Question

• Explain the physical and mechanical features of the GI tract and where they take place?
• What enzymes and digestive juices are released into the small intestines and for what purpose? Where do they come from?

Answer 1

The human gastrointestinal tract applies to the stomach and intestines, and sometimes all the structures from the mouth to till excretory organ. Digestion is required for nutrient absorption from food and occurs through two processes: mechanical and chemical digestion.Digestion begins when food enters the mouth (oral cavity).

The mechanical and chemical digestions occur in the mouth at first. The Teeth grinds and breaks up the food which marks the mechanical action, while an amylase enzyme in saliva begins to break down carbohydrates which is known as the chemical action. After the chewed food (bolus) is swallowed moves down the esophagus.

The esophagus acts as a link between the mouth and the stomach, but no digestion occurs at esophagus. The esophagus acts as a link between the mouth and the stomach, but there is no digestion here. The bolus then reaches the stomach, where more mechanical and chemical digestion takes place. The muscles in the stomach walls churn the bolus (mechanical) and allow it to be mixed with digestive enzymes and gastric (chemical) acids. This process converts the bolus to a liquid called chyme.

Digestion continues in the stomach for several hours. During this time, an enzyme called pepsin breaks down most of the protein in the food.The chyme is gradually transported into the small intestine, where most chemical digestion occurs.

Bile, produced in the liver, is released from the gallbladder to help in digesting fats. Furthermore, pancreatic enzymes and intestinal walls interact with chyme to initiate the final part of digestion.

Much absorption of the nutrients occurs in the small intestine.Nutrients are absorbed into the circulatory system through its walls, and only water and non-digestible compounds are left behind by the time the chyme leaves the small intestine. Instead, the chyme joins the large intestine.

There, water is extracted and some indigestible materials are breached by bacteria, creating essential compounds (such as vitamin K).The residual accumulated waste material is called feces, which is passed through the excretory organ into the rectum and removed from the body.

Now for the second part of your question,

The secretions released into the small intestine are bile, pancreatic fluid, and intestinal juice. Movements created by the small intestine layer of muscularis Helps the food to be thoroughly mixed with various secretions in the intestine and thus promote digestion.

The pancreatic juice contains enzymes such as trypsinogen, carboxypeptidases, pancreatic amalysis, nucleases, chymotrypsinogen and pancreatic lipases. Trypsinogen is activated by enterokinase, an enzyme which is secreted into active trypsin by the intestinal mucosa, which in effect activates the enzyme chymotrypsinogen in the pancreatic water. The bile includes bile pigments (bilirubin and biliverdin), which break down hemoglobin products of dead RBCs, bile salts, cholesterol and phospholipids but has no enzymes. Bile helps in emulsification of fats. These bile salts reduce greatly the surface tension of fatty droplets and they also break them fats into small globules. Bile also activates lipases to digest lipids.

Proteins and partially digested proteins in the intestinal chemistry act by proteolytic enzymes of pancreatic juice. Trypsin hydrolyses proteins into polypeptides and peptones, while chymotrypsin hydrolyses peptide bonds associated with specific amino acids.

The pancreatic amylase converts glycogen and starch into maltose. Lipase acts on emulsified fat also known as triglycrides hydrolyses them into two substances such as free fatty acid and monoglycerides. Monoglycerides are further hydrolysed to fatty acid and glycerol. Nucleases in the pancreatic juice break the nucleic acid into nucleotides and nucleosides.

Brunner's gland secretions along with intestinal gland secretions make up the intestinal juice or succus entericus. Intestinal juice enzymes such as maltase, lactase, sucrase (invertase), dipeptidases, lipases, nucleosidases function on bile breakdown products and pancreatic digestion.