| Info: Human Organ Systems – Digestive System (Salivary Glands, Oesophagus, Stomach. Liver, Gallbladder, Pancreas, Intestines, Rectum and Anus
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones. In mammals, food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva from the salivary glands. This is called mastication. Then it travels down the oesophagus into the stomach, where hydrochloric acid kills most contaminating microorganisms and begins mechanical break down of some food (e.g., denaturation of protein), and chemical alteration of some. The hydrochloric acid also has a low pH, which is great for enzymes. After some time (typically an hour or two in humans, 4–6 hours in dogs, somewhat shorter duration in house cats, ...), the resulting thick liquid is called chyme. Chyme will go through the small intestine, where 95% of absorption of nutrients occurs, through the large intestine, and are eliminated during defecation. Salivary Glands The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose. In other organisms such as insects, salivary glands are often used to produce biologically important proteins like silk or glues, and fly salivary glands contain polytene chromosomes that have been useful in genetic research. Oesophagus The oesophagus, sometimes known as the gullet, is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing food passes from the mouth through the pharynx into the oesophagus and travels via peristalsis to the stomach. The word oesophagus is derived from the Latin oesophagus, which derives from the Greek word oesophagus, lit. "entrance for eating." In humans the oesophagus is continuous with the laryngeal part of the pharynx at the level of the C6 vertebra. The oesophagus passes through posterior mediastinum in thorax and enters abdomen through a hole in the diaphragm at the level of the tenth thoracic vertebrae (T10). It is usually about 25–30 cm long and connects the mouth to the stomach. It is divided into cervical, thoracic and abdominal parts. Stomach In some animals, including vertebrates, echinoderms, insects (mid-gut) and molluscs, the stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract. It is involved in the second phase of digestion, following mastication (chewing). The stomach is located between the oesophagus and the small intestine. It secretes protein-digesting enzymes and strong acids to aid in food digestion, (sent to it via oesophageal peristalsis) through smooth muscular contortions (called segmentation) before sending partially-digested food (chyme) to the small intestines. Liver The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion. The liver is necessary for survival; there is currently no way to compensate for the absence of liver function. This organ plays a major role in metabolism and has a number of functions in the body, including glycogen storage, decomposition of red blood cells, plasma protein synthesis, hormone production, and detoxification. It lies below the diaphragm in the abdominal-pelvic region of the abdomen. It produces bile, an alkaline compound which aids in digestion via the emulsification of lipids. The liver's highly specialized tissues regulate a wide variety of high-volume biochemical reactions, including the synthesis and breakdown of small and complex molecules, many of which are necessary for normal vital functions. Gallbladder In vertebrates the gallbladder (cholecyst, gall bladder) is a small organ that aids digestion and stores bile produced by the liver. In humans the loss of the gallbladder is usually easily tolerated. Pancreas The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to the small intestine. These enzymes help to further break down the carbohydrates, proteins, and fats in the chyme. Intestine In human anatomy, the intestine (or bowel) is the segment of the alimentary canal extending from the mouth via stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine. In humans, the small intestine is further subdivided into the duodenum, jejunum and ileum while the large intestine is subdivided into the caecum and colon. Rectum The rectum (from the Latin rectum intestinum, meaning straight intestine) is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long. Its calibre is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla. Anus The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of faeces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as bones; food material after all the nutrients have been extracted, for example cellulose or lignin; ingested matter which would be toxic if it remained in the digestive tract; and dead or excess gut bacteria and other endosymbionts. Amphibians, reptiles, and birds use the same orifice for excreting liquid and solid wastes, and for copulation and egg-laying; this orifice is known as the cloaca. Monotreme mammals also have a cloaca, which is thought to be a feature inherited from the earliest amniotes via the therapsids. Marsupials have two nether orifices: one for excreting both solids and liquids; the other for reproduction, which appears as a vagina in females and a penis in males. Female placental mammals have completely separate orifices for defecation, urination, and reproduction; males have one opening for defecation and another for both urination and reproduction, although the channels flowing to that orifice are almost completely separate. The development of the anus was an important stage in the evolution of multicellular animals. In fact it appears to have happened at least twice, following different paths in protostomes and deuterostomes. This accompanied or facilitated other important evolutionary developments: the bilaterian body plan; the coelom, an internal cavity that provided space for a circulatory system and, in some animals, formed a hydrostatic skeleton which enables worm-like animals to burrow; metamerism, in which the body was built of repeated "modules" which could later specialize, for example the heads of most arthropods are composed of fused, specialized segments. |
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