Digestive System Part 1

Digestive Function• The digestive system has

one goal only: to put nutrients into the bloodstream so that all our cells can access those nutrients– Many excess nutrients

can be stored• The digestive system has

many “preparation steps” to prepare food for this process

The Most Vital Organ• Only one organ performs

almost all the absorption of nutrients (excepting water): the small intestine– So called because it has a

small diameter, though it is several meters long

• If we drank liquids containing only single molecules of nutrients, we would need no other digestive organ!– This is what they inject into

your blood when you’re in a coma

Digestive Organs

• The digestive system is divided into two types of organs:– The alimentary organs that

make up the alimentary canal– Accessory organs that do not

make up the alimentary canal

• The alimentary canal is the tube running through your body

Alimentary Organs

• Mouth• Pharynx• Esophagus• Stomach• Small intestine• Large intestine• Anus

Movement

• Organs are moved from one to the next through a process called peristalsis (the pushing of food through the alimentary canal by smooth muscle)– In the esophagus, swallowing is

initiated by skeletal muscle but completed by smooth muscle and qualifies as peristalsis

– The stomach, small and large intestine all have peristalsis but it is much slower

Mouth and Esophagus• The mouth uses the teeth and

tongue to grind up food before swallowing it by pushing it back to the pharynx– Teeth are made with a lot of calcium

phosphate, which can be dissolved with acid (such as in soda or bacterial waste products)

• Swallowing is initiated voluntarily but is completed by reflex to get the food through the esophagus to the stomach

The Stomach• The stomach is a highly

muscular grinding chamber• The stomach produces gastric

juice, which contains the enzyme pepsin for protein digestion as well as other chemicals– This includes hydrochloric acid,

which makes the stomach environment very acid

• The acid itself is not for digesting the food– Kills bacteria– Enables pepsin to work

Stomach Ulcers

• Stomach ulcers were once thought to be caused by stress but are now known to be a bacterial infection (but stress does block the immune system)

• The scientist who discovered the bacterium that causes ulcers was eventually awarded the Nobel Prize– No one had believed him initially so

he drank a flask of the bacterium to gain ulcers

Gastric Juice

• Gastric juice is the secretion of the stomach• Gastric juice is very acidic and can damage

tissues other than the stomach lining– Repeated vomiting can cause gastric juice to

erode the enamel of the teeth– If gastric juice is produced in excess it can

overflow into the esophagus, causing “heartburn”• Once gastric juice mixes with and digests food

it is called chyme

Processing of Chyme

• Chyme is the partially digested food of the stomach mixed with acid

• Chyme is delivered into the small intestine very slowly through the pyloric sphincter at the bottom– Excess acid could damage the

small intestine– Helps the intestine absorb all the

nutrients

Accessory Organs

• The small intestine has enzymes on its absorptive cells but also receives digestive juices from two accessory organs

• The pancreas produces a massive battery of digestive enzymes to finish all digestion of food

• The liver produces bile, which is stored in the gallbladder until chyme enters the small intestine

Bile• Bile does not contain

digestive enzymes and does not directly perform chemical digestion

• Bile is instead an emulsifying agent, much like a detergent– To emulsify is to allow oil and

water to mix• Bile is therefore vital for

digesting and absorbing fats from food

Anatomy of Small Intestine• The lead part of the small

intestine is the duodenum, which accepts chyme from the stomach as well as the bile and the pancreatic juices

• The duodenum becomes the jejunum which then becomes the longest part, the ileum

• The entire small intestine is typically 10-15 feet long– Has enormous surface area so it

can absorb all the nutrients

Anatomy of Large Intestine

• Any indigestible materials pass from the ileum to the large intestine

• Most of large intestine is the colon (three parts are ascending, transverse and descending)– Also consists of the appendix and

the rectum– The rectum stores feces, and as it

swells it stimulates the brain to want to defecate

Hopefully that wasn’t too much to digest…

• Next Monday is digestion part 2: chemical digestion and absorption!