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.

Intrinsic factor, a molecule necessary for the absorption of dietary B12, is produced by the parietal cells of the stomach. Its release stimulated by the same factors that stimulate release of hydrochloric acid: histamine, gastrin, and acetylcholine. 

Gastric chief cells secrete pepsinogen and chymosin, while beta islet cells of the pancreas secrete insulin. Brunner's glands secrete a mucus that protects th lining of the intestine.

The kidneys receive more blood flow than any single organ, after the liver. 20-25% of the cardiac output of the heart goes to the kidneys for filtration (the liver receives closer to 28%). This is especially significant considering that the kidneys account for approximately 0.5% of total body weight (while the liver makes up closer to 3% of total body weight). 

The basic functional unit of the kidney is the nephron. The nephron is composed of the glomerulus, which is encased in the Bowman's capsule, the proximal convoluted tubule, the loop of Henle, and distal convoluted tubule.

99% of glucose and amino acids are resorbed by the proximal tubule of the nephron. The loop of Henle and the distal tubule primarily resorb electrolytes and water, while the glomerulus is strictly for filtration and does not resorb solutes. 

The GFR represents the rate at which blood is filtered through the glomeruli of the kidneys. Creatinine is a waste product of muscle metabolism. Healthy glomeruli are capable of filtering creatinine very effectively, wherein creatinine is excreted in the urine. Generally an elevated serum creatinine will be accompanied by a decreased glomerular filtration rate and is strongly predictive of kidney dysfunction.