Structure and Function of Digestive Organs; Mouth, Oesophagus, Stomach, Intestines, Rectum, Anus-6

Digestion and Absorption of Carbohydrates, Fats and Proteins

Digestion of Carbohydrate

Before food enters the small intestine, very little digestion of carbohydrates (starches and sugars) takes place. Salivary amylase breaks down starch into maltose in the mouth. Because many of us swallow our food quickly, salivary amylase typically has little time to do its job. There are no enzymes for breaking down carbohydrates in the stomach juice. As a result, by the time the chyme leaves the stomach, very little starch has been broken down. The starches and sugars in the food are broken down by pancreatic and intestinal juice enzymes once it reaches the small intestine.

Additionally, a pancreatic amylase broke down starch, a polysaccharide, into maltose and maltotriose (a trisaccharide). Maltase, sucrase, and lactase, three intestinal enzymes, break down double sugar (disaccharide) into simple sugars (monosaccharides). Maltose (malt sugar) and maltotriose are each converted by maltase into three molecules of glucose. Galactose and glucose are produced as a result of sucrose, a common cane sugar. The final byproduct of carbohydrate digestion is glucose, also known as simple sugar.

Digestion of Protein

In this stomach, the digestion of proteins begins. The pepsin in the gastric juice degrades proteins into peptides after being triggered from pepsinogen by hydrochloric acid. Proteins are split into peptides by the pancreatic juice's trypsin, chymotripsin, and carboxypeptidase enzymes. Numerous amino acids are fused together to form each protein molecule. The digestion of proteins is finished when all these enzymes break down large protein molecules into their individual amino acids. As a result, amino acids are the byproduct of protein digestion.

Digestion of Fats (lipids)

Lingual lipase, gastric lipase, and pancreatic lipase are the three different types of lipases that can take part in the digestion of lipids. Although some lipids are digested by lingual and gastric lipase, the majority of lipids are broken down into a number of tiny lipid globules by the action of bile in the duodenum. Fat molecules (triglycerides) are then broken down by pancreatic lipase into fatty acids and monoglycerides. Therefore, fatty acids and monoglycerides are the final byproducts of the digestion of fat (lipid).

Large Intestine

The large intestine starts at the ceacum in the right iliac fossa and ends at the rectum and anal canal, measuring approximately 6.5 cm (2.5 inches) in diameter and 1.5 m (5 feet) in length. The large intestine ascends on the right side from the ceacum, makes an oblique left cross, and then falls into the pelvis. It opens up to the outside of the body as the anus at its distal end. The ileocecal sphincter, a mucus membrane fold, prevents the ileum from entering the large intestine.

The following characteristics set the large intestine apart from the small intestine:

• Omental appendices: Small, fatty, omentum-like projection of visceral peritoneum
• Teniae coli: Three distinct longitudinal bands:
  • Mesocolic tenia, to which the transverse and sigmoid mesocolons attach.
  • Omental tenia, to which the omental appendices attach.
  • Free tenia (L. t. libera), to which neither mesocolons nor omental appendices are attached.
• Haustra: Colonic wall sacculations that occur in between teniae. Much more powerful caliber (internal diameter).

Water and electrolytes still present in the alimentary canal are absorbed in the large intestine. Furthermore, it recycles and reabsorbs water and leftover digestive secretions. Feces are also produced and stored in the large intestine.

Parts of the Large Intestine

The large intestine is divided into: 

• Cecum
• Colon
• Rectum
• Anal canal.

Cecum

This is where the big intestine starts. At the intersection of the ileum and colon on the right iliac fossa, there is a blind sac-like, dilated, pouch-like structure hanging down that is 2 to 3 inches long. The ileocecal valve is located at the cecum's upper border and keeps feces from passing back into the small intestine.

The vermiform appendix rises an inch behind the ileocecal valve and is a fine tubular structure that ranges in length from 2 to 20 cm (average: 9 cm). There is no known digestive function for the human appendix. However, lymphatic tissue is present. It comes from the cecum's posteromedial region, which is located below the ileo-cecal junction. The meso-appendix, a short triangular mesentery that originates from the terminal ileum's posterior side, is located in the appendix.

The ileocolic artery, the terminal branch of the SMA, supplies the cecum with blood. The ileocolic artery's branch, the appendicular artery, feeds blood to the appendix. The ileocolic vein, a tributary of the SMV, receives venous drainage from the caecum and appendix.

