After you have finished this chapter you should be able to:
– To list in sequence each structure through which a bite of food passes on its way through the digestive tract; label a diagram of the digestive system
– Describe in general terms the following steps in processing food; ingestion, digestion, absorption, and elimination
– Describe the wall of the digestive tract, distinguish between the visceral and parietal peritoneums, and describe their major folds
– Describe the structures of the mouth, including teeth and give their functions
Describe the structure and functions of the pharynx and esophagus
Describe the structure of the stomach and its role in processing food
Identify the three main region of the small intestine and give the function of small intestine
Summarize the function of pancreas and liver
Summarize the carbohydrate, lipid, and protein digestion
Describe the structure of an intestinal villus, and explain the role villi in absorption of nutrient
Describe the structure and function of large intestine
List the nutrients that make up a balanced diet and summarize the function of each
Introduction
Why we need to have digestive system?
Principally, our body need energy to run the machinery of the body
In order to fulfill this energy, we need nutrients as sources of energy
The nutrients can be obtained in foods
Other function of nutrients are:
– As building blocks: used to make new cells
– As Ingredient to make chemical compounds needed for metabolism
The foods that we eat are still raw (they are not ready to be used for nourishment
In order to be ready, they must be broken down into simpler compounds or into a form that can be delivered to the cells and then used by the cells
This require a digestive system that will process the foods
This chapter will focus on the long and eventful journey that foods take through the digestive system
The components of the digestive system
The digestive system consists of:
– The digestive tract (also called alimentary canal)
This is a long tube (about 8 m long)
It extend from the mouth (where the food is taken in) to the anus (where the unused food is eliminated) (Fig. 15.5)
Below the diaphragm, the digestive tract is often called GI (gastrointestinal) tract
– Accessory organs
Three types of accessory digestive glands: salivary glands, liver and pancreas (they secrete juice to the digestive tract)
The food processing in the digestive system
The digestive system is responsible for 4 major processes:
– Ingestion
Involves taking food into the mouth, chewing it, and swallowing it
– Digestion
The break down of the food into simple molecules
This involve specific enzymes
– Absorption
The transfer of digested food through the wall of intestine and the circulatory system
The circulatory system transport the food molecules to the liver (many are removed and stored)
Those remain in the blood are transported to the cells
– Elimination
Remove undigested and unabsorbed food from the body
The wall of the digestive tract
From esophagus to anus, the wall of the digestive tract consists of 4 layers:
– The mucosa
The lining of the digestive tract
Consist of epithelial tissue
This tissue is resting upon a layer of loose connective tissue
The epithelium in the esophagus and anal canal is specialized for protection of underlying tissue
Those found in other region is specialized for secretion of mucus and for absorption of nutrients
In the stomach and small intestine the mucosa is thrown into folds
This increase the surface area for digestion and absorption
– Submucosa
Located beneath the mucosa (a layer of connective tissue)
Rich in blood vessel and nerve
– Muscle
This muscle contract in a wavelike motion called peristalsis
This will push the food along through the digestive tract.
– Connective tissue
The outer coat of the wall of the digestive tract
Below the level of the diaphragm, this layer is called Visceral peritoneum
By various folds, it connect to the parietal peritoneum
Between the visceral and the parietal peritoneum is a potential space (peritoneal cavity)
Inflammation of the peritoneum is called Peritonitis
This can have a very serious consequences, because infection can easily spread to all the adjoining organs
The mouth
The mouth is also called oral cavity
The foods are ingested here
The mouth prepare the foods for digestion
On the floor of the mouth, there is a flexible, muscular tongue
The tongue has its taste buds
These buds enable us to taste foods (salty, sweet, sour, or bitter)
Teeth are also found in this mouth cavity
The teeth
They are bony ridges that project from the mandible and maxilla
Each tooth consists of:
– Crown (the part above the gum)
– One or more roots (the portion beneath the gum line) (Fig. 15.4)
Cross section of a tooth:
– Each is composed mainly of dentin
A calcified connective tissue that imparts shape and rigidity of the tooth
In the crown region, dentin is protected by a tough covering enamel
Enamel protect the teeth against chemical that may dissolve the dentin
– A pulp cavity
This part of the teeth is enclosed by the dentin
It is filled with pulp (the most sensitive tissue that contains blood vessels and nerves)
– Root canals
These are narrow extensions of the pulp cavity
These canals pass through the root of the tooth
Each root canal has an opening at its base
This opening is the site through which the nerves, blood, and lymph vessel enter the tooth
The deciduous teeth
These are temporary teeth
These teeth are fairly small
Their first appearance is at about 6 month of age
They are also called as baby teeth of milk teeth
After 6 months of age, a new tooth appear every a few weeks until a set of 20 deciduous teeth is present (reached at the age of 2 years old)
By the age of 6 and 13 years they are slowly shed and replaced by larger permanent teeth
Teeth of an adult
These are permanent teeth
A set of these teeth consist of a maximum of 32 teeth
These permanent teeth consist of:
– Incisor (8 pieces, 4 on the top and 4 on the bottom)
They are specialized for biting and cutting
– Canines (4 pieces, 2 on the top, and 2 on the bottom)
Located laterally of the incisor
In humans they assist the incisors in biting
In some mammals they are enlarged and adapted for stabbing and tearing prey
– Premolars (8 pieces)
They are for grinding and crushing
– Molars (12 pieces)
They are also modified for grinding and crushing
The third molar is also called the wisdom teeth (often appear after age 18)
In many persons, their jaw is too small to accommodate the wisdom teeth
They may embedded
Sometimes they cause pain and must be surgically removed
The salivary glands
The three pairs of the salivary glands are:
– Parotid gland
The largest salivary glands
Located in the tissue inferior and anterior to the ears
This gland can get infection by mump virus (it become swollen)
– Submandibular glands
These glands lies below the jaw
– Sublingual glands
Located under the tongue
All glands above produce saliva that consist of:
– A thin watery secretion containing amylase
– A mucus secretion (lubricates the mouth)
– Anti bacteria (salt, antibodies, & other substances)
The functions of the saliva:
– To lubricates the tissues of the mouth and pharynx
– This make it easier to talk and chewing
– By moistening the foods, the saliva the tongue to convert the mouthful of food to a semisolid mass that is called a bolus
– In the form of bolus, the food is easier to be swallowed.
