The enzymes for starches is pancreatic amylase and it does the same thing ptyalin in the saliva does. It digests starches to disaccharides - double sugars. These enzymes act only on starches, which are large molecules made by hooking glucose molecules together. These enzymes break starches down into maltose, two glucose molecules hooked together.

Fat is digested by pancreatic lipase. This enzyme breaks down neutral fat - most triglycerides - into single fatty acids, and the glycerol into monoglycerides, which are only one fatty acid still attached to glycerol. That is the reason why you cannot swallow specialized fats such as lecithin and have lecithin enter your bloodstream. It will be digested into its components fatty acid and glycerol parts.

Once the pancreatic enzymes have acted on the fats, they can then be absorbed through the wall of your small intestine. Fatty acids pass through the wall rapidly and raise your blood fat level, but it may take 3 days for cholesterol to pass through your intestinal wall into your bloodstream.

Protein is digested by the pancreatic enzyme trypsin. It breaks small protein particles into smaller units of several amino acids, but not all the way into the single amino acids. It is the next step after pepsin has started the initial dismantling of the long chain of amino acids that forms proteins.

From 700 - 100ml of bile are formed every day. Bile contains bile salts, which aid in the absorption of fat. Bile acids are formed inside your liver cells from cholesterol. The bile acids form salts, and these emulsify fatty acids and cholesterol, much like laundry detergent emulsifies oil and grease. That helps absorb fatty acids into the circulation. Without bile salts, as much as 40% of fats would be lost in the stools.

Also, the fat soluble vitamins A, D, E and K depend on the absorption of fat to enter your internal body. Since the body has abundant stores of all these except vitamin K, the big initial danger is a Vitamin K deficiency. That will limit the liver's ability to form substances essential for blood clotting, and is one reason why patients with liver disease may tend to bleed easily.

Bile salts are not eliminated with your stools. Rather, 94% is reabsorbed into the circulation and returned to the liver to be used again. However, some medicines used to lower cholesterol take advantage of the fact that bile acids are formed from cholesterol. Bile also contains cholesterol. In fact, the major portion of the cholesterol in your bloodstream is from cholesterol formed by the liver and reabsorbed into your circulation.

Bile contains bilirubin, derived from the hemoglobin pigment in your red blood cells. That pigment is what gives your stools their color. Cholesterol itself is insoluble in water and it's the bile salts that keep it in solution in your bile. When bile is stored in the gallbladder, if there is too much cholesterol or not enough bile salts, cholesterol gallstones will form. An overproduction of cholesterol, as in some people consuming too much calories or too much fat, can lead to gallstones.

Despite all these actions by pancreatic juice and bile, much work remains to be done by the small intestine itself. The fingerlike projections or villi from the wall of the small intestine are essential for the final phase of digestion of both protein and carbohydrate. At the edge of the larger cells are tiny cells, almost like a brush, and these are called the brush border cells. Within these cells, not in the intestinal fluid, are more enzymes.

Maltase enzyme splits maltose and sucrase enzyme splits sucrose (table sugar). Lactase enzyme splits the double sugar lactose into glucose and galactose so they can be absorbed. Lactase deficiency is a common cause of gaseous distention, abdominal cramps and diarrhea. The undigested lactose sugar is the cause. Some small peptides are also split by peptidases into individual amino acids.

In addition to its chemical role in digestion, the small intestine plays a major role in emptying your stomach, through both hormones and reflex action. Whenever your duodenum is overly distended, it sends a reflex signal to slow down the stomach's action. If the chyme contains too much acid or there is irritation of the duodenum, the same mechanism is invoked. Sometimes the amount of salts is not the right concentration and again the duodenum says, 'Slow down until I can correct the situation'. A fatty meal also delays emptying of the stomach.

In general, minerals and vitamins are not digested. There are many factors that affect their absorption through the intestinal wall. That includes what else you have eaten. Both iron and calcium absorption can be decreased by coffee and tea. The presence of phytic acid and oxalic acid in many vegetables will form insoluble calcium salts and reduce absorption. There is also an internal mechanism that controls how much iron and calcium is absorbed. If your blood calcium or iron levels are high, absorption is significantly decreased.

Iron is another important element. There are two types, elemental iron, such as ferrous gluconate or ferrous sulfate, and heme iron, which is present in meats. Heme iron is rapidly absorbed in the first part of the small intestine. A much higher percentage of iron is absorbed from heme iron compounds than from iron salts that you find in iron pills or non animated food sources. Often the reason a person has an iron deficiency anemia is because there is not enough heme iron in the diet.

The Colon's Action

Although the colon is blamed for many things, its role in digestion is relatively limited compared to the rest of the digestive tube. The amount of fluid that passes into your digestive tube each day from saliva, gastric secretions, the pancreas, bile and the small intestine is nearly seven liters and people ingest about another 1,5liters of fluid a day for a total of nearly 9 liters. Most of this is reabsorbed by the last part of the small intestine. Only about 1,500ml of chyme is left to pass through the ileocecal valve into the colon. This is fluid to semi-solid slush.

The first half of the colon is actively engaged in recovering even that fluid and balancing the concentration of salts and water with the rest of the body. This part is loaded with bacteria. This is not all bad because under normal circumstances these are mostly friendly bacteria, which compete with harmful bacteria and help to protect you from them. They also further digest various food items that were not digested earlier. It is here that undigested lactose is fermented to release gas. But the bacteria are also responsible for formation of Vitamin K and Vitamin B12. The normal diet alone would not provide sufficient Vitamin K, but the needed amounts are obtained from bacterial action.

The final fecal mass is mostly water, about three fourths of its weight. The other one fourth is solid matter. The amount of stool depends greatly on the amount of bulk in the diet. Those who consume very little bulk usually have small stools and not infrequently have problems with motility of the colon and constipation. About 30% of the weight of the solid matter in the stool is dead bacteria. Here again, the friendly bacteria help by providing needed bulk.

The final act in the completion of digestion is the elimination of the undigested residue, along with the dead bacteria. - defecation. There is a weak contraction of the muscles in the lower colon (sigmoid) where it joins the rectum - about 20cm from the anus. That keeps the rectum empty under normal circumstances. But reflex action can fill the rectum. This most often occurs when the stomach and small intestines are filling up immediately after eating.

The main reflex that causes this is the gastro colic reflex. There is a sudden strong peristaltic rush over the last half of the colon, the storage portion. As the peristaltic rush occurs, and the junction of sigmoid colon and rectum opens, the stored fecal material moves into the rectum. The distention of the rectum initiates the well recognized urge to have a bowel movement. Then the complex mechanism of opening the anal sphincter and the actual bowel movement occurs. This latter stage is normally under voluntary control in the adult. But before the pathways to the brain are fully developed in babies, or in a person with a spinal chord injury, voluntary control is absent.

It is clear from this discussion why so many things can go wrong with the digestive sequence. But for most people, who follow a sensible diet and a good lifestyle, the digestive system can perform its vital functions for their health completely unnoticed, except for the pleasure of the eating itself.