Human Digestive System

Study Notes

Consists of a long muscular tube and several accessory organs: salivary glands, pancreas, and gall bladder.
Responsible for food ingestion, digestion, absorption of digestion products, and elimination of undigested materials.

Food is physically broken down by teeth into smaller pieces.
Presence of food triggers a nervous reflex, causing salivary glands to deliver saliva to the mouth.
Saliva moistens and lubricates food, making swallowing easier, and dissolves some food for tasting.
Saliva contains a digestive enzyme called amylase, which begins chemical digestion of starches.

Swallowing reflex moves food into the oesophagus.
Waves of muscular contractions (peristalsis) move food down the thin-walled tube to the stomach.

Both physical and chemical digestion occurs within the stomach.
Muscular walls of the stomach mix food with gastric juice, a digestive fluid produced by stomach cells.
Gastric juice is highly acidic and contains enzymes that break down protein components of food.
Food is eventually reduced to a creamy paste called chyme.

Structure at the bottom of the stomach (pyloric sphincter) controls the entry of chyme into the duodenum (first part of the small intestine).
Ducts from the gall bladder and pancreas feed fluids rich in bile salts and digestive enzymes into the duodenum.
Cells lining the small intestine produce a fluid called 'succus entericus' (mostly water, mucus, and sodium bicarbonate).
These fluids help lubricate partially digested food, neutralize acidic chyme, emulsify fats and oils, and enzymatically digest protein, carbohydrate, and fatty acids.
Breaking down of large molecules into small molecules enables absorption into the bloodstream.
Small intestine is 5-6m in length, with most chemical digestion occurring in the first meter.

Millions of tiny finger-like structures called villi project inwards from the lining of the small intestine, increasing the surface area for absorption.
Absorbed molecules are transported to the liver by the hepatic portal vein.

4 main functions: absorbing water, electrolytes, and vitamins, and storing and eliminating waste.
Over 500 species of bacteria are present in the large intestine, performing functions like metabolizing undigested carbohydrates to short-chain fatty acids and producing vitamins.
Undigested material accumulates in the rectum, stimulating a response that leads to evacuation of waste through the anus.