister phylum Nematomorpha has kept the ancestral protostome Hox genotype, which shows that the reduction has occurred within the nematode phylum.

Classical conditioning (also respondent conditioning and Pavlovian conditioning) is a behavioral procedure in which a biologically potent stimulus (e.g. food, a puff of air on the eye, a potential rival) is paired with a neutral stimulus (e.g. the sound of a musical triangle). The term classical conditioning refers to the process of an automatic, conditioned response that is paired with a specific stimulus. It is essentially equivalent to a signal.

Ivan Pavlov, the Russian physiologist, studied classical conditioning with detailed experiments with dogs, and published the experimental results in 1897. In the study of digestion, Pavlov observed that the experimental dogs salivated when fed red meat. Pavlovian conditioning is distinct from operant conditioning (instrumental conditioning), through which the strength of a voluntary behavior is modified, either by reinforcement or by punishment. However, classical conditioning can affect operant conditioning; classically conditioned stimuli can reinforce operant responses.

Milk is a white liquid food produced by the mammary glands of lactating mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digest solid food. Milk contains many nutrients, including calcium and protein, as well as lactose and saturated fat; the enzyme lactase is needed to break down lactose. Immune factors and immune-modulating components in milk contribute to milk immunity. The first milk, which is called colostrum, contains antibodies and immune-modulating components that strengthen the immune system against many diseases.

As an agricultural product, milk is collected from farm animals, mostly cattle, on a dairy. It is used by humans as a drink and as the base ingredient for dairy products. The US CDC recommends that children over the age of 12 months (the minimum age to stop giving breast milk or formula) should have two servings of milk products a day, and more than six billion people worldwide consume milk and milk products. The ability for adult humans to digest milk relies on lactase persistence, so lactose intolerant individuals have trouble digesting lactose.

Metabolism (/məˈtæbəlɪzəm/, from Greek: μεταβολή metabolē, "change") refers to the set of life-sustaining chemical reactions that occur within living organisms. The three main functions of metabolism are the conversion of energy in food into a usable form for cellular processes; the conversion of food to building blocks of macromolecules (biopolymers) such as proteins, lipids, nucleic acids, and some carbohydrates; and the excretion of metabolic wastes. These enzyme-catalyzed reactions allow organisms to grow, reproduce, maintain their structures, and respond to their environments. The word metabolism can also refer to all chemical reactions that occur in living organisms, including digestion and the transportation of substances into and between different cells. In a broader sense, the set of reactions occurring within the cells is called intermediary (or intermediate) metabolism.

Metabolic reactions may be categorized as catabolic—the breaking down of compounds (for example, of glucose to pyruvate by cellular respiration); or anabolic—the building up (biosynthesis) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy.

The bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of very high metabolic rates and oxygen supply, permit the bird to fly. The development of a beak has led to evolution of a specially adapted digestive system.

The liver is a major metabolic organ exclusively found in vertebrates, which performs many essential biological functions such as detoxification of the organism, and the synthesis of various proteins and various other biochemicals necessary for digestion and growth. In humans, it is located in the right upper quadrant of the abdomen, below the diaphragm and mostly shielded by the lower right rib cage. Its other metabolic roles include carbohydrate metabolism, the production of a number of hormones, conversion and storage of nutrients such as glucose and glycogen, and the decomposition of red blood cells. Anatomical and medical terminology often use the prefix hepat- from ἡπατο-, from the Greek word for liver, such as hepatology, and hepatitis.

The liver is also an accessory digestive organ that produces bile, an alkaline fluid containing cholesterol and bile acids, which emulsifies and aids the breakdown of dietary fat. The gallbladder, a small hollow pouch that sits just under the right lobe of liver, stores and concentrates the bile produced by the liver, which is later excreted to the duodenum to help with digestion. The liver's highly specialized tissue, consisting mostly of hepatocytes, regulates a wide variety of high-volume biochemical reactions, including the synthesis and breakdown of small and complex organic molecules, many of which are necessary for normal vital functions. Estimates regarding the organ's total number of functions vary, but is generally cited as being around 500. For this reason, the liver has sometimes been described as the body's chemical factory.

Decomposers are organisms that break down dead organisms and release the nutrients from the dead matter into the environment around them. Decomposition relies on chemical processes similar to digestion in animals; in fact, many sources use the words digestion and decomposition interchangeably. In both processes, complex molecules are chemically broken down by enzymes into simpler, smaller ones. The term "digestion," however, is commonly used to refer to food breakdown that occurs within animal bodies, and results in the absorption of nutrients from the gut into the animal's bloodstream. This is contrasted with external digestion, meaning that, rather than swallowing food and then digesting it using enzymes located within a GI tract, an organism instead releases enzymes directly onto the food source, which is what decomposers do as compared to animals. After allowing the enzymes time to digest the material, the decomposer then absorbs the nutrients from the environment into its cells. Decomposition is often erroneously conflated with this process of external digestion, probably because of the strong association between fungi, which are external digesters, and decomposition.

The term "decomposer" refers to a role in an ecosystem, not to a particular class or type of organism, or even to a specific capacity of those organisms. The definition of "decomposer" therefore centers on the outcome of the decomposition process, rather than the types of organisms performing it. At the center of this definition are the organisms that benefit most directly from the increase in nutrient availability that results from decomposition; plants and other non-mobile (sessile) autotrophs cannot travel to seek out nutrients, and most cannot digest other organisms themselves. They must therefore rely on decomposers to free up nutrients from dead matter that they can then absorb.

