Internal structure of Birds (digestive, circulatory, nervous, excretory systems)

Digestive system

The digestive system of birds, like other vertebrates, is represented by the digestive tract and glands.
Digestive tract: Mouth – pharynx – esophagus – goiter (in granivorous birds) – stomach (glandular and muscular sections) – intestines (small and rectum) – cloaca.
Digestive glands – Secrete substances (enzymes) necessary for the digestion of food.

  • salivary glands – secrete saliva;
  • liver – secretes bile;
  • pancreas – secretes pancreatic juice.

The digestive system begins with a beak. The beak limits the mouth. There is a tongue in the mouth. No teeth.
Food passes through the pharynx and then into the esophagus. In many birds, the esophagus has an expansion – a goiter, where food accumulates and softens.
The stomach is divided into glandular and muscular sections. In the glandular part of the stomach, food is digested under the action of enzymes, in the muscular part it is rubbed by the walls of the stomach and small stones swallowed by a bird.
From the stomach, food travels to the small intestine, where it is processed by the digestive enzymes of the liver and pancreas. The small intestine passes directly into the rectum, which opens into the cloaca.
The structural features of the digestive system (absence of teeth, pretreatment of food in the goiter, structure of the stomach, short rectum) is an adaptation to lighten the body weight of a bird (which is important for flight).

Circulatory system

The circulatory system is closed and consists of a four-chambered heart and blood vessels.
Two circles of blood circulation.
The heart of a bird has a complete septum and consists of 4 chambers: two atria and two ventricles. Aortic arch – right only (left reduced).
In the heart, the blood does not mix, it is completely divided into venous (on the right side of the heart) and arterial (on the left side of the heart).
When moving through the body, the blood passes through two circles:
Large circle – from the left ventricle of the heart throughout the body to the right atrium.
Small (pulmonary) circle – from the right ventricle of the heart through the lungs to the left atrium.


Birds have a high metabolic rate.
Due to the complete separation of arterial and venous blood, oxygen-rich blood enters all organs of the body, which contributes to the maintenance of a constant high body temperature, therefore Birds are warm-blooded animals.
Warm-blooded animals are animals with a constant body temperature that does not depend on the ambient temperature.

Excretory system

The excretory organs are represented by two pelvic kidneys. From the kidneys, the ureters depart, opening into the cloaca. No bladder (adaptation to lighten body weight).
The excretion product is uric acid.

Nervous system

The nervous system of birds is well developed. The brain and its mass increase. The visual hillocks of the midbrain and cerebellum develop.
The development of the cerebellum is associated with complex coordination of movements.

Sense organs

The organs of sight and hearing are well developed.
The eyes are large, have two leathery eyelids and a blinking transparent membrane.
Vision in birds is colored, in most cases monocular. In owls, binocular vision.
Birds have a high visual acuity, since they have double accommodation – a change in the curvature of the lens and a change in the distance between the lens and the retina.
Birds hear well. The auditory opening is located closer to the back of the head and is hidden by feathers.
The hearing organs consist of the inner and middle ear with the tympanic membrane. The auditory bone is one.
The olfactory organs are poorly developed.

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