Sophie Bennett
Chickens have a variety of organs that work together to support their digestive, urinary, and reproductive systems. The digestive tract of a chicken includes several important organs, such as the proventriculus (true stomach), gizzard, small intestine, and cloaca. The gizzard, a highly muscular organ, is particularly interesting for its role in grinding and crushing ingested food, especially tough substances like seeds and grains. This organ is found not only in chickens but also in various other animals, including birds, fish, and crustaceans. The small intestine, which is relatively long and has a constant diameter, is where much of the food digestion and nutrient absorption occurs. The cloaca is where digestive wastes mix with urinary wastes, and it also serves as the exit point for the reproductive tract. Understanding the unique characteristics and functions of these organs provides insights into the adaptations and biology of chickens.
| Characteristics | Values |
|---|---|
| Organ responsible for grinding food | Gizzard |
| Location of gizzard | Between the proventriculus (or glandular stomach) and the intestine |
| Function of gizzard | Grinds and crushes ingested food, especially tough materials like seeds and grains |
| Gizzard structure | Thick, muscular walls with a multi-layered membrane made of koilin |
| Gizzard membrane function | Protects the muscles of the gizzard and aids in the mechanical breakdown of food |
| Glandular stomach | Proventriculus |
| Proventriculus function | Secrete digestive enzymes, hydrochloric acid, and hormones like gastrin |
| Glandular stomach alternative name | True stomach |
| Gizzard alternative names | Ventriculus, gastric mill, gigerium |
| Gizzard in other animals | Found in archosaurs (birds, crocodiles, alligators, pterosaurs), earthworms, some fish, crustaceans, and molluscs |
| Gizzard as food | Popular in grilled, fried, and soup dishes in various cuisines |
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What You'll Learn
The digestive system
The chicken's digestive system is highly efficient, and, despite their lack of teeth, chickens are omnivores, eating vegetation, insects, and even small mammals. The chicken's digestive tract, or gastrointestinal tract (GI tract), begins at the mouth, or beak, and ends at the cloaca.
Food is moistened by saliva, which contains enzymes such as amylase, to begin the process of digestion. The food then moves from the mouth into the oesophagus, where it is transported to the crop, a pouch that acts as an expandable storage compartment. The crop can hold food for up to 12 hours before it trickles into the stomach.
The stomach, or proventriculus, is where the first major enzyme activity takes place. Hydrochloric acid and digestive enzymes, such as pepsin, are added to the food, breaking it down. The food then moves to the gizzard, a muscular part of the stomach, which grinds the food into smaller particles, aided by small particles of stone or grit.
From the gizzard, food passes into the small intestine, where the majority of digestion and nutrient absorption takes place. The small intestine is composed of three parts: the duodenum, jejunum, and ileum. The duodenum is the only section that can be easily distinguished in the fowl. The bile ducts from the gallbladder, attached to the liver, and the pancreatic ducts, enter the small intestine at the caudal end of the duodenum. The pancreas is a vital organ in the digestion of food.
After the small intestine, the food moves to the large intestine, where water is absorbed, and indigestible foods are dried out. The remaining residue passes through the cloaca, where digestive waste mixes with waste from the urinary system. This waste exits the chicken at the vent, the external opening of the cloaca.
The chicken's digestive system is also home to beneficial bacteria, yeast, and other microflora, which aid in digestion. However, an upset in the balance of this microflora can lead to intestinal diseases such as enteritis or necrotic enteritis, resulting in diarrhoea, dehydration, and weight loss.
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The female reproductive system
The ovary is well-endowed with blood vessels to ensure there is no hindrance to the transport of nutrients to the developing yolk. The yolks remain in the ovary until they are released into the oviduct, which is when ovulation occurs. The oviduct is a twisted tube that is 25 to 27 inches long when fully developed and is divided into five major sections: the infundibulum, magnum, isthmus, shell gland, and vagina. The first part of the oviduct, the infundibulum, is a funnel that engulfs the yolk released from the ovary. The yolk remains in the infundibulum for 15 to 17 minutes, during which fertilization takes place if the ovum is to be fertilized.
In the oviduct, glands secrete substances that form other parts of the egg, such as the albumen (egg white) and the shell. The total time taken for a hen’s body to transform a yolk into a fully developed egg and lay that egg is about 25 to 26 hours. Typically, about 30 to 75 minutes after a hen lays an egg, the ovary releases the next yolk. The female reproductive system also produces hormones such as androgen, oestrogen, and progesterone, which aid in controlling body functions.
