Digestion Chapter 8 Section 8.4 - 1

Small Intestine Physical Digestion = Segmentation Small Diameter – 2.5cm 4X longer than Large Intestine Physical Digestion = Segmentation Segments = bands of circular muscle that briefly contract, chopping food into segments. Peristalsis helps food move along.

Main Function of Small Intestine Fully digest macromolecules and absorb its subunits for use in the body. Macromolecules are digested by hydrolysis (adding water).

Regions/Structures Small intestine can be divided into 3 regions: 1. Duodenum 2. Jejunum 3. Ileum

Duodenum The first 25 – 30 cm of small intestine. U – Shaped. Shortest and widest part. Channels from LIVER and PANCREAS converge into ONE CHANNEL that enters here.

Duodenum Con’t Its Function: To increase surface area for digestion. To increase surface area it has many hills and ridges. ON TOP OF RIDGES are VILLI (s. villus) Finger-like projections that extend into small intestine to further increase surface area. MICROVILLI on top of villi Microscopic villi (looks fuzzy) that also help to increase surface area.

Ridges of Duodenum

Duodenum Con’t. Each villus has a capillary network and lymph vessels called lacteals. They conduct absorbed substances from small intestine into the bloodstream and lymphatic system.

Jejunum 2.5m long More folds and secretory glands than duodenum. Continues to break down food.

Ileum 3m long Contains fewer and smaller villi. Absorbs nutrients. Pushes undigested food into large intestine.

Accessory Organs There are 3 accessory organs to the small intestine: PANCREAS, LIVER and GALL BLADDER. Their purpose is to secrete (produce) substances that aid in the digestion of food.

Pancreas Secretes ~1L of fluid to duodenum everyday. Pancreatic fluid contains many enzymes: Proteases Carbohydrases Lipase

Pancreatic Enzymes Proteases Carbohydrases Lipase Digests proteins 2 proteases: Trypsin and Erepsin Trypsin is activated by the enzyme Enterokinase Enterokinase changes Trypsinogen  Trypsin Carbohydrases Digests carbohydrates 1 carbohydrase: Pancreatic Amylase Lipase Digests lipids or fat

Pancreatic Enzymes They are secreted in their inactive form. They are activated by other enzymes secreted by the duodenal lining. Pancreas also secretes a bicarbonate (HCO3-) Neutralizes the HCl and chyme from the stomach that enters the small intestine.

Liver Largest organ in body. Main secretion: BILE Bile is the liver’s waste product that will be excreted in feces eventually. It assists lipases in small intestine by breaking down fats.

How does bile help lipase? Bile disperses large fat droplets into smaller droplets. This creates a greater surface area for lipase to act on.

Gall Bladder Stores bile sent from liver in between meals. Fat-containing chyme (thick liquid from stomach) in duodenum stimulates gall bladder to CONTRACT. This causes bile to be injected into the duodenum.