Digestion And Absorption

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Human digestive system

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1. The human digestive system helps in digestion of food and absorption of nutrients.
2. The alimentary canal is divided into buccal cavity, food pipe, stomach, accessory organs, small intestine, large intestine, and anus.
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Gastrin

Gastrin is released from the mucosa of pyloric (stomach) and targets the stomach. It is stimulated by the distension (enlarged or swollen due to internal pressure) of the stomach on food entry. Its role is to:
1. Stimulate gastric glands to secrete and release gastric juice
2. Stimulate gastric mobility and HCl secretion
3. Constrict cardiac sphincter
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Teeth

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Teeth are the hardest substances in the human body. Besides being essential for chewing, the teeth play an important role in speech. There are different kinds of teeth. They are as follows:
1. Incisors
2. Canines
3. Premolars
4. Molars
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Accessory organs

The accessory organs of the digestive system are the Liver, Gall bladder, and Pancreas.
Liver:
1. Liver is an accessory organ. 
2. It is the largest gland of the body. 
3. Its main function is to produce bile (a digestive juice).
4. Bile helps in the digestion of fat and its elimination.
Gall Bladder:
1. Gall bladder is a small, pear-shaped organ. 
2. It stores and releases bile. 
3. When the food reaches the small intestine, the gallbladder contracts and releases bile.
Pancreas:
1. The pancreas secretes enzymes into the small intestine.
2. It helps in the chemical digestion of the food.
3. The enzymes secreted by the pancreas helps in breaking down proteins, fats and carbohydrates.
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Function of liver

Liver is the reddish brown organ located in the upper right side of the abdomen just below the diaphragm.
Function of liver are as follows:
1. Control of blood sugar levels
2. Control of amino acids levels
3. Synthesis of foetal red blood cells
4. Production of fibrinogen, prothrombin and heparin
5. Regulates blood volume
6. Destroys dead red blood cells
7. Stores vitamins
8. Detoxifies substances including drugs and alcohol
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Pancreas- Origin, location and structure

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The pancreas is a composite gland with both exocrine and endocrine glands.

Origin
1. Develops from the endoderm of the embryo.

Location
1. Located behind the stomach in the upper left abdomen.

Structure
1. The pancreas has an elongated and tapered structure that can be divided into 4 parts; head, neck, body and tail
2. The head lies in the curve of the duodenum where the stomach meets the first part of the small intestine.
3. The neck and the body extend and end in the tail which is close to the spleen.
4. The pancreas has both endocrine and exocrine glands. 
5. The endocrine part of the pancreas is small (is only 1-2% of pancreatic tissue) is made of 1-2 million Islets of Langerhans.
6. Islets of Langerhans are made of the -cells (25%) and the -cells (60%), the -cells (10%) and pancreatic polypeptide cells or the -cells.
7. -cells produce glucagon, the -cells produce insulin, -cells secrete the pancreatic polypeptide and the -cells secrete the somatostatin. function of the pancreatic polypeptide cells is not known
8. The exocrine part of the pancreas is made of acinar cells which secrete digestive enzymes into the duodenum. The pancreatic epithelial cells secrete bicarbonate 
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Carbohydrate digestion in small intestine

1. Digestion of most of the nutrients takes place in the duodenum under the action of various enzymes.
2. The pancreatic juice contains starch digesting enzyme called amylase which converts starch into maltose, isomaltose and dextrins.
3. Intestinal juice contains maltase, isomaltase, sucrase, lactase and dextrinase which act as follows:
Maltose = Glucose + Glucose
Isomaltose = Glucose + Glucose
Sucrose = Glucose + Fructose
Lactose = Glucose + Galactose
Dextrins = Glucose
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Protein digestion in small intestine

1. Pancreatic juice contains proenzymes trypsinogen, chymotrypsinogen and procarboxypeptidase and enzyme elastase.
2. The bile provides an alkaline medium for various reactions.
3. The intestinal juice contains aminopeptidases and dipeptidases, and enterokinase or enteropeptidase.  
4. Out of these enterokinase activates the trypsinogen. Aminopeptidase hydrolyses the terminal amino group from peptide bonds to release the last amino acid from the peptides thus making the peptide shorter. 
5. Dipeptidase acts on dipeptides to release the individual amino acids.
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Summary of absorption in different parts of alimentary canal

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Digestion of fats

1. Bile juice contains bile salts, which are secreted by the liver in the bile. Bile salts break down the bigger molecules of fat globules into smaller droplets by reducing the surface tension of fat droplets. This process is known as emulsification of fats.
2. Lipase is the enzyme that acts on emulsified fats. It is present both in the pancreatic juice and intestinal juice.
3. Lipase converts emulsified fats into diglycerides and monoglycerides releasing fatty acids at each step.
4. At the end of digestion, all fats are converted into fatty acids, glycerol and monoglycerides.
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Assimilation of protein, carbohydrate and fat

Proteins
1. Proteins are used for growth repair etc.
2. Excess amino acid is converted into glucose and then to fat.
3. Amino acids are also converted to glucose, during this process amino acids are deaminated.

Carbohydrates
1. Excess monosaccharides are stored as glycogen in liver and muscle cells.
2. Glucose is converted into fats and stored.

Fats
1. Fat is stored in the fat deposits of the body, such as subcutaneous layers, mesenteries etc.