Digestion and Absorption neet notes

 The digestive system

• The digestive system includes the gastrointestinal tract (mouth to anus) and the glandular organs.
• This system serves to transfer organic molecules, salts and water from the external environment to the body’s internal environment. 
• Most of the food taken into the mouth are large particles containing macromolecules such as polysaccharides and proteins. 
• As such they cannot be absorbed by the intestinal wall. They must be dissolved and broken down into much smaller molecules. 
• This process is named digestion. Digestion is accomplished by substances called enzymes produced from the digestive glands. 
• The enzymes are biocatalysts in the food breakdown process.

The process of digestion 

Mouth

• In the mouth, digestion starts with chewing. It breaks up large pieces of food into smaller particles that can be swallowed without choking. 
• It is accomplished by teeth, tongue, jaws and saliva. Chewing is controlled by the somatic nerves to the skeletal muscles of the mouth and jaw. 
• Rhythmic chewing motions are reflexly activated by the pressure of food against the gums, hard palate at the roof of the mouth and tongue.

Saliva

• The saliva is secreted by three pairs of exocrine glands, namely : the parotid, the submandibular and the sublingual. 
• The daily secretion of saliva ranges from 1000 to 1500ml. It contains the organic substances amylase and mucin. 
• Salivary amylase or ptyalin can act on starch. It converts cooked starch into the disaccharide, maltose. Mucin is a glycoprotein.
•  It helps in the lubrication of food. 
• The lubricated, swallowable form of food is called the bolus. The salivary secretion is controlled by reflex activities.

Swallowing

• It is complex reflex activity. It is controlled by the swallowing centre in the medulla oblongata. During swallowing the soft palate is elevated, the larynx gets raised. 
• The tongue forces the food back into the pharynx, the epiglottis closes the glottis and the food slowly passes into the oesophagus.
• The oesophageal phase begins with relaxation of the upper oesophageal sphincter. In the oesophagus, the food is moved towards the stomach by a progressive wave of muscle contractions that proceed down to the stomach. 
• Such waves of contraction in the muscle layer surrounding a tube are known as peristaltic waves. In the oesophagus one peristaltic wave takes about 9 seconds to reach the stomach. 
• Due to peristaltic waves, swallowing can occur even while a person is upside down.

The stomach 

• It is a wide chamber, located below the diaphragm. The size and shape of the stomach depending on the food inside it. The stomach volume during feeding may increase up to 1.5 lit. 
• The stomach’s primary contractile action will produce peristaltic waves. Each wave begins in the body of the stomach and proceeds towards the pyloric region. 
• The initial wave influences the muscles to close the pyloric sphincter, a ring of smooth muscles between the stomach and the duodenum.
• The inner wall of the stomach is lined with gastric glands. There are nearly 40 million glands engaged in producing gastric juice. 
• The chief cells of the lining of the stomach secrete enzymes and the parietal cells (Oxyntic cells) produce HCl to create an acidic medium for enzymes.
•  The enzymes of the stomach are pepsin and rennin. Pepsin is secreted in an inactive precursor form known as pepsinogen.
•  The activity due to HCl converts pepsinogen into pepsin. Pepsin hydrolyses the proteins into short polypeptide chains and peptones. It is most effective in an acidic environment.
• Rennin acts on soluble milk protein caesinogen and converts it into insoluble casein. 
• In the presence of calcium ions, casein is precipitated as insoluble calcium-casein compound (curds). 
• Repeated peristaltic waves in the stomach help to soften the food. The frequency of contraction is determined by the basic electrical rhythm and remains essentially constant. 
• It is also aided by neural and hormonal influences. The food leaves the stomach in the form of chyme and enters the upper small intestine at periodic intervals.

The small intestine

• It is about 5-7 metres long. It is divided into three segments namely the initial short segment the duodenum, the jejunum and the longest segment the ileum.
• The food is propelled down into the duodenum due to the peristaltic the action of the stomach wall. 
• The pyloric sphincter located at the junction of the stomach and duodenum regulates the movement of the chyme.
• The food in the small intestine is mixed with three juices namely bile juice, pancreatic juice and intestinal juice.

Bile juice

• It is a brownish-green, alkaline secretion of the liver. It is stored in the gall bladder and poured into the duodenum via the bile duct. 
• The bile contains water, mucus, inorganic salts, cholesterol and bile salts. The bile salts emulsify fats and help enzymes like lipase to act upon fats. 
• During emulsification, the bile salts convert bigger fat particles into smaller fat globules.

Pancreatic juice

• It is an alkaline fluid (pH 7 to 8). It is transported to the duodenum through the pancreatic duct. It contains water, mineral salts and a variety of enzymes like trypsin, chymotrypsin which are secreted in the form of inactive precursors trypsinogen and chymotrypsinogen. 
• The precursors are activated by enterokinase of the intestinal juice, the amylytic enzyme amylase, the pancreatic lipase (steapesin), carboxypeptidase and nuclease. The enzyme trypsin hydrolyses proteins into polypeptides and peptones.
• Chymotrypsin hydrolyses peptide bonds associated with specific amino acids like phenylalanine, tyrosine or tryptophan. It results in large peptides.
• Carboxypeptidase is an exopeptidase. It attacks the peptide bonds at the carboxyl end of the polypeptide chain resulting in di-, tripeptides and amino acids
• . The pancreatic amylase converts starch into maltose. The lipase acts on emulsified fat (triglycerides) and hydrolyses them into free fatty acids and monoglycerides. 
• Monoglycerides may be further hydrolysed to fatty acid and glycerol.

Intestinal juice: (Succus entericus) 

Absorption and assimilation 

• As a result of digestion, all macromolecules of food are converted into their corresponding monomeric units. 
• Carbohydrates are broken into monosaccharides such as glucose and fructose. Proteins are hydrolysed into amino acids. Lipids get broken into glycerol and fatty acids. 
• The simpler organic molecules along with minerals, vitamins and water enter into body fluids through the villi. 
• The villi are small microscopic finger-like projections. Each villus is an absorbing unit consisting of a lacteal duct in the middle surrounded by a fine network of blood capillaries. 
• While the fatty acids are absorbed by the lymph duct, other materials are absorbed either actively or passively by the capillaries of the villi. 
• From the lumen of the alimentary canal absorbed food materials are carried to the liver through the hepatic portal vein. 
• From the liver, materials are transported to all other regions of the body for utilisation. This conversion of food into energy and cellular organisation is called assimilation.