Small Intestine By the time food (chyme) reaches the small intestine, it is broken down to its simplest form and is ready for absorption Almost every nutrient digested is absorbed into the body through the walls of the small intestine 80% of all absorption occurs here, while the rest (20%) occurs in the stomach and large intestine It takes food about 5 hrs to pass through the small intestine The total surface area available for absorption of nutrients is about 300 m 2  about the size of a tennis court!

Villi (singular: villus) Villi are finger-like tubes in small intestines They are lined by thread-like microvilli They increase surface area for faster absorption of nutrients At the core of the villus is a lymph vessel (called a lacteal) that accesses the blood system in order to distribute nutrients throughout the body Some nutrients are also absorbed through diffusion

Villi

Liver This is the second largest organ in the body (skin – 1 st ) It performs over 500 different functions There are four main functions of the liver: 1. Synthesizing bile Liver cells produce about 1 L of bile daily 2. Detoxifies blood (removes toxins like alcohol) All blood leaving the stomach and the intestines flows through the liver before entering circulation, so the liver takes nutrients and toxins 3. Converts glucose to glycogen (the stored form of glucose) 4. Breaks down hemoglobin from red blood cells  brown feces

Bile This is a greenish fluid that contains cholesterol and bile salts Bile salts speed up fat digestion It is stored in the gall bladder when the stomach is empty When fat enters duodenum, cells release a hormone that causes the gallbladder to contract and sends bile down Bile salts break down fat into small droplets If there are very large crystals of cholesterol, this will lead to gall stones

Emulsification (breakdown) of Fat by Bile It is a physical process (not chemical  no bonds broken) Smaller fat droplets increase surface area for lipase action There are 3 stages: 1. Fats present in small intestine 2. Gall bladder releases bile 3. Bile salts break down large globs of fat into smaller ones, allowing that greater surface area for the fat-digesting enzymes to work on

Large Intestine A.k.a. the colon It is puckered – not smooth It does not contain villi It’s about 1.5m long and made up of the following: Caecum Ascending colon Transverse colon Descending colon Sigmoid colon Rectum

Large Intestine It has four functions: 1. Absorbs water and vitamins 2. Houses E. coli bacteria that use waste to make vitamins E.g.: vitamins B & K is assembled in the large intestine 3. Forms feces Solid waste 4. Moves feces for excretion

Defecation This is controlled by two anal sphincters Usually occurs once or twice daily in humans Three-fourths is water and one-fourth is solid matter