Quaestio: How do humans obtain and process nutrients?

Nunc Agenda: What is the difference between

mechanical and chemical digestion?

Review Types of Digestion :

• 1. Mechanical Digestion: Physical breakdown of food.

• 2. Chemical Digestion: Chemical breakdown of food through hydrolysis.

HUMAN DIGESTIVE SYSTEM

• Human digestive tract = GI (gastrointestinal)

• Consists of a continuous one way food tube (mouth to anus)

The Path food takes:1. Mouth: (oral cavity) ingests

food2. Teeth: function in

mechanical breakdown of food, – increases surface area of food

for enzyme action

3.Tongue: acts as a plunger to push food back into the throat (pharynx) taste buds are located on the surface of the tongue

Saliva

• Salivary glands are located in the mouth– Produce and secrete salivary amylase which starts

the chemical digestion of starch– Provides lubrication for the chewed food– Contains enzymes to kill harmful microbes

• Chewed food is called a bolus

The Path food takes:

4. Pharynx = throat– food is pushed by tongue

to back of throat, initiates swallowing – food is now in the form of a bolus

• Epiglottis: flap of tissue that covers the entrance to the trachea– prevents choking

A typical swallow (food is pink).

Path of food

5. Esophagus: muscular tube that moves food from mouth to the stomach by process of:

• peristalsis: wave-like muscular contractions that moves chewed food in one direction– involuntary

The Path food takes:

6. Stomach: J-shaped, sac-like , muscular organ

• - mechanically churns food into a liquid

• - begins chemical digestion of protein

• - lining secretes gastric juice (enzymes and HCL) and mucus

• - food now liquid called chyme

The Stomach’s Environment• Gastric Juice: The digestive secretion of glands in

the stomach, containing hydrochloric acid and pepsin.

– Hydrochloric acid (HCl) makes gastric juice highly acidic (pH between 1.5 and 2.5).

– HCl kills most microorganisms that reach the stomach from swallowed food.

– Pepsin is the enzyme that breaks down protein. – Pepsin is secreted in the form of inactive pepsinogen,

which is converted to active pepsin when it comes into contact with HCl.

The Stomach secretes gastric juice, a combo of enzymes (I.e. pepsin) and hydrochloric acid (HCl)

The stomach and accessory organs

Absorption of nutrients 7. Small Intestine: site of

chemical digestion and nutrient absorption

• - long twisted tube with small diameter ~21 ft

• -main function is absorption of the digested molecules into the blood

• Villi: fingerlike projections where nutrient absorption takes place – increase surface area

The Parts of the Small Intestine.

• The duodenum: – 25 centimeters long.– Ducts from the liver, pancreas, and gallbladder open into

the duodenum.

• The jejunum:– 2.5 meters long.

• The ileum– 4 meters long.

Immense Surface Area

• The small intestine’s surface area is about 250 square meters. That is the surface area of a tennis court!– This allows for maximum chemical digestion and

absorption.

The small intestine seen here in cross section shows the numerous villi that greatly increase the surface area for the digestion and absorption of nutrients. SEM X30

Dr. Richard Kessel & Dr. Gene Shih/Visuals Unlimited, Inc.

Villi Line The Small Intestine

The Small Intestine’s Environment

• Unlike the stomach (acidic), the small intestine is alkaline (basic).– Different secretions account for this.

• Secretions include:– Pancreatic Juice (from Pancreas)– Bile (from Gallbladder, Originating in Liver)– Intestinal Juice (from Intestine).

Accessory organs• when chyme enters the

duodenum (first section of s.i.),it mixes with enzymes and digestive fluids

1. Pancreas – produces Pancreatic juice:– enzymes that break down

carbs, protiens, lipids, and nucleic acids

– sodium bicarbonate that neutralizes HCL acid from stomach

Accessory organs2. Liver: digestion of fats• - produces bile

– (NOT an enzyme,) • emulsifies lipids- breaks them

into smaller droplets to increase the surface area for enzymes (lipases) to act on fats

3.Gallbladder: small storage sac for bile, carries bile to small intestine

Overview of Small Intestine Inputs

The Path food takes:8. Large Intestine: - No digestion • Functions:

1. reabsorption of water and salts2. absorption of vitamins produced by bacteria that live in the LI

- vitamin K and B3. storage and elimination of fecal matter

- egestion – removal of undigested waste

• sections• Cecum• Colon• rectum

Some Digestive Homeostasis Disorders

1.Constipation - too much water absorbed in l.i.2. Diarrhea– not enough water absorbed, can lead to

dehydration3. Ulcer – sores that develop when the mucus layer in

the stomach breaks down4. Gall stones– small hard particles made of

cholesterol which form & collect in gall bladder- may block the bile duct and cause pain

5. acid reflux -backflow of stomach contents upward into esophagus

6. appendicitis- inflammation of appendix

Pensa

• 30.3 # 1-4