The Anatomy and Physiology of the Chicken

The digestive system of the chicken starts at the mouth and ends at the cloaca, with multiple intermediate organs. Each organ in the digestive system plays a critical role and though a chicken may be able to survive with the damage or loss of certain organs, the resulting effects of such damage or loss could be fatal.
Among chicken illnesses, the most common ailments tend to take root in the digestive system of the bird. Included wuold be cocci, impacted crop, sour crop, impacted gizzard, intestinal parasites, and blockages of the intestine.
Seeing as a chicken's energy comes from food and water and the digestion of nutrients transported in those forms, it is imperative to understand the structure and functions of the entire digestive system. Knowing what is normal in regards to the digestive system should help cultivate your understanding of the nutritional requirements of your chicken as well as when something is wrong with the digestive tract -hopefully before it is too late.

Diagram 4 shows the illustrated digestive system of a chicken


Whether a chicken forages for their food or is fed a complete diet, the bird still has to ingest and digest the feed. This process takes a while and the food travels through many organs, changes shape and texture and is utilized differently in different sections of the digestive tract.

The digestive tract begins with the beak, but digestion begins with the mouth. Although chickens pick up feed with their beaks, the feed enters the digestive system through the mouth. The mouth is made up of glands and enzymes. The glands in the mouth secrete saliva, which contains the digestive enzyme amylase that starts to digest the feed upon contact. The tongue's function is to push the feed to the back of the mouth so that it can be swallowed.
The feed then enters the esophagus -a flexible tube which connects the mouth with the rest of the digestive tract. The esophagus is long and is present in two transitional areas -from the mouth to the crop, and from the crop to the proventriculus. The function of the esophagus is to transport food between organs in the first half of the digestive system.
Next, the feed reaches the crop, where it is stored for up to 12 hours. The crop is located on the upper right side of the breast of the chicken. The crop can expand to store copious amounts of water and food. Little to no digestion takes place in the crop.
From the crop, the feed travels again through the esophagus into the proventriculus, the glandular stomach where digestion begins. Hydrochloric acid and the digestive enzyme Pepsin are added to the feed in the proventriculus and further breakdown of the feed takes place.
The feed and digestive juices then pass from the proventriculus to the gizzard, which is the mechanical stomach. Here, the food is crushed, mixed and mashed. The gizzard is made up of two sets of strong muscles. These muscles serve the same function that teeth serve us and other mammals who have them. Stored in the gizzard are small insoluble stones, or grit, that aid in the mechanical breakdown of feed.
The now mushy feed passes from the gizzard into the small intestine. The small intestine's function is to absorb the large majority of the nutrients that are useful to the bird. Nutrients such as calcium, lipids, carbohydrates, sugars, potassium, sodium, vitamins and minerals diffuse into the surrounding capillaries. Some water absorption also takes place in the small intestine. The small intestine is made up of the duodenum, and the lower small intestine. The duodenum receives digestive enzymes and bicarbonate from the pancreas and bile from the gall bladder. Protein digestion is possible with the digestive enzymes produced by the pancreas. The pancreas also serves to regulate the bird's blood sugar by the secretion of insulin and glycogen which raise and lower blood sugar accordingly. Bile is important to the digestion of lipids and the absorption of fat soluble vitamins A, D, E and K. The remainder of the digestion occurs primarily in the duodenum. The remaining nutrients are absorbed mostly in the jejunum and ileum. Remaining nutrients and their juices are then stored in the ceca, to be deposited later.
The large intestine is the next part of the digestive tract and has little to nothing to do with nutrient absorption. By this point, almost all of the nutrients are absorbed. The large intestine's role is to reabsorb the large amounts of water in the fecal matter, drying up the fecal matter and working to prevent dehydration in the chicken. Urates join the fecal matter in the Cloaca and both urates and fecal matter are deposited at once and removed from the digestive tract.

A comprehensive video, produced and owned by Nutrena, of the chicken's digestive tract is posted below for your learning convenience.

The next page will discuss the Respiratory System.​