4.3 The Digestive System

The digestive system converts the food you eat into molecules small enough for your cells to absorb and use. The TEAS tests this system from two angles: the anatomy (which organ does what, in what order) and the biochemistry (which enzyme breaks down which nutrient into which product). A clean mental model separates the system into two groups: the GI (gastrointestinal) tract — the continuous tube from mouth to anus that food actually travels through — and the accessory organs — the liver, gallbladder, and pancreas, which sit outside the tube but secrete substances into it.

Food never passes through the accessory organs; it only receives their secretions.

The GI Tract: An Assembly Line

The alimentary canal is one long tube about 9 meters in an adult. Each segment performs a specialized job before passing material along.

Food moves along by peristalsis, the wave-like contraction of smooth muscle in the tube walls. It is involuntary, which is why you can swallow even while upside down.

Mechanical vs. Chemical Digestion

The exam expects you to distinguish two complementary processes that occur simultaneously.

Mechanical digestion increases surface area so chemical digestion can work faster — the two are partners, not rivals. Bile's emulsification of fat is a useful borderline case: it physically breaks large fat globules into tiny droplets (mechanical in effect) but is not an enzyme, so it does not chemically digest the fat itself.

Accessory Organs

A classic test trap: the liver makes bile, the gallbladder stores it. If a question asks where bile is produced, the answer is the liver, not the gallbladder.

Digestive Enzymes

Enzymes are named for the nutrient they break down, usually ending in -ase. This pattern is your shortcut: amylase acts on starch (amyl-), lipase acts on fats (lip-), and protease acts on protein.

Note that pepsin works only in the acidic stomach (created by hydrochloric acid), while the pancreatic enzymes need the alkaline environment that pancreatic bicarbonate creates in the small intestine.

Absorption: The Job of the Small Intestine

The small intestine — divided into the duodenum, jejunum, and ileum — is where the vast majority of nutrients enter the bloodstream. Its inner wall is folded and covered with finger-like villi, and each villus cell is covered with tiny microvilli (the brush border). These two structures multiply the surface area enormously, turning a tube only a few centimeters wide into an absorptive surface the size of a small room.

The Large Intestine and Nutrients

The large intestine (colon) receives the watery residue the small intestine leaves behind. Its main jobs are to reabsorb water, compact the remaining material into feces, and host beneficial bacteria that synthesize vitamin K and several B vitamins. It performs little nutrient digestion of its own.

Finally, the TEAS expects familiarity with the six macronutrients and micronutrients: carbohydrates (quick energy), proteins (build and repair tissue, made of amino acids), lipids/fats (energy storage and cell membranes), vitamins and minerals (regulate body processes), and water (the solvent for nearly every reaction).

Worked Example: A person drinks a glass of milk but experiences bloating and cramps because they lack the enzyme lactase. Trace what goes wrong. Lactase normally breaks lactose (milk sugar) into glucose and galactose in the small intestine. Without it, undigested lactose passes into the large intestine, where bacteria ferment it, producing gas and drawing in water — the cause of the symptoms. This shows the rule “enzyme-name = substrate + ase” in action: no lactase means no lactose digestion.