Human Digestive System : Internal Food Processing Unit of Human

Position and Different Organs of Digestive System

DIGESTIVE SYSTEM:

The activities in the digestive system can be grouped under 5 main heading.

1) INGESTION:

This is the taking of food into the alimentary tract i.e. eating and drinking.

2) PROPULSION:

This mixes and moves the contents along the alimentary tract.

3) DIGESTION:

This consist of :

a) Mechanical Breakdown of Food e.g : Mastication ( Chewing)

b) Chemical digestion of food into small molecules by enzyme present in secretions produced by glands and accessory organs of the digestive system.

4) ABSORPTION:

This is the process by which digested food substances pass through the wall of some organs of the alimentary canal into the blood and lymph capillaries for the circulation and use by body cell.

5) ELIMINATION:

Food subtances that have been eaten but cannot be digested and absorbed are excreted from the alimentary canal as faeces by the process of defaecation.

ORGANS OF THE DIGESTIVE SYSTEM:

1) Alimentary tract:

Also known as gastrointestinal (GI) tract, This is the long tube through which the food passes.It commences at the mouth and terminates at anus,and the various parts are given separate names, although structurally they are remarkably similar. The parts are:

a) MOUTH

b) PHARYNX

c) OESOPHAGUS

e) STOMACH
d) SMALL INTESTINE

f) LARGE INTESTINE

g) RECTUM AND ANAL CANAL.

2) Accessory organs:

Various secretion are poured into the alimentary tract, some by glands in the lining memebrane of the organs,e.g. gastric juice secreted by glands in the lining of the stomach,and some by glands situated outside the tract.The latter are the accessory organs of digestion and their secretions pass through ducts to enter the tract.

They consist of:

a) 3 pairs of salivary glands

b) The liver and biliary tract.

The organs and glands are linked physiologically as well as anatomically in that digestion and absorption occur in stages,each stage being dependent upon previous stage or stages.

Mouth and Oesophagus of Human with all Parts Name

OESOPHAGUS & MOUTH:

Structure:

there are 4 layers of tissue , The oesophagus is almost entirely in the thorax the outer covering,the adventitia,consist of elastic fibrous tissue that attaches the oesophagus to the surrounding stuctures.The proximal third is lined by saritified squamous epithelium and distal third by columnar epithelium.The middle third is lined by mixture of two.

Function of mouth pharynx and Oesophagus:

Formation of a Bolus:

When food is taken into the mouth it is masticated,or chewed by the teeth and moved run the mouth by the tongue and muscles of the cheeks.It is with saliva and formed into soft mass or bolus ready for swallowing.The length of time that food remains in the mouth depends. To a large extent, on the consistency of the food.Some foods need to be chewed longer than others before the individual feels that the bolus is ready for swallowing.

Mouth and Oesophagus of Human

Swallowing(deglutition):

This occurs in 3 stages after mastication is complete and the bolus has been formed.It is initiated voluntarily but completed by a reflex(involuntary) action.

1) The mouth is closed and the voluntary muscles of the tongue and cheeks push the bolus backwards into the pharynx.

2) The muscles of the pharynx are stimulated by a reflex action initiated in the walls of the  oropharynx and coordinated in the medulla and lower pons in the brain stem.Involuntry contraction of these muscle propels the bolus down into the oesophagus.All other routes that the bolus take are closed.The soft palate rises up and closes of the nasopharynx;the tongue and pharyngeal folds block the way back into the mouth; and larynx is lifted up and forward so that its opening

is occluded by the over hanging epiglottis preventing entry into the airway(trachea).

3) The presence of the bolus in the pharynx stimulates a wave of peristalsis that propels the bolus through the oesophagus to the stomach.

Human Stomach with its Part Names

STOMACH:

The stomach is a J-shaped dilated portion of the alimentary tract situated in the epigastric,umbilical and left hypocondriac regions of the abdominal cavity.

Organs associated with stomach:

Anteriorly- Left lobe of liver and anterior abdominal wall

Posteriorly- Abdominal aorta,pancreas,spleen,left kidney and adrenal gland.

Superiorly - Diaphragm,oesophagus and left lobe of liver.

Inferiorly - Transverse colon and small insestine.

Top to left- Diaphragm and spleen.

To the right- Liver and duodenum.

