Digestive SystemDigestive System

The digestive system prepares food for use by hundreds of millions of body cells. Food when eaten cannot reach the cells (because it cannot pass through the intestinal walls to the bloodstream and, if it could not be in a useful chemical state. The gut modifies food physically and chemically and disposes unusable waste.

Purpose:

The digestive system uses mechanical and chemical methods to break food down into nutrient molecules that can be absorbed into the blood.

Types of Digestive System in AnimalsTypes of Digestive System in Animals

There are two types of plans and locations of digestion :

Sac-like plans are found in many invertebrates, who have a single opening for food intake and the discharge of wastes.

Coelenteronic: incomplete digestive system, having a single opening that serves as the mouth and the anus.

OR

Vertebrates use the more efficient tube-within-a-tube-plan with food entering through one opening (the mouth) and wastes leaving through another (the anus).

OREnteronic: complete type of digestive system, having an anterior opening called mouth and posterior opening called anus

Types of DigestionTypes of Digestion

Intracellular Digestion: food is taken into cells by phagocytosis with digestive enzymes being secreted into the phagocytic vesicles occurs in sponges, coelenterates and most protozoans.

Extracellular Digestion: occurs in the lumen (opening) of the digestive system, with the nutrient molecules being transferred to the blood or body fluid; occurs in chordates, annelids, and crustaceans.

DigestionDigestion is the process of breaking down food into is the process of breaking down food into molecules small enough for the body to absorb. molecules small enough for the body to absorb. Proteins, carbohydrates, and fat in our diets must Proteins, carbohydrates, and fat in our diets must be broken down and later, reassembled in forms be broken down and later, reassembled in forms useful to our body.useful to our body.

herbivores: organisms which feed on plants

carnivores: organisms which feed on flesh of animals

omnivores: organisms which feed on both plants and animals

Stages in the Digestive ProcessStages in the Digestive Process

movement:movement: propels food through the propels food through the digestive system digestive system

secretionsecretion: release of digestive juices in : release of digestive juices in response to a specific stimulus response to a specific stimulus

digestiondigestion: breakdown of food into molecular : breakdown of food into molecular components small enough to cross the components small enough to cross the plasma membrane plasma membrane

absorptionabsorption: passage of the molecules into the : passage of the molecules into the body's interior and their passage throughout body's interior and their passage throughout the body the body

elimination:elimination: removal of undigested removal of undigested

Components of the Digestive System Components of the Digestive System

The human digestive system The human digestive system is a coiled, muscular tube (6-is a coiled, muscular tube (6-9 meters long when fully 9 meters long when fully extended) extending from extended) extending from the mouth to the anus. the mouth to the anus. Several specialized Several specialized compartments occur along compartments occur along this length: mouth, pharynx, this length: mouth, pharynx, esophagus, stomach, small esophagus, stomach, small intestine, large intestine, and intestine, large intestine, and anus. Accessory digestive anus. Accessory digestive organs are connected to the organs are connected to the main system by a series of main system by a series of ducts: salivary glands, parts ducts: salivary glands, parts of the pancreas, and the liver of the pancreas, and the liver and gall bladder and gall bladder ((bilary bilary systemsystem).).

The Anatomy and Physiology of The Anatomy and Physiology of DigestionDigestion

Food enters the digestive Food enters the digestive system via the mouth or oral system via the mouth or oral cavity, mucous membrane cavity, mucous membrane lined. The lips (labia) protect lined. The lips (labia) protect its outer opening, cheeks its outer opening, cheeks form lateral walls, hard form lateral walls, hard palate and soft palate form palate and soft palate form anterior / posterior roof. The anterior / posterior roof. The floor of the mouth is the floor of the mouth is the muscular tongue.muscular tongue.

Food is first processed Food is first processed (bitten off) by teeth, (bitten off) by teeth, especially the anterior especially the anterior incisors. Suitably sized incisors. Suitably sized portions then retained in portions then retained in closed mouth and chewed closed mouth and chewed or masticated (especially or masticated (especially by cheek teeth, premolars, by cheek teeth, premolars, molars) aided by saliva molars) aided by saliva Ducted salivary glands Ducted salivary glands open at various points open at various points into mouth. This process into mouth. This process involves teeth (muscles of involves teeth (muscles of mastication move jaws) mastication move jaws) and tongue (extrinsic and and tongue (extrinsic and intrinsic muscles intrinsic muscles

Mouth and Teeth

Structure of a toothStructure of a tooth Enamel:Enamel: material covering the material covering the

crown which is the hardest crown which is the hardest substance in the bodysubstance in the body

Crown:Crown: portion above the gum portion above the gum Dentine:Dentine: bulk of the teeth bulk of the teeth Root:Root: portion embedded in a portion embedded in a

socket of jawbonesocket of jawbone Neck:Neck: slightly constricted region slightly constricted region

between the root and the crown between the root and the crown and is covered by the gum or and is covered by the gum or gingivagingiva

Periodontal membrane:Periodontal membrane: covers the covers the roof and hold the tooth firmly in roof and hold the tooth firmly in the socketthe socket

Pulp:Pulp: consists of connective tissue consists of connective tissue containing a dense network of containing a dense network of capillaries, nerves, and lymph capillaries, nerves, and lymph vesselsvessels

Number Name Number Name

1 Enamel 7 Periodontal

2 Crown 8 Root

3 Dentine 9 Alveolar bone

4 Gingival sulcus

10 Apex/ical forame

n

5 Neck 11 Pulp cavity

6 Gingiva

SwallowingSwallowingSwallowing moves food from the mouth through the pharynx into Swallowing moves food from the mouth through the pharynx into

the esophagus and then to the stomach.the esophagus and then to the stomach.

