Digestive system

Nutrition

In the sum of all those activities which are concerned with ingestion, digestion, absorption of digested food into blood or lymph, oxidation of simple food to produce energy for growth, repair, synthesis of bio molecules & egestion.

Types of Nutrition

On the basis of their method of food procurement, the living organisms show two modes of nutrition.

A.    Autotrophic Nutrition: In this, organisms synthesize the organic compounds from inorganic compounds themselves. Autotrophic organisms may be Phototrophic or Chemosynthetic in nature. Former ones, use CO₂ & water in presence of sunlight & Chlorophyll pigment to manufacture their food. E g. plants. Latter manufacture their flood in the presence of energy derived from the oxidation of simple inorganic compounds of iron, sulphur, etc. eg. Nitrifying bacteria, etc.

B.    Heterotrophic Nutrition: In this, readymade organic food in take by organisms as their can’t synthesis the food themselves.

The process of nutrition involves following steps:

1.    Ingestion: Intaking of food.

2.    Digestion: Breaking of complex food components into simple soluble components.

3.    Absorption: Passing of digested food through the wall of small intestine into the blood and lymph.

4.    Assimilation: Use of simple food components in the synthesis of complex components in different body cells.

5.    Excretion: Elimination of undigested food as faeces.

HUMAN DIGESTIVE SYSTEM

Digestive System of man consists of two parts:

A.    Alimentary Canal                                                       

B.    Digestive Glands

Alimentary Canal

It’s a long sized (8 – 10 metres in length) tube of varying diameter and is complete. Its formed of following parts:

1.    Mouth          

2.    Buccophrangeal Cavity : Its divisible into three parts.

a)    Vestibule: Space bounded by lips & cheeks externally & gums & teeth internally. Its for temporary storage of food.

b)    Buccal (Oral) Cavity: It’s the space bounded by palate above & laterally by joins having teeth.

(c)   Pharynx: Its small sized conical part where food and air passages cross each others.

3.    Oesophagus: It a tubular structure running downward through the neck behind the trachea, passes through diaphragm & opens in the stomach in the abdomen. Its inner mucosa is raised into longitudinal folds, oesophageal Rugae, which expand to receive food-bolus.It conducts food to stomach by peristalsis.

4.    Stomach: It’s the most distensible portion of alimentary canal.

5.    Small Intestine: It is the longest part of alimentary canal. Its divided into 3 parts.

        -       Duodenum: It is widest, shortest & most flexed part of small intestine. Its receives bile pancreatic duct.

        -       Jejunum: It is the middle part.

        -       Ileum: It is the longest part of small intestine. It finally opens in the caecum in the lower part of abdominal cavity. The opening is guarded by an flood area value which prevents regurgitation of food from caecum. Both ileum of jejunum are greatly coiled.

6.    Large Intestine: It is shorter but wider than small intestine consisting of 3 parts.

(a)   Caecum: It’s a small pouch like structure which ends into a tubular structure called Vermiform Appendix. Caecum & Vermi form Appendix are vestigial organs.

(b)   Colon: It’s a long, sacculated structure which is differentiated into four regions: Ascending Colon; Transverse Colon, Descending Colon & sigmoid or Pelvic Colon which is S-shaped & enters pelvis & joins the rectum.

        Its is concerned with conservation of water, sodium or other mineral and formation of faeces.

(c)   Rectum: Its concerned with temporary storage of faeces. It leads the anal canal which opens to out by anus present at the base of trunk & guarded by 2 Anal Sphincter Muscles.

DIGESTIVE GLANDS

Digestive Glands are those glands which secrete digestive juices for the digestion of food. These are of following types:

1.    Salivary Glands: These are 3 pairs of glands which secrete saliva. The saliva is carried to buccal cavity by the salivary ducts. These are of 3 types:-

2.    Gastric Glands: these are present in mucosa of stomach. These are of 3 types: Cardiac glands (secrete an alkaline mucus), Pyloric glands (secrete alkaline mucus) & feindic glands. Latter has 4 types of cells.

-       Chief/Peptic/Zymogen Cells: These secrete 2 proenzymes: Pepsinogen & Prorennin & an enzyme gastric Lipase.

-       Oxyntic/Parietal Cells:- Secrete HCL.

-       Goblet Cells: Secrete Mucus.

-       Argentaffin Cells: Secrete hormone: GASTRI.

