[Biology Class Notes] on Alimentary Canal Pdf

The alimentary canal is a continual tube that starts from the mouth and ends at the anus, carrying food and other nutrition throughout various organs of the digestive system as well as allowing waste to leave from the body. The alimentary canal differs broadly in living beings, yet is only found in organisms that are bilaterally symmetrical. Different regions of the alimentary canal consist of cells that envelop digestive enzymes, enabling food to be broken down. 

Other distinct cells permit for the ingestion of substances into the body. The alimentary canal is divided into distinct organs and tissues in human and other highly complex animals. These tissues and organs originated from the alimentary canal of our foremost predecessors that probably consisted of a simple pathway linking the mouth and the anus.

Foregut

The foregut is the foremost region of the alimentary canal and stretches from the mouth to the duodenum at the opening of the bile duct. It is connected with the abdominal walls by mesentery above the stomach. The foregut starts from the endoderm, evolving from the folding primary gut, and is progressively different from the midgut and hindgut. 

Despite the expression foregut is usually used to refer to the foremost region of the primary gut, the constituents of the adult gut can also be explained with this expression. The pain in the epigastric area, which lies at the bottom of the junction of the ribs, generally is in reference to the frameworks in the adult foregut. 

The Components of the Foregut are as Follows:

Esophagus – Esophagus is a muscular passage via which tiny lumps of food materials pass from the mouth through the stomach. It is distinctly twisted at the area where it intersects with the stomach. This avoids the backward motion of the food materials from the stomach into it. The food materials in the digestive tract pass with the help of an involuntary motion of the alternate relaxation and contraction of the muscles referred to as peristalsis.

Stomach – The outset of the primary stomach can be seen around the end of the first month. During the beginning, it is slanted in the central plane and the mesogastrium or dorsal mesentery suspends it from the dorsal wall of the abdominal cavity.  

During the developmental process, the stomach moves 90 degrees in a clockwise direction across its longitudinal axis, keeping the left vagus nerve across its foremost area and the right vagus nerve cross its backward area.  This movement of the stomach gives rise to lesser peritoneal sac or the omental bursa.

Duodenum – The duodenum has its C-shaped loop from the rotational motion of the stomach. The branches of both the superior mesenteric artery and the celiac trunk support the duodenum due to its location at the intersection of the foregut and midgut.

Pancreas – The two outgrowths of the dorsal pancreatic bud, the endodermal epithelium and the ventral pancreatic bud are responsible for the formation of the pancreas.  The primitive parts gather to create a lone pancreas during the rotational motion of the gut.  

The uncinate procedure and a certain portion of the head are created by the ventral pancreatic bud, while the dorsal pancreatic bud creates the body and tail of the pancreas as well as the remaining portion of the head.  The tubes of the pancreatic buds combine to create the chief pancreatic tube, yet the adjacent region of the tube of the dorsal pancreatic bud might continue to remain as an ancillary pancreatic tube.

Liver – The liver grows from the endodermal cells which create the hepatic diverticulum. It develops within the vicinity of the septum transversum that further becomes a component of the diaphragm.  With the subsequent development of the liver, the hepatic diverticulum splits into a cranial region. The cranial region gives rise to the caudal region and the parenchyma of the liver. Further, the caudal region creates the cystic duct and the gallbladder.  

The mesenchyme, present in the septum transversum, is responsible for the formation of the conjugative tissue, the Kupffer cells, and the hemopoietic cells of the liver.  The embryonic liver is huge in size and occupies a majority of the abdominal cavity throughout the second month of the evolution. The formation of blood, also called hemopoiesis, starts in the liver after the first month of the evolution, and the formation of bile takes place in the third month.

Spleen – The mesodermal dorsal mesentery forms the spleen. However, the spleen shares the blood supply same as that of a majority of the adult structures that emerge from the foregut, that is, the celiac artery.

Midgut

The midgut is that region of the embryo from where a majority of the intestines evolve. It is referred to as the midgut loop after it twists about the superior mesenteric artery. The midgut consists of that region of the alimentary canal that starts from the terminal of the foregut at the beginning of the bile tube to the hindgut, nearly two-thirds of the path via the transverse colon.

The Components of Midgut are as Follows:

• Duodenum (distal half of the 2nd, 3rd and 4th parts)

• Jejunum

• Ileum

• Caecum

• Appendix

• Ascending colon

• Hepatic flexure of colon

• Transverse colon (about two-thirds)

Duodenum (distal half of the 2nd, 3rd and 4th parts) – It is exposed to the hepatopancreatic duct created by the combination of the pancreatic and bile ducts. 

Jejunum – It is the middle region of the small intestine having thick walls and is more vasculature. 

Ileum – It is the bottom region of the small intestine having thin walls and is less vasculature. 

Caecum – It is a tiny sac-like region at the intersection of the small and large intestines.

Colon – It consists of sigmoid colon, transverse colon, ascending colon and descending colon.

Rectum – It leads into the anus.

Hindgut

Also known as the epigaster, the hindgut is the caudal region of the alimentary canal. The hindgut, in mammals, consists of the distal third of the splenic flexure and the transverse colon, the sigmoid colon, descending colon and the rectum. In the field of zoology, the expression hindgut is also associated with the ascending colon and the caecum. 

Blood Supply – The inferior mesenteric artery is responsible for the arterial supply. While venous drainage is associated with the portal venous system, the lymphatic drainage is associated with the chyle cistern. 

Nerve Supply – The inferior mesenteric plexus stimulates the hindgut. Sympathetic reflex arises from the
Lumbar splanchnic nerves (L1-L2), while the parasympathetic reflex arises from S2-S4.

Digestion Process

The process of digestion consists of a sequence of reactions of food substances with the digestive juices and enzymes. This mechanism begins right from the oral cavity.