Human Digestive System

The pyloric valve, or pyloric sphincter, is a ring of smooth muscle that regulates the passage of partially digested food from the stomach into the duodenum. Mitral and pulmonary valves are found in the heart, while the diencephalon is a region of the brain.

Trypsin is a pancreatic enzyme secreted into the small intestine where it hydrolyzes proteins into smaller peptides and peptones. Other enzymes like ptyalin and amylase digest carbohydrates, while lipase digests fats.

Hydrochloric acid (HCl) is secreted by the parietal cells in the stomach lining, creating an acidic environment necessary for the activation of pepsin and the digestion of food.

Peristalsis is the wave-like muscle contractions that move food through the digestive tract. This involuntary movement occurs in the oesophagus, stomach, and large and small intestines. Therefore, all the listed organs exhibit peristaltic movement.

Ghrelin is a hormone produced primarily by the stomach that signals hunger to the brain, thereby increasing appetite and hunger pangs. Leptin, on the other hand, is produced by fat cells and acts to decrease appetite.

The digestive system breaks down food into nutrients, which are then absorbed into the bloodstream of the circulatory system to be transported to cells throughout the body. Making food is not a human biological process, saliva is produced by salivary glands, and waste disposal is primarily handled by the excretory system.

Pancreatic amylase digests starch into the disaccharide maltose (and dextrins) in the small intestine. Maltose is subsequently broken down into glucose by maltase, which is an enzyme bound to the microvilli of the small intestine (not a pancreatic enzyme).

Villi and microvilli are structural adaptations of the small intestine that vastly increase the surface area available for the absorption of digested nutrients. Assimilation is the subsequent integration of these nutrients into body tissues, which occurs after absorption.

Pancreatic lipase is produced in the pancreas but is secreted into and functions within the lumen of the small intestine, where it hydrolyzes lipids (triglycerides) into fatty acids and glycerol.

Pepsin is an endopeptidase secreted in the stomach that hydrolyzes proteins into smaller polypeptides and eventually amino acids. While pepsin primarily cleaves proteins into shorter peptide chains rather than individual amino acids, option D is the only option that correctly identifies proteins as the substrate.

The pancreas is an accessory organ of the digestive system that produces and secretes digestive enzymes, such as pancreatic amylase, into the duodenum of the small intestine to chemically break down carbohydrates. Food does not pass through the pancreas itself, and peristalsis is carried out by the muscular walls of the digestive tract. Pepsin is secreted by the stomach, not the pancreas.

The digestive system is composed of the gastrointestinal tract, which includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Its primary function is to ingest food, break it down mechanically and chemically, absorb nutrients, and excrete waste.