ZLY 106
Digestive system
Dr. Wasantha Sena Weliange
Senior Lecturer
Department of Zoology
Digestive system
1. Digestion and the alimentary tract
2. Modifications of the alimentary tract in
herbivores
3. Functions of liver
15 hr (L)
• Plants
– Photosynthesis
– Autotrophs
• Animals
–
–
–
–
–
not synthesizing food
Heterotrophs
from their surrounding
Some ingest food and later absorbs the nutrients
others directly absorb nutrients
• Animals eat molecules (carbohydrates, proteins, lipids,
vitamins etc,).
• They are broken down into smaller molecules (monomers)
to be absorbed and transported.
• Single-celled organisms
– directly take in nutrients from outside environment.
• Multi-cellular animals
– specialized structures for obtaining, breaking &
transporting.
– known as the DIGESTIVE SYSTEM.
• Digestive system of higher animals made of
1. Mouth
- Feeding
2. Alimentary Canal - Digestion
Digestion occurs in two ways
• Intracellular digestion
– sponges, coelenterates (corals, hydras and their relatives) and
most protozoans
– food particles are taken into body cell by phagocytosis
– later digested by enzymes being secreted into the phagocytic
vesicles
• Extracellular digestion
– chordates, annelids, and crustaceans
– occurs outside the cells but in the lumen (or opening) of a
digestive system by enzymes.
– The digested nutrients are transferred to the blood, transported
5 steps in digestion
1. Movement
• propels through the digestive system
2. Secretion
• release enzymes in response to a specific stimulus
3. Digestion
• Breaking into molecules to cross the plasma membrane
4. Absorption
• passage of the molecules into the body's interior and their
passage throughout the body
5. Elimination
• removal of undigested food and wastes
The complete process of digestion has
three main parts
•Digestion
• is the mechanical and chemical breakdown of food into
particles/molecules small enough to pass into the blood.
•Absorption
• is the passage of food monomers into the blood stream.
•Assimilation
• is the passage of the food molecules into body cells.
Units of the human digestive system
• The human digestive system is a coiled, muscular tube (6-9
meters long when fully extended) stretching from the mouth to
the anus.
• Accessory parts
• Major parts
– Salivary glands
– Mouth
– Parts of the pancreas
– Pharynx
– Liver
– Esophagus
– Gall bladder
– Stomach
– Small intestine
– Large intestine
– Anus
The Mouth and Pharynx
• Beginning
STARCH
• Ingestion
• Mechanical breakdown - teeth,
jaws and the tongue
GLUCOSE
• Digestion starts
ACIDS IN
• Salivary glands secrete salivary FOODS
amylase.
• Mammals have taste buds
Bicarbonate in saliva
clustered on their tongues.
NEUTRALIZES
The Mouth and Pharynx
• Mucus
– Lubrication effect ----> moistens food
• Tongue
– Manipulate while chewing & swallowing
– Swallowing moves food from mouth through the
pharynx into the esophagus
The Mouth and Pharynx
• Epiglottis
– flap-like structure at the back of the throat
– closes over the trachea preventing food from entering
it.
• Oesophagus
– 10 inches muscular tube
– pharynx to stomach
– Secreting mucus
– moves food from the throat to the stomach using
muscle movement called peristalis.
– If acid from the stomach gets in here that’s heartburn.
Peristalsis and the movement of food from the mouth to the
stomach
Stomach
•
•
•
•
J-shaped muscular bag
stores the food and digests
Mechanical and chemical break down
empty capacity of 50-100 milliliters to fills to
maximum capacity of 2 liters or more
• Food in the stomach is called chyme
• The epithelial cells in the stomach secrete 2 liters of
gastric juices per day
– Hydrochloric Acid (HCl)
– Pepsinogen
– Mucus
Stomach
• Carbohydrate digestion continues
• Hydrochloric acid
– kills microorganisms
– lowers the pH to 1.5 - 2.5 and activates pepsin
• Pepsin
– proteolytic enzym
– Proteins  peptides
• lower part of the stomach
• bolus is broken down into acid chyme and allowing the
stomach's acidity to inhibit further carbohydrate breakdown.
Stomach to small intestine
• The peristaltic waves in the lower part of the
stomach propel the acid-chyme against the
pyloric sphincter.
• acid-chyme go through the pyloric sphincter to
small intestine
• Stomach empties over a 1 to 2 hour period.
• High fat diets significantly increase this time
period.
• The secretions of the gastric juice are controlled
by nervous (smells, thoughts, and caffeine) and
endocrine signals.
Small intestine
• coiled tube, 3 meters long / 2-3 cm wide
• three parts
– upper duodenum
– middle  jejeunum
– lower  ileum
• duodenum  most active in digestion
– secrete watery mucus
– secretions from the liver and pancreas (protease) enter
» protein  amino acids
– Pancreas secrets stomach acid-neutralizing bicarbonate
– Pancreases regulates blood sugar by producing insulin.
Small intestine
 Most absorption occurs in the duodenum and
jejeunum
• Absorptions
– Maltose, sucrose, and lactose
– amino acids
Liver
• produces bile, send to gall bladder via hepatic duct
and store,  entering the bile duct  duodenum
• Bile emulsifies fats, facilitating breakdown into
smaller fat globules until they can be acted upon by
lipase enzymes.
• Bile contains cholesterol, phospholipids, bilirubin,
and a mix of salts.
• Fats are completely digested in the small intestine.
The
The liver and associated organs and their connections to the
digestive system
Other functions of liver
1.
2.
3.
4.
detoxification of blood
synthesis of blood proteins
destruction of old erythrocytes
conversion of hemoglobin into a component of
bile
5. production of bile
6. storage of glucose as glycogen, and release when
blood sugar levels drop
7. production of urea from amino groups and
ammonia.
Large Intestine
• Four parts
– colon, cecum, appendix, and rectum.
• 5 feet long.
• The large intestine performs three basic functions
1. recovery of water
2. formation and storage of feces
3. microbial fermentation
• Material in the large intestine is mostly indigestible
residue and liquid.
Large Intestine
• Water, salts, and vitamins are absorbed, the
remaining contents in the lumen form feces
(mostly cellulose, bacteria, bilirubin).
• Rectum holds feces for a short term before it
is expelled.
Gut microbes
• Bacteria in the large intestine, such as E. coli,
produce vitamins (including vitamin K) that are
absorbed.
• Those microbes produce enzymes that can digest
many of molecules indigestible by vertebrates.
• Secretions in the large intestine are alkaline
mucus that protects epithelial tissues and
neutralizes acids produced by bacterial
metabolism.