Question

Describe in details the passage of food starting from the mouth to the Anus. Please include both Chemical and Mechanical Digestions.

Answer 1

Digestion of food

Mechanical digestion of food

Each part of your digestive system helps to move food and liquid through your GI tract, break food and liquid into smaller parts, or both. Once foods are broken into small enough parts, your body can absorb and move the nutrients to where they are needed. Your large intestine absorbs water, and the waste products of digestion become stool. Nerves and hormones help control the digestive process.

Mouth. Food starts to move through your GI tract when you eat. When you swallow, your tongue pushes the food into your throat. A small flap of tissue, called the epiglottis, folds over your windpipe to prevent choking and the food passes into your esophagus.

Esophagus. Once you begin swallowing, the process becomes automatic. Your brain signals the muscles of the esophagus and peristalsis begins.

Lower esophageal sphincter. When food reaches the end of your esophagus, a ringlike muscle—called the lower esophageal sphincter —relaxes and lets food pass into your stomach. This sphincter usually stays closed to keep what’s in your stomach from flowing back into your esophagus.

Stomach. After food enters your stomach, the stomach muscles mix the food and liquid with digestive juices. The stomach slowly empties its contents, called chyme, into your small intestine.

Small intestine. The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine, and push the mixture forward for further digestion. The walls of the small intestine absorb water and the digested nutrients into your bloodstream. As peristalsis continues, the waste products of the digestive process move into the large intestine.

Large intestine. Waste products from the digestive process include undigested parts of food, fluid, and older cells from the lining of your GI tract. The large intestine absorbs water and changes the waste from liquid into stool. Peristalsis helps move the stool into your rectum.

Rectum. The lower end of your large intestine, the rectum, stores stool until it pushes stool out of your body

Chemical digestion of food

Mouth. The digestive process starts in your mouth when you chew. Your salivary glands make saliva, a digestive juice, which moistens food so it moves more easily through your esophagus into your stomach.

• lingual lipase: Lipid digestion initiates in the mouth. Lingual lipase starts the digestion of the lipids/fats.
• Salivary amylase: or ptylin Carbohydrate digestion also initiates in the mouth. Amylase, produced by the salivary glands, breaks complex carbohydrates to smaller chains, or even simple sugars.
• lysozyme: food contains more than just essential nutrients, e.g. bacteria or viruses, the lysozyme offers a limited and non-specific, yet beneficial antiseptic function in digestion.

Esophagus. After you swallow, peristalsis pushes the food down your esophagus into your stomach.

Stomach. Glands in your stomach lining make stomach acid and enzymes that break down food. Muscles of your stomach mix the food with these digestive juices.

• Pepsin is the main gastric enzyme. It is produced by the stomach cells called "chief cells" in its inactive form pepsinogen, which is a zymogen. Pepsinogen is then activated by the stomach acid into its active form, pepsin. Pepsin breaks down the protein in the food into smaller particles, such as peptide fragments and amino acids.
• Gastric lipase: Gastric lipase is an acidic lipase secreted by the gastric chief cells in the fundic mucosa in the stomach. It has a pH optimum of 3–6. Gastric lipase, together with lingual lipase, comprise the two acidic lipases. These lipases, unlike alkaline lipases (such as pancreatic lipase), do not require bile acid or colipase for optimal enzymatic activity.

Hormones or compounds produced by the stomach and their respective function:

• Hydrochloric acid (HCl): This is in essence positively charged hydrogen atoms (H+), or in lay-terms stomach acid, and is produced by the cells of the stomach called parietal cells. HCl mainly functions to denature the proteins ingested, to destroy any bacteria or virus that remains in the food, and also to activate pepsinogen into pepsin.
• Intrinsic factor (IF): Intrinsic factor is produced by the parietal cells of the stomach. Vitamin B12 (Vit. B12) is an important vitamin that requires assistance for absorption in terminal ileum. Initially in the saliva, haptocorrin secreted by salivary glands binds Vit. B, creating a Vit. B12-Haptocorrin complex. The purpose of this complex is to protect Vitamin B12 from hydrochloric acid produced in the stomach. Once the stomach content exits the stomach into the duodenum, haptocorrin is cleaved with pancreatic enzymes, releasing the intact vitamin B12. Intrinsic factor (IF) produced by the parietal cells then binds Vitamin B12, creating a Vit. B12-IF complex. This complex is then absorbed at the terminal portion of the ileum.
• Mucin: The stomach has a priority to destroy the bacteria and viruses using its highly acidic environment but also has a duty to protect its own lining from its acid. The way that the stomach achieves this is by secreting mucin and bicarbonate via its mucous cells, and also by having a rapid cell turn-over.
• Gastrin: This is an important hormone produced by the "G cells" of the stomach. G cells produce gastrin in response to stomach stretching occurring after food enters it, and also after stomach exposure to protein. Gastrin is an endocrine hormone and therefore enters the bloodstream and eventually returns to the stomach where it stimulates parietal cells to produce hydrochloric acid (HCl) and Intrinsic factor (IF).

