Chapter III - Digestive System

SMALL INTESTINE

In biology the small intestine is the part of the gastrointestinal tract between the stomach and the large intestine (colon). In humans over 5 years old it is about 7 m long. It is divided into three structural parts: duodenum, jejunum and ileum. Food from the stomach is allowed in to the duodenum by a muscle called the pylorus, or pyloric sphincter, and is then pushed through the small intestine by a process of muscular contractions called peristalsis.

The small intestine is the site where most of the nutrients from ingested food are absorbed. There are microscopic finger-like projections called villi covering the small intestinal walls, which increase surface area for absorption. Each villus contains a lacteal and capillaries. The lacteal absorbs the digested fat into the lymphatic system, which will eventually drain into the circulatory system. The capillaries absorb all other digested nutrients.

Functions
In the small intestine, proteins are changed into amino acids; fats are changed into fatty acids; and carbohydrates are changed into sugars. The small intestine is also where most of the nutrients from ingested food are absorbed.

Small Intestine Disorders
Small intestine obstruction ("high" mechanic ileus)
Volvulus
Obstruction from external pressure
Obstruction by masses in the lumen (foreign bodies, bezoar, gallstones)
Paralytic ileus
Crohn's disease
Celiac disease
Carcinoid
Meckel's diverticulum
Gastric dumping syndrome
Infectious diseases
Giardiasis
Ascaridosis
Tropical sprue
Tapeworm infection
Mesenteric ischemia
Short bowel syndrome

DUODENUM
In anatomy of the digestive system, the duodenum is a hollow jointed tube connecting the stomach to the jejunum. It is the first part of the small intestine. It starts with the duodenal bulb and ends at the ligament of Treitz. Two very important ducts open into the duodenum, namely the bile duct and the pancreatic duct.

The duodenum is largely responsible for the breakdown of food in the small intestine. Brunner’s glands are only found in the duodenum and they secrete mucus.

The duodenum is divided into four sections for the purposes of description. The duodenum is almost all retroperitoneal.

JEJUNUM
In anatomy of the digestive system, the jejunum is the central of the three divisions of the small intestine and lies between the duodenum and the ileum. In adult humans, it is usually between 2-8m (06' 07"-26' 03") long. The pH in the jejunum is usually between 7 and 8 (neutral or slightly alkaline). The jejunum and the ileum are suspended by mesentery which gives the bowel great mobility within the abdomen.

The inner surface of the jejunum, its mucous membrane, is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients from the gut contents. It differs from the duodenum due to lack of Brunner's glands. It is also different from the ileum due to less goblet cells and generally lacks Peyer's patches.

ILEUM
In anatomy of the digestive system, the ileum (not to be confused with the ilium, a pelvic bone), is the final section of the small intestine. It is about 4m long in humans, follows the jejunum and duodenum, and is separated from the cecum by the ileocecal valve (ICV). The pH in the ileum is usually between 7 and 8 (neutral or slightly alkaline).

Its function is to absorb vitamin B12 and bile salts.

COLON (ANATOMY)
In anatomy of the digestive system, the colon, also called the large intestine or large bowel, is the part of the intestine from the cecum to the rectum. Its primary purpose is to extract water from feces. In mammals, it consists of the cecum, ascending colon and approximately the first two-thirds of the transverse colon on the right (or proximal) side and the last third of the transverse colon to the splenic flexure, the descending colon, the sigmoid colon, and the rectum on the left (or distal) side.

Role in digestion
The large intestine comes after the small intestine in the digestive tract and measures approximately 1.5 meters in length. Although there are differences in the large intestine between different organisms, the large intestine is mainly responsible for storing waste, reclaiming water, maintaining the water balance, and absorbing some vitamins, such as vitamin K.

By the time the chyme has reached this tube, almost all nutrients have been absorbed by the body and only water and some electrolytes like sodium and chloride are left. As the chyme moves through the large intestine, water is removed, while the chyme is mixed with mucus and bacteria known as gut flora, and becomes feces. The large intestine produces no digestive enzymes - chemical digestion is completed in the small intestine before the chyme reaches the large intestine. The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral).

Diseases of the colon
Angiodysplasia of the colon
Colitis
Colon cancer
Constipation
Crohn’s disease
Diarrhea
Diverticulitis
Hirschsprung's disease (aganglionosis)
Irritable bowel syndrome (spastic colon)
Polyposis (see also polyp)
Pseudomembranous colitis
Ulcerative colitis and toxic megacolon

CECUM
In anatomy of the digestive system, the cecum or caecum is a pouch connected to the large intestine between the ileum. It is separated from the ileum by the ileocecal valve (ICV) or Bauhin's valve, and is considered to be the beginning of the large intestine and part of the colon.

Its primary function is to absorb water and salts from undigested food. It has a muscular wall that can knead the contents to enhance absorption.

The appendix is a branch of the cecum. Like the appendix, the cecum was once believed to have no function.

RECTUM
The rectum (from the Latin verb regere "to straighten, correct, rule" hence the rectum is "that which is ruled, controlled") is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus.

For the diagnosis of certain ailments, a rectal exam may be done. Suppositories may be inserted into the rectum as a route of administration for medicine.

Body temperature can also be taken in the rectum. Rectal temperature can be taken by inserting a mercury thermometer for 3 to 5 minutes,

ANUS
In anatomy, the anus (from Latin anus "ring, anus") is the external opening of the rectum. Closure is controlled by sphincter muscles. Feces are expelled from the body through the anus during the act of defecation, which is the primary function of the anus.

Role in defecation
When the rectum is full the increase in intrarectal pressure forces the walls of the anal canal apart allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincters of the anus allow the feces to be passed by muscles pulling the anus up over the exiting feces.

Pathology
Anal cancer, abscess, warts, fistula, fissure, itching and hemorrhoids are among the diseases of the anus that benefit from medical intervention. Birth defects of the anus include stenosis and imperforation. The anus is also a frequent site of sexually transmitted infections.