Structure and Function of Digestive Organs; Mouth, Oesophagus, Stomach, Intestines, Rectum, Anus-1

Organs of the Digestive System

The digestive tract (alimentary canal) and its supporting organs make up the two components of the digestive system.

• The digestive tract is a continuous passageway that starts at the mouth, where food is consumed, and ends at the anus, where the body's solid wastes from digestion are discharged. This tube is around 8 to 10 meters long. The digestive system is composed of:
  • Mouth
  • Pharynx
  • Oesophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum and anal canal
• The accessory organs despite not being a direct component of the digestive tract, are essential for the digestive process. The physiological and anatomical connections between the glands and organs play a role in digestion and absorption. Through ducts, they release substances into the digestive system. These include
  • Three pairs of salivary gland
  • The pancreas
  • The Liver
  • The gall bladder

The Walls of the Digestive Tract

The structure of the digestive tract's wall is consistent from the esophagus to the anus. From the innermost to the outermost surface, this wall's layers are:

• Mucous membrane layer / mucosa: The mucous membrane, which is the innermost layer and is made up of stratified squamous epithelium, has a large number of cells that secrete mucus. The muscularis mucosae, lamina propria, and mucous membrane make up its three layers of tissue.
• Mucous membrane: (Epithelial lining) varies in structure depending on how the digestive tract functions and is subject to heavy mechanical friction, such as in the oral cavity, pharynx, esophagus, and anal canal. Squamous epithelium that is stratified and has glands that secrete mucus make up this layer. It serves three primary purposes: absorption, secretion, and protection.
• Lamina Propria: The epithelium is supported by this layer of connective tissue. Lamina propria contains lymphatics and blood arteries.
• Muscularis Mucosa: It is a thin layer of smooth muscle on the surface. In some areas of the alimentary tract, the smooth muscle creates folds by making small, localized movements in the mucosa.
• Sub-mucosa: This layer, which holds the muscle layer to the mucosa, is made up of connective tissue with collagen and some elastic fibers. It contains lymphoid tissues, blood vessels, lymphatics, and some of the nerves that control digestive function.
• Muscle Layer: With certain exceptions, this layer is made up of two layers of smooth muscle (involuntary muscle). Circular fibers make up the inner layer, whereas longitudinal fibers make up the outer layer. Blood vessels, lymphatic vessels, and a plexus (network) of sympathetic and parasympathetic nerves known as the myenteric or Aurbach's plexus are present between these two muscle layers. These nerves supply the blood vessels and nearby smooth muscle. The wave-like motion that propels food through the digestive tract and blends it with digestive juices is produced by the alternate contractions of these muscles. We refer to this movement as peristalsis.
• Adventitia or serosa: Serous membrane or adventitia make up the outermost layer of the alimentary canal. The wall of the digestive tract is primarily made up of serous membrane below the diaphragm and fibrous connective tissue above. Serous membrane, the outermost layer of the digestive tract and also known as the peritoneum, is found in additional layers over the abdominal organs.

The Peritoneum

The majority of the abdominal organs are covered by a thin, shiny serous membrane that lines the abdominal cavity. The visceral peritoneum covers the organ and is distinguished from the parietal peritoneum by the fact that it lines the abdomen.

• Parietal Peritoneum: The portion of the wall it borders as well as the parietal peritoneum get the same blood and lymphatic vasculature and somatic nerve supplies. The peritoneum lining the interior of the body wall is sensitive to pressure, discomfort, heat, and cold, as well as laceration, just like the skin above it. Except for the inferior surface of the central part of the diaphragm, where the phrenic nerves provide innervation, pain from the parietal peritoneum is typically well localized; irritation here is frequently referred to the C3-C5 dermatomes over the shoulder.
• Visceral Peritoneum: The same blood and lymphatic vasculature as well as the visceral nerve supply supply the visceral peritoneum and the organs it protects. The visceral peritoneum is largely triggered by stretching and chemical stimulation; it is unresponsive to touch, heat, cold, and laceration. The dermatomes of the spinal ganglia that supply the sensory fibers are where most of the midline portions of these dermatomes are where the pain is most strongly localized. As a result, pain from foregut derivatives typically affects the epigastric area, midgut derivatives affect the umbilical area, and hindgut derivatives affect the pubic area.

The peritoneal cavity is the space that exists between the two layers of peritoneum. The cavity is filled with a serous fluid that the cells secrete to stop the layers from rubbing against one another. While the uterine tubes open into the peritoneal cavity in females, it is completely closed in males. There are a number of double-layered peritoneal structures in addition to these single-layered sections that carry nerves, blood vessels, and lymph vessels. These structures can also serve as ligaments to support the organs.

The Mesentery

It is a section of the peritoneum that is double-layered and somewhat fan-shaped. The extended long edge is joined to the small intestine, while the handle section is attached to the back wall. The mesentery, which is located between the two layers of membrane, houses the nerves, lymphatic vessels, and blood vessels that supply the intestine. The meso-colon is the region of the peritoneum that connects the colon to the back wall. The greater omentum is a large, four-layered peritoneal fold that protrudes from the proximal duodenum and the greater curvature of the stomach. It hangs down like an apron. Following descent, it folds back and affixes to the meso-colon and anterior surface of the transverse colon. The lesser omentum, which connects the lesser curvature of the stomach and the proximal part of the duodenum to the liver, is a much smaller, double-layered peritoneal fold. Additionally, it links the stomach to a trio of organs that are located in the lesser omentum's free edge and run between the duodenum and liver.

Blood and Nerve Supply of the Gastrointestinal Tract

Blood Supply

• Arterial blood supply: Pairs of esophageal arteries, which are branches of the thoracic aorta, supply the esophagus in the thorax. The celiac artery, superior mesenteric artery, and inferior mesenteric artery are branches of the aorta that supply blood to the gastrointestinal tract, pancreas, liver, and biliary tract in the abdomen and pelvis.
• Venous Drainage: The azygous vein and hemiazygus veins in the thorax receive venous blood from the esophagus through the esophageal veins. Azygous and hemiazygous veins combine to form the superior vein and bracheocephalic vein. A small amount of blood drains into the left gastric vein from the bottom section of the esophagus. The portal vein is formed in the abdomen and pelvis by the veins that drain blood from the lower portion of the esophagus, stomach, pancreas, small and large intestines, and a portion of the rectum. Blood is drained from the anal canal and the lower part of the rectum by the internal illiac veins.

Nerve supply

Nerves from both divisions of the autonomic nervous system feed the alimentary canal.

• The pair of vagus nerves supply the majority of the digestive system with parasympathetic blood flow. Smooth muscle contracts in response to stimulation, and digestive juices are released. The sacral nerve provides parasympathetic feed to the tract's furthest reaches (the rectum and anal canal).
• The nerve from the spinal cord in the thoracic and lumber areas supplies the sympathetic supply. The sympathetic nerves that supply the alimentary canal organs pass through sympathetic nerve plexuses in the thorax, abdomen, and pelvis. Their effects lessen glandular secretion and smooth muscle contraction.
• Two nerve plexuses located within the tract's walls are where sympathetic and parasympathetic fibers are disseminated. The smooth muscles of the muscular layer are supplied by the myenteric plexus. The secretary glands in the mucosa are supplied by the submucosal or Meissner's plexus.