Any cell receives the substances necessary for life through simple diffusion, active transfer and facilitated diffusion. How does this happen? In unicellular organisms, this is facilitated by the movement of the cytoplasm. But with the advent of multicellularity, the circulatory system first appeared. In plants, the transport function is performed by the vessels of xylem or phloem. In the animal kingdom, for the first time in evolution, the circulatory system appears in nemerthins, a subspecies of annelids.
Types of circulatory systems
In the process of evolution, an open type of transport system first appears. This is a network of vessels in which part of the pathway the blood passes through the tissue space (lacunae and sinuses). In the process of complicating organisms, a closed type of circulatory system appears first. In this case, the blood circulates only through the blood vessels. Later, the arterial circulatory system of organs appeared for the first time in arthropods with a pulsating organ - a harbinger of the heart.
Evolutionary significance
The closed vascular network system is the largest achievement in the evolutionary development of chordates, although the circulatory system first appeared in invertebrates. In the embryonic period, it is formed 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 is the first occurrence of cephalization in animals (development of the brain and nervous system).
Open circulatory system
First appeared in insects. The transport system, for example, a cockroach is very simple. From a single aorta (a large blood vessel), blood simply spills onto the organs into the body cavity. Through simple diffusion, it then enters the pericardial sac, from where it is absorbed through 12 slots in the heart and pushed again into the aorta. Blood is colorless, as it is not involved in gas exchange. This process is carried out through trachea. A complete revolution of the blood flow takes 20-25 minutes. Such a system is characteristic of most invertebrate animals.
Closed circulatory system
An improvement in the form of isolation of the circulatory system appears for the first time in annelids and cephalopods. An ordinary earthworm has a dorsal vessel (dorsal), which leads to 5 paired hearts. Of these, blood enters the abdominal vessel (ventral). Oxygen saturation occurs in the surface capillaries, as the worm breathes through the entire surface of the body.
Two-chamber fish heart
Nature went further and the circulatory system of organs first appeared in fish with a contractile organ from two chambers. This is the atrium and ventricle. Venous blood is collected in the atrium. From here it is pushed into the ventricle, and then through the arteries to the gills. Saturation of the circulatory system first appeared in fish in gill capillaries. In this case, the blood greatly slows down its movement. Then it flows throughout the body. This is one body circle of blood circulation.
More difficult
In amphibians, a second circle of blood circulation appears. 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-lungs-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 difficulties required a third chamber in the heart - another atrium for venous blood. And in the organ itself appears a partition. It partially prevents the mixing of venous and arterial blood.
Four-hearted heart
Already in reptiles, the heart septum completely divides the ventricle into two halves - venous and arterial. In crocodiles, the blood is still partially mixed. But in animals located higher in the phylogenetic tree, it is already completely divided. Two pumps (ventricle-atrium) drive the blood into small and large circles, while the left pump is always more powerful than the right. After all, it is he who drives blood throughout the body and to each individual capillary. In humans, for example, the wall of the left ventricle is almost two times thicker than the right (15 mm versus 8 mm). The cardiac cycle begins with a reduction in the atria (in humans - 0.1 second), then the ventricles (0.3 seconds) and ends with a pause (0.4 seconds).
On the heart is bone
Surprisingly, a real bone is present in the heart of a deer and a camel. Its size is only a few centimeters. But precisely because of its presence, these animals withstand a very long, exhausting movement. This bone, but smaller, is also found in cows. After all, in 24 hours, the heart of the animal that gives milk should pump about 17 tons of blood
Two hearts are no vice
There are amazing people among us - they have two hearts. Usually they are smaller than the average normal human heart. But such people are more resilient, although they require a longer period of rest after exertion. There are also "mirror" people whose heart is not on the left, but on the right.