Digestive System — Set 2

Correct Answer: C. Large Intestine

• **Large Intestine** = approximately 1.5 m long, its colon absorbs 90% of the remaining water from the liquid chyme delivered by the small intestine, along with electrolytes like sodium and potassium, converting waste into semi-solid feces. • **Gut microbiota** — the large intestine harbors trillions of bacteria that ferment undigested carbohydrates, produce vitamins K and B12, and further compact the waste material before elimination. • 💡 Option A (Stomach) is wrong because the stomach primarily digests food and absorbs very little water; Option B (Small Intestine) is wrong because while it absorbs some water, its primary role is nutrient absorption, not water reabsorption; Option D (Mouth) is wrong because the mouth only begins mechanical and chemical digestion — no significant absorption occurs there.

Correct Answer: D. Trypsin

• **Trypsin** = a serine protease in pancreatic juice, secreted as inactive trypsinogen and activated by enterokinase (enteropeptidase) in the duodenum; it cleaves proteins at lysine and arginine residues, continuing protein digestion that began with pepsin in the stomach. • **Autocatalytic activation** — once a small amount of trypsin is formed, it can activate more trypsinogen as well as other pancreatic zymogens like chymotrypsinogen and proelastase, creating a digestive cascade. • 💡 Option A (Ptyalin) is wrong because ptyalin is another name for salivary amylase, which digests starch, not proteins; Option B (Sucrase) is wrong because sucrase is a brush-border enzyme that splits sucrose into glucose and fructose; Option C (Lactase) is wrong because lactase breaks down lactose (milk sugar) into glucose and galactose.

Correct Answer: C. Ingestion

• **Ingestion** = the first step of nutrition, referring specifically to the act of taking food or liquid into the body through the mouth, initiating the digestive process that includes mastication and mixing with saliva. • **Five stages of nutrition** — Ingestion → Digestion → Absorption → Assimilation → Egestion; these follow a strict sequence, and ingestion is the entry point that triggers all subsequent steps. • 💡 Option A (Egestion) is wrong because egestion is the final step — the elimination of undigested waste from the body; Option B (Digestion) is wrong because digestion refers to the breakdown of food into absorbable molecules, not the act of taking food in; Option D (Absorption) is wrong because absorption is the uptake of digested nutrients into the bloodstream or lymph.

Correct Answer: B. Oxyntic cells

• **Oxyntic cells** = also called parietal cells, these are located in the fundus and body of the stomach; they use a proton pump (H⁺/K⁺-ATPase) to secrete H⁺ ions into the stomach lumen, forming HCl that maintains the highly acidic gastric environment. • **Intrinsic factor** — oxyntic cells also secrete intrinsic factor, a glycoprotein essential for Vitamin B12 absorption in the ileum; its absence leads to pernicious anemia. • 💡 Option A (Peptic cells) is wrong because peptic cells (chief cells) secrete pepsinogen, not HCl; Option C (Goblet cells) is wrong because goblet cells secrete mucus to protect the intestinal lining; Option D (Mucous cells) is wrong because mucous neck cells in the stomach secrete protective mucus, not acid.

Correct Answer: D. Duodenum

• **Duodenum** = the Sphincter of Oddi is a muscular valve at the junction of the common bile duct and pancreatic duct where they open into the duodenum at the ampulla of Vater; it regulates the flow of both bile and pancreatic juice into the duodenum during digestion. • **Hormonal regulation** — cholecystokinin (CCK) released during fat digestion relaxes the Sphincter of Oddi, allowing bile and pancreatic enzymes to flow into the duodenum precisely when needed. • 💡 Option A (Stomach) is wrong because the stomach has its own pyloric sphincter controlling chyme exit; Option B (Jejunum) is wrong because the Sphincter of Oddi opens at the beginning of the small intestine (duodenum), not the middle section; Option C (Ileum) is wrong because the ileum is the last section of the small intestine, far from where the Sphincter of Oddi is located.

