Correct Answer: D

Rationale: The correct answer is D: portal blood system. After absorption, carbohydrates and proteins are transported via the portal blood system to the liver for further processing and distribution to the rest of the body. Choices A, B, and C are incorrect. Enterohepatic circulation refers to the recycling of compounds between the liver and the intestines, not the direct route for absorbed nutrients. Gastrointestinal circulation is a vague and non-specific term, not specifically related to the transport of absorbed nutrients. The common bile duct is involved in transporting bile from the liver and gallbladder to the small intestine, not the end products of carbohydrate and protein digestion.

Correct Answer: A

Rationale: The parotid, submandibular, and sublingual glands are the main salivary glands located in the mouth. They play a crucial role in producing saliva, which aids in digestion. The stomach (choice B) is where gastric juices are produced, the pancreas (choice C) secretes digestive enzymes into the small intestine, and the duodenum (choice D) is the first part of the small intestine where most chemical digestion occurs. Therefore, the correct answer is the mouth (choice A) as it is where these salivary glands are situated.

Correct Answer: B

Rationale: The rate at which the stomach empties its contents is primarily determined by the composition of the food consumed. Foods high in fat and protein tend to slow down gastric emptying, while foods high in carbohydrates usually pass through more quickly. Therefore, the composition of food consumed directly impacts the rate of gastric emptying. Choices A, C, and D are incorrect as they do not directly affect the rate of gastric emptying.

Correct Answer: B

Rationale: The correct answer is B: Carbohydrates and fats. Carbohydrates are the body's primary and most efficient source of energy, providing fuel for daily activities. Fats are also a significant energy source, especially during times of prolonged or intense exercise. Proteins are primarily used for building and repairing tissues, not as a major energy source. Vitamins and minerals are essential for various bodily functions but do not serve as direct sources of energy.

Correct Answer: D

Rationale: The correct answer is D: Hydrochloric acid. Hydrochloric acid in the stomach plays a crucial role in converting pepsinogen into its active form, pepsin, which is necessary for digesting proteins. Bile (Choice A) is involved in the emulsification of fats, not in the activation of pepsinogen. Gastrin (Choice B) is a hormone that stimulates the secretion of gastric acid but does not directly activate pepsinogen. Secretin (Choice C) is a hormone that regulates the pH of the duodenum but is not responsible for the activation of pepsinogen.