Chapter 14 – Part 4 The Digestive System

Accessory Digestive Organs Salivary glands Teeth Pancreas Liver Gall bladder

Salivary Glands Salivary Glands – empty their secretions (saliva) into the mouth Three Pairs: Parotid glands – located anterior to ears Submandibular and Sublingual glands – Empty secretions into the floor of the mouth through tiny ducts

Saliva Mixture of mucus and serous fluids Moistens and helps to bind food together into a mass called a bolus Contains the enzyme salivary amylase, which begins starch digestion in the mouth Dissolves food chemicals so they can be tasted

Teeth The role is to masticate (chew) food The teeth tear and grind the food, breaking it down into smaller fragments Humans have two sets of teeth Deciduous (baby) teeth Permanent teeth

Teeth Deciduous (baby) teeth Permanent teeth Begin to erupt around 6 months Full set (20 teeth) by the age of 2 years Permanent teeth Replace deciduous teeth beginning between the ages of 6 to 12 A full set is 32 teeth, but some people do not have wisdom teeth

Classification of Teeth Incisors – chisel-shaped; adapted for cuttin Canines – Fanglike; adapted for tearing or piercing Premolars (bicuspids) and Molars – broad crowns with rounded cusps (tips); best suited for grinding

Regions of a Tooth: Crown Crown – exposed part of the tooth above the gum Covered with enamel, the hardest substance in the body Enamel is heavily mineralized with calcium salts

Regions of a Tooth: Crown Dentin - A bonelike material that underlies the enamel and forms the bulk of the tooth Pulp cavity – Cavity that contains blood vessels and nerves and that is surrounded by dentin Supplies nutrients and provides for tooth sensation

Regions of a Tooth: Root Root – Portion of the tooth embedded in the jawbone Periodontal membrane (ligament) holds the tooth in place in the bony jaw The root canal carries the blood vessels and nerves

Regions of a Tooth: Neck Neck – Connects the crown to the root Region in contact with the gum

Impacted Teeth Impacted teeth – when teeth remain embedded in the jawbone Exert pressure and cause a good deal of pain and must be removed

Pancreas Produces a wide spectrum of digestive enzymes that break down all categories of food Enzymes are secreted into the duodenum Enzymes are secreted in an alkaline fluid, which neutralizes the acidic chyme coming in from the stomach The pancreas also has an endocrine function. It produces the following hormones: Insulin Glucagons

Liver Largest gland in the body Located on the right side of the body under the diaphragm; Overlies and almost completely covers the stomach Consists of four lobes and is suspended from the diaphragm Many functions; However, its digestive function is to produce bile

Bile Produced by cells in the liver Bile leaves the liver through the common hepatic duct and enters the duodenum through the bile duct Function: Its bile salts emulsify fats by physically breaking large fat globules into smaller ones Provides more surface area for the fat-digesting enzymes to work on Does not contain enzymes

Composition of Bile Yellow-to-green watery solution containing: Bile salts Bile pigments Cholesterol Phospholipids Electrolytes

Gall Bladder Small, thin-walled green sac found in a shallow fossa in the inferior surface of the liver Function: Store bile from the liver When food digestion is not occurring: Bile backs up the cystic duct and enters the gallbladder to be stored When food digestion is occurring: Stored bile is spurt out into the duodenum in the presence of fatty food

Gallstones Gallstones – The crystallization of cholesterol found in bile Occurs when bile is stored for too long in the gallbladder or when too much water is removed Tend to be quite sharp; Agonizing pain may occur

Jaundice Jaundice - The yellowing of the body tissues caused by bile pigments circulating through the body Bile salts and bile pigments begin to enter the bloodstream Can be caused by: Blockage of the common hepatic or bile ducts (example: wedged gallstones) Liver problems such as hepatitis or cirrhosis

Liver Problems Hepatitis - Inflammation of the liver Most often due to viral infections resulting from drinking contaminated water or transmitted in blood via transfusion or contaminated needles Cirrhosis - Chronic, inflammatory condition in which the liver is severely damaged and becomes hard and fibrous Almost guaranteed when one drinks alcoholic beverages in excess for many years Is a common consequence of severe hepatitis

Processes of the Digestive System The essential activities of the GI tract include the following six processes: Ingestion Propulsion Food breakdown: mechanical digestion Food breakdown: chemical digestion Absorption Defecation

Processes of the Digestive System Ingestion – Getting food into the mouth Active, voluntary process

Processes of the Digestive System Propulsion – Moving foods from one region of the digestive system to another Peristalsis – Involuntary alternating waves of contraction and relaxation; Net effect is to squeeze the food along the tract Segmentation – Moving food back and forth across the internal wall of the organ; Aids in mixing it with digestive juices

Processes of the Digestive System Food Breakdown: Mechanical digestion Prepares food for further degradation by enzymes Examples include: Mixing of food in the mouth by the tongue Churning of food in the stomach Segmentation in the small intestine

Processes of the Digestive System Food Breakdown: Chemical Digestion Enzymes break down food molecules into their building blocks Each major food group uses different enzymes: Carbohydrates are broken down into simple sugars Proteins are broken down into amino acids Fats are broken down to fatty acids and glycerol (an alcohol)

Processes of the Digestive System Absorption End products of digestion are absorbed in the blood or lymph The small intestine is the major site of absorption Defecation Elimination of indigestible substances from the body via the anus in the form of feces

Processes of the Digestive System

Control of Digestive Activity The body must maintain a constant internal environment, especially in the blood, since it comes into contact with all body cells The digestive system is mostly controlled by reflexes via the parasympathetic division of the autonomic nervous system Chemical and mechanical receptors are located in organ walls that trigger reflexes

Control of Digestive Activity The sensors involved in these reflexes respond to a number of stimuli: Stretch of the organ by food in its lumen pH of the contents Presence of certain breakdown products of digestion Reflexes include: Activation or inhibition of glandular secretions (digestive juices or hormones) Smooth muscle activity (mixing or propagation of food)