Any cell gets the necessarythe vital activity of the substance through simple diffusion, active transport and facilitated diffusion. How does this happen? In unicellular organisms this is facilitated by the movement of the cytoplasm. But with the emergence of multicellularity, the circulatory system first appeared. In plants, the transport function is performed by vessels of xylem or phloem. In the animal kingdom for the first time in the evolution of the circulatory system appears in nemertine - a subspecies of annelids.
In the process of evolution for the first time appearsnon-closed type of transport systems. It is a network of vessels in which part of the way the blood passes in the tissue space (lacunae and sinuses). In the process of complication of organisms, a closed type of circulatory system first appears. In this case, the blood circulates only through the blood vessels. And only later the circulatory system of organs in arthropods with a pulsating organ - the harbinger of the heart - appears for the first time.
The closed-circuit network of vessels is the largestachievement in the evolutionary development of chordates, although for the first time the circulatory system appeared in invertebrates. In the embryonic period, it forms the first of the mesoderm. In vertebrates, it consists of a pulsating organ (heart) and a network of closed vessels with valves and, in fact, blood with its shaped elements. It provides nutrition to the embryo and continues to perform a transport function throughout the life of the body. One of the consequences of the development of the circulatory system - first appeared in animals cephalization (development of the brain and nervous system).
It first appeared in insects.The transport system, for example, a cockroach is very simple. From a single aorta (a large blood vessel), the blood simply pours on the organs into the body cavity. Through simple diffusion, it then enters the pericardial bag, from which it is sucked through 12 cracks in the heart and is pushed again into the aorta. The blood is colorless, since it does not participate in gas exchange. This process is carried out by trachea. A complete turnover of blood flow commits in 20-25 minutes. Such a system is characteristic of most invertebrates.
Improvement in the form of closed bloodsystem appears for the first time in annelids and cephalopods. A normal earthworm has a dorsal vessel (dorsal), which leads to 5 twin hearts. Of these, blood enters the ventral vessel (ventral). Saturation with oxygen occurs in the surface capillaries, as the worm breathes the entire surface of the body.
Nature went further and first appearedcirculatory system of organs in fish with contractile organ of two chambers. This is the atrium and ventricle. Venous blood is collected into the atrium. From here she is pushed out into the ventricle, and then along the arteries to the gills. Saturation of the circulatory system first appeared in fish in gill capillaries. In this case, blood greatly slows down its movement. Then it flows all over the body. This is a single bodily circle of blood circulation.
У земноводных появляется второй круг blood circulation. Their lungs were formed not from gills, but from outgrowths in the throat. Now the blood is saturated with oxygen in a small circle (heart-lung-heart), and gives oxygen in a large circle (heart-vessels of the body-heart). It does not lose speed, but rather quickly flows to organs and tissues. Such complications required a third chamber in the heart - another atrium for venous blood. And in the organ itself there is a septum. It partially prevents the mixing of venous and arterial blood.
Already in reptiles the cardiac septumcompletely divides the ventricle into two halves - venous and arterial. In crocodiles, the blood is still partially mixed. But in animals that are higher along the phylogenetic tree, it is already completely divided. Two pumps (ventricle-atrium) drive blood into small and large circles, with the left pump always more powerful than the right one. After all, it is he who drives the blood throughout the body and to each individual capillary. In humans, for example, the wall of the left ventricle is almost twice as thick as the right wall (15 mm to 8 mm). The cardiac cycle begins with a contraction of the atria (0.1 second in humans), then ventricles (0.3 seconds) and ends with a pause (0.4 seconds).
Surprisingly, in the heart of a deer and a camelthere is a real bone. Its size is only a few centimeters. But it is due to its presence that these animals endure a very long and exhausting movement. This bone, but smaller, is also found in cows. After all for 24 hours the heart of the animal giving milk should pump about 17 tons of blood
There are amazing people among us - they have twoheart. Usually they are smaller than the average ordinary human heart. But such people are more endurance, although they require a longer period of rest after exercise. Also there are "mirror" people, whose heart is not on the left, but on the right.
