5 Digestive System.

Introduction to Digestion

• All animals require food to provide energy and building blocks for new molecules.

• Essential nutrients needed include vitamins and minerals.

Digestive System Anatomy

Major Organs and Structures

• Salivary Glands: Moisten food into a bolus; begin digestion of polysaccharides.

• Esophagus: Transports food to the stomach.

• Liver: Detoxifies, stores vitamins/iron/glycogen, and synthesizes bile.

• Stomach: Stores and churns food; begins protein digestion.

• Gall Bladder: Stores and concentrates bile.

• Pancreas: Releases digestive enzymes and hormones (e.g. insulin).

• Small Intestine: Absorbs nutrients.

• Large Intestine: Absorbs water and ions; involved in waste storage and expulsion.

• Rectum/Anus: Stores and expels feces.

Digestion Processes

Types of Digestion

1. Mechanical Digestion: Physically breaks food into smaller pieces (e.g., chewing in the mouth, stomach churning).

2. Chemical Digestion: Breaks down food into smaller molecules using enzymes; essential for nutrient absorption.

Details of Mechanical Digestion

• Happens in the Mouth with teeth and tongue, in the Stomach with muscle movement, and in the Small Intestine with bile salts.

• Increases surface area for enzyme activity.

Details of Chemical Digestion

• Enzymes are utilized to break food into building blocks that can pass through cell membranes.

The Mouth

• The oral cavity is covered by a multilayered epithelium called mucosa.

• Involves teeth and tongue in both digestion and speech.

• Saliva: Contains enzyme amylase for starch breakdown.

• Tongue: It enables the perception of taste, mixes food ground

  by teeth with saliva to form a bolus, and sends it to the pharynx.

  Teeth and the tongue assist not only in digestion but also in

  speech.

Pharynx and Esophagus

Pharynx

1. Connects oral and nasal cavities to esophagus and trachea.

2. Lined with multilayered epithelium; NO digestion occurs here.

3. Epiglottis prevents food from entering trachea (airway) during swallowing.

4. Swallowed food and drink move from the pharynx,

   1. into the esophagus, and

      2. into the stomach.

5. During swallowing,

   1. the tip of the larynx moves upward,

   2. epiglottis preventing the food from entering the trachea.

Esophagus

• Approximately 25 cm long; lined with smooth muscles.

• Food moves down via peristalsis, a coordinated muscle contraction.

• Sliding down of food from esophagus is called as peristaltic movement.

Movement of Food

Peristalsis

• Takes place in

• Oesophagus

• Stomach

• Gut

• Continuous segments of longitudinal smooth muscle rhythmically contract and relax

• Food is moved unidirectionally along the alimentary canal (mouth to anus)

The Stomach

• Can stretch and store up to 2 liters of food.

• Both mechanical and chemical digestion occur; gastric juice contains mucus, pepsinogen, and HCl.

• Pepsinogen converts to active pepsin for protein digestion.

• Protective mechanisms: secretion of pepsinogen and mucus protects stomach lining from HCl.

Stomach layers

• Mucosa layer (inner layer)

• Smooth muscle layer (middle layer)

• Connective tissue layer (outer layer) have

• peritoneum.

Small Intestine

• Major organ for chemical digestion and nutrient absorption, 6 meters long.

• Divided into three regions:

  • Duodenum: First 25 cm, where chyme mixes with digestive juices.

  • Jejunum: Most digestion and nutrient absorption.

  • Ileum: Regulates food movement into the large intestine.

• Surface area for absorption increased by folds, villi, and microvilli.

Digestive Hormones

• Cholecystokinin (CCK) and Secretin are released from the duodenum, stimulating bile and enzyme release.

Absorption in the Small Intestine

• Nutrients absorbed through diffusion and against concentration gradients.

• Fatty acids/glycerol are transported by lymph vessels; sugars/amino acids enter the bloodstream via capillaries.

The Large Intestine

• Approximately 1.5 m long; divided into regions: caecum, ascending, transverse, descending colon, sigmoid colon, rectum.

• Functions include water absorption, mineral absorption, and waste expulsion.

ACCESSORY ORGANS AND STRUCTURES

Salivary glands

• Moisten, soften, and dissolve food

• Clean the mouth and teeth

• It consists of,

• Water

• mineral salts

• enzyme: salivary amylase

• Mucus

Liver

• Regulates glucose concentration in the blood

• Produces bile

• Modifies substances absorbed in the digestive tract into less toxic

  forms

• Synthesizes many blood proteins including heparin albumin, globulin,

  and fibrinogen (blood-clotting protein)

• Stores A-D-E-K vitamins

• Stores Fe mineral

• Provides the conversion of nutrients to each other (converts extra lipids

  to glycogen)

Gallbladder: Stores and concentrates bile until it is needed in the small intestine.

Pancreas

• Secretes digestive enzymes and bicarbonate to aid in digestion

• Neutralize stomach acid

• Secretes insulin and glucagon hormones, releasing them directly into the

  bloodstream.

  

Digestive Disorders

• Gastritis: Inflammation of stomach lining.

• Reflux: Acid backing into esophagus.

• Ulcer: Sores in stomach/intestine.

• Constipation: Difficulty in bowel movement.

• Diarrhea: Frequent watery stools.

• Hemorrhoids: Swollen veins in the rectum causing pain and bleeding

Functions of Digestive Organs

• Mouth: Ingestion, mechanical digestion.

• Pharynx: Propels food to esophagus.

• Esophagus: Transports food to stomach.

• Stomach: Mixes food with gastric juices for digestion.

• Small Intestine: Major site of digestion/absorption.

• Accessory organs (Liver, Gallbladder, Pancreas): Produce and store enzymes and bile for digestion.

• Large Intestine: Absorbs water, electrolytes, and vitamins; processes waste for elimination.