Hierarch in Biology

The living world is organized in a series of hierarchical levels from less complex to more complex

Atom Molecule Organelle Cell Tissue Organ Organ system

Digestive System

Cells can perform all the necessary functions of life

For nutrition and digestion a single cell acquires nutrients through diffusion, osmosis and active transport

Multicellular organisms require a more specialized system for digestion

Role of Digestion

The human body requires six types of nutrients in order to function

The organs of the digestive system allow you to extract useful nutrients from food, deliver them to your cells and eliminate wastes

Basic Structure

The disgestive tract (alimentary canal) is basically a muscular tube open at both ends

The inner surface of tube is continuous with outer surface of body

Food enters one end, products of digestion are absorbed and waste products are eliminated

Structure of Wall of Digestive Tract Layers of the gastrointestinal wall are

basically the same throughout

Types of Mucosa Tissue

Steps in Digestion

There are four basic steps in digestion Ingestion Digestion (mechanical and chemical) Absorption Elimination

Ingestion Digestive tract is approx. 6.5 to 9 m long It stores and breaks down organic

molecules into simpler components Digestion begins in the mouth where food

is taken in, chewed by the teeth and formed into a bolus by the tongue (physical digestion)

Saliva begins the chemical digestion of food

Movement through the esophagus is regulated by contractions of smooth muscles called peristalsis

http://www.westga.edu/~lkral/peristalsis/

Digestion - Stomach Digestion begins in the stomach Gastric fluids (mucus, HCl, and

pepsinogins) aid in digestion Mucus provides the stomach with a

protective coating against the effects of HCl and pepsinogins

HCl kills harmful substances ingested with food and converts pepsinogin into its active form pepsin

Pepsin is a protein digesting enzyme

Digestion – Small Intestines Most digestion occurs in the small

intestine (~7 m in length) within the first 25 – 30 cm (the duodenum)

Pancreatic fluids are stimulated by the conversion of secretin from prosecretin in the small intestine

Pancreatic enzymes begin the chemical digestion of lipids, proteins and carbohydrates

Absorption (stomach and small intestines) The stomach absorbs some water, specific

vitamins, some medicines and alcohol Most absorption takes place within the

small intestine Long fingerlike tubes (villi) greatly increase

the absorptive surface area Cells that make up the lining of each villus

have microvilli which further increase the surface area

Each villus contains blood capillaries and lymph vessels called lacteals

Some nutrients are absorbed by diffusion while others are actively transported

Carbohydrates and amino acids are absorbed into the capillaries

Fats are absorbed into the lacteals

Absorption (large intestine) Large intestine (~ 1.5 m long) stores

wastes long enough to reabsorb water out of the wastes

Large intestine contains bacteria such as E. coli which are essential to life and use waste material to synthesize vitamins B and K

Elimination As wastes build up in the large intestine,

receptors in the intestine wall provide information to the central nervous system

This results in the prompting of a bowel movement resulting in the removal of potentially toxic wastes from the body

Ingestion of indigestible cellulose (fibre) increases the occurrence of bowel movements

Components of Digestion System

Accessory Organs in Digestion

Salivary Glands secrete saliva and mucus which:

lubricate food contain salivary amylase to begin

carbohydrate breakdown Dissolves food particles

Liver Liver synthesizes bile Bile is a mixture of bile salts, bile acids,

cholesterol, phospholipids, fatty acids and water

Liver also extracts absorbed nutrients or toxins from blood

decomposes toxins such as hydrogen peroxide, ammonia and alcohol

Converts glucose into glycogen

Gallbladder

Muscular sac that stores and concentrates bile from liver

When fat enters duodenum endocrine cells in duodenum release hormone cholecystokinin (CCK)

CCK causes gallbladder to contract and send bile into duodenum to emulsify fats and aid in absorption of lipids

Pancreas

Secretes a number of substances essential to digestion such as:

bicarbonate, lipases, carbohydrases and proteases

Homeostatsis

Means the maintenance of a steady internal state (such as blood pressure, respiration rate, body temperature and blood sugar levels)

Negative feedback relates of homeostasis 1. a variable rises above or below a normal

range 2. receptors detect the change and signal

other parts to respond 3. Organs receive the signal and respond

accordingly to return variable to normal