Chemical Digestion

Lots of organs of the digestive system sercrete different substances. These secretions play different roles in the chemical digestion of food, enzymes break down large food molecules by the process of hydrolysis. During hydrolosis, a water molecule is added to the site and a bond is broken.

Mouth

• The mouth begins the process of carbohydrate digestion through the action of saliva. Salivary amylase, the enzyme found in saliva, starts the chemical digestion of starch, converting it from a polsaccharide to the disaccharide maltose.

Stomach

• The wall of the stomach is lined with millions of gastric glands, which together secrete 400-800mL of gastric juice at each meal. Three kinds of cells are found in the gastric glands:
• Parietal Cells - secrete hydrochloric acid and intrinsic factor (aids in absorption of vitamin B12 by the large intestine)
• Cheif Cells - secrete pepsinogen
• Mucous Secreting Cells - secrete mucous
• A very important constituent of gastric juices is the substance pepsinogen. Pepsinogen is not an enzyme but a forerunner of the enzyme pepsin. The hydrocholoric acid secreted increases the acidity of the stomach contents to a pH of 2 or lower. At this pH, pepsinogen is converted to pepsin, an active enzyme which begins the chemical digestion of proteins into peptides.
• Salivary amylase functions best at pH 6 or 7. Therefore, it becomes inactive when it reaches the stomch, and consequently, digestion of starch does not take place there.
• The mucous coats the cells lining the stomach and protects them from the digestive action of enzyme pepsin.

Small Intestine

• Final digestion of proteins and carbohydrates must occur, and fats have not yet been digested. Villi have cells that produce intestinal enzymes which complete the digestion of peptides and sugars.
• The upper part, the duodenum, is the most acitve in digestion. Secretions from the liver and pancreas are used for digestion in the duodenum. Epithelial cells of the duodenum secrete a watery mucous which acts to protect its lining from pepsin and HCI. The pancreas secreted digestive enzymes and stomach acid neutralizing sodium bicarbonate. The sodium bicarbonate neutralizes the acidic chyme, allowing the enzymes in the small intestine to function and protect the intestinal wall.
• The liver produces bile, which is stored in the gall bladder before entering the bile duct into the duodenum. Bile contains cholesterol, phospholipids, bilirubin, and a mix of salts. Bile salts emulsify - breaking them down into progressivly smaller fat globules until they can be acted upon by fat digesting enzymes. They also aid in neutralizing the chyme from the stomach.
• The chemical digestion that begins in the small intestine is a result of pancreatic secretions. Starch and glycogen are broken down into maltose through the action of pancreatic amylase. Proteases continue the breakdown of protein that began in the stomach and form small peptide fragments and some amino acids. Lipase break down fats into fatty acids and glycerol.
• Finally, the villi in the small intestine itself, release enzymes which complete the digestive process. Maltease, lactose and sucrase are three carbohydarate digesting enzymes which break down the maltose, sucrose and lactose into monosaccaraids.
• Lactose intolerence results from a lack of the enzyme lactose. Peptidase breaks down peptides into amino acids and nuclease breaks down nucleic acids into sugars and nitrogen bases.

Mechanical Digestion

Mechanical digestion involves the physical breakdown of food. It occurs in the following locations:

• Mouth - teeth, tongue, and palates

• Esophagus - peristalsis

• Stomach - muscular churning action

• Small Intestine - bile and peristalsis

Anatomy of the Digestive System

Mouth
Inside the mouth, there are several things that are present:

1) Salivary Glands: There are 3 pairs of glands. The collections of the secretions from the glands is known as saliva. This saliva contains amylase, which is an enzyme for starch digestion. The water in saliva moistens and helps dissolve some of the food. It aids chemical digestion, allows us to taste, and the chewing process. Mucous helps make the chewed food smooth and easy to swallow.

2) Teeth: Teeth are responsible for chewing and crushing food. We have 4 different types of teeth. They include:

                            *Canines- tear and shred food

                            *Incisors- biting and cutting food

                            *Premolars- grinding and chewing food

                            *Molars- grinding and chewing food

3)Tongue: The tongue is held responsible for 4 jobs. They include:

                            1) Mixes food with saliva
                            2) Rough surface aids in mechanical digestion
                            3) Positions food for chewing
                            4) positions food to initiate swallowing reflex

4) Uvula: The uvula is a soft plate composed of muscles. It closes off nasal passage during swallowing.

5) Pharynx: the pharynx is located behind your mouth, below the soft palate. It connects the mouth to the esophagus. The lower part of the pharynx connects to two tubes; the esophagus, which delivers food to the stomach, and the trachea, which carries air to the lungs.  

Esophagus:

• the inside of the esophagus is covered with a film of slippery mucous, which helps food pass freely
• layers of muscles: circular muscles, which encircle the esophagus; and longitudinal muscles, which run the length
• peristalsis: rhythmic wave of contraction throughout tract; and propels bolus down esophagus
• sole purpose is the conduction of food
• esophageal sphincter (entrance to stomach)

Stomach:

• thick walled, J-shaped organ
• lies on the left side of abdomen
• rugae folds in wall: mechanical digestion friction; and allows expansion
• three muscle layers:
1. longitudinal
2. oblique
3. circular
• gastric glands:
• found in the bottom of the rugae folds
• produce gastric juice - which contain hydrochloric acid and digestive enzymes. Secretions of these glands are controlled by nervous signals such as smells, thoughts and hormone signals.
• pepsinogen - protein digestion
• HCI - activates pepsinogen to pepsin (protein digestion)
• mucous cells
• produce thick protective mucus layer
• mixture of gastric juice, mucous and food is called chyme, and acidic paste with a pH of about 2
• moves into intestine through the pyloric sphincter

Small Intestine:

• duodenum
• the first 25cm of the small intestine
• principal site of digestion of nutrients
• receives bile, greenish-yellow liquid, from the liver (which was stored in the gallbladder)
• break down of fats
• receives pancreatic juice from pancreas
• many carbohydrate, lipid and protein digesting enzymes for digestion of nutrients
• bicarbonate with bile nuetralizes chyme from stomach
• receives intestinal juice
• secretes carbohydrate, lipid, and protein digesting enzymes
• jejunum and ileum
• principal site of absorption of nutrients
• lining has villi
• increases surface area
• outer layer of villi have microvilli (more surface area)
• villi contain lymph lacteals
• absorb fatty acids and glycerol
• villi contain blood capillaries
• absorb sugars and amino acids

Large Intestine:

• the large intestine or colon is about 1.5m long and 8cm in diameter. Its shape resembles an inverted "U"
• the ileocecal valve, occurs at the small intestine/large intestine junction. Its function is to control the rate at which the contents of the small intestine pass into the colon
• the main function of the large intestine is to eliminate undigested food wastes and to reabsorb water.
• The Cecum
• beginning of the large intestine
• Appendix
• protection of cecum
• may play role in fighting infections
• rupture may cause peritonitis
• Colon
• ascending, transverse and descending portions
• absorbing water and salts
• terminates at the rectum

Rectum/Anus

• as a result of the reabsorption of water the remnants of digestion become drier and more solid. This is called feces. It is stored in the rectum, the last 20cm of the large intestine. The anal sphincter, keeps the lower portion of the rectum closed.
• when this valve opens, waste materials from digestion are released from the body through the anus