DIGESTION IN THE STOMACH

 

Middle part of the gut showing the stomach and duodenum

              

The site at which the oesophagus and empties joins and empties into the stomach is called cardiac sphincter. On the peristaltic reflex, the cardiac sphincter relaxes and allows the food to enter into the stomach. The walls of the stomach have three layers of muscles which are responsible for grinding or churning action of the stomach. The inner lining of the stomach consists of tubular gastric glands which secrete gastric juice. The gastric juice is poured into the stomach cavity by means of small ducts.

 

GASTRIC JUICE

              The gastric juice is a mixture of serval substance, mainly water, mucin, pepsin and hydrochloric acid.

 

MUCIN

       It forms a coating over the stomach lining. It acts as a buffer, neutralizes the acid and inhibits the enzymatic action of pepsin. Thus it prevents digestion of stomach wall which is also composed of protein. Mucin coating is steadily renewed and only slowly digested. Moreover when enzymes are secreted, they are in an inactive from.

 

PEPSIN

     It is an enzyme secreted in an inactive from called pepsinogen by special glandular cells. Pepsinogen is activated o pepsin when exposed to the acid medium of the stomach or to some already activated pepsin. Pepsin hydrolyes protiens to form smaller pieces or polypeptides and peptides.

 

HYDROCHLORIC ACID

       It is the most unusual component of the gastric juice. It is produced in concentrated from having a pH of about 1.3. It provides acidic conditions necessary for the activation of pepsinogen to pepsin and enables the pepsin to work. The very low pH of the acid is also responsible for the precipitation of many soluble proteins. The strong acid also functions to kill many micro-organisms.

 

RENIN

      This enzyme is present in the gastric juice of many mammals in infant stage but is hardly distinguishable from pepsin in the human infants. It helps to precipitate milk proteins.

   Due to the mechanical and biochemical processes of the stomach, food is changed into a nearly homogeneous semi-fluid called chyme. Stay time of food in the stomach depends upon the consistency and quality of the food. fluid pass on in two minutes to be converted to chyme while spicy, fatty and rich food may take 3-4 hours.

 

CAUSES FOR THE SECRETION OF GASTRIC JUICE

      Gastric juice is secreted due to chemical and nervous stimulation.

SMELL AND SIGHT OF FOOD

       When some body is hungry, the smell and sight of the food may cause small amount of gastric juice to be secreted just like the watering of the mouth.

PRESENCE OF FOOD IN THE MOUTH

      When the food is, in the oral cavity, undergoing the process of mastication, the nerves in the cheeks and tongue are stimulated and impulses are carried to the brain. Then brain sends massage to stimulate the gastric glands in the stomach wall to secrete a small amount of gastric juice before the food arrives.

PRESENCE OF FOOD IN THE STOMACH

      When food touches the lining of the stomach more gastric juice is poured depending upon the composition of food. when only starch food is eaten relatively little amount of gastric juice is secreted while for meat and proteinaceous food, a larger amount of gastric juice is secreted.

GASTRIN

      Small protein molecules some of the cells of the stomach lining which secreted a hormone known as gastrin. The gastrin diffuse into blood and is carried by the capillaries back to the stomach where it stimulates the gastric glands to secrete more gastric juice.