Pharynx, Oesophagus and Stomach
Protein digestion in stomach1. Protein digestion begins in the stomach by gastric juice.
2. It contains hydrochloric acid, pepsinogen and prorennin. The H ions present in HCl converts pepsinogen into pepsin.
3. In the presence of HCl, the pepsinogen and prorennin is converted into their active forms, i.e., pepsin and rennin respectively.
4. Rennin acts on the casein protein of milk and converts it into paracasein, which in the presence of calcium ions forms calcium paracaseinate (curdling of milk).
5. The function of rennin is then taken over by pepsin and other milk-coagulating enzymes.
MouthMouth is also called the oral or buccal cavity.
Food is chewed (broken down into smaller pieces) by teeth and swallowed by the tongue.
Carbohydrates, such as starch, are broken down or digested to form sugar.
Hence, digestion starts from the mouth.
PharynxThe pharynx is the part of the digestive system which acts as a common pathway for air and food. It receives food from the mouth. The food/bolus is carried to the stomach via esophagus (food pipe).The tongue along with the soft palate pushes the food forward into the pharynx.The pharynx closes off the trachea with the help of a cartilaginous flap called epiglottis, during swallowing. Thus, the bolus enters the esophagus.
OesophagusThe esophagus or the food pipe is a muscular tube and has the following features:
- It starts from the pharynx to the stomach. The pharynx is present behind the trachea.
- Food/bolus is pushed through the esophagus into the stomach by means of a series of contractions called peristalsis. These contractions occur in the muscles in its wall.
- There is no digestion in this part.
- It is also called as gullet.
- The opening of the esophagus into the stomach is regulated by a muscular sphincter called gastro-esophageal sphincter.
StomachStructure of the stomach
It is a highly muscular, J-shaped organ.
It is located in the upper left portion of the abdominal cavity.
The stomach has three parts:
The opening of the stomach into the duodenum is guarded by the pyloric sphincter.
- Cardiac portion: The esophagus opens here.
- Fundic region
- Pyloric portion: It opens in the first part of small intestine.
The innermost layer lining the lumen is the mucosa. This layer forms irregular folds called rugae in the stomach.
Mucosa form gastric glands in the stomach.
Function of the stomach
The gastric glands present in its walls secrete gastric juice containing hydrochloric acid (HCl) and enzymes, like pepsinogen.
HCl activates pepsinogen into pepsin and kills bacteria.
Proteins are broken into smaller fragments called peptones by the enzyme called pepsin.
It eventually leads to the formation of chyme, which is moved to the intestine through pylorus.