The intestine is that portion of the gastrointestinal tract that lies between the stomach and the anus. The intestine comprises two parts: small intestine and large intestine. Here, let's focus on the small intestine.
Small Intestine Anatomy
The small intestine stretches from the stomach to the beginning of the large intestine. It measures over 6 meters (20 feet) in length and is coiled in the central portion of the abdominal cavity.
However, despite the 6 meter length, the small intestine's surface area is not adequate to absorb all body nutrients required by the body, as it is narrow in width. Thus, the small intestine features myriads of microscopic folds, which increase the surface area available for absorption.
The inner walls of the small intestine (jejunum) are made up of two kinds of folds called plicae circulares and rugae. The rugae permit extra tissue for the small intestine to contract and distend when needed. The plicae circulares are fixed structures located on the intestinal wall that again comprise two structures called villi and microvilli.
These two structures are frond-like protrusions working together to maximize surface area for absorption. Blood vessels present in the villi are responsible for the transportation of nutrients absorbed by the surface cells. 'Mucosa' the inner lining of small intestine features three kinds of cells: epithelial cells, endocrine cells and secretory cells.
Moreover, the 6 meter long small intestine is divided into three distinct zones: duodenum (25 cm long), jejunum (2.5 m long) and ilium (3.5 m long). However, the difference in anatomical structure is not very evident, as the structural differences are microscopic. Nevertheless, each part is associated with a specific function.
What is the Function of the Small Intestine?
As far as the main function of small intestine is concerned, it is the part where the 'most extensive part of digestion' occurs. The large intestine is most responsible for absorption of water and excretion of solid wastes, thus, works more towards dealing with the bulk and its removal from the body.
Since the large intestine deals with bulk, it is shorter and wider, while the small intestine's narrow width corresponds to the absorption and digestion process carried out.
Food ingested via the mouth enters the stomach and is allowed to enter the duodenum, by a muscle called pyloric sphincter. The ingested food is then pushed through the small intestine by a muscular-wavelike process called peristalsis.
Most of the chemical digestion takes place in the small intestine (duodenum). In fact, the duodenum is only responsible for digestion of food and not absorption. The pancreas secretes digestive enzymes, which enter the small intestine via the pancreatic duct.
Moreover, the pancreas also releases bicarbonate into the small intestine under the influence of hormone secretin, to neutralize potentially deleterious acids coming from the stomach. The major classes of nutrients undergoing digestion in the small intestine are carbohydrates, proteins and lipids.
In the small intestine, carbohydrates are broken down into simpler sugars (monosaccharides - glucose). For example, carbohydrates are degraded from oligosaccharides to monosaccharides by pancreatic amylase, after which two other enzymes: dextrinase and glucoamylase further break them down.
The gallbladder also releases bile into the duodenum, which is responsible for the breakdown of dietary fats, along with pancreatic lipase. Bile envelops the fat molecules and forms clumps known as micelles, which can be now absorbed by the cells lining the small intestine.
Proteins and peptides, on the other hand, are broken down into amino acids. The degradation of protein commences in the stomach and continues to take place in the small intestine.
Proteolytic enzymes secreted by the pancreas break down peptides into smaller peptides. Moreover, the pancreatic brush border enzyme called carboxypeptidase split one amino acid at a time. Pancreatic lipase degrades triglycerides into monoglycerides and free fatty acids.
Once the food has been digested, it is ready to pass into the blood vessels situated in the intestinal walls, by a process known as diffusion. Active, passive and facilitated diffusion of nutrients (including vitamins and minerals) takes place in the small intestine.
Moreover, the mucosal lining in the intestinal walls featuring plicae circulares and rugae absorb maximum nutrients as possible from the food that is passed through the small intestine.
The absorbed nutrients are then transported to different organs of the body, via blood vessels, wherein, they are used to build proteins and other substances required by the body. This process is known as assimilation.
Most of the nutrients are absorbed by the jejunum of the small intestine and nutrients not absorbed by the jejunum are absorbed by the ilium. The remaining undigested food is passed to the next part of the digestive system i.e. to the large intestine.
The small intestine function is quite intricate and interesting. The mentioned process was only a summary of all that the 6 feet convoluted tube in our abdominal cavity does.