Human Stomach

Structure of stomach:

The stomach is continuous with the oesophagus at the cardiac sphincter and with the duodenum at the pyrolic sphincter it curves upward to complete the J shape.Where the oesophagus join the stomach the anterior region angles acutely upwards,curves downwards forming the great curvature and then slightly upwards towards the pyrolic sphinter. The stomach is divided into 3 regions: The fundus, The body and the antrum.At the distal end of the pyrolic antrum is the pyrolic sphincter, guarding the opening between the stomach and the duodenum.When the stomach is inactive the pyrolic sphincter is relaxed and open, and when the stomach contains food the sphincter is closed.

Functions of  the stomach and gastric juice:

Stomach size varies with the volume of food it contains,which may be 1.5 litres or more in an adult. When a meal has been eaten the food accumulates in the stomach in layers,the part of the meal remaining in the fundus for some time.Mixing with the gastric juice takes place gradually and it  may be some time before the food is sufficiently acidified to stop the action of salivary amylase. Gastric muscle contraction consists of a churning movement that breaks down the bolus and mixes it with gastric juice and peristaltic waves that propel the stomach contain towards the pyrolus.When the stomach is active the pyrolic sphinter closes.Strong peristalic contraction of the pyrolic antrum forces chyme,gastric contents after they sufficiently liquefied,through the pyrolus into the duodenum in small spurts.Parasympathetic stimulation increses the mortality of the stomach and secretion of the gastric juice; sympathetic stimulation has the opposite effect.

Gastric juice:

About 2 litres of gastric juice are secreted daily by specialised secretory glands in the mucosa  consists of:

Water : Further liquefies the food swallowed.

Mineral salts

Mucus secreted by goblet cells in the glands and on the stomach surface.It prevent the mechanical injury to the stomach by lubricating the contents.

Hydrochloric acid: Acidifies the food and stops the action of salivary amylase,kills ingested microbes,provides acid environment needed for effective digestion by pepsin.                 

Intrinsic factor

Inactive enzyme Precursors: pepsinogens secreted by chief cells in the glands.They begin the digestion of protein,breaking them into smaller molecules.

SMALL AND LARGE INTESTINE:

Human Small and Large Intestine with  its Parts Names

The small intestine comprise 3 main sections continuos with each other.

1) Duodenum: it is about 25 cm long and curves around the head of the pancreas secretion from the gall bladder and pancreas are released into the duodenum through a common structure the hepato-pancreatic ampulla and the opening into the duodenum is guarded by the hepatopancreatic sphincter.

2) Jejunum: It is the middle section of the small intestine and is about 2 metres long.

3) Ileum: The terminal section,is about 3 metres long and ends at theileocaecal valve,which cintrols the flow of material from the ileum to the caecum, the first part of the large intestine and prevent regurgitation.

Human Small and Large Intestine

Structure of small intestine:

The walls of small intestine are composed of the 4 layers of tissue

a) Peritoneum: A double layer peritneum called the mesentry attaches the jejunum and ileum to the posterior abdominal wall.

b) Mucosa: The suface area of the small intestine mucosa is greatly incresed by permanent circular folds,villi and microvilli.

Function of the small intestine:

1) Upward movement of its contents by peristalsis which is increased by parasympathetic stimulation.

2) Secretion of intestinal juice,also increased by parasympathetic stimulation

3) Completion of chemical digestion of carbohydrates protein and fats in the enterocytes of the villi

4) Protection aganist infection by microbes that have survived the antimicobial action of the hydrochloric acid in the stomach,by the solitary lymph follicles and aggregated lymph follicles.

5) Secretion of the hormone cholecystokinin and secretin

6) Absorption of nutrients.

Human Digestive System

LARGE INTESTINE:

This is about 1.5 metres long,beginning at the caecum in the right iliac fossa and terminating at the rectum and anal canal deep in the pelvis. Its lumen is about 6.5 cm in diameter, larger than that of the small intestine. The colon is divided into the 5 parts

1)The caecum: The 1st part of the colon. It is a dilated region which has a blind end inferiorly and it is continuous with the ascending colon superiorly.

2)The ascending colon: This passes upwards from the caecum to the level of the liver where it curves acutely to the left at the hepatic flexure to become transverse colon.

3) Transverse colon: This is the loop colon that extend  across the abdominal cavity in front of the duodenum and stomach to the area of the spleen where it forms splenic flexure nad curves acutely downwards to become the descending colon.

4) The descending colon: This passes down the left side of the abdominal cavity then cuves towards the midline.After it enters the true pelvis it is known as sigmoid colon.

5) The sigmoid colon: This part describes an s-shaped curve in the pelvis that continues downwards to become the rectum.

6) Rectum: This slightly dilated section of the colon about 13 cm long.Its leads from the sigmoid colon about 13 cm long.It leads from the sigmoid colon and terminates in the anal canal.