Cnidaria (/nɪˈdɛəriə, naɪ-/ nih-DAIR-ee-ə, ny-) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in freshwater and marine environments (predominantly the latter), including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites. Their distinguishing features are an uncentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable organelles used mainly for envenomation and capturing prey. Their bodies consist of mesoglea, a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Many cnidarian species can reproduce both sexually and asexually.

Cnidarians mostly have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes, which are specialized stinging cells used to capture prey. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both (hence they are trimorphic). Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Cnidarians also have rhopalia, which are involved in gravity sensing and sometimes chemoreception. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce sexually, but many species have complex life cycles of asexual polyp stages and sexual medusae stages. Some, however, omit either the polyp or the medusa stage, and the parasitic classes evolved to have neither form.

The gastrointestinal tract (also called the GI tract, digestive tract, and the alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is one of the largest of the body's systems. The GI tract contains all the major organs of the digestive system, in humans and other animals, including the esophagus, stomach, and intestines. Food taken in through the mouth is digested to extract nutrients and absorb energy, and the waste expelled at the anus as feces. Gastrointestinal is an adjective meaning of or pertaining to the stomach and intestines.

Most animals have a "through-gut" or complete digestive tract. Exceptions are more primitive ones: sponges have small pores (ostia) throughout their body for digestion and a larger dorsal pore (osculum) for excretion, comb jellies have both a ventral mouth and dorsal anal pores, while cnidarians and acoels have a single pore for both digestion and excretion.

The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.

The first stage, the cephalic phase of digestion, begins with secretions from gastric glands in response to the sight and smell of food, and continues in the mouth with the mechanical breakdown of food by chewing, and the chemical breakdown by digestive enzymes in the saliva. Saliva contains amylase, and lingual lipase, secreted by the salivary glands, and serous glands on the tongue. Chewing mixes the food with saliva to produce a bolus to be swallowed down the esophagus to enter the stomach. The second stage, the gastric phase, takes place in the stomach, where the food is further broken down by mixing with gastric juice until it passes into the duodenum, the first part of the small intestine. The intestinal phase where the partially digested food is mixed with pancreatic digestive enzymes completes the process of digestion.

The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the pancreatic duct to aid in digestion. The small intestine is about 6.5 metres (21 feet) long and folds many times to fit in the abdomen. Although it is longer than the large intestine, it is called the small intestine because it is narrower in diameter.

The small intestine has three distinct regions – the duodenum, jejunum, and ileum. The duodenum, the shortest, is where preparation for absorption through small finger-like protrusions called intestinal villi begins. The jejunum is specialized for the absorption through its lining by enterocytes of small nutrient particles which have been previously digested by enzymes in the duodenum. The main function of the ileum is to absorb vitamin B₁₂, bile salts, and whatever products of digestion were not absorbed by the jejunum.

The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces before being removed by defecation. The colon (progressing from the ascending colon to the transverse, the descending and finally the sigmoid colon) is the longest portion of the large intestine, and the terms "large intestine" and "colon" are often used interchangeably, but most sources define the large intestine as the combination of the cecum, colon, rectum, and anal canal. Some other sources exclude the anal canal.

In humans, the large intestine begins in the right iliac region of the pelvis, just at or below the waist, where it is joined to the end of the small intestine at the cecum, via the ileocecal valve. It then continues as the colon ascending the abdomen, across the width of the abdominal cavity as the transverse colon, and then descending to the rectum and its endpoint at the anal canal. Overall, in humans, the large intestine is about 1.5 metres (5 ft) long, which is about one-fifth of the whole length of the human gastrointestinal tract.

In mammals, invertebrates and most fish, the anus (pl.: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the exit end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facilitate the expulsion of waste that remains after digestion.

Bowel contents that pass through the anus include the gaseous flatus and the semi-solid feces, which (depending on the type of animal) include: indigestible matter such as bones, hair pellets, endozoochorous seeds and digestive rocks; residual food material after the digestible nutrients have been extracted, for example cellulose or lignin; ingested matter which would be toxic if it remained in the digestive tract; excreted metabolites like bilirubin-containing bile; and dead mucosal epithelia or excess gut bacteria and other endosymbionts. Passage of feces through the anus is typically controlled by muscular sphincters, and failure to stop unwanted passages results in fecal incontinence.

Defecation (or defaecation) follows digestion and is the necessary biological process by which organisms eliminate a solid, semisolid, or liquid waste material known as feces (or faeces) from the digestive tract via the anus or cloaca. The act has a variety of names, ranging from the technical (e.g. bowel movement), to the common (like pooping or crapping), to the obscene (shitting), to the euphemistic ("doing number two", "dropping a deuce" or "taking a dump"), to the juvenile ("going poo-poo" or "making doo-doo"). The topic, usually avoided in polite company, forms the basis of scatological humor.

Humans expel feces with a frequency varying from a few times daily to a few times weekly. Waves of muscular contraction (known as peristalsis) in the walls of the colon move fecal matter through the digestive tract towards the rectum. Flatus may also be expulsed. Undigested food may also be expelled within the feces, in a process called egestion. When birds defecate, they also expel urine and urates in the same mass, whereas other animals may also simultaneously urinate during defecation, but the processes are spatially separated. Defecation may also accompany childbirth and death. Babies defecate a unique substance called meconium prior to eating external foods.