1. Small Intestine
Food in liquid form enters the small intestine. The small intestine is about 20 feet long and is broken up into three regions, duodenum, jejunum, and ileum. It is lined with mucin and adapted for absorption of food through a highly molded inner surface. Each fold is called a villus.
a.Functions
1) Chemical digestion
2) Absorption
b. Enzyme Producing Organs
These secrete the enzymes into the small intestine.
1) Liver: The liver secretes bile into the duodenum. Bile is a fat emulsifier. Bile is stored in the gall bladder and reached the duodenum of the small intestine via the bile duct. Where it is joined by basic fluids from the pancreatic duct. Bile reduces the size of fat globules so lipases, enzymes that break down lipids, can break them down. If the bile duct is blocked, one becomes jaundiced because bile ends up in the blood stream.
2) Pancreas: The pancreas is a glandular organ that lies in the first turn of the small intestine. The pancreas secretes bicarbonate which neutralized acid. The pancreas also secretes enzyemes that brea down carbohydrates, proteins, fats, and nucleic acid. These enzymes from the pancreas carry out much of the digestive process. The acid of the chyme activates the stomach to produce secretin, a hormone that stimulates the pancreas to release bicarbonate.
The digestion of starch, begun in the mouth, is continued in the small intestine by the addition of pancreatic amylase, which hydrolyzes starch into the disaccharide maltose. Maltase will break down maltose into glucose monomers.
Proteins are broken down by trypsin and chymotrypsin. These break down the polypeptides into the smaller polypeptides. Carboxypeptidases split one amino acid at a time off the carboxyl end while the aminopeptidases work in the opposite direction. These enzymes are secreted by the pancreas as inactive zymogens. Enterokinase activates zymogens in the small intestine.
Nucleases hydrolyze DNA and RNA
All this occurs in the duodenum of the small intestine. The two remaining regions are specialized for absorption.
c.Absorption in the Small Intestine
Each villus is covered with epithelial cells that contain microvilli. These projections increase the surface area of the small intestine. Within each villus is a capillary bed and a lacteal. The nutrients enter directly into the blood stream from the small intestine.
The small intestine is capable of vigorous movement which mizes food and enzymes together. This movement also helps with absorption.
Nutrients are absorbed across the epithelial layer into the capillaries or lacteals. Sometimes the transport is passive while other times it is active. Amino acids, vitamins, glucose, are pumped against the gradient by the epithelial membranes. The absorption of some nutrients appears to be soupled with the active transport of sodium across the membrane. The sodium is actively transported out of the cell, nutrients are co-transported.
2. Large Intestine
The small intestine joins the large intestine on the right side of the body. Here, the cecum forms a pouch which ha a finger like projection called the appendix. There is a one way valve at the junction of the large and small intestine called the ileocecal valve which insures that there is no backflow into the small intestine.
The large intestine consists of several parts: cecum, ascending colon, transeverse colon, descending colon, sigmoid colon, rectum, anal canal, and anus. The fun tions of the large intestine include the absorption of water and minerals into the blood, preparation of feces, and the housing of bacteria which produce vitamin K, biotin, folic acid, and methane. Approximately 2/3 of fecal matter is dead bacteria. The large intestine secretes mucus to lubricate for easier export. The rectal valves support the feces until defecation. The wastes go through the anal sphincter, under voluntary control and out the anus. It takes 12-24 hours for material to travel the length of the large intestine.