Stomach transit time varies widely: it can take anywhere from ten minutes to four hours for food to pass through the pyloric sphincter into the duodenum. Transit time in the stomach is affected by the composition of the meal, and by the individual's rate of gastric emptying. Transit time may be shortened due to consumption of liquid or simple-carbohydrate foods, or lengthened in the case of consumption of solid food that has a high protein or fiber content.

Normal transit time of food through the small intestine is three to eight hours. This is significantly longer than transit time through the stomach due to the wider variety of digestive processes that occur in the small intestine, including the catabolism of proteins, lipids, nucleic acids, and simple and complex carbohydrates.

The average colon transit time for both adults and children is between 32 and 48 hours. This is significantly longer than the transit times of food through the stomach and small intestine, which averages 4-12 hours total. A slow transit time through the colon can be increased by various lifestyle factors such as exercise and hydration, and  dietary changes such as an increase in fiber-rich foods and avoidance of food allergies or other constipation triggers.

The main artery supplying blood to the small intestine is the superior mesenteric artery. This vessel branches off the abdominal aorta and supplies the lower part of the duodenum, the ileum, the jejunum, and two-thirds of the transverse colon. The rest of the colon and the rectum are supplied by the inferior mesenteric artery, while the right hepatic and left gastric supply the liver and stomach, respectively.

We have three main types of salivary glands: the parotid glands, the submandibular glands, and the sublingual glands. The thymus gland is not a salivary gland, and is located in the chest just superior to the heart. 

The primary signaling molecule that acts to increase saliva production by acinar cells in salivary glands is acetylcholine (ACh). ACh is released by parasympathetic innervation and triggers muscarinic receptors in salivary cells, leading to increased saliva secretion. While serotonin, epinephrine, and dopamine may have some effect on salivation, they are not the primary molecules through which the process is controlled. 

The swallowing center is not a specific anatomical structure, rather an area of physiological control. The swallowing reflex is primary controlled by the medulla, and to a lesser degree by the pons. None of the other structures listed play a significant role in deglutition. 

Dietary vitamin B12 must first form a complex with intrinsic factor, a gastric glycoprotein, before it can be absorbed into the hepatic portal system. Individuals who are unable to produce intrinsic factor are at risk for developing B12-related macrocytic anemia. While the other molecules listed are necessary for a wide variety of digestive and nervous system function, they do not directly control vitamin B12 absorption.

Lipoprotein lipase hydrolyzes triglycerides into two free fatty acids and one molecule of monoacylglycerol.

Hepatic lipase aids in the regeneration of LDL. Colipase is secreted by the pancreas and is an important co-enzyme for optimal function of pancreatic lipase. Lingual lipase is a fat-digesting enzyme that is secreted in the saliva and aids in the break down of dietary lipids before they are stored in chylomicron form. 

All of the bacterial strains listed have been shown to have beneficial effects on gut and immune health except for Clostridium difficile, a pathogenic bacteria commonly seen in hospital settings after prolonged antibiotic use. C. difficile has been associated with chronic diarrhea, behavioral disorders, and colitis.