Download Absorption - biology3u

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 m2  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 – 1st)
 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
Detoxifies blood (removes toxins like alcohol)
2.

All blood leaving the stomach and the intestines flows through
the liver before entering circulation, so the liver takes nutrients
and toxins
Converts glucose to glycogen (the stored form of glucose)
4. Breaks down hemoglobin from red blood cells  brown
feces
3.
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
Forms feces
3.

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