More Organs of Digestion
The lower GI tract—from small intestine to anus—is responsible for the final breakdown of food, its absorption into the body, and the elimination of waste products.
Small Intestine
The small intestine is the longest portion of the alimentary canal, approximately 20 feet long. It absorbs the raw digested materials into the blood and lymphatic system. It is divided into three regions. The first, and the shortest, is the duodenum, which is connected to the stomach. This region has an abundance of cells that produce an acid-neutralizing mucus. The jejunum is the middle region of the small intestine and is approximately equal in length to the last portion of the small intestine, the ileum.
To aid in absorption, the luminal mucosa has been expanded and folded to increase surface area. The folds or finlike protrusions are called plicae circularis and function much like the blades on the inside of a clothes dryer (which help move material around). Smaller folds called intestinal villi are present over the entire mucosal surface and are where absorptive cells are located. Additionally, apical modifications called microvilli on the lining cells maximize the absorptive area.
In addition to increasing surface area, the intestinal villi provide a space for vascular and lymphatic capillaries, allowing material to pass from the intestinal tract into the rest of the body.
Between the intestinal villi and extending downward into the lamina propria are extensions of the mucosal epithelium. These are similar to the gastric pits in the stomach and are called the crypts of Lieberkühn (intestinal crypts). Here you will find cells with functions other than absorption, including the secretion of hormones, enzymes, and acid-neutralizing mucus.
Cells
The predominant cell type found lining the intestinal tract is the surface absorptive cell (SAC). With numerous microvilli, these cells are well suited to absorb material from the intestinal lumen. With tight junctional complexes between cells, they ensure that no material in the lumen passes into the deeper tissues of the intestine.
As material moves along the alimentary canal, it becomes progressively less hydrated and, as a result, more difficult to move without causing damage to the tissues. Therefore, mucus-secreting cells (goblet cells) become more abundant in the mucosal lining.
The crypts are populated with enteroendocrine cells, which produce hormones. These cells also produce gastric inhibitory peptide (to stop the production of HCl) and cholecystokinin (causing peristaltic contractions of the gall bladder to expel bile into the alimentary canal).
The ileum (final segment) of the small intestine contains lymphoid nodules (Peyer’s patches) and M (microfold) cells. There are antigen-presenting cells, much like macrophages. Large Paneth cells present in the base of the intestinal crypts are triggered by pathogens to release their secretory materials. Paneth cells produce a variety of antimicrobial enzymes and agents such as lysozyme and also release several immune system cytokines that are essential for immune system function.
Large Intestine
Although the large intestine (colon) focuses on absorption, it mostly absorbs water (approximately 1400 ml/day) and compacts the material into solid waste (feces; 100 ml/day), which is stored in the lower portion of the colon until eliminated from the body. Digested material passes from the ileum of the small intestine through a muscular sphincter (ileocolic valve) and into the beginning of the colon.
The colon does not have intestinal villi. The crypts are still present, but become shorter as the fecal material gets closer to the rectum (end of the large intestine). While the colon is not the longest portion of the alimentary canal, it stretches for approximately 5 feet and is larger in diameter than the small intestine (3 inches versus 1 inch).
Another important histological, as well as gross anatomical, difference is that the outer longitudinal muscle is only present as three bands of smooth muscle called taenia coli. These maintain a certain base level of tension on the colon and results in the folds (sacculations) of the large intestine. Additionally, spasmodic contractions help move the fecal material farther along the colon.
Cecum and Appendix
The beginning of the colon is a blind-ended pocket that is inferior to the ileocolic valve called the cecum. Material is moved upward into the first ascending portion of the colon; however, some material is invariably trapped in the cecum. Projecting off the cecum is a wormlike appendage called the appendix (vermiform appendix) where lymphoid nodules can be found in the lamina propria.
Colon
From the cecum, the ascending colon rises superiorly on the right side of the abdomen before making a 90° bend and extending across the body as the transverse (across) colon. Once on the left side of the body, the ascending colon makes another 90° turn and forms the descending colon, which extends to the lower left quadrant of the abdominal cavity. To align with the midline of the body, the colon makes an S-shaped bend as the sigmoid colon and then continues straight downward as the rectum.