The pharynx
Also called throat
It is a muscular tube about 12 cm long
It serve as the hallway of both the respiratory and digestive system
The pharynx is important in swallowing
This process moves the bolus from the mouth through the pharynx and down the esophagus
The three regions of the pharynx:
– Oropharynx (Posterior to the mouth)
– Nasopharynx (Posterior to the nose)
– Laryngopharynx (Opens into the larynx and esophagus
The esophagus
Extends from the pharynx through the thoracic cavity
It passes through the diaphragm and empties into the stomach
The bolus is swept through the pharynx and into the esophagus by a wave of contraction called the peristaltic contraction
Similar contraction works in the esophagus that push the bolus into the stomach
This contraction is caused by two layers of muscles
At the lower end of the esophagus is a sphincter muscle (cardiac muscle)
This is a circular muscle that constrict the tube so that the entrance stomach is generally closed
This prevent the acidic gastric juice from splashing up into the esophagus
Sometimes the splashing occurs, and this cause the wall of the esophagus to become irritated
– To list in sequence each structure through which a bite of food passes on its way through the digestive tract; label a diagram of the digestive system
– Describe in general terms the following steps in processing food; ingestion, digestion, absorption, and elimination
– Describe the wall of the digestive tract, distinguish between the visceral and parietal peritoneums, and describe their major folds
– Describe the structures of the mouth, including teeth and give their functions
Describe the structure and functions of the pharynx and esophagus
Describe the structure of the stomach and its role in processing food
Identify the three main region of the small intestine and give the function of small intestine
Summarize the function of pancreas and liver
Summarize the carbohydrate, lipid, and protein digestion
Describe the structure of an intestinal villus, and explain the role villi in absorption of nutrient
Describe the structure and function of large intestine
List the nutrients that make up a balanced diet and summarize the function of each
Introduction
Why we need to have digestive system?
Principally, our body need energy to run the machinery of the body
In order to fulfill this energy, we need nutrients as sources of energy
The nutrients can be obtained in foods
Other function of nutrients are:
– As building blocks: used to make new cells
– As Ingredient to make chemical compounds needed for metabolism
The foods that we eat are still raw (they are not ready to be used for nourishment
In order to be ready, they must be broken down into simpler compounds or into a form that can be delivered to the cells and then used by the cells
This require a digestive system that will process the foods
This chapter will focus on the long and eventful journey that foods take through the digestive system
The components of the digestive system
The digestive system consists of:
– The digestive tract (also called alimentary canal)
This is a long tube (about 8 m long)
It extend from the mouth (where the food is taken in) to the anus (where the unused food is eliminated) (Fig. 15.5)
Below the diaphragm, the digestive tract is often called GI (gastrointestinal) tract
– Accessory organs
Three types of accessory digestive glands: salivary glands, liver and pancreas (they secrete juice to the digestive tract)
The food processing in the digestive system
The digestive system is responsible for 4 major processes:
– Ingestion
Involves taking food into the mouth, chewing it, and swallowing it
– Digestion
The break down of the food into simple molecules
This involve specific enzymes
– Absorption
The transfer of digested food through the wall of intestine and the circulatory system
The circulatory system transport the food molecules to the liver (many are removed and stored)
Those remain in the blood are transported to the cells
– Elimination
Remove undigested and unabsorbed food from the body
The wall of the digestive tract
From esophagus to anus, the wall of the digestive tract consists of 4 layers:
– The mucosa
The lining of the digestive tract
Consist of epithelial tissue
This tissue is resting upon a layer of loose connective tissue
The epithelium in the esophagus and anal canal is specialized for protection of underlying tissue
Those found in other region is specialized for secretion of mucus and for absorption of nutrients
In the stomach and small intestine the mucosa is thrown into folds
This increase the surface area for digestion and absorption
– Submucosa
Located beneath the mucosa (a layer of connective tissue)
Rich in blood vessel and nerve
– Muscle
This muscle contract in a wavelike motion called peristalsis
This will push the food along through the digestive tract.
