Name the internal organs of fish and tell us about their functions.

The swimbladder is a sac-shaped organ filled with air. The swim bladder is capable of contracting and expanding. By adjusting the volume of air in the swim bladder, the fish is kept at a certain depth, emerges or sinks.
The digestive system of fish begins with the mouth. From the oral cavity, food enters the pharynx, then into the esophagus, the voluminous stomach and intestines. Undigested residues are excreted through the anus. Fish breathe oxygen dissolved in water. Together with water, it enters the oral cavity, and from it into the pharynx. There are gill slits in its walls. Passing through them, water washes the gills – the respiratory organs of fish. The gills are located on the sides of the head. The circulatory system of fish consists of the heart and blood vessels. The heart of fish has two chambers – the atrium and the ventricle. Such a heart is called two-chambered. The vessels that leave the heart are called arteries. The vessels that carry blood to the heart are called veins. Large vessels are called aortas.
Blood from the atrium is pushed into the ventricle and then into the abdominal aorta. From it, blood flows through the arteries to the gills. This blood is saturated with carbon dioxide and has a dark color. This blood is called venous blood. Gas exchange takes place in the gills. There is scarlet blood in the vessels extending from the gills. It is enriched with oxygen. This blood is called arterial. Through the dorsal aorta, arterial blood from the gills is carried throughout the body. The blood moves in the body of the fish in a vicious circle. Therefore, the circulatory system of fish is called closed. The excretory organs of fish are the kidneys, ureters and bladder.
The nervous system of fish is divided into central and peripheral. The central nervous system is represented by the brain and spinal cord. The fish brain is usually small and protected by the bones of the skull. Many nerves radiate from the brain and spinal cord. With their help, the work of muscles, fins and internal organs is coordinated. The nerves that connect the brain and spinal cord to parts and organs of the body are called the peripheral nervous system.
In the environment, fish navigate using their senses. The organs of vision of fish are rather large and mobile eyes. The hearing organs of fish inside the skull and outside are not visible. The organ of balance is also located inside the skull of fish. Thanks to him, fish sense the position of their body in space. Fish have a very fine sense of smell. They feel the taste by clusters of sensitive cells located in the oral cavity and in the skin. The sense of touch is carried out by the tactile cells scattered throughout the body. An important sense organ in fish is the lateral line.

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