Related organs

The liver secretes bile into the small intestine via the gallbladder as a reservoir. The pancreas secretes an isosmotic fluid containing bicarbonate and several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes, into the small intestine. Both these secretory organs aid in digestion.

Physiology

Digestion and excretion

Main article: Digestion

In the mouth, food is broken down mechanically by the teeth and tongue, and also broken down chemically to some extent by saliva. It then passes through the esophagus by means of peristalsis to the stomach. Here, the process of breakdown continues. This is still mostly mechanical, as relatively large parts of food (now called "bolus") are reduced to smaller portions. However, slight amounts of chemical processing also take place at this point, especially on protein, by the action of enzymes present in the stomach. The food then passes to the small intestine where further breakdown occurs, by enzymes and with the aid of bacteria, and the useful particles are absorbed into the bloodstream. The remaining particles pass through the large intestine and are ultimately expelled as feces.

Digestion is regulated both hormonally and by the autonomic nervous system:

• The major hormones that control the functions of the digestive system are produced and released by cells in the mucosa of the stomach and small intestine. These hormones, such as secretin, gastrin and cholecystokinin, are released into the blood by the digestive tract and stimulate digestive juices and cause organ movement.
• The two arms of the autonomic nervous system both influence the digestive process; parasympathetic nerves stimulate secretions and peristalsis while the sympathetic influence is more inhibitory.

Specialization of organs

Four organs are subject to specialization in the kingdom Animalia.

• The first organ is the tongue which is only present in the phylum Chordata.
• The second organ is the esophagus. The crop is an enlargement of the esophagus in birds, insects and other invertebrates that is used to store food temporarily.
• The third organ is the stomach. In addition to a glandular stomach (proventriculus), birds have a muscular "stomach" called the ventriculus or "gizzard." The gizzard is used to mechanically grind up food.
• The fourth organ is the large intestine. An outpouching of the large intestine called the cecum is present in non-ruminant herbivores such as rabbits. It aids in digestion of plant material such as cellulose.

Immune function

The gastrointestinal tract is also a prominent part of the immune system (Coico et al 2003). The low pH (ranging from 1 to 4) of the stomach is fatal for many microorganisms that enter it. Similarly, mucus (containing IgA antibodies) neutralize many of these microorganisms. Other factors in the GI tract help with immune function as well, including enzyme in the saliva and bile. Health enhancing intestinal bacteria serve to prevent the overgrowth of potentially harmful bacteria in the gut.