ZLY 106Digestive system

Dr. Wasantha Sena WeliangeSenior Lecturer

Department of Zoology

Digestive system

1. Digestion and the alimentary tract2. Modifications of the alimentary tract in

herbivores3. 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 of1. Mouth - Feeding2. 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

to the cells by body fluids.

– In mammals digestion occurs mechanically and chemically

5 steps in digestion1. 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 body5. 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.

• Major parts– Mouth– Pharynx– Esophagus– Stomach– Small intestine– Large intestine– Anus

• Accessory parts– Salivary glands– Parts of the pancreas– Liver– Gall bladder

The Mouth and Pharynx• Beginning• Ingestion• Mechanical breakdown - teeth,

jaws and the tongue • Digestion starts• Salivary glands secrete salivary

amylase.• Mammals have taste buds

clustered on their tongues.

NEUTRALIZES

Bicarbonate in saliva

ACIDS IN FOODS

GLUCOSE

STARCH

• 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

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 liver and associated organs and their connections to the digestive system

The

Other functions of liver

1. detoxification of blood2. synthesis of blood proteins3. destruction of old erythrocytes 4. conversion of hemoglobin into a component of bile5. production of bile6. storage of glucose as glycogen, and release when

blood sugar levels drop7. 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 water2. formation and storage of feces3. 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.