7) The anal canal: This is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior

Human Digestive System

Functions of the large intestine,rectum and anal canal:

a) Absorption: The contents of the ileum which pass through the ileocaecal valve into the caecum are fluid,even through some water has been absorbed in the small intestine.

b) Microbial activity: The large intestine is heavily colonised by certain types of bacteria,which synthesis vitamin K and Folic acid.They include E.coli,Enterobactor aerogenes, Streptococcus faecalis, Clostridium perfringens.

c) Mass movement: The large intestine does not exhibit peristaltic movement as in other parts of  digestive tract.

d) Defaecation: Usually the rectum is empty,but when the mass movement forces the contents of the sigmoid colon into the rectum the nerve endings in its walls are stimulated by strech.In the infants ,defaecation occurs by reflex(involuntary) action.However,during the 2nd and 3rd year of the life the ability to override the defaecation reflex is developed.

Constituents of faeces:

Fibre(Indigestable cellular plant and animal material) Dead and live microbes.

Epithelial cell shed from the walls of tract Fatty acids Mucus secreted by epithelial linning of the large intestine    

Human Pancreas with its Part Names

PANCREAS:

This pancreas is a pale grey gland weighing about 60 grams.It is about 12 to 15 cm long and is situated in the epigastric and left hypochondriac regions  of the abdominal, It Consist fo a board head and body and a narrow tail.

Human Pancreas

The head lies in the curve of a duodenum,The body behind the stomach and tail lies in front of  the left kidney and just reaches the spleen.The abdominal aorta and the inferior vena cava lie behind the gland.It both an exocrine and endocrine gland. 

Human Liver with its Parts Name

LIVER:
The liver is the largest gland in the body, weighing between 1 and 2.3 kg.It is situated in the upper part of the abdominal cavity occupying the greater part of the right hypochondric  region,part of the epigastric region and extending into the left hypochondriac region. Its upper and anterior surface is smooth and curved to fit the under surface of the diaphragm;its posterior surface is irregular in outline.

Anterior Part of The Human Liver

Organs associated with the liver:
Superiorly & anteriorly- Diaphragm and anterior abdominal wall.


Inferiorly- Stomach,bile duct,duodenum,hepatic fixture of colon right kidney and adrenal gland.


Posteriorly-Oesophagus,inferior vena cava,aorta,gall bladder,vertebral column and diaphragm.


Laterally- Lower ribs and diaphragm.


Liver enclosed in a thin inelastic capsule and incompletely covered by a layer of peritonium.Folds of peritoneum from supporting ligaments attaching the liver to the inferior surface of the diaphragm.It is held in position partly by these ligaments and partly by the pressure of the organs in the abdominal cavity.Liver has 4 lobes.The 2 most ovious are the large right lobe and the smaller,wedge-shaped,left lobe.The other 2,The caudate and quadrate lobes,are areas on the posterior surface.

Posterior Part of The Human Liver

Functions of the liver:
1) Carbohydrate Metabolism


2) Fat metabolism


3) Protein metabolism


4)Breakdown of erythrocytes and defence aganist microbes


5) Detoxification of drugs and noxious substances


6) Inactivation of hormones: This include insulin,glucagon,cortisol,aldosterone,thyroid and sex hormone.


7) Production of heat


8)Secretion of heat


9)Secretion of bile


10)Storage 

Salivary Gland

SALIVARY GLANDS:

Its release their secretions into ducts that lead to the mouth.There are 3 main pairs;the parotid glands, the submandibular glands and the sublingual glands.There are also numerous smaller salivary glands scattered around the mouth.

SALIVARY GLANDS

Parotid glands:

These are situated one on each side of the face just below the external acoustic meatus. Each gland has a parotid duct opening into the mouth at the level of the 2nd upper molar tooth.

Submandibular glands:  

These lie on each side of the face under the angle of the jaw.The 2 submandibular ducts open on the floor of the mouth,one on each side of the frenulum of the tongue.

Sublingual gland:

These glands lie under the mucous membrane of the floor of the mouth in front of the submandibular glands.They have numerous small ducts that open into the floor of the mouth.

Composition of saliva:

1.5 litres of saliva is produced daily and it consist of:

Water

Mineral salts

Salivary amylase

mucus

Lysozyme

Immunoglobulins

Blood -clotting factors.

Functions of saliva:

1) Chemical digestion of polysaccharides:

2) Lubrication of foods

3) Cleaning and lubricating

4) Non-specific defence

5)Taste

                                                                                    Animation Showing Different organs of Human Digestive System.