The Human Digestive System - The Human Digestive System Part 2

First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. The stomach has a capacity of 1 liter, from an empty capacity of 50-100 milliliters, but the stomach can distend to hold 2 liters or more. With most people, the stomach mucosa (lining) is able to resist the juices in the stomach, although food and other tissues of the body cannot. The stomach starts to churn the food by muscular layers to form chyme. Epithelial cells line inner surface of the stomach, and secrete about 2 liters of gastric juices per day. Gastric juice contains hydrochloric acid, pepsinogen, and mucus. Hydrochloric acid (HCl) lowers pH of the stomach so that pepsin can is activated and also gives the stomach the right acidity to inhibit further carbohydrate breakdown. The enzyme pepsin splits off fragments of peptides from protein molecules, and begin the break down of protein. Chyme, the mix of acid and food in the stomach, then leaves the stomach and enters the small intestine.

Liver: Duodenum: Gallbladder: Pancreas:

These 4 organs do various tasks involved with digestion, the main one being the production and secretion of enzymes needed to break down Chyme. Bile, a watery greenish fluid that is produced by the liver and secreted via the hepatic duct and cystic duct to the gall bladder for storage. At mealtime, it is squeezed out of the gallbladder into the Duodenum via the bile duct. The Duodenum(upper third of small intestine) releases Bile, controlled by pyloric sphincter, into the small intestine as food enters it.

Bile contains cholesterol, phospholipids, bilirubin, bile pigments, and a mix of salts. Bile lowers pH of the chyme, Bile salts and phospholipids emulsify fats, facilitating their breakdown into progressively smaller fat globules until they can be acted upon by lipases (enzyme secreted by the small intestine). After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.

The pancreas secretes digestive enzymes and stomach acid-neutralizing bicarbonate. Proteases (enzymes secreted from the pancreas) continue the breakdown of protein into small peptide fragments and some amino acids.

Small Intestines:

As the food is digested in the small intestine and dissolved into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward.

Digestion of carbohydrates, proteins, and fats continues in the small intestine. Starch and glycogen are broken down into maltose. Maltose, sucrose, and lactose are the main carbohydrates present in the small intestine; they are absorbed by the microvilli.

Jejunum(8 feet)(second third of the small intestine) Final digestion of proteins and carbohydrates must occur, and fats have not yet been digested. Starch is broken down into two-glucose units(maltose).

Ileum(13 feet)(lower third) - Fats are completely digested, digested fats are not very soluble. Bile salts surround fats to form micelles that can pass into the epithelial cells. The bile salts return to the lumen to repeat the process. Fat digestion is usually completed by the time the food reaches the ileum of the small intestine. Bile salts are in turn absorbed in the ileum and are recycled by the liver and gall bladder. Fats pass from the epithelial cells to the small lymph vessel that also runs through the villus. The small intestine drains fat and other nutrient rich lymph into it via the hepatic portal system.

Villi (intestinal folds) have cells that produce intestinal enzymes which complete the digestion of peptides and sugars and push the remaining contents into the Caecum.

Large intestines:

Caecum
Ascending Colon
Transverse Colon
Descending Colon
Sighnoid Colon

The first part of the large intestine is the caecum. It is a repository for bacteria and other organisms able to digest cellulose. Enzyme lactase produced by the intestinal walls helps to break down lactose. Secretions in the large intestine are an alkaline mucus that protects epithelial tissues and neutralizes acids produced by bacterial metabolism. Water, salts, and vitamins are absorbed, the remaining contents in the lumen form feces (mostly cellulose, bacteria, bilirubin). Bacteria in the large intestine, such as E. coli, produce vitamins (including vitamin K) that are absorbed. where they remain, usually for a day or two, until the feces are expelled by a bowel movement. Anus has voluntary and involuntary sphincter and ability to distinguish whether contents are gas or solid.

Urinary system:

The urinary system is a whole other system in itself, but is a part of the digestion process. Liquids are absorbed in the intestines and sent thru the blood stream and empty off into the beginning of the urinary system. But that’s a whole other journey.

Conclusion:

In conclusion, even though this is only a brief explanation of the basics of the digestive system, you can see how incredibly complex it is. The true miracle is how every part works in perfect harmony with one each other, "a well oiled machine" if you will. The total time it takes for a meal to take the entire journey depends on what was eaten. The time it takes to fully digest a meal is anywhere from 2 hours to over 24 hours. High fat diets make this time longer. It’s important to take care of your digestive system. Neglect can lead to digestive system diseases, mal-nutrition and poor a quality of life.