The Digestive System

The Digestive System

• Breaks down food into smaller particles so cells can use it

• Built around alimentary canal (one-way tube passing through body)

• Digestive system includes: mouth, pharynx, esophagus, stomach, small intestine, and large intestine

• Accessory structures include: salivary glands, pancreas, and liver

The Digestive System

Mouth

Salivary glands

Stomach

Pancreas (behind stomach)Large intestine

Small intestine

Rectum

Gallbladder (behind liver)

Liver

Esophagus

Pharynx

The Mouth

• Chewing (mastication) begins process of mechanical digestion = physical break down of large pieces of food into smaller pieces

• Saliva, a digestive enzyme, begins breaking down food molecules (starches like crackers) into different molecules = chemical digestion

• Saliva also moistens food and makes it easier to chew/swallow and helps fight infections by killing some bacteria on food

The Esophagus

• Clump of food (bolus) goes down throat and into esophagus

• Bolus moves through esophagus by peristalsis – contractions of smooth muscle surround esophagus squeeze food down esophagus

• “Heartburn” is caused by stomach acid entering the esophagus and irritating the lining of the esophagus

Swallowing

The Stomach

• Bolus empties into the stomach from the esophagus

• Stomach continues mechanical and chemical digestion of food

– Chemical digestion – some glands in the stomach produce mucus to protect the wall of stomach and other glands produce hydrochloric acid (HCl). The acid activates an enzyme called pepsin (made by another set of glands). Pepsin and HCl together chemically break down food molecules

– Mechanical digestion – stomach muscles contract and churn to mix food with stomach juices (mixture now called chyme). After an hour or two, chyme moves into the small intestine

The Small Intestine, Pancreas, and Liver

• Chyme enters first part of small intestine called duodenum

• Location where most chemical digestion takes place (small intestine)

• Small intestine is 6.5m long and 2.5cm wide• Enzyme from pancreas and liver enter duodenum

– Pancreas releases two enzymes – one to help chemically digest food and the other to neutralize the stomach acid

– Liver releases bile to help break down fats from food

The Small Intestine, Pancreas, and Liver (continued)

• After duodenum, most chemical digestion is completed and the rest of the small intestine absorbs nutrients

• Projections called villi create and enormous surface area for absorption of nutrients

• Contractions of smooth muscle move chyme along

The Small Intestine

Small Intestine Villus

Circular folds

Villi

Epithelial cells

Capillaries

Lacteal

Vein

Artery

The Liver and Pancreas

Liver

Gallbladder

Duodenum

Bile duct

Pancreas

Pancreatic duct

To small intestine

The Large Intestine (Colon)

• Large intestine is 2m long and 7cm wide• Removes water from undigested material left

over from small intestine• Bacteria in large intestine helps digestion• Concentrated waste material remaining after

water removal is eliminated from body– What is the condition called when too much water is

reabsorbed in the large intestine?• Constipation

– What is the condition called when not enough water is reabsorbed in the large intestine?

• Diarrhea

Path of Digestion = mouth – esophagus, stomach, small intestine, large intestine, anus