Human Digestive System

Present in the inner lining of the intestine

After descending through the esophagus, food enters the stomach. The duodenum is the first part of the small intestine (stomach comes before it). The liver and pancreas are accessory organs that secrete into the duodenum but food doesn't enter them directly.

Pepsin is produced in the stomach as pepsinogen (inactive precursor) and activated by stomach acid to digest proteins. Trypsin comes from the pancreas, amylase from salivary glands and pancreas (not liver), and maltase is an intestinal brush-border enzyme.

The efficacy of an antacid depends upon its duration of stay in the stomach and so the neutralization process causes the prevention of activation of pepsin and secretion of excess HCl.

Recto and Verso refer to the front and back sides of a leaf of paper, commonly used in manuscripts, books, and printing. Recto is the right-hand page (front), verso is the left-hand page (back).

Herbivorous animals have symbiotic bacteria and protozoa in their digestive tract (especially in the rumen or cecum) that produce cellulase enzymes to break down cellulose. Humans lack these microbial symbionts in their digestive system, making cellulose indigestible. Cellulose digestion is not mechanical but enzymatic.

Frogs possess digestive enzymes like maltase, lactase, and pepsin. However, rennin (chymosin) - the enzyme that curdles milk in mammalian infants - is absent since frogs don't nurse their young with milk.

Gastrin, secreted by G cells in the stomach lining, stimulates parietal cells to release HCl and chief cells to release pepsinogen. Pancreozymin and cholecystokinin stimulate pancreatic enzymes, while secretin stimulates bicarbonate secretion - these don't directly control gastric juice.

Activity of hormone sensitive lipase is inhibited by insulin, reducing the release of glycerol and fatty acids.

Choanocytes (or collar cells) are the feeding cells in sponges. Each choanocyte has a flagellum surrounded by a microvillar collar that creates water currents and traps food particles (bacteria, organic debris) which are then ingested by phagocytosis. Porocytes are pore cells, pinacocytes are covering cells, and trophocytes are nutritive cells in other animals.

Assimilation is the metabolic process where absorbed nutrients are converted into complex protoplasmic molecules for growth and repair. Digestion breaks down food, absorption moves nutrients into blood, and assimilation builds body tissues.

Ptyalin (salivary amylase) is the digestive enzyme secreted by salivary glands present in the buccal cavity. Trypsin and pepsin are proteases found in the stomach and small intestine, while lipase is primarily pancreatic.

Secretin is the hormone released from the small intestine that stimulates the pancreas to secrete bicarbonate-rich fluid to neutralize acidic chyme from the stomach. Insulin and glucagon regulate blood sugar, while oxytocin is involved in reproduction.

The intestinal lining has a brush border composed of microvilli that massively increase surface area for nutrient absorption. It is neither ciliated nor keratinized - those features would impair absorptive function.