The large intestine

The small intestine joins a wider tube, the large intestine (large because it is wider than the small intestine) at the cecum (ileocecal junction), in the right lower abdominal cavity. The cecum is a saclike pouch with only one opening; the cavity in which the large intestine begins and into which the ileum opens. The cecum absorbs water and salt from undigested food. From the lower end of the cecum extends a 10 cm long blind ended tube, the appendix, which, just like the cecum, has no major function in the digestive process.
The approximately 1.5 meters long large intestine or colon is responsible for absorption of minerals, chemicals, nutrients and water from the digested food, and excretion of solid waste material. Similarly to other parts of the digestive tract, the walls of the large intestine are muscular, and contract to move material along its length. Unlike the small intestine, the large intestine does not produce digestive enzymes. The pH in the colon varies between 5.5 and 7 (neutral or slightly acidic). Chemical digestion is completed in the small intestine before the chyme reaches the large intestine. Substances which have not been absorbed in the small intestine enter the large intestine in the form of liquid and fiber. As the chyme moves through the large intestine, the remaining nutrients are absorbed and water is removed, while the chyme is mixed with mucus and bacteria, and becomes feces.
The large intestine consists of the colon, rectum, and anal canal. The colon consists of ascending, transverse, descending, and sigmoid portions. The ascending colon extends from the cecum upward, along the right abdominal wall, to the under surface of the right lobe of the liver, where it bends abruptly forward and to the left (this bend is called the right colic flexure). This is where the transverse colon, the longest part of the colon starts. It passes across the abdomen with a downward convexity, curves sharply beneath the lower end of the spleen (this curve is the left colic or splenic flexure) into the descending colon. The descending colon passes downward along the left abdominal wall to the pelvic region (the lower part of the abdomen located between the hip bones). The colon then forms an approximately 40 cm long loop, the sigmoid colon, which normally lies within the pelvis. The remaining materials from the digested food move from the colon into the rectum. The rectum is a short (about 12 cm long) muscular tube, that acts as a temporary storage facility for feces.
It then bends sharply backward into the anal canal.
The last 2 to 3 cm of the digestive tract is the anal canal, which continues from the rectum and opens to the outside at the anus. The anus is the opening where stool exits the body through a ring of muscles, the anal sphincter. The sphincter keeps the anus closed as stool collects in the rectum. Eventually the pressure on the rectum wall causes the anal sphincter to relax, allowing stool to pass out of the body through the anus.

The small intestine

Once food has been stored in the stomach long enough to be partially digested, contractions of the stomach push partially digested food into the small intestine, where digestion is completed and nutrients are absorbed by the blood. This narrow, muscular, hoselike portion of the gastrointestinal tract is about 6-7 meters long, completely folded up within the abdominal cavity. The small intestine is divided into 3 sections: the duodenum, the jejunum and the ileum. The duodenum is the beginning portion of the small intestine, an 18 cm long receiving area for partially digested food from the stomach. The stomach is connected to the duodenum by the pyloric sphincter, where the food enters the small intestine. From here it is pushed through the intestines by peristalsis. The environment in the duodenum is slightly acidic, with the pH being 6.0 to 6.5.
The jejunum is the 2-8 meters long central division of the small intestine, where most of the nutrients are absorbed into the bloodstream. The pH in the jejunum is usually between 7 and 8, which is neutral or slightly alkaline.
The ileum is the final section of the small intestine. It is about 4 meters long. This is where the remaining nutrients are absorbed before moving into the large intestine. The pH in the ileum, similarly to the the jejunum is 7-8 (neutral or slightly alkaline).
For best absorption of nutrients the small intestinal walls are covered with fingerlike vascular projections, called villi. Each villus has additional cell membrane extensions, called microvilli. The villi and microvilli together increase intestinal absorptive surface area providing exceptionally efficient absorption of nutrients.
The liver, gallbladder, and pancreas are accessory organs of the digestive system that are closely associated with the small intestine. The lining of the small intestine secretes a hormone called secretin, which stimulates the pancreas to produce digestive enzymes. The duodenum receives secretions from the pancreas and liver via the pancreatic and common bile ducts. Bile is produced in the liver and stored in the gallbladder. When food enters the duodenum, the gallbladder contracts and discharges the bile into the common bile duct, that carries it to the small intestine, where it helps with digestion. The gallbladder and the ducts that carry bile and other digestive enzymes from the liver, gallbladder, and pancreas to the small intestine are called the biliary system.

The stomach

The stomach is a hollow, muscular, sac-like, pear-shaped organ. It is located between the esophagus and the small intestine, lying crosswise in the abdominal cavity, beneath the diaphragm. An adult's stomach is approximately 30.5 cm long and 15.2 cm wide. It can change size and shape according to the amount of food inside. Its capacity is about 0.94 liter.
The stomach stores, dissolves, and partially digests the contents of a meal, then delivers this partially digested food to the small intestine. Simple sugars and alcohol are absorbed directly through the stomach wall.
Food enters the stomach from the esophagus via the cardiac sphincter which prevents food from passing back to the esophagus.
The stomach consists of layers of muscles and nerves. These strong muscles contract and move and mix the contents of the stomach, mashing the food into a wet pulp.
Gastric glands on the epithelium of the stomach lumen secrete gastric acid (also called hydrochloric acid), which contains acids, mucus and enzymes. The gastric acid causes the stomach contents to have a pH of around 1 to 3. During a meal, the rate of hydrochloric acid production increases markedly. Seeing, smelling, tasting, and chewing food sends information through the vagus nerve to the gastric glands, causing them to increase acid production. On average, the stomach produces 2 liters of gastric acid daily. The gastric acid kills most of the bacteria in the food and stimulates hunger. The highly acidic environment in the stomach causes food proteins (large molecules, one of the three types of nutrients used as energy sources by the body) to lose their characteristic folded structure and break down to their components so they can be easily absorbed later in the intestines. The main enzyme in the stomach that functions to break proteins into smaller pieces is called pepsin. So that pepsin doesn't digest the cell that makes it, it is synthesized and secreted in an inactive form called pepsinogen.
Other glands on the epithelium of the stomach secrete mucus, a viscous, slippery fluid, that lubricates and protects the stomach from self-digestion.
The muscular action of the stomach and the digestive action of the gastric acid convert food in the stomach into a semiliquid mixture called chyme, which passes from the stomach into the small intestine.