Step 1:Step 1: A mass of chewed, moistened food, a bolus, is moved to A mass of chewed, moistened food, a bolus, is moved to the back of the moth by the tongue. In the pharynx, the bolus the back of the moth by the tongue. In the pharynx, the bolus triggers an involuntary swallowing reflex that prevents food from triggers an involuntary swallowing reflex that prevents food from entering the lungs, and directs the bolus into the esophagus. entering the lungs, and directs the bolus into the esophagus.

Step 2:Step 2: Muscles in the esophagus propel the bolus by waves of Muscles in the esophagus propel the bolus by waves of involuntary muscular contractions involuntary muscular contractions (peristalsis)(peristalsis) of smooth muscle of smooth muscle lining the esophagus. lining the esophagus.

Step 3:Step 3: The bolus passes through the gastroesophogeal sphincter, The bolus passes through the gastroesophogeal sphincter, into the stomach. Heartburn results from irritation of the into the stomach. Heartburn results from irritation of the esophagus by gastric juices that leak through this sphincter. esophagus by gastric juices that leak through this sphincter.

Structure of the throat and the mechanics of swallowing. Structure of the throat and the mechanics of swallowing.

Peristalsis and the movement of food from Peristalsis and the movement of food from the mouth to the stomach. the mouth to the stomach.

The muscles in the esophagus squeeze the food downward with a wavelike motion, a process referred to as peristalsis.

EsophagusEsophagus

The esophagus (about 10") is the first part of the digestive tract The esophagus (about 10") is the first part of the digestive tract proper and shares its distinctive structure. proper and shares its distinctive structure.

Basic tissue layers of the gut are:Basic tissue layers of the gut are:

mucosa.mucosa. Innermost, moist lining membrane. Epithelium (friction Innermost, moist lining membrane. Epithelium (friction resistant stratified squamous in esophagus, simple beyond) plus a resistant stratified squamous in esophagus, simple beyond) plus a little connective tissue and smooth muscle. little connective tissue and smooth muscle.

submucosa.submucosa. Soft connective tissue layer, blood vessels, nerves, Soft connective tissue layer, blood vessels, nerves, lymphatics lymphatics

muscularis externa.muscularis externa. Typically circular inner layer, longitudinal outer Typically circular inner layer, longitudinal outer layer of smooth muscle layer of smooth muscle

serosal fluid producing single layer.serosal fluid producing single layer.

StomachStomach

The stomach is a J-shaped, expandable sack, normally The stomach is a J-shaped, expandable sack, normally on the left side of the upper abdomen. Several muscle on the left side of the upper abdomen. Several muscle layers surround the stomach, serving to churn food. layers surround the stomach, serving to churn food. The stomach can expand to hold about 2 L of food (= The stomach can expand to hold about 2 L of food (= 1/2 gal). The stomach contains 1/2 gal). The stomach contains hydrochloric acidhydrochloric acid (HCl (HCl) ) strong enough to dissolve metal (pH about 1.5 to 3, strong enough to dissolve metal (pH about 1.5 to 3, usually around 2), which kills bacteria and helps usually around 2), which kills bacteria and helps denature the proteins in our food, making them more denature the proteins in our food, making them more vulnerable to attack by pepsin. The stomach secretes vulnerable to attack by pepsin. The stomach secretes mucus to protect itself from being digested by its own mucus to protect itself from being digested by its own acid and enzymes. The stomach also manufactures acid and enzymes. The stomach also manufactures pepsinpepsin, an enzyme to digest protein., an enzyme to digest protein.

SummarySummaryThe digestive process begins with the senses. When we see and smell food, especially when we are hungry, our mouths begin to "water." Saliva is secreted into the mouth from salivary glands located in the cheeks and jaw. Saliva contains the first enzymes that will act upon our food.

The first step of digestion is mastication, the chewing process. The food particles must be broken down into very tiny pieces and mixed with saliva in order for the enzymes to do their job. Many digestive disturbances could be avoided with proper chewing. In our fast-paced society most people eat too fast and swallow their food without proper mastication. When chewed properly food will be in an almost liquid form when swallowed.

From the mouth, the food moves to the stomach by way of the esophagus, a muscular tube which passes through a hole in the diaphragm to enter the abdominal area. The muscles in the esophagus squeeze the food downward with a wavelike motion, a process referred to as peristalsis. Because of this muscular action, food can be swallowed in zero gravity, or even when the body is upside down.

Whereas the pH (acidity) in the mouth and esophagus is very basic (high pH), the pH in the stomach is very acidic (low pH). This low or acid pH is the result of the secretion of hydrochloric acid (HCL), and is necessary for the action of the enzymes in the stomach that are responsible for breaking down protein. The stomach is protected from this acid by a mucus lining. The stomach acid is kept out of the esophagus by the action of the cardiac valve, a sphincter muscle which opens when food passes through, but which immediately closes to prevent the contents of the stomach from backing up into the esophagus.

Another sphincter valve, the pyloric sphincter, keeps the food in the stomach until it has been properly churned and mixed. Once the enzymes that require an acid environment have had time to do their job, the pyloric sphincter opens and the food passes into the small intestines where most of the absorption of nutrients takes place.