Claire Thompson
Chickens are fascinating creatures with a unique anatomy. Their internal organs are designed to support their ability to fly, lay eggs, and digest a variety of foods. The digestive and reproductive systems are of utmost importance when it comes to chicken anatomy. The alimentary canal, which includes the gastrointestinal tract, begins at the beak and ends at the cloaca, also known as the vent. The cloaca serves as the only opening for both the reproductive and digestive systems. Intestinal issues in chickens can be identified through wet feathers or caked faeces around the vent. The gizzard, an organ unique to birds, is essential for grinding and mixing food, as chickens do not have teeth. The small intestine, consisting of the duodenum, jejunum, and ileum, leads to the ileo-caecal junction, where bacterial colonisation occurs. The liver, spleen, and heart are also vital organs in the chicken's anatomy. Understanding the internal organs of chickens is crucial for recognising and addressing any health issues that may arise.
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What You'll Learn
The digestive system
The first part of the digestive tract is the alimentary canal, a long tube-like organ that starts at the beak and ends at the vent or cloaca. The alimentary canal is lined with mucous membranes and glands that produce digestive juices. Food is ingested and passes into the crop, an organ in the neck where grain and other food is stored. The food then moves to the proventriculus, where the entrance to the proventriculus and the exit to the duodenum are close together.
After the proventriculus is the gizzard, a muscular organ that grinds and mixes food with the help of small rocks or grit. The gizzard is followed by the small intestine, which is made up of three sections: the duodenum, jejunum, and ileum. The duodenum receives digestive enzymes and bicarbonate from the pancreas, as well as bile from the liver via the gall bladder. The bile is important for the digestion of lipids and the absorption of fat-soluble vitamins. The remainder of digestion occurs in the duodenum, and the released nutrients are absorbed mainly in the lower small intestine, which includes the jejunum and ileum.
The small intestine leads to the ileo-caecal junction, where the two large caeca are found. The caeca are emptied between 2-4 times a day, and the consistency and colour of the caecal contents can vary with age and diet. From the caeca, the digestive tract continues to the colon and ends at the cloaca, where waste is eliminated from the body.
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The reproductive system
The reproductive process begins with mating. Mating occurs when the male mounts the female and they have a 'cloacal kiss', in which the cloaca of both chickens touch briefly, allowing the transfer of sperm from the male to the female. The cloaca is a combined intestinal and sexual opening, situated under the tail.
The female reproductive system consists of two parts: the ovary and the oviduct. The ovary contains thousands of ova, which develop into the yolk and eventually an egg. When an ova reaches its final size, it breaks away from the ovary in a process called ovulation. The ova then enters the oviduct, where the albumen (white) and shell are added. The egg then passes through the cloaca, without coming into contact with faeces. The uterus turns inside out beyond the cloaca, ensuring the egg does not touch any waste.
The egg serves to protect and provide nutrients to the developing embryo. As the embryo receives no additional nutrients from the hen, the egg must contain all the nutrients essential for life. The process of egg-laying is triggered by hormones in the hen's body. The pituitary gland in her eye produces these hormones when stimulated by light. Chickens need about 14-16 hours of light per day to lay eggs and will usually stop laying between November and January in the UK.
Hens do not need roosters to lay eggs. In fact, most hens raised on commercial farms have never seen a rooster. However, after a rooster inseminates a hen, her eggs will be fertilised for up to four weeks. This is because the sperm remains viable for about 30 days, stored in 'sperm nests' along the hen's oviduct.
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The skeletal system
The chicken's skeleton is composed of bones that are homologous to those found in other animals, but they have evolved over time to enhance the bird's ability to fly. While chickens are not strong fliers, they retain this ability to some extent. The chicken's bones are also designed to withstand the shock of taking off and landing.
One notable feature of the chicken skeleton is its pneumatic bones, which are somewhat hollow. The prefix "pneu" refers to the containment of air, as these bones aid in the high respiratory demands of a flying bird. The skull, humerus (upper wing), clavicle (collarbone), keel (breastbone), pelvic girdle (hip bones), and lumbar/sacral vertebrae (backbones) are all pneumatic bones. The air sacs of the respiratory system connect with the cavity of some of these long bones.
Another adaptation for flight is the fusion of some vertebral sections, providing the rigidity required for flight. The sternum, or keel bone, provides a large surface area for the attachment of the main muscles used for flight. The chicken's long neck helps protect the delicate tissues of the brain from jarring during landing.
The chicken's wings are formed by the bones of the forelimb, which are modified to facilitate flight. The radius and ulna are widely spaced, with the carpals, metacarpals, and digits reduced to form a stiff skeletal unit for the anchorage of primary flight feathers. The three digits of the wing correspond to digits 2, 3, and 4 in other animals. The elbow is the joint between the humerus and the radius/ulna, similar to that of humans.
