Most digestion and absorption of digested food happens in the small intestine. These processes can take 3 to 6 hours.
The duodenum, which is the 1st section of the small intestines, is the hub of the gastrointestinal system: receiving food from the stomach, enzyme-rich juice from the pancreas, and bile from the liver. Pancreatic juice has enzymes that break down sugars and starches into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol. Bile breaks down fat from globules to tiny droplets which enzymes can then act on. The autonomic nerve complex that services the duodenum is sometimes called the solar plexus.
The small intestine is a twisting tube, ~2.4 cm in diameter and ~6 meters long. As with stomach, food is transported through the small and large intestines by peristalsis: waves of involuntary muscle contractions.
The lining of the intestinal wall is a single layer of elongated cells that encounter food on one side and the bloodstream on the other. The intake side absorbs nutrients while the output side releases them into the blood.
The 2 sides of the cell differ in their composition of genic instructions (messenger RNA (mRNA)), and in the organelles (ribosomes) that make the enzymes needed for digestion. There are twice as many ribosomes on the food-facing side of the cell than on the bloodstream-facing side, making the food-facing side much more efficient in enzyme production.
When food enters the intestine, cells in the intestinal lining ramp production of ribosomes, particularly in the food-facing part of the cell. To accelerate the process a cell dispatches large numbers of mRNAs to provide instructions for ribosome production. This part of the cell becomes an intensive factory: generating ribosomes which then produce the needed enzymes.
For most of the night and day, cells in the lining of the intestines just loll around, but once food appears, they must instantly step into action. Generating new mRNA molecules from DNA to make new proteins would take the cells about half an hour. Instead, they can increase production of certain proteins within minutes by moving the mRNA molecules encoding the relevant proteins into the side of the cell that is rich with ribosomes. This strategy enables them to deal with the arrival of food in a fast and efficient manner. ~ Israeli systems biologist Shalev Itzkovitz
The surface area for nutrient absorption in the small intestine is enormous. The small intestine is lined with millions of fingerlike projections called villi. Each villus is 0.5–1.5 mm long and covered with a layer of cells.
In an application of fractals in Nature, the villi are covered with even tinier microvilli fingers, which vastly increases the surface area, multiplying capacity for digestive absorption ~150 times.
Behind a villus’ single layer of cells are capillaries of the bloodstream and lymphatic systems. These capillaries transport nutrients throughout the body. Simple sugars and amino acids enter the bloodstream, while fatty acids and glycerol are transported by the lymphatic system.
The huge expanse of surface area in the small intestine for absorbing food also constitutes a portal for pollutants and unwanted microbes. Normally the lining of the small intestine functions as a filter: letting nutrients through but not undesirable materials. A weakened system stops being an effective guard, whether from pesticides, herbicides, drugs, irritation from stimulants or harsh spices, or from bad bacteria.
Some absorption is passive. Sugar concentration, for example, may be so high in the intestine as to force infusion across the intestinal wall. In other instances, absorption is active: metabolic systems pull nutrients from the intestinal tract and into the bloodstream or lymphatic system.
The digestive process does not proceed smoothly if food does not remain in contact with the intestinal wall long enough. The speed of digestive processes is governed by the nervous system, and so heavily influenced by the mind and emotional flux.