Gastrointestinal tract
The gastrointestinal tract 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 throughout their body for digestion and a larger dorsal pore 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 gastrointestinal tract consists of the esophagus, stomach, and intestines, and is divided into the upper and lower gastrointestinal tracts. The GI tract includes all structures between the mouth and the anus, forming a continuous passageway that includes the main organs of digestion, namely, the stomach, small intestine, and large intestine. The complete human digestive system is made up of the gastrointestinal tract plus the accessory organs of digestion. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment. The whole human GI tract is about long at autopsy. It is considerably shorter in the living body because the intestines, which are tubes of smooth muscle tissue, maintain constant muscle tone in a halfway-tense state but can relax in different areas to allow for local distension and peristalsis.
The human gut microbiota is made up of around 4,000 different strains of bacteria, archaea, viruses and eukaryotes, with diverse roles in the maintenance of immune health and metabolism. Enteroendocrine cells of the GI tract release hormones to help regulate the digestive process. These digestive hormones, including gastrin, secretin, cholecystokinin, and ghrelin, are mediated through either intracrine or autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution.
Human gastrointestinal tract
Structure
The structure and function of the GI tract can be described both by gross anatomy and microscopic anatomy. The tract itself is divided into upper and lower tracts, and the intestines into small and large intestines.Upper gastrointestinal tract
The upper gastrointestinal tract consists of the mouth, pharynx, esophagus, stomach, and duodenum.The exact demarcation between the upper and lower tracts is the suspensory muscle of the duodenum. This differentiates the embryonic borders between the foregut and midgut, and is also the division commonly used by clinicians to describe gastrointestinal bleeding as being of either "upper" or "lower" origin. Upon dissection, the duodenum may appear to be a unified organ, but it is divided into four segments based on function, location, and internal anatomy. The four segments of the duodenum are as follows : bulb, descending, horizontal, and ascending.The suspensory muscle of the duodenum suspends the superior border of the ascending duodenum from the diaphragm, and serves as an important anatomical landmark showing the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively. This is a thin muscle which is derived from the embryonic mesoderm.
Lower gastrointestinal tract
The lower gastrointestinal tract includes most of the small intestine and all of the large intestine. In human anatomy, the intestine is the segment of the gastrointestinal tract extending from the pyloric sphincter of the stomach to the anus and as in 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. The large intestine is subdivided into the cecum, and ascending, transverse, descending, and sigmoid colons, rectum, and anal canal.Small intestine
The small intestine is a tubular structure around 6 to 7 m long, that begins at the duodenum, and ends at the ileum. Its mucosal area in an adult human is about. The combination of the circular folds, the villi, and the microvilli increases the absorptive area of the mucosa about 600-fold, making a total area of about for the entire small intestine. Its main function is to absorb the products of digestion into the bloodstream. There are three major divisions:- Duodenum: A short structure that receives chyme from the stomach, together with pancreatic juice containing digestive enzymes and bile from the gall bladder. The digestive enzymes break down proteins, and bile emulsifies fats into micelles. The duodenum contains Brunner's glands which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in the chyme.
- Jejunum: This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about long and contains the circular folds also known as plicae circulares and villi that increase its surface area. Products of digestion are absorbed into the bloodstream here.
- Ileum: The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients.
Large intestine
The large intestine begins at the cecum, where the appendix is located. This is also the start of the colon as the ascending colon in the back wall of the abdomen. At the right colic flexure it runs across the abdomen in the transverse colon, passing below the diaphragm. At the left colic flexure the flexed portion of the transverse and descending colon, it descends down the left side of the abdomen.
It reaches the sigmoid colon which is a loop of the colon closest to the rectum and continues to the rectum and anal canal.
Development
The gut is an endoderm-derived structure. At approximately the sixteenth day of human development, the embryo begins to fold ventrally in two directions: the sides of the embryo fold in on each other and the head and tail fold toward one another. The result is that a piece of the yolk sac, an endoderm-lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut. The yolk sac remains connected to the gut tube via the vitelline duct. Usually, this structure regresses during development; in cases where it does not, it is known as Meckel's diverticulum.During fetal life, the primitive gut is gradually patterned into three segments: foregut, midgut, and hindgut. Although these terms are often used in reference to segments of the primitive gut, they are also used regularly to describe regions of the definitive gut as well.
Each segment of the gut is further specified and gives rise to specific gut and gut-related structures in later development. Components derived from the gut proper, including the stomach and colon, develop as swellings or dilatations in the cells of the primitive gut. In contrast, gut-related derivatives — that is, those structures that derive from the primitive gut but are not part of the gut proper, in general, develop as out-pouchings of the primitive gut. The blood vessels supplying these structures remain constant throughout development.
Histology
The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy. The GI tract can be divided into four concentric layers in the following order:- Mucosa
- Submucosa
- Muscular layer
- Adventitia or serosa
Mucosa
- Epithelium – innermost layer. Responsible for most digestive, absorptive and secretory processes.
- Lamina propria – a layer of loose connective tissue. Unusually cellular compared to most connective tissue
- Muscularis mucosae – a thin layer of smooth muscle that aids the passing of material and enhances the interaction between the epithelial layer and the contents of the lumen by agitation and peristalsis
Submucosa
The submucosa consists of a layer of dense irregular connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and muscular layer. It contains the submucosal plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.Muscular layer
The muscular layer consists of an inner circular layer and a longitudinal outer layer. The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch. Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food. The muscularis externa of the stomach is composed of the inner oblique layer, middle circular layer, and the outer longitudinal layer.Between the circular and longitudinal muscle layers is the myenteric plexus. This controls peristalsis. Activity is initiated by the pacemaker cells,. The gut has intrinsic peristaltic activity due to its self-contained enteric nervous system. The rate can be modulated by the rest of the autonomic nervous system.
The coordinated contractions of these layers is called peristalsis and propels the food through the tract. Food in the GI tract is called a bolus from the mouth down to the stomach. After the stomach, the food is partially digested and semi-liquid, and is referred to as chyme. In the large intestine, the remaining semi-solid substance is referred to as feces.