Internal structure of fish

Digestive system

The digestive system is well differentiated into sections: mouth (with teeth) – pharynx – esophagus – stomach – intestine – anus.

The fish have a liver with a gallbladder and a pancreas, and their juices help the digestion of food in the intestines.

Respiratory system

The respiratory system is located in the pharyngeal region. In the pharynx there are gill slits, separated by intergill septa, on which the gills (respiratory organs) are located.
Gill plates, divided into fringed gill lobes, are attached to four pairs of vertical bony branchial arches (which perform the function of support). Thin-walled blood vessels branching into capillaries pass inside them. Gas exchange takes place through the walls of the capillaries: the absorption of oxygen from the water and the release of carbon dioxide. Water moves between the gill lobes due to the contraction of the pharyngeal musculature and the movement of the gill covers.
From the side of the pharynx, the bony branchial arches bear the branchial stamens. They protect delicate gills from clogging with food particles.

Circulatory system

The circulatory system of fish is closed.
The heart is two-chambered, consisting of 1 atrium and 1 ventricle.
Venous blood (containing carbon dioxide) passes through the heart.
The blood is saturated with oxygen and becomes arterial in the gills.
Fish have 1 circle of blood circulation:
venous blood from the ventricle of the heart through the abdominal aorta through the supplying branchial arteries enters the gills, where the blood becomes arterial (gives off carbon dioxide and is enriched with oxygen).
Arterial blood through the outflowing branchial arteries enters the dorsal aorta, which supplies blood to the internal organs.
In organs and tissues, the blood gives off oxygen, is saturated with carbon dioxide (becomes venous) and enters the atrium of the heart through the veins.

Nervous system

The central nervous system (CNS) consists of the brain and spinal cord.

The brain has five divisions:
* forebrain;
* diencephalon;
* midbrain;
* cerebellum;
* medulla.

In fish, the diencephalon and midbrain, as well as the cerebellum, are well developed. The forebrain is less developed than that of the higher classes of animals.
Each part of the brain performs its own function. In different parts of the brain there are different centers: in the front – sense of smell, control of animal behavior and reflexes; on average – vision, in the cerebellum – coordination of movements and balance, in the oblong – hearing and touch, as well as the centers of regulation of respiration, blood circulation, and digestion.
The medulla oblongata gradually passes into the spinal cord, which is a long white cord. It is located in the canal of the spine. This canal is formed by the holes of the vertebrae connected to each other.
The cranial nerves branch off from the brain. They ensure the functioning of the senses and some internal organs.
The spinal nerves branch off from the spinal cord. They regulate the coordinated work of the muscles of the body, organs of movement, and internal organs.

Excretory system

The excretory organs are represented by ribbon-like primary kidneys.
The process of urine excretion consists of the following stages. The blood passes through the blood vessels of the kidneys, from which harmful substances are filtered out and urine is formed. Urine flows through the ureters into the bladder, and from it through the urethra is excreted from the body.

In the overwhelming majority of teleost fishes, ammonia is the final decay product of nitrogenous (including protein) compounds removed from the body (as in most invertebrates).

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