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The digestive system is a sequential processing tract that mechanically and chemically converts food into absorbable molecules while expelling waste. The major segments are the mouth (mastication, salivary amylase for starch), esophagus (peristaltic transport), stomach (acid denaturation at pH 1.5–3.5, pepsin for protein), small intestine (primary digestion and absorption), and large intestine (water and electrolyte reabsorption, microbial fermentation of fiber). Accessory organs provide essential secretions: the liver produces bile salts for fat emulsification and detoxifies absorbed substances; the pancreas secretes proteases, lipase, and amylase plus bicarbonate for luminal neutralization. The enteric nervous system coordinates motility largely independently of the brain.
Follow the fate of each macronutrient class through every segment: carbohydrates (starch → disaccharides in mouth/stomach → monosaccharides in small intestine); proteins (denatured in stomach → peptides by pancreatic proteases → amino acids by brush-border enzymes); fats (emulsified by bile → monoglycerides + fatty acids by lipase → absorbed as micelles). Build a segment-by-segment table.
The digestive system works like an assembly line in reverse — instead of building something, it systematically dismantles food into molecules small enough to cross the intestinal epithelium into the bloodstream. Understanding the system means tracking what happens to each macronutrient class at each station along the tract.
The process starts in the mouth, where mechanical chewing increases surface area and salivary amylase begins hydrolyzing starch into shorter chains. The esophagus does nothing chemically — it just moves boluses to the stomach via peristaltic contractions. The stomach's main contributions are mechanical churning, acidification (pH 1.5–3.5 by parietal cell HCl), and activation of pepsinogen to pepsin, which makes the first cuts in dietary proteins. The acid also kills most ingested microbes, making the stomach a significant immunological checkpoint.
The small intestine — duodenum, jejunum, ileum — is where essentially all nutrient absorption happens. The pancreas delivers a powerful secretion into the duodenum: proteases (trypsin, chymotrypsin), lipase, amylase, and bicarbonate to neutralize the stomach acid. The liver contributes bile salts stored in the gallbladder; these are detergent-like molecules that emulsify dietary fats, breaking fat globules into tiny droplets and dramatically increasing the surface area available to lipase. The products of lipid hydrolysis (fatty acids and monoglycerides) are packaged into micelles that diffuse to the brush-border membrane for absorption. For proteins and carbohydrates, brush-border enzymes (peptidases, disaccharidases) complete the final hydrolysis steps, and dedicated transporters carry amino acids, monosaccharides, and di/tripeptides across the epithelium.
A critical distinction: the stomach does not absorb nutrients to any meaningful degree, and neither does the large intestine absorb calories. The large intestine's main jobs are reabsorbing water and electrolytes from the remaining luminal contents (thus concentrating stool) and providing habitat for the gut microbiome, which ferments indigestible fibers to produce short-chain fatty acids. When students picture "digestion and absorption" as happening in the stomach, they are mislocating the most important work by several feet of intestine.
Finally, the whole system is coordinated by the enteric nervous system — roughly 500 million neurons embedded in the gut wall — which regulates motility largely independently of the brain. Hormonal signals (gastrin, secretin, CCK) from enteroendocrine cells fine-tune acid and enzyme secretion in response to the composition of each meal. This feedback architecture, which you studied in homeostasis, is why the gut adjusts its digestive output based on what you have actually eaten rather than running at a single fixed rate.