The colon extracts water and salts while forming feces. The colon also absorbs some nutrients that made it through the small intestine without being absorbed, but its main job is water and salt absorbtion.

The large intestine is the last organ in the digestive tract. The food mass exits the stomach as chyme and enters the small intestine, where 90% of chemical digestion occurs. The digested chyme is transported to the large intestine where most of the water and undigested nutrients in the chyme are reabsorbed; therefore, the majority of the digestion occurs in the small intestine, while the majority of water reabsorption occurs in the large intestine. This makes statement I false.

The large intestine also contains E. coli, a bacterial species that thrives in the anaerobic environment of the large intestine. Presence of E. coli is beneficial to humans because the bacteria synthesize vital vitamins such as vitamin B12. This makes statement II true.

Recall that the stomach is characterized by its highly acidic environment. This means that the lumen of the stomach contains a low pH and, therefore, a high concentration of hydrogen ions. Other organs involved in digestion, such as the large intestine, do not contain a highly acidic environment. The pH of the large intestine is higher than that of the stomach and, consequently, it has a lower concentration of hydrogen ions than stomach. This makes statements III true.

The most important function of the large intestine is the resorption of water that comes from intake or the pancreas. Waste is still hydrated while in the small intestine.

The gastrointestinal, or digestive, system is primarily designed to break down (catabolize) nutrients and absorb them into the blood for use by the body. Digestion of nutrients by stomach and pancreatic enzymes allows for their absorption in the small intestine. The large intestine is responsible for water absorption.

Filtration occurs in three primary areas in the body: the kidney, the liver, and the spleen/lymph nodes. The kidney is part of the excretory system, in which fluids are filtered from the blood to remove solute waste from the body. The liver filters the blood in circulation, and is responsible for removing toxins that may have been absorbed during digestion. It is not, however, part of the gastrointestinal tract. The spleen and lymph nodes filter the interstitial fluid to screen for antigens and pathogens.

Diarrhea is a condition that causes living organisms to have loose bowel movements. This means that the bowel movements contain a high amount of liquid water. Recall that the main function of the large intestine is to reabsorb water back into the body. If there is a decrease in water reabsorption, then more water will be excreted via bowel movements; therefore, decreased water reabsorption in the large intestine will lead to loose bowel movements and, subsequently, diarrhea.

Most of the pathogens are not killed in the large intestine; they are killed in the acidic environment of the stomach. The E. coli found in large intestine function to release vitamins, such as vitamin B12. They do not participate in water reabsorption; lack of microbes (E. coli) in the large intestine does not cause diarrhea.

Fat-soluble vitamins include A, D, E, and K. Water-soluble vitamins include B and C.

The digestive system begins in the mouth with fragmentation. Mucous helps roll the food into a bolus and salivary amylase begins the process of chemical breakdown.

The primary roles of the large intestine are water and electrolyte absorption. If this role is impeded by a pathogen, diarrhea results. Dehydration can also be a secondary effect of large intestine damage, and can lead to death.

The small intestine is primarily involved in nutrient absorption and chemical digestion. The stomach is involved in mechanical digestion, and some chemical digestion. The liver produces bile, which can be secreted to emulsify fats.

The myenteric interstitial cells of Cajal act as the "pacemaker" of the gastrointestinal tract. They cause spontaneous smooth muscle cell contractions in the stomach, small intestine, and large intestine. These contractions play and integral role in mechanical digestion and motility of digestates.

The stimulus is spread to the gastrointestinal tract via gap junctions; however, the action potential does not originate from the junctions themselves and must be initiated by the cells of Cajal. Acinar cells are found in several regions and are defined by their clustered formation to form an exocrine gland. Acinar cells secrete pancreatic enzymes and saliva in the mouth. The cells of Cajal act via spontaneous depolarization and are not under the direct control of gastrointestinal neurotransmitters.

The esophagus contains both striated (skeletal) muscle and smooth muscle. The two types of muscle, therefore, allow two distinct functions. Skeletal muscle, in the upper part of the esophagus, allows for voluntary control of swallowing; thus, when a bolus of food has been broken down by chewing, a person can consciously make a decision to begin the swallowing process. Once in the esophagus and past the striated muscle of the upper esophagus, the muscular control is smooth muscle. Smooth muscle is autonomically regulated, meaning that no voluntary control is needed to continue swallowing. Once the food enters the esophagus, the smooth muscle will propel it downwards towards the stomach. The mix of striated and smooth muscle allows for voluntary control of beginning to swallow, but automatic control for swallow completion.