Pancreas. Your pancreas makes a digestive juice that has enzymes that break down carbohydrates, fats, and proteins. The pancreas delivers the digestive juice to the small intestine through small tubes called ducts.

Two of the population of cells in the pancreatic parenchyma make up its digestive enzymes:

• Ductal cells: Mainly responsible for production of bicarbonate (HCO3), which acts to neutralize the acidity of the stomach chyme entering duodenum through the pylorus. Ductal cells of the pancreas are stimulated by the hormone secretin to produce their bicarbonate-rich secretions, in what is in essence a bio-feedback mechanism; highly acidic stomach chyme entering the duodenum stimulates duodenal cells called "S cells" to produce the hormone secretin and release to the bloodstream. Secretin having entered the blood eventually comes into contact with the pancreatic ductal cells, stimulating them to produce their bicarbonate-rich juice.
• Acinar cells: Mainly responsible for production of the inactive pancreatic enzymes (zymogens) that, once present in the small bowel, become activated and perform their major digestive functions by breaking down proteins, fat, and DNA/RNA. Acinar cells are stimulated by cholecystokinin (CCK), which is a hormone/neurotransmitter produced by the intestinal cells (I cells) in the duodenum. CCK stimulates production of the pancreatic zymogens.

Pancreatic juice, composed of the secretions of both ductal and acinar cells, contains the following digestive enzymes:[2]

• Trypsinogen, which is an inactive(zymogenic) protease that, once activated in the duodenum into trypsin, breaks down proteins at the basic amino acids. Trypsinogen is activated via the duodenal enzyme enterokinase into its active form trypsin.
• Chymotrypsinogen, which is an inactive (zymogenic) protease that, once activated by duodenal enterokinase, turns into chymotrypsin and breaks down proteins at their aromatic amino acids. Chymotrypsinogen can also be activated by trypsin.
• Carboxypeptidase, which is a protease that takes off the terminal amino acid group from a protein
• Several elastases that degrade the protein elastin and some other proteins.
• Pancreatic lipase that degrades triglycerides into two fatty acids and a monoglyceride.[3]
• Sterol esterase
• Phospholipase
• Several nucleases that degrade nucleic acids, like DNAase and RNAase
• Pancreatic amylase that breaks down starch and glycogen which are alpha-linked glucose polymers. Humans lack the cellulases to digest the carbohydrate cellulose which is a beta-linked glucose polymer.

Liver. Your liver makes a digestive juice called bile that helps digest fats and some vitamins. Bile ducts carry bile from your liver to your gallbladder for storage, or to the small intestine for use.

Gallbladder. Your gallbladder stores bile between meals. When you eat, your gallbladder squeezes bile through the bile ducts into your small intestine.

Small intestine. Your small intestine makes digestive juice, which mixes with bile and pancreatic juice to complete the breakdown of proteins, carbohydrates, and fats. Bacteria in your small intestine make some of the enzymes you need to digest carbohydrates. Your small intestine moves water from your bloodstream into your GI tract to help break down food. Your small intestine also absorbs water with other nutrients.

The following enzymes/hormones are produced in the duodenum:

• secretin: This is an endocrine hormone produced by the duodenal "S cells" in response to the acidity of the gastric chyme.
• Cholecystokinin (CCK) is a unique peptide released by the duodenal "I cells" in response to chyme containing high fat or protein content. Unlike secretin, which is an endocrine hormone, CCK actually works via stimulation of a neuronal circuit, the end-result of which is stimulation of the acinar cells to release their content.CCK also increases gallbladder contraction, causing release of pre-stored bile into the cystic duct, and eventually into the common bile duct and via the ampulla of Vater into the second anatomic position of the duodenum. CCK also decreases the tone of the sphincter of Oddi, which is the sphincter that regulates flow through the ampulla of Vater.
• Gastric inhibitory peptide (GIP): This peptide decreases gastric motility and is produced by duodenal mucosal cells.
• motilin: This substance increases gastro-intestinal motility via specialized receptors called "motilin receptors".
• somatostatin: This hormone is produced by duodenal mucosa and also by the delta cells of the pancreas. Its main function is to inhibit a variety of secretory mechanisms.

Throughout the lining of the small intestine there are numerous brush border enzymes whose function is to further break down the chyme released from the stomach into absorbable particles. These enzymes are absorbed whilst peristalsis occurs. Some of these enzymes include:

• Erepsin: converts peptones and polypeptides into amino acids.
• Maltase: converts maltose into glucose.
• Lactase: This is a significant enzyme that converts lactose into glucose and galactose.
• Sucrase: converts sucrose into glucose and fructose.
• Other disaccharidases

Large intestine. In your large intestine, more water moves from your GI tract into your bloodstream. Bacteria in your large intestine help break down remaining nutrients and make vitamin K NIH external link. Waste products of digestion, including parts of food that are still too large, become stool.