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The male reproductive system
Flock fertility is dependent on the reproductive status of the chickens, which is the level of egg and semen production, combined with the chickens' interest and capability of mating. Fertility in both male and female chickens tends to decrease as they get older. In males, this is due to a decline in sperm quality and mating activity.
During mating, the male climbs on the back of the female, placing a foot on each wing, forcing the tail feathers upwards to press their cloacas together in what is called a "cloacal kiss". In these few seconds, sperm is transferred from the male cloaca to the cloaca of the female.
Caponization is a process that can be performed on male chickens to improve the quality of their meat as they age. Caponized males grow more slowly than intact male chickens and accumulate more body fat, resulting in meat that is more tender and juicy.
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The muscular system
The gizzard is a highly muscular organ in chickens that is used for grinding up food. It is located between the proventriculus (the glandular stomach) and the intestine. Its primary role is to mechanically break down food, particularly tough materials such as seeds and grains, through powerful muscular contractions. This is essential because chickens do not have teeth to chew their food. The gizzard also works in conjunction with grit (small, hard particles) that the chicken ingests, which assists in the grinding process.
The alimentary canal is a long tube-like organ that starts at the beak and ends with the vent or cloaca in the abdominal region. It generally has layers of muscle that run lengthwise and around it and is lined with mucous membranes. Glands that produce important digestive juices are found in different locations along the canal. The nutrients from the food are absorbed through the wall of the alimentary canal into the circulatory system for transport to the liver or other parts of the body.
The biceps brachii, triceps brachii, deltoideus, superficial pectoral (pectoralis major), and supracoracoideus (pectoralis minor) are some of the muscles that help with wing movement. The semimembranosus muscle is the deepest and innermost of the hamstring muscles, found on the inner side of the back of the thigh. It extends the thigh and flexes the knee. The adductor longus is one of the main muscles that receive nerves from the lumbar plexus and helps maintain balance.
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The skeletal system
Chicken bones are homologous to bones found in other animals, but they have some key adaptations for flight. These include pneumatic bones, which are somewhat hollow as the air sacs of the respiratory system connect with the cavity of some of the long bones. The prefix "pneu" implies the ability to contain air. Other bones include the skull, humerus (upper wing), clavicle (collar bone), keel (breastbone), pelvic girdle (hip bones), and lumbar/sacral vertebrae (backbones). The crisscrossing of struts or trusses in these bones makes for a strong but light framework on which to attach the muscles of flight.
The sternum (breastbone or keel) is a noticeable feature of the chicken skeleton, providing a large surface area for the attachment of the main flight muscles. The keel is cartilaginous in young poultry and ossifies to harder bone as the bird ages. There are seven pairs of ribs that originate from the thoracic vertebrae, forming the rib cage. All but the first and second pairs attach to the sternum in a unique fashion, with a strong uncinate process to avoid collapse of the thoracic cavity during flight.
The chicken skeleton also has a fusion of some vertebral sections to provide the rigidity required for flight. The size of the head has been reduced over time, as a large head would be a hindrance when flying. The neck is quite long in most bird species to protect the delicate tissues of the brain from too much jarring when landing.
The chicken's skeleton is a major reservoir of calcium and phosphorus, with 99% of body calcium and 80% of body phosphorus stored in the skeleton. Young chickens are very sensitive to vitamin D deficiency, which can lead to a calcium deficiency and result in rickets. Vitamin D is important for the assimilation and use of calcium, especially in layer hens where eggshell formation demands a lot of calcium. Most bones of the fowl go through a cartilaginous stage, with some bones, such as the skull, omitting this stage. The ossification process hardens the cartilage when the bone takes up minerals, mainly calcium carbonate. Long bones increase in length through this process and are usually hollow, filled with bone marrow and extensions of the air sacs.
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Frequently asked questions
The alimentary and reproductive tracts are the most important organ systems in a chicken.
Organs in the alimentary tract include the crop, proventriculus, gizzard, small intestine, ceca, and cloaca.
Organs in the reproductive tract include the ovary, oviduct, testes, and cloaca.
The gizzard is a muscular part of the digestive system that contains small stones (grit) to help macerate grain and other large food particles.
The cloaca is a combined intestinal and sexual opening. During mating, the cloaca of both chickens touch briefly, allowing the transfer of sperm from the male to the female.
