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 correct answer shows the proper order, and you should pay special attention to the names of the two sphincters.  The cardiac sphincter is by the heart, and comes before the stomach.  The pyloric sphincter is at the bottom of the belly and comes after the stomach.  Food does not pass through the larynx, which is used in breathing and speaking.

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.

Lipid digestion begins to occur in the mouth because of the enzyme lipase. Saliva in the mouth incorporates both lipase, to begin lipid digestion, and amylase, to being carbohydrate digestion. Lipids are also broken down later in the digestive pathway by lipase in the small intestine. Protein digestion does not begin until food enters the stomach.

Vitamins A, D, E, and K (ADEK) are fat soluble vitamins, and require the ingestion and absorption of fat in order to maintain adequate stores of the vitamins in the body. Any problems with bile salt function or absorption of bile salt and fat miscelles in the small intestine can lead to a deficiency in vitamins A, D, E, and K. All other vitamins can be absorbed without the help of fat.

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.

The hormone aldosterone is most frequently associated with the kidney and nephron function, but has effects on water balance in other regions of the body as well. In the mouth, aldosterone acts on salivary glands to promote sodium and water absorption. The sodium is pumped across the epithelial cells of the mouth, and is able to directly enter the blood via sodium-potassium ATPase function. Potassium is secreted as a result.

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.

The parasympathetic nervous system, controlled in the gastrointestinal system by the vagus and pubic nerves, serves to decrease the rate of digestion. Rhythmic contractions of the small intestine and large intestine are slowed under parasympathetic control, leading to decreased digestion and absorption of food contents. This represents the "rest and digest" portion of the parasympathetic nervous system, in contrast to the "fight or flight" portion of the sympathetic nervous system.

The parasympathetic nervous system slows digestion, allowing for more absorption of nutrients. The sympathetic response increases the rate of flow through the digestive tract, limiting the amount of absorption that can occur.

The interstitial cells of Cajal link the parasympathetic nervous system with the smooth muscle of the gastrointestinal system. They are responsible for the rhythmic contractions of the gastrointestinal system that propel digested food, and are found in the stomach, small intestine, and large intestine. Interference with their "pacemaker-like" function can lead to constipation and gastroparesis (failure of the gastrointestinal system to contract).

Absorption of fats, iron, and proteins occurs in the small intestine with the aid of microvilli.