Correct Answer: B. Fats

• **Fats** = pancreatic lipase is the main enzyme responsible for fat digestion; it acts on emulsified fat droplets in the small intestine, hydrolyzing triglycerides into two fatty acids and one monoglyceride, which are then absorbed by intestinal cells. • **Bile dependency** — lipase requires bile salts to first emulsify fats into tiny droplets, giving it a larger surface area to work on; without bile, fat digestion is severely impaired. • 💡 Option A (Carbohydrates) is wrong because carbohydrates are digested by amylase and brush-border enzymes like maltase and sucrase; Option C (Proteins) is wrong because proteins are broken down by proteases such as pepsin, trypsin, and chymotrypsin; Option D (Vitamins) is wrong because vitamins are not digested — they are absorbed directly, fat-soluble ones aided by bile.

Correct Answer: B. Pharynx

• **Pharynx** = a muscular funnel-shaped cavity (~13 cm) situated behind the nasal and oral cavities; it serves as a shared passageway for both food (leading to the esophagus) and air (leading to the larynx and trachea), making it unique in the human body. • **Epiglottis** — during swallowing, the epiglottis flaps down over the larynx to prevent food or liquid from entering the airway (aspiration), a critical protective reflex managed at the pharyngeal level. • 💡 Option A (Larynx) is wrong because the larynx is a voice box that conducts only air to the trachea and is not a food passage; Option C (Esophagus) is wrong because the esophagus carries only food and liquid from the pharynx to the stomach; Option D (Trachea) is wrong because the trachea is the windpipe carrying only air to the lungs.

Correct Answer: A. Caecum

• **Caecum** = a pouch-like structure that forms the first part of the large intestine; it receives chyme from the ileum through the ileocecal valve and is the site where the vermiform appendix is attached, placing it firmly outside the small intestine. • **Small intestine = Duodenum + Jejunum + Ileum** — these three segments together measure ~6–7 m; the duodenum receives digestive secretions, the jejunum absorbs most nutrients, and the ileum absorbs bile salts and vitamin B12. • 💡 Option B (Duodenum) is wrong as an answer because duodenum IS part of the small intestine; Option C (Ileum) is wrong as an answer because ileum IS the last segment of the small intestine; Option D (Jejunum) is wrong as an answer because jejunum IS the middle segment of the small intestine.

Correct Answer: D. Pancreas

• **Pancreas** = the beta (β) cells within the Islets of Langerhans in the pancreas produce insulin, a peptide hormone that stimulates cells throughout the body to absorb glucose from the blood, lowering blood sugar levels after a meal. • **Dual function organ** — the pancreas is both an exocrine gland (secreting digestive enzymes into the duodenum) and an endocrine gland (secreting insulin and glucagon directly into the bloodstream); damage to beta cells causes Type 1 diabetes. • 💡 Option A (Thyroid gland) is wrong because the thyroid produces thyroxine (T3/T4), which regulates metabolism but not blood sugar; Option B (Liver) is wrong because the liver stores and releases glucose but does not produce insulin; Option C (Adrenal gland) is wrong because the adrenal gland produces adrenaline and cortisol, which raise blood sugar rather than lower it.

Correct Answer: D. Chyme

• **Chyme** = formed in the stomach after 2–4 hours of mechanical churning (peristalsis) and chemical digestion (by HCl and pepsin); it is a thick, acidic (pH ~2), semi-fluid mixture that is released into the duodenum in small spurts through the pyloric sphincter. • **Acid neutralization** — once chyme enters the duodenum, sodium bicarbonate from pancreatic juice rapidly neutralizes its acidity to a pH of ~7–8, creating the optimal environment for pancreatic enzymes. • 💡 Option A (Bolus) is wrong because a bolus is the ball of food formed in the mouth after chewing and before swallowing — it has not yet been chemically processed in the stomach; Option B (Succus) is wrong because succus refers to intestinal juice (Succus Entericus), not partially digested stomach content; Option C (Chyle) is wrong because chyle is the milky fluid in the lymphatic vessels (lacteals) after fat absorption in the small intestine.