The oesophagus is a tube that carries the feed from the mouth to the stomach. A series of muscle contractions push the feed towards the stomach. Swallowing is the first of these contractions. At the end of the oesophagus is the cardiac valve, which prevents feed from passing from the stomach back into the oesophagus.
The Stomach
The pig has a simple stomach with a capacity of about 8 litres, which functions not only as an organ for the digestion of food but also for storage. The monogastric stomach of a pig is similar to the abomasum, the fourth and final stomach compartment of ruminants.
The stomach is a reaction chamber where chemicals are added to the feed. Once in the stomach, the food is mixed and moistened with digestive enzymes. The ingested food enters the stomach through the oesophageal opening (cardia) and mixing takes place through contractions of the stomach wall. During this period of mixing the pylorus (the opening from the stomach into the duodenum) remains closed, but as soon as sufficient digestive juices have been secreted and digestion has advanced far enough, the pylorus opens and small quantities of stomach content (or chyme) is forced into the duodenum (the first part of the small intestine)
Gastric glands in the mucosal lining of the stomach (i.e., certain cells along the stomach wall) secrete certain substances collectively called gastric juice. Gastric juice is a clear fluid which consists of water, mucus, salts, hydrochloric acid (HCl) and enzymes (renin and pepsinogen). The hydrochloric acid and enzymes help digest feed into smaller particles of carbohydrates, protein and fats.
Gastric glands include:
Cardiac and pyloric glands.
They secrete mucus to protect the stomach lining against hydrochloric acid.
Fundic glands
They secrete hydrochloric acid, pepsinogen and renin.
Functions of hydrochloric acid in the stomach:
- It activates and converts the enzyme pepsinogen to the enzyme pepsin.
- It neutralizes the alkaline reaction of the saliva.
- It supplies the acid medium which is required for the optimal functioning of the enzyme’s renin and pepsin.
- It converts sucrose to glucose.
- It has an antiseptic action and therefore aids in the prevention of putrefaction (the process of decay or rotting) in the stomach
Some of the digested particles are absorbed from the stomach into the bloodstream. Other particles which the stomach cannot absorb, pass on to the small intestine through the pyloric valve.
Carbohydrate digesting enzymes (amylase) from the pancreas and intestinal mucosa (sucrase, maltase and lactase) complete carbohydrate digestion in the stomach.
Protein-digesting enzymes (pepsin) and hydrochloric acid in the stomach begin significant protein digestion in the stomach. protein digestion is completed in the small intestine. As mentioned before, pepsinogen is first changed to pepsin, which then reacts on the proteins in the stomach content and partly digests them into simpler substances called peptones.
In young nursing piglets, the enzyme rennin coagulates (solidify or semi-solid state) milk to allow for more complete digestion. The curdling (or coagulation) of milk occurs when the soluble caseinogen of milk is changed to insoluble casein. Renin plays a very important role in the stomachs of young suckling animals
A fat-digesting enzyme (lipase) begins fat digestion in the stomach. Fat digestion is completed in the small intestine.
The monogastric stomach
