Mammals and birds have four chamber hearts (composed of right atrium, right ventricle, left atrium and left ventricle), but frogs have only three chambers - two atriums and one ventricle. Generally speaking, the heart absorbs anoxic blood from the body, transports it to the lungs for oxygen, and pumps it throughout the body to provide oxygen for body organs. For humans, the four chamber heart stores oxygenated blood and deoxygenated blood in different chambers, but in the frog’s body, a groove called trabeculae separates oxygenated blood from anoxic blood in the ventricle.
Frogs can get oxygen not only from the lungs, but also from the skin. The frog’s heart survived based on this strange evolutionary phenomenon. When hypoxic blood enters the right atrium, then enters the ventricle, and then outputs it to the lungs and skin for oxygen. Oxygenated blood returns to the heart through the left atrium, then enters the ventricle and is transported to major organs and tissues.
According to a research report published in the American Journal of physiology in 1989, the most strange heart structure among frog species is cold resistant frogs, including tree frogs. When tree frogs hibernate, their bodies will freeze and their hearts will completely stop beating, but their hearts will resume beating after thawing in less than an hour.
Whale
The heart of the blue whale is the largest heart structure among the species on earth today. The heart of the blue whale is the size of a small car and weighs about 430kg. Like other mammals, the heart of the whale has four chambers. The heart is responsible for delivering blood to the whale body twice the length of the school bus. The thickness of the active vein wall is similar to that of an iPhone 6 plus mobile phone, with a thickness of at least 15cm, This is a very thick blood vessel.
When blue whales dive into the deep sea, their heart rate drops to 4 beats per minute, which helps them prolong diving time and even alleviate diving decompression sickness. In 2021, a study reported that the lower heart rate of blue whales reduced the blood entering the compressed lung channels, and reducing nitrogen intake may alleviate diving decompression sickness.
Cephalopods
Cephalopods are tentacled marine creatures, including octopus, squid and cuttlefish. They all have three hearts. The two arm like hearts on both sides of the cephalopods’ body deliver blood to the gill blood vessels to make the blood absorb oxygen. The system heart in the center of the body pumps oxygenated blood from the gills to other parts of the body.
The blood of cephalopods is blue because their blood contains copper. Human blood is red because of the iron element in hemoglobin. Just as rust is red, the iron element in human hemoglobin will also appear red during oxygenation, but in cephalopods, oxygenated blood will turn blue.
Cockroaches
Like other insects, cockroaches have an open body circulation system, which means that their blood does not fill blood vessels. Cockroach blood flows through a single structure with 12-13 chambers. The dorsal sinus of cockroaches is located at the top of the body. The dorsal sinus can deliver oxygenated blood to all chambers of the heart, but the heart cannot deliver oxygenated blood to all parts of the body.
Cockroaches and other insects breathe through vents (open structures on the body surface) rather than mammalian lungs, and then transmit oxygen to all parts of the body, so cockroach blood does not need to transport oxygen from one place to another.
On the contrary, the cockroach’s body is a blood substance called hemolymph, which contains nutrients and appears white or yellow. Cockroach’s heart cannot beat alone. It must expand and contract muscles in the body cavity to help the heart transport hemolymph to other parts of the body. Usually, the heart of wingless cockroaches is smaller than that of winged cockroaches. The heart rate of cockroaches is very close to that of humans.
Earthworm
Earthworms have no heart, but there are five pseudo hearts around their esophagus. These pseudo hearts do not pump blood, but squeeze blood vessels to promote blood circulation in earthworms. At the same time, earthworms have no lungs, but they absorb oxygen through moist skin. The air in the soil, or the earthworms drill out of the soil after the rain and come into contact with the moist air, will dissolve in the skin mucus, and then the oxygen will be sucked into the cells and blood system and pumped everywhere in the body.
Earthworm’s blood is red and contains hemoglobin. This protein carries oxygen, but unlike humans, earthworm has an open circulatory system, and the hemoglobin in its blood is only floating.
Fish
Scientists have found that if 20% of zebrafish’s myocardium is damaged, it can completely regenerate myocardium in two months. Humans can regenerate liver, amphibians and some lizards can regenerate tail. In addition, frogs receiving special drug cocktail therapy can even regenerate their legs, but the regenerative ability of zebrafish has become the main model for scientists to study heart growth.
However, fish has a unique heart. In addition to an atrium and a ventricle, fish also has two body structures that humans do not have - the sinus venosus and the bulbus arteriosus. The sinus venosus is a cystic structure in front of the atrium and the bulbus arteriosus is a tubular structure behind the ventricle. Like other animals, the heart drives the blood to flow through the whole body. The anoxic blood enters the venous sinus and then flows into the atrium. Then the atrium will pump the blood into the ventricle. The ventricular wall of the fish is thick, which will pump the blood into the bulbar artery. When the blood flows through the capillaries around the gill, the bulbar artery will regulate the pressure of the blood, and finally the oxygen will exchange into the blood through the cell membrane in the gill.
But why do fish need a balloon to regulate blood pressure? Because the fish gill is very fragile and thin-walled, if the blood pressure is too high, the fish gill will be damaged. Compared with the nature of ventricular muscle, the arterial ball itself is obviously a highly elastic chamber. (Ye Qingcheng)
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