Rectum
The final straight segment of the colon is the rectum, which functions in the storage of feces and its elimination (defecation). Just before the anus (external opening of the rectum), two muscular sphincters retain the material internally until voluntarily released. The internal anal sphincter is under involuntary control and is always in a state of contraction. It is triggered by fecal pressure. The external sphincter may be controlled voluntarily or involuntarily, and is made up of skeletal muscle.
Pancreas
Considered one of the accessory digestive glands, the pancreas is located in the curve of the duodenum near the pylorus of the stomach. This is a perfect location, since the exocrine secretions from the pancreas pass through the pancreatic duct, into the common bile duct, and enter the duodenum through the sphincter of Oddi.
Triggered by hormones like cholecystokinin (produced by enteroendocrine cells of the intestine) and the neurotransmitter acetylcholine (active during the rest and digest phase of the autonomic nervous system), the pancreatic cells secrete a solution of digestive enzymes as pancreatic juice, which contains enzymes that further degrade carbohydrates, proteins, and fat.
Endocrine cells are the characteristic feature of the pancreas. Aggregated into small masses, these cells form the islet of Langerhans (pancreatic islets) and are surrounded by the exocrine cells of the pancreas.
Liver
The liver, the largest gland in the body, is positioned in the superior portion of the abdominal cavity just superior to the stomach. It is divided into two major lobes (right and left) and two minor lobes (the quadrate lobe is located near the gall bladder, while the caudate lobe is near the entry of the hepatic portal vein). Blood from the digestive tract is brought to the liver via the hepatic portal vein and enters the liver at the junction of the four hepatic lobes, called the porta hepatis. This nutrient-rich blood is spread throughout the open spaces of the liver (sinusoids) and forced to make contact with the liver cells (hepatocytes), where they can metabolize this material.
The regenerating organ
The liver is the most highly regenerative tissue in the body. Two-thirds of the liver may be lost and the remainder of the liver will regenerate the portion that was removed.
Structure
Think of the liver as a high-efficiency filter. Blood is emptied into long channels between rows of liver cells (hepatic cords) and flows toward a central vein. With these rows of cells and sinusoids arranged in series, it creates what resembles a 6-sided wheel with the spokes (rows of cells) radiating toward the axle (central vein). At three or more of the peripheral points on the hexagon, there is an arrangement of a hepatic vein, a hepatic artery, and a bile duct. These always occur together at these points and are thus referred to as a hepatic triad.
As this blood travels through the hepatic sinusoids, it is in contact with the capillaries that line the spaces and separate the blood cells from the underlying liver cells. The endothelial cells of these capillaries are filled with large holes that resemble Swiss cheese. These large holes allow all material in the blood, with the exception of cells and platelets, to pour into a space (the space of Disse) between the sinusoidal capillaries and the hepatocytes. It is within this space that the material is cleaned. Resident macrophages, called Kupffer cells, are abundant in the liver sinusoids and remove any substances that may be detrimental to the body.
Functions
In addition to blood-filtering and bile-producing activities, the liver is a storage facility for carbohydrate (glycogen) and vitamins (A, D, and B). This requires the liver to process carbohydrates. It can polymerize glucose into glycogen, and conversely break down the glycogen it stores and return glucose to the blood stream.
The liver also plays an important role in protein metabolism and composition of plasma. The liver produces albumin, the most abundant plasma protein, which is critical for the maintenance of osmotic pressure in the blood stream. A by-product of red blood cell destruction and hemoglobin metabolism is bilirubin, which is further processed by the liver into a water-soluble form that can be eliminated through the urine and feces.
Fats are also metabolized and regulated by the liver, and cholesterol and lipoproteins are produced by the liver.
Lastly, the liver detoxifies the blood. It enzymatically processes toxins into a less harmful form. Ethyl alcohol is the best example of this. If you consume alcohol faster than your liver can metabolize it, your blood alcohol levels increase and coordination and judgment decrease.