VMBO Year 2 · HAVO/VWO Years 2–3 · Nutrition and digestion · Section 2.3
Digestive organs
Mouth, throat and oesophagus
Digestion starts in the mouth. Salivary glands produce saliva, containing water, mucus and an enzyme that begins starch digestion. Mucus lubricates food and helps it slide along.
During swallowing, the soft palate closes the passage towards the nose, while the epiglottis and other protective movements help keep food out of the airway. Food travels through the oesophagus to the stomach.
Stomach
The stomach temporarily stores food, churns it and mixes it with gastric juice. This contains water, hydrochloric acid and enzymes that begin protein digestion. The acid kills many swallowed microorganisms and provides suitable conditions for the enzymes.
The pyloric sphincter, a ring of muscle at the stomach's exit, controls passage into the duodenum.
Duodenum, liver, gallbladder and pancreas
In the duodenum, bile and pancreatic juice are added to the food mixture. The liver makes bile, which is stored in the gallbladder and passes along the bile duct.
Bile contains no digestive enzymes. It emulsifies fats, dividing large droplets into smaller ones and increasing the surface area for enzymes.
The pancreas produces pancreatic juice containing enzymes that digest proteins, carbohydrates and fats.
Small intestine
The small intestine is several metres long, often described as about six metres in an adult. Intestinal secretions and enzymes at the lining help complete digestion, including the breakdown of proteins and carbohydrates.
Folds and villi greatly increase the surface area for absorption. The lining is thin, and blood vessels in the villi absorb many nutrients and water. Most absorbed fats first enter lymph vessels within the villi before reaching the blood.
Large intestine, caecum and rectum
The large intestine absorbs water and salts from the remaining contents. When too much water remains in the stools, diarrhoea may result.
Gut bacteria ferment some dietary fibre, producing substances that can be absorbed. Humans do not themselves produce cellulose-digesting enzymes, and cellulose is only poorly fermented compared with some other fibres.
The caecum is the pouch at the start of the large intestine. The appendix projects from it; appendicitis is inflammation of this appendix, not of the whole caecum.
The rectum stores faeces before elimination. Anal sphincters help control the passage of faeces through the anus.
Glossary
- Pancreas
- An organ producing pancreatic juice and releasing it into the duodenum.
- Pancreatic juice
- A digestive fluid containing enzymes for proteins, carbohydrates and fats.
- Anus
- The opening at the end of the digestive tract, controlled by sphincter muscles.
- Appendix
- A narrow, worm-shaped structure attached to the caecum that becomes inflamed in appendicitis.
- Caecum
- A pouch at the beginning of the large intestine; it is not a major site of human food digestion.
- Alimentary canal
- The passage from mouth to anus through which food travels.
- Intestinal folds
- Folds in the small-intestinal lining that increase the absorption surface.
- Intestinal juice
- Secretions from the intestinal lining that help the digestive process.
- Intestinal glands
- Glands in the small-intestinal wall producing secretions.
- Villi
- Small projections of the intestinal lining that increase the absorption surface and contain blood and lymph vessels.
- Large intestine
- The part of the digestive tract that absorbs water and salts and helps form faeces.
- Emulsification
- Dividing large fat droplets into smaller droplets so enzymes can act more effectively.
- Bile
- A fluid helping emulsify fats during digestion.
- Gallbladder
- An organ storing and concentrating bile before release into the duodenum.
- Bile duct
- A tube carrying bile towards the duodenum.
- Liver
- An organ with many functions, including making bile.
- Stomach
- An organ that stores and churns food and mixes it with gastric juice.
- Gastric juice
- Fluid containing acid and digestive enzymes; the acid kills many microorganisms and enzymes begin protein digestion.
- Gastric glands
- Glands in the stomach wall producing gastric juice.
- Pyloric sphincter
- The ring of muscle controlling passage from stomach to duodenum.
- Faeces
- Material eliminated through the anus, including undigested food residues, bacteria and water.
- Oesophagus
- The muscular tube moving swallowed food from the throat to the stomach.
- Saliva
- Fluid containing water, mucus and enzymes that lubricates food and begins starch digestion.
- Salivary glands
- Glands producing saliva in the mouth.
- Duodenum
- The first part of the small intestine, where bile and pancreatic juice enter.
- Digestive juices
- Secretions helping digestion, many of which contain enzymes.