Respiration is skin-pulmonary. There are no trachea and bronchi. The respiratory surface of the lungs is small.
The larvae have gill respiration.
For the first time, the ability to breathe atmospheric air appeared in amphibians. The respiratory organs in adult amphibians are the lungs.
They are thin-walled hollow bags entwined with a dense network of blood vessels, capillaries, in which gas exchange takes place.
Amphibians have internal nostrils (choanas) that conduct air into the lungs.
The frog’s throat is pulled down several times a second, thereby creating a rarefied space in the oral cavity. Then the frog makes a swallowing movement, and the air penetrates through the nostrils into the oral cavity, and from there into the lungs.
But in amphibians, the lungs are still poorly developed, and pulmonary respiration alone is not enough for them. An additional very important respiratory organ in frogs is the skin.
To ensure skin respiration, the skin must be devoid of protective formations (scales, etc.). The frog’s skin is tender, covered with mucus, rich in blood capillary vessels; oxygen of the air easily penetrates into these vessels, if the skin is moist. At the same time, the frog can breathe through the skin under water, absorbing oxygen dissolved in the water.
The role of skin in the general gas exchange is relatively higher in amphibians with a long and narrow body (newts, salamanders, worms). Some tailed amphibians have completely lost their lungs and completely switched to skin respiration (a family of lungless salamanders that live mainly in America). In tailless amphibians, pulmonary respiration predominates.
On the diagrams of the circulatory system of animals, blue represents venous blood, red represents arterial blood, and purple represents mixed blood.
The circulatory system is closed. Two circles of blood circulation – large and small (pulmonary).
A large circle is the movement of blood from the ventricle to all cells of the body (except for the lungs), and then to the right atrium.
Mixed blood flows through the arteries of the systemic circulation, and only the brain is supplied with arterial blood.
A small circle is the movement of blood from the ventricle to the lungs and to the skin, and then to the left atrium.
Mixed blood is supplied to the cells of organs and tissues in all amphibians, therefore, these animals have a relatively low level of metabolism (that is, amphibians are cold-blooded animals, that is, they do not maintain a constant body temperature).
The heart in adult animals is three-chambered, in the larvae (in the frog – the tadpole) – two-chambered.
Venous blood from organs and tissues is collected in the veins and enters the right atrium, and then into the ventricle.
From the lungs, arterial blood is collected in the left atrium.
The right atrium is filled with venous blood, the left one is filled with arterial blood. There is mixed blood in the ventricle.