Sophie Bennett
The chicken digestive tract is an incredibly efficient system, allowing chickens to eat almost anything, from vegetation to meat. Food is ingested through the beak and travels down the oesophagus to the crop, where it is stored temporarily before moving to the proventriculus (true stomach) for chemical digestion. The proventriculus releases hydrochloric acid and digestive enzymes to break down food, which then passes to the gizzard, where physical grinding occurs. From here, food moves to the small intestine, where chemical digestion occurs through enzymes released by the pancreas and bile from the liver. The small intestine is the longest part of the digestive tract and is where most nutrients are absorbed.
| Characteristics | Values |
|---|---|
| Where chemical digestion occurs | The duodenum, a section of the small intestine |
| Length of digestive tract | 2 to 2.5m long |
| Time taken to pass through digestive tract | 4 to 8 hours |
| Where digestive, urinary and reproductive systems meet | Cloaca |
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What You'll Learn
The role of the proventriculus
The proventriculus, also known as the true stomach or glandular stomach, is a crucial part of a chicken's digestive system. It is a rod-shaped organ located between the oesophagus and the gizzard or ventriculus. The proventriculus is a glandular stomach that produces digestive enzymes and hydrochloric acid (HCl), which are essential for breaking down food particles.
The gastric glands in the proventriculus secrete both HCl and pepsinogen, which is then activated to pepsin by the HCl. Pepsin plays a critical role in breaking down peptide bonds in peptides and proteins. This secretion process is influenced by hormones such as gastrin, bombesin, avian pancreatic polypeptide, and cholecystokinin, which regulate the amount and concentration of the secretions. The proventriculus also contains an area called the proventriculus bulb, which includes hair-like or tooth-like structures that help filter food and facilitate digestion.
The proventriculus is an essential component of the avian compound stomach, and its function differs from that of insects and other invertebrates. In chickens, the proventriculus is responsible for commencing the digestion process by secreting digestive enzymes and HCl into the digestive compartments. This process softens the food, which is then passed to the gizzard for further grinding and mixing. The gizzard, or ventriculus, is composed of strong muscles that act as the bird's teeth, and it works in conjunction with the proventriculus to break down food mechanically and chemically.
The proventriculus is also associated with the liver and spleen, and it plays a role in regulating the passage of food into the digestive tract. The size and shape of the proventriculus can vary among different avian species, with carnivorous and piscivorous species having relatively larger proventriculi than granivorous species. The proventriculus is a unique organ in the avian digestive system, and its functions are distinct from those of mammals, which have separate cell types for producing HCl and pepsinogen.
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The small intestine and duodenum
The small intestine is the longest part of the chicken's gastrointestinal system. It is composed of the duodenum (or duodenal loop), the jejunum, and the ileum. The duodenum is the middle part of the small intestine, while the jejunum and ileum form the lower small intestine.
The remainder of digestion occurs in the duodenum, where digestive enzymes and bicarbonate from the pancreas, as well as bile from the liver, are received. The duodenum is also where most glucose and vitamins are absorbed. The jejunum is responsible for the absorption of amino acids, while fatty acids are absorbed in the distal part of the jejunum and the start of the ileum.
The small intestine is populated by beneficial organisms (bacteria, yeast, etc.), referred to as microflora, which aid in digestion. The presence of villi – long, flattened, finger-like projections that extend into the lumen (inside) of the intestine – increases the surface area of the intestinal wall, facilitating efficient nutrient absorption.
The duodenum receives secretions from the pancreas, which is essential in the process of digesting food. The pancreas is attached to each side of the duodenal loop and lies between the two arms. The bile ducts from the gallbladder, attached to the liver, enter the small intestine at the caudal end (closest to the rear) of the duodenum.
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The role of the gizzard
Chickens, like other birds, do not have teeth. The gizzard, a muscle in the digestive tract, therefore serves the purpose of grinding food down into smaller pieces. This grinding action is achieved through powerful muscle contractions, which, together with grit (small pieces of gravel, stones, or rocks), act as teeth to break down food into digestible particles. The gizzard is attached to the lower part of the true stomach (proventriculus) and the beginning of the small intestine.
The gizzard receives food from the true stomach, which is where digestion begins in earnest. In the true stomach, hydrochloric acid and digestive enzymes like pepsin are added to the food to break it down. However, it is in the gizzard that food is physically ground down. This grinding action allows nutrients to be absorbed into the chicken's body.
Grit is essential for the gizzard to function effectively. Chickens swallow grit, which lodges in the gizzard. When food arrives, the gizzard's muscles contract, squeezing the grit and pulverising the food. Commercial feeds are water-soluble, meaning they break down before reaching the gizzard, so additional grit is not required in these cases. However, when chickens consume whole foods, grit must be available to aid the grinding process.
Chicken gizzards are also consumed as food and are considered a delicacy in some parts of the world. They are often sold pre-cleaned, but if bought fresh, they need to be thoroughly rinsed to remove any remaining grit, dirt, or gravel. Gizzards have a flavour similar to dark chicken meat and a chewy texture, so they are typically cooked slowly over low heat to avoid toughness. They are a versatile ingredient, used in dishes like tacos, enchiladas, bolognese sauce, and fried rice, as well as in soups and chillies.
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The large intestine and ceca
The large intestine in chickens is shorter than the small intestine. It is where the last of the water reabsorption occurs. The large intestine ends in the cloaca, a tubular cavity that is common to the digestive and urogenital tracts. The cloaca is where digestive wastes mix with wastes from the urinary system (urates). Chickens do not urinate; instead, they void fecal material as digestive waste with uric acid crystals on the outer surface. The appearance of the faeces, including colour and texture, can indicate the health status of the chicken's digestive tract.
The ceca are blind pouches located where the small and large intestines meet. Birds have two ceca, which are about 7 inches (17 cm) long. Food from the small intestine is first stored in the ceca to reabsorb water. A fermentation process in the ceca breaks down crude fibres, and bacteria in the ceca produce several fatty acids and B vitamins, including thiamine, riboflavin, pantothenic acid, and vitamin B12. However, because the ceca are located so close to the end of the digestive tract, few of the produced nutrients are absorbed and available to the chicken. The contents of the ceca are discharged a couple of times a day as cecal droppings, which have a distinct smell and pasty consistency.
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The cloaca
- Copradaeum: A continuation of the colon-rectum.
- Urodaeum: The middle part, into which the ureters and genital ducts open.
- Proctodaeum: Opens to the exterior of the vent.
Birds under one year old have a dorsal opening leading to a blind, rounded sac called the bursa of fabricius. The cloaca is where digestive wastes mix with wastes from the urinary system (urates). Chickens do not urinate; instead, they void fecal material as digestive waste with uric acid crystals on the outer surface. The colour and texture of chicken fecal material can indicate the health status of the digestive tract. The white, pasty material coating chicken fecal matter is uric acid, the avian form of urine, and is normal. The contents of the caeca are also discharged periodically as discrete masses of brown, glutinous material.
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Frequently asked questions
Chemical digestion in chickens occurs in the small intestine, specifically in the duodenum, where enzymes are released by the pancreas and bile is released by the liver through the gallbladder.
The small intestine aids in the digestive process by reducing complex food compounds into simple compounds or building blocks that can be absorbed through the intestinal wall for transport to other parts of the body.
Food that is not digested by enzymes in the small intestine is subjected to bacterial breakdown in the ceca, where additional nutrients are extracted through fermentation.
