Topic 6: Food Processing and Digestion

Emulsification

the breakdown of large fat globules into smaller, uniformly distributed particles. It is accomplished mainly by bile acids in the small intestine. Emulsification is the first preparation of fat for chemical digestion by specific enzymes

Mucosal folds

"gastric mucosa" mucous membrane layer of the stomach which contains the glands and the gastric pits.

Large Intestine

= colon, additional water reabsorption. Wastes are consolidated in the rectum (final 12 inches) before elimination through the anus. Home to a large population of resident bacteria (colonic flora, gut flora).

Bile duct

A bile duct is any of a number of long tube-like structures that carry bile. Bile, required for the digestion of food, is secreted by the liver into passages that carry bile toward the hepatic duct, which joins with the cystic duct (carrying bile to and from the gallbladder) to form the common bile duct, which opens into the intestine.

Esophagus

A bolus of chewed food is swallowed and moves by peristalsis (coordinated muscle contraction) down the esophagus into the stomach. Once food is in the stomach, the lower esophageal sphincter (muscular valve) closes, preventing the acidic contents of the stomach from entering the esophagus (when the sphincter does malfunction, you get heartburn or acid reflux)

Bolus

A mass of food that has been chewed at the moment of swallowing

Blood capillary

Absorb and transport hydrophilic substances. Gather into the hepatic portal vein, which goes to the liver for processing.

Lymph capillary

Absorb and transport most hydrophobic substances, circulate through body tissues before joining the blood circulatory system. Bypasses the liver initially.

Small Intestine

Acid chyme from the stomach enters the duodenum one small squirt at a time, it is then neutralized by bicarbonate coming from the pancreas, and pH returns to near neutral or slightly basic. It mixes with enzymes (amylases, proteases, and lipases) made by the pancreas and by the intestinal lining itself. It mixes with bile, made in the liver, stored in the gall bladder until needed. Nearly all chemical breakdown occurs in the small intestine, nearly all absorption of the breakdown products also occurs in the small intestine. 3 Levels of topography increase the surface area of the intestinal mucosa: large ridges, villi, and microvilli. Nutrients cross the intestinal mucosa by diffusion, facilitated diffusion, or active transport. Each villus is supplied by blood capillaries and lymph capillaries. Blood capillaries absorb and transport hydrophilic substances that gather into the hepatic portal vein, which goes to the liver for processing. Lymph capillaries (lacteals) absorb and transport most hydrophobic substances. Circulate through body tissues before joining the blood circulatory system. Bypasses the liver initially. Some water absorption takes place.

Amylase

An enzyme that catalyzes the hydrolysis of starch into sugars, found in saliva and begins the chemical process of digestion. Digests carbs.

Bicarbonate

Bicarbonate also acts to regulate pH in the small intestine. It is released from the pancreas in response to the hormone secretin to neutralize the acidic chyme entering the duodenum from the stomach. Bicarbonate (HCO3−) is alkaline, and a vital component of the pH buffering system[3] of the human body

Stomach

Can be closed tightly at both ends by the lower esophageal sphincter and pyloric sphincter. Food then mixes with gastric juices to become acid chyme. gastric secretions include- 1. water (food becomes a wet slurry), 2. acid (hydrochloric acid lowers pH to between pH 1 and 2, the acidity attacks many carbon based molecules, especially proteins and loosens them up for further attacks by enzymes) 3. Pepsin: an enzyme that catalyzes the partial breakdown of proteins, secreted in an inactive form called pepsinogen, which is activated by the acid environment of the stomach interior (the lumen) 4. Mucus:lubricates and provides a protective coating for the stomach lining 5. Gastric lipase: a fat digesting enzyme that breaks down a very small amount of fat. Food is then churned by stomach contractions with sphincters closed, and furthers the mechanical breakdown as food mixes with gastric juices.

Mouth

Chewing mechanically disrupts food and results in a greater surface area for more efficient attack by digestive enzymes. Saliva is secreted by the salivary glands and it wets food, mixes food with amylase (which breaks down a small amount of starch), mixeds food with lingual lipase, which breaks down a little fat- although insignificant in adults, and some glucose can be absorbed through the mouth, but the amount absorbed is inconsequential compared to that in the small intestine.

Peristalsis

Coordinated muscle contractions that moves food to different stations in the digestive tract.

Lipase

Digests fats/oils. Is an enzyme produced mainly by the pancreas (an organ located near the stomach) and secreted into the small intestine, where it helps breaks down fats (lipids) we eat into fatty acids and glycerol.