– Connective tissue
The outer coat of the wall of the digestive tract
Below the level of the diaphragm, this layer is called Visceral peritoneum
By various folds, it connect to the parietal peritoneum
Between the visceral and the parietal peritoneum is a potential space (peritoneal cavity)
Inflammation of the peritoneum is called Peritonitis
This can have a very serious consequences, because infection can easily spread to all the adjoining organs
The mouth
The mouth is also called oral cavity
The foods are ingested here
The mouth prepare the foods for digestion
On the floor of the mouth, there is a flexible, muscular tongue
The tongue has its taste buds
These buds enable us to taste foods (salty, sweet, sour, or bitter)
Teeth are also found in this mouth cavity
The teeth
They are bony ridges that project from the mandible and maxilla
Each tooth consists of:
– Crown (the part above the gum)
– One or more roots (the portion beneath the gum line) (Fig. 15.4)
Cross section of a tooth:
– Each is composed mainly of dentin
A calcified connective tissue that imparts shape and rigidity of the tooth
In the crown region, dentin is protected by a tough covering enamel
Enamel protect the teeth against chemical that may dissolve the dentin
– A pulp cavity
This part of the teeth is enclosed by the dentin
It is filled with pulp (the most sensitive tissue that contains blood vessels and nerves)
– Root canals
These are narrow extensions of the pulp cavity
These canals pass through the root of the tooth
Each root canal has an opening at its base
This opening is the site through which the nerves, blood, and lymph vessel enter the tooth
The deciduous teeth
These are temporary teeth
These teeth are fairly small
Their first appearance is at about 6 month of age
They are also called as baby teeth of milk teeth
After 6 months of age, a new tooth appear every a few weeks until a set of 20 deciduous teeth is present (reached at the age of 2 years old)
By the age of 6 and 13 years they are slowly shed and replaced by larger permanent teeth
Teeth of an adult
These are permanent teeth
A set of these teeth consist of a maximum of 32 teeth
These permanent teeth consist of:
– Incisor (8 pieces, 4 on the top and 4 on the bottom)
They are specialized for biting and cutting
– Canines (4 pieces, 2 on the top, and 2 on the bottom)
Located laterally of the incisor
In humans they assist the incisors in biting
In some mammals they are enlarged and adapted for stabbing and tearing prey
– Premolars (8 pieces)
They are for grinding and crushing
– Molars (12 pieces)
They are also modified for grinding and crushing
The third molar is also called the wisdom teeth (often appear after age 18)
In many persons, their jaw is too small to accommodate the wisdom teeth
They may embedded
Sometimes they cause pain and must be surgically removed
The salivary glands
The three pairs of the salivary glands are:
– Parotid gland
The largest salivary glands
Located in the tissue inferior and anterior to the ears
This gland can get infection by mump virus (it become swollen)
– Submandibular glands
These glands lies below the jaw
– Sublingual glands
Located under the tongue
All glands above produce saliva that consist of:
– A thin watery secretion containing amylase
– A mucus secretion (lubricates the mouth)
– Anti bacteria (salt, antibodies, & other substances)
The functions of the saliva:
– To lubricates the tissues of the mouth and pharynx
– This make it easier to talk and chewing
– By moistening the foods, the saliva the tongue to convert the mouthful of food to a semisolid mass that is called a bolus
– In the form of bolus, the food is easier to be swallowed.
The pharynx
Also called throat
It is a muscular tube about 12 cm long
It serve as the hallway of both the respiratory and digestive system
The pharynx is important in swallowing
This process moves the bolus from the mouth through the pharynx and down the esophagus
The three regions of the pharynx:
– Oropharynx (Posterior to the mouth)
– Nasopharynx (Posterior to the nose)
– Laryngopharynx (Opens into the larynx and esophagus
The esophagus
Extends from the pharynx through the thoracic cavity
It passes through the diaphragm and empties into the stomach
The bolus is swept through the pharynx and into the esophagus by a wave of contraction called the peristaltic contraction
Similar contraction works in the esophagus that push the bolus into the stomach
This contraction is caused by two layers of muscles
At the lower end of the esophagus is a sphincter muscle (cardiac muscle)
This is a circular muscle that constrict the tube so that the entrance stomach is generally closed
This prevent the acidic gastric juice from splashing up into the esophagus
Sometimes the splashing occurs, and this cause the wall of the esophagus to become irritated
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