Colon

The colon is further separated into four identically structured sections. Along the right side of the abdomen, the ascending colon climbs upward toward the liver. The transverse colon is created when the large intestine bends and crosses the abdomen. The descending colon is formed at this point by a sharp bend and an extension of the left side of the abdomen into the pelvis. The lower portion of the colon continues to descend as the sigmoid colon after bending posteriorly in an "S" shape. The rectum is where the sigmoid colon empties.

Rectum

It is a 10- to 15-cm long, slightly dilated portion of the large intestine that acts as a temporary holding space for food residue that is indigestible or unabsorbable. The anal canal is where it ends after emerging from the sigmoid colon.

Anal Canal

The final 2.5 to 4.0 cm of the large intestine make up this area. Through an orifice known as the anus, this leads to the outside of the body. The anus is protected by two sphincter muscles: an internal anal sphincter muscle made of smooth muscle that is controlled involuntarily and an external anal sphincter muscle made of skeletal muscle that is controlled voluntarily.

Structure of the Large Intestine

There are four tissue layers in the large intestine:

• The serosa layer: The large intestine's surface is protected from friction with the surrounding muscles and abdominal organs by the serosa, a thin layer of simple squamous epithelial tissue. The serosa secretes watery serous fluid to lubricate the large intestine's surface.
• The muscular layer: Numerous layers of visceral smooth muscle cells that contract and move the large intestine are found in the muscularis layer, which encircles the submucosa. Instead of forming a continuous layer of smooth tissues around the colon, they are instead gathered into three bands called taenia coli that are spaced evenly apart. These bands give the large intestine its characteristic wrinkled appearance because they are shorter than the other layers.
• The sub-mucous layer: The areolar connection tissue that surrounds the mucosa includes blood vessels, lymph vessels, lymphoid tissue, and autonomic nervous system nerves. It supports the large intestine's other layers.
• The mucusa layer: This is the deepest layer, which is made up of goblet cells and straightforward columnar epithelial tissue. The big intestine's mucosa is smooth and devoid of the villi that are present in the small intestine. To lubricate its surface and shield it from abrasive food particles, the large intestine's hollow lumen is filled with numerous mucous glands that secrete mucus.

Functions of the Large Intestine

• Absorption: Water, minerals, salts, and some medications are absorbed into the blood capillaries in the colon.
• Secretion: It secretes mucin to lubricate and ease the passage of feces through the rectum and anus.
• Digestion: It aids in the digestion of various food residues that the small intestine was unable to digest or absorb.
• Excretion: It facilitates the removal of excess calcium, iron, and heavy metal medications, like bismuth, from the walls of the large intestine and mixing them with feces.
• Microbial activity: The large intestine contains a variety of intestinal flora microbes that produce folic acid and vitamin K.

Blood Supply of the Large Intestine

The superior and inferior mesenteric arteries supply arterial blood, and the superior and inferior mesenteric veins drain venous blood.

Nerve Supply of the Large Intestine

The large intestine is supplied by both the sympathetic and parasympathetic nervous systems.

Digestion and Absorption in the Large Intestine

The crucial processes of turning food into feces, absorbing important vitamins produced by gut bacteria, and recovering water from feces are all carried out by the large intestine. Chyme goes through the large intestine where it is combined with bacteria that have colonized the large intestine as it enters the large intestine from the small intestine via the ileocecal sphincter. Chyme is converted into feces by bacterial fermentation, which also releases vitamins K and B. In order for blood to clot properly, vitamin K, which is almost exclusively produced by gut bacteria, must be present. As a byproduct of bacterial fermentation, gases like carbon dioxide and methane are also produced, which causes flatulence, or gas passed through the anus.

In addition to aiding in the solidification of feces, the large intestine's water absorption helps to keep the body's water balance. Additionally, some vitamins and ions (sodium, potassium) are absorbed in the large intestine. The undigested food particles, shed epithelial cells, inorganic salts, and water that make up the dried fecal matter are finally stored in the rectum and sigmoid colon until they can be expelled from the body through defecation.

Defecation

Defecation is the act of passing feces from the body or emptying the rectum. This is accomplished by the gastro-colic reflex, which, in infants, acts reflexively whereas, in adults, it is controlled by the will and is carried out in response to the desire to empty the bowel brought on by the rectum becoming distended with feces.