Rectum

Ending of the large intestine (last 12 cm)

Duodenum

First 12 inches of the small intestine, It receives partially digested food (known as chyme) from the stomach and plays a vital role in the chemical digestion of chyme in preparation for absorption in the small intestine. Many chemical secretions from the pancreas, liver and gallbladder mix with the chyme in the duodenum to facilitate chemical digestion.

Hydrochloric acid

Gastric acid is one of the main secretions of the stomach. It consists mainly of hydrochloric acid and acidifies the stomach content to a pH of 1 to 2. Gastric acid acts as a barrier against microorganisms to prevent infections and is important for the digestion of food. Its low pH denatures proteins and thereby makes them susceptible to degradation by digestive enzymes such as pepsin. The low pH also activates the enzyme precursor pepsinogen into the active enzyme pepsin by self-cleavage. After leaving the stomach, the hydrochloric acid of the chyme is neutralized in the duodenum by sodium bicarbonate.[30]

Protease

Is any enzyme that performs proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in a polypeptide chain. Proteases are involved in digesting long protein chains into shorter fragments by splitting the peptide bonds that link amino acid residues.

Liver

It is an accessory digestive gland and produces bile, an alkaline compound which aids in digestion via the emulsification of lipids. The liver's highly specialized tissue consisting of mostly hepatocytes regulates a wide variety of high-volume biochemical reactions, including the synthesis and breakdown of small and complex molecules, many of which are necessary for normal vital functions.

Microvilli

Microvilli function as the primary surface of nutrient absorption in the gastrointestinal tract. Because of this vital function, the microvillar membrane is packed with enzymes that aid in the breakdown of complex nutrients into simpler compounds that are more easily absorbed. For example, enzymes that digest carbohydrates called glycosidases are present at high concentrations on the surface of enterocyte microvilli. Thus, microvilli not only increase the cellular surface area for absorption, they also increase the number of digestive enzymes that can be present on the cell surface.

Villi

Small, fingerlike projections that are multicellular. One of the minute, wormlike processes on certain membranes, especially on the mucous membrane of the small intestine, where they serve in absorbing nutriment.

Lymphatic system

The lymphatic system is part of the circulatory system, comprising a network of lymphatic vessels that carry a clear fluid called lymph (from Latin lympha meaning water directionally towards the heart.

Lower esophageal Sphincter

The most important valve for keeping stomach contents out of the esophagus and airway is the lower esophageal sphincter.

Pancreas

The pancreas is also a digestive organ, secreting pancreatic juice containing digestive enzymes that assist digestion and absorption of nutrients in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.

Pyloric sphincter

When the pyloric sphincter is contracted, it holds food in the stomach, allowing the digestive juices to do their work. This breaks down the food into a substance called "chyme." Once the food has broken down, the sphincter opens and allows it to enter the duodenum. The time the food spends in the stomach allows the body to absorb more of the nutrients.

Hepatic portal vein

a blood vessel that conducts blood from the gastrointestinal tract and spleen to the liver. This blood is rich in nutrients that have been extracted from food, and the liver processes these nutrients; it also filters toxins that may have been ingested with the food. The liver receives about 75% of its blood through the hepatic portal vein

Pancreatic duct

is a duct joining the pancreas to the common bile duct to supply pancreatic juices which aid in digestion provided by the exocrine pancreas. The pancreatic duct joins the common bile duct just prior to the ampulla of Vater, after which both ducts perforate the medial side of the second portion of the duodenum at the

Mucus

is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water

Gall bladder

is a small organ where bile (a fluid produced by the liver) is stored and concentrated before it is released into the small intestine.

Pepsin

is an enzyme whose zymogen (pepsinogen) is released by the chief cells in the stomach and that degrades food proteins into peptides.

Bile

made up of bile acids, modified from cholesterol. Partly hydrophilic, partly hydrophobic. Emulsifies fats: keeps small droplets from coalescing into larger ones (soap/detergents disperse fats). Maintains large surface area for lipases to attack.

Chyme

the semifluid mass of partly digested food that is expelled by the stomach into the duodenum and moves through the intestines during digestion.

Gastric juice

thin, strongly acidic ( p H varying from 1 to 3), almost colorless liquid secreted by the glands in the lining of the stomach. Its essential constituents are the digestive enzymes pepsin and rennin (see rennet), hydrochloric acid, and mucus.