        All these juice are collectively form gastric juices. Daily secretion is 2-3 litres. They work at 1.2 – 1.8 pH. Secretion of gastric juices is under nervous as well as hormonal control.

3.    Liver: Its is largest sized, reddish brown gland of body. Its present in posterion concavity of diaphragm.

Gall Bladder stores bile secreted by liver. Bile is drained from liver by a Bile duct which is formed by joining of Cystic duct from Gall Bladder & a Common, Hepatic duct from different Liver Lobes.

4.    Pancreas: Pancreas is a heterocrine gland. Its oxocrine part, secrets PANCREATIC JUICE. Latter is drained by pancreatic duct which joins bile duct.

5.    Intestinal Glands: These are numerous, microscopic glands present in mucosa of smell intestine.

        Besides these above five types of glands, whole mucosa of alimentary canal is lived by goblet cells. These secrete mucus which keeps alimentary canal lubricant helping in resistless & protecting put mucosa from injury.

Digestion and absorption of proteins

Digestion of proteins in initiated in the stomach by the action of the enzyme pepsin. Pepsin is secreted by the chief cells of stomach is an inactive form, pepsinogen. Pepsinogen is converted into pepsin in the presence of H+ which is provided by HCl. HCl is secreted by the parietal cells. Pepsin attacks the peptide linkages adjacent to the aromatic amino acids and breaks the protein into proteoses, peptones, and polypeptides.

The milk of the diet is clotted by another gastric enzyme known as rennin; the coagulated caesin (a milk protein) is then acted upon by the pepsin. Rennin is found in large quantities in infants.

When the semi digested proteins (proteoses, peptones, and polypeptides) reach the small intestine they are acted upon by trypsin and Chymotrypsin, which are simultaneously poured from pancreas and broken into small fragments. The polypeptides thus formed are finally hydrolysed by peptidases, which break the terminal bonds and release the individual amino acids. Peptidases are secreted by intestine and pancreas.

At the end of protein digestion although most of the protein are converted to amino acids, a part is left in the form of small polypeptides.

The free amino acids are absorbed both by diffusion at transport. From the absorptive cells of the intestine. The amino acids enter the blood from where these go to the liver. In certain cases, some of the unchanged proteins may act as antigens for producing antibodies. When sufficient quantities of antibodies are formed further absorption of protein produces a reaction in the form of ashes or asthmatic conditions. An individual is then said to be allergic to a particular protein.

Digestion and Absorption of Fats

The digestion of fat is also a hydrolytic process in which the lipids are broken into fatty acids, glycerol and glycerides. Fats are digested by the enzyme lipase secreted in the stomach, pancreas, and intestine. There is no bulk digestion of fat. Small part of the fat is digested in the stomach with the help of gastric lipase. But most of the fat digestion takes place in the small intestine.

When the fat enters the small intestine they are acted upon by the bile salts simultaneously released from liver, which have a detergent action, they emulsify the fat into small globulins known as micelles which offer a greater surface area for lipases to act, and are further degraded by intestinal and pancreatic lipases into the product of fat digestion.

Digestion and Absorption of Carbohydrates

In carbohydrate digestion the complex carbohydrate molecules are broken down into monosaccharides by the addition of water. This process is known as hydrolysis.

The first step, in the digestion of starch is its conversion to maltose with the help of amylase; a large amount (40%) is digested in the mouth by salivary amylase. When the ‘chyme’ enters  the stomach, part of the undigested starch is hydrolysed by the HCl present in the gastric juice. The total digestion is accomplished in the intestine, where it is exposed both to pancreatic and intestinal amylase.

The other carbohydrates that are the disaccharide sucrose, lactose, and maltose are digested in the intestine by the enzymes Invertase. b-glucosidose, and a-galactosidase respectively. Evidently glucose is the most abundant end product (80%) of carbohydrate digestion. The other are 10% galactose, and 10% fructose. The enzymes responsible for starch digestion are intracellular enzymes released when mucosal cells break apart.

The digested carbohydrates are absorbed through the intestinal villi. There are two mechanisms of absorption in the intestine i.e. diffusion and active absorption. Fructose is transported by simple diffusion in response to a concentration gradient by the absorptive cells of the villi. Glucose and galactose are actively transported. Of the three monosaccharides, glucose is immediately transported throughout the body fluids.