Every cell in the body requires nutrition. This includes not only oxygen, but also other nutrients, which must be ingested. Most foods are composed of larger molecules that cannot be readily absorbed; therefore, they must be broken down into smaller molecules
The digestive process involves both mechanical action such as that provided by the teeth, stomach and intestines, and chemical action provided by fluids that help digestion. Additionally, the cells produce waste that must be removed. These functions of nutrition and waste removal are performed by the systemic or peripheral circulatory system, and final removal of waste or “cleaning” of the blood is performed by the kidneys. It is important to understand that the digestive system is an element of the human body that is crucial for the function of all anatomical systems.
Peristalsis is the rhythmic contraction of circular and longitudinal smooth muscles used to propel contents through the digestive tract. In much of the gastrointestinal tract , smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food (called bolus while in the esophagus and gastrointestinal tract and chyme in the stomach ) along the gastrointestinal tract. Peristaltic movement is initiated by circular smooth muscles contracting behind the chewed material to prevent it from moving back into the mouth, followed by a contraction of longitudinal smooth muscles which pushes the digested food forward.
In biology and medicine, epithelium is a tissue composed of a layer of cell s. The lamina propria (more correctly lamina propria mucosae ) is a thin layer of loose connective tissue which lies beneath the epithelium and together with the epithelium constitutes the mucosa . the thin layer of smooth muscle found in most parts of the gastrointestinal tract , located outside the lamina propria mucosae and separating it from the submucosa .
Peritoneal cavity The peritoneal cavity is a potential space between the parietal peritoneum and visceral peritoneum.
The cavity bound by the peritoneum is known as the peritoneal cavity (Figure 8). The space outside the peritoneum, but still within the abdominal cavity, is known as the extraperitoneal space. The extraperitoneal space can be divided into three areas, all connected: an anterior, a lateral, and a posterior area. The anterior region is known as the preperitoneal space, the posterior region is known as the retroperitoneal space (Figure 8.) All organs found in the retroperitoneal space are known as retroperitoneal organs. As a very general rule, all retroperitoneal organs -some of which may be related to the peritoneum -do not have a mesentery, therefore they are not mobile. An organ behind the parietal peritoneum is classified as being retroperitoneal. Retroperitoneal organs are fixed to the posterior abdominal wall. Also as a very general rule, most organs of the digestive system contained within the peritoneal cavity do have a mesentery and are therefore mobile. The presence of mobility defines an organ as intraperitoneal.
The mesenteries are double layers of serous membrane, composed of peritoneal membranes which have folded against each other. These mesenteries connect and hold gastrointestinal organs in place and attach blood vessels and nerves. They also, with their fatty coverings, protect and insulate the organs. The greater omentum , for instance, hangs in front of the intestines acting as an insulator and shock absorber.
Seen here is a loop of bowel attached via the mesentery. Note the extent of the veins and arteries. There is an extensive anastomosing arterial blood supply to the bowel, making it more difficult to infarct. Also, the extensive venous drainage is incorporated into the portal venous system heading to the liver.
The stomach : The stomach is composed of several regions and structures: The gastroesophageal region (a.k.a. cardia) mentioned above. The fundus is the blind portion of the stomach above its junction with the Esophagus. This portion is thin walled compared to the rest of the stomach and has few secretory cells. As the bolus of food enters this area first some action of salivary amylase may continue briefly. The body of the stomach. This is where extensive gastric pits located which possess the secretory cells of the stomach. The pylorus . This narrowed region leads through the pyloric sphincter into the duodenum . 3-layered muscularis - an oblique layer in addition to the longitudinal and transverse layers. The three layers produce a churning and liquefying effect on the chyme in the stomach.
The pyloric sphincter is a structural sphincter which regulates the onward progression of materials from the stomach into the duodenum, and helps to prevent their return to the stomach.
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The appendix, part of the cecum, the transverse colon and the sigmoid colon present with a mesentery or meso , and they are easily moved out of the way during surgery.
The ascending and the descending colon do not have a mesentery or meso but are attached to the posterior abdominal wall by a reflection of their serosal covering from which they must be separated during surgery.
The mesorectum is actually a misnomer as the attachment of the rectum is not covered by peritoneum, but the name persists due to common usage.
The rectum is an “S” shaped reservoir found in the pelvis.
Most of the rectum has only 3 tissue layers—mucosa, submucosa, and muscularis—since all but the upper anterior rectum is below the visceral peritoneal covering (extra- and infra-peritoneal) and, therefore, has no serosa.
The adipose tissue contained within the fascia propria posterior to the rectum is ( incorrectly) referred to as the mesorectum .
The tenia coli completely surround the rectum, having merged in the rectosigmoid from 3 distinct columns in the colon.
For surgical purposes, the rectum is divided into proximal, middle, and distal rectum.
The rectal curvatures result in inner mucosal infoldings, known as Valves of Houston , which can make traversing the rectum in retrograde fashion difficult during endoscopy or surgery.
The branches of the superior and inferior mesenteric arteries split and, with the exception of the rectal arteries, create a long artery that runs along the inner margin of the colon and is called the Marginal Artery of Drummond .
The middle rectal arteries (when present) and inferior rectal arteries supply the lower portion of the rectum, the anal canal and the anal sphincter.
The middle and inferior rectal arteries arise from the internal iliac arteries , which is important especially in cancer surgery as the proximal blood supply would have been divided . Due to inconsistencies in the blood supply to the lower rectum, it is often better to anastomose lower rather than higher as long as there is no tension.