In summary, the chicken's skeletal system is designed to support flight, with hollow bones to reduce weight, fused vertebrae for rigidity, and a large sternum for muscle attachment. The skeleton also provides protection for the bird's internal organs and tissues.
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The respiratory system
The chicken respiratory system is quite different from that of mammals. Birds, including chickens, have an incomplete diaphragm and chest muscles, and their sternum (keel) does not allow for the expansion that occurs in mammals. As such, a bird's lungs operate differently.
The chicken respiratory system begins at the head region, with the nasal openings, nasal cavities, and the pharyngeal region of the mouth. The cranial larynx, located in the pharyngeal region, is the opening to the trachea (windpipe). The pharyngeal region also has the openings of the oesophagus. The cranial larynx is normally open to allow air passage, but it closes when food is passing down the throat, so that the food goes down the oesophagus and does not enter the trachea. After passing through the cranial larynx, air continues through the trachea, which is made up of cartilaginous rings that keep it from collapsing due to the negative pressure when a chicken breathes in. The trachea in medium-sized adult chickens measures between 15 and 18 centimetres. It is held open by 108 to 125 cartilaginous rings, which prevent collapse or compression but allow elongation and flexion.
The trachea is lined with muco-ciliary epithelium, a special type of epithelium where hair-like cilia move foreign materials, such as dust, up and out of the trachea. The larynx only has three cartilages: arytenoids, prearytenoids, and cricoids. The syrinx is the phonation organ and produces sounds through vibrations.
Each bronchus enters a lung. Chicken lungs are relatively small, are firmly attached to the ribs, and do not expand. The lungs contain parabronchi, which are continuous tubes that allow air to pass through the lung in one direction, and air sacs. The parabronchi are laced with blood capillaries, and it is here that gas exchange occurs. The air sacs fill a large proportion of the chest and abdominal cavity of a bird. They are balloon-like structures at the ends of the airway system and act as bellows to suck air in and blow it out, and to hold part of the total air volume. Air moves in and out of the avian respiratory system through distention and compression of these air sacs, not the lungs. At any given moment, air may be flowing into and out of the lung and being “parked” in the air sacs.
The air sacs are connected to the air spaces in some of a bird's hollow bones, which then act as part of the avian respiratory system. These bones are called pneumatic bones and include the skull, humerus, clavicle, keel, pelvic girdle, and lumbar and sacral vertebrae.
The avian respiratory system is protected by three defensive mechanisms: cilia, mucus secretions, and the presence of scavenging cells that consume bacteria. The mucosal glands of the nasal cavity produce mucus that helps to keep foreign material from entering the body through the respiratory system. Mucus secretions also help to reduce the electrolyte content of body fluids. The nasal glands are small and located on either side of the nasal cavity.
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The circulatory system
The system consists of the heart, blood vessels, spleen, bone marrow, and blood and lymph vessels. The heart is a unique cardiac muscle, which has high-energy demands influenced by the need for flight. The chicken heart has four chambers, allowing for the complete separation of oxygenated and deoxygenated blood. The left side of the heart pumps blood at a higher pressure, so the walls of the left ventricle are thicker than those of the right ventricle.
The blood vessels include arteries, veins, and capillaries. Arteries carry oxygenated blood from the heart to the capillaries, which are only one cell in diameter. The capillaries then interact with the tissues, exchanging gases, nutrients, and wastes. The venules and veins then conduct the blood back to the heart.
The spleen assists the circulatory system by filtering the blood, removing old red blood cells, and contributing to the chicken's immune system. The lymphatic system is connected to the circulatory system, and it drains the body of fluid left behind by the blood vessels.
The vascular system in chickens is designed to conserve heat. Arteries and veins lie next to each other, so that warm blood leaving the heart warms the cooled blood returning to it, helping to maintain the chicken's body temperature of over 104°F.
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Frequently asked questions
The digestive system of a chicken, also known as the gastrointestinal tract or GI tract, begins at the beak and ends at the cloaca. The cloaca, also known as the vent, is the only hole for the reproductive and digestive system of a chicken. Chickens do not urinate and instead void fecal material with uric acid crystals on the outer surface. The digestive tract includes several important organs, including the gizzard, which is used to grind and mix food since chickens do not have teeth. The small intestine is made up of the duodenum, jejunum, and ileum.
The reproductive tract exits through the cloaca/vent. The egg is laid through the cloaca without coming into contact with feces or urine as the vagina folds over. The oviduct is an organ that looks like a tube along the backbone between the ovary and the tail, and the egg yolk is developed in the ovary. The testes in male chickens are much smaller but situated at the top of the kidneys.
Other internal organs in a chicken include the liver, spleen, heart, gall bladder, kidneys, and bursa of fabricius.
Intestinal disease in chickens can occur when the balance of normal microflora is upset, leading to inflammation of the intestines or necrotic enteritis. Coccidiosis is indicated by thickened red patches in the gut lining, and intestinal worms may also be present. Infectious bursal disease causes redness and jelly-like swelling of the bursa.
