The bloodstream , and circulation , circulation , vascular system or blood stream called, is the flow system of the blood in the body of humans and most animals , starting from the heart to the organs and back to the heart. The heart and bloodstream together form the cardiovascular system ( cardiovascular system ). Its task is to supply the organs with nutrients, signaling substances and other things, the disposal of metabolic products and, in most groups with a circulatory system, also the supply of oxygenand the removal of carbon dioxide . Depending on the animal group, further tasks can be added.
Blood vessels that carry blood to the heart are called veins , and those that carry blood from the heart to organs are called arteries . These terms apply regardless of whether the blood in the respective vessel is oxygen-poor or oxygen-rich. The more the blood vessels branch, the smaller their diameter becomes. In animals with a closed circulatory system, arteries first become arterioles and these become capillaries , in which most of the exchange of substances with the tissues takes place. These in turn merge and form the postcapillary venules that unite to form veins.
In animals with an open circulatory system, the fluid, which is not called blood here but hemolymph , pours out of arterial blood vessels into the body cavity to flow around the organs. It flows through the body cavity to venous vessels or directly back to the heart.
In many groups of multicellular animals , a cycle ensures the survival of the organism by enabling metabolism in all parts of the body and maintaining the chemical and physiological properties of the body fluids .
In most circulatory animals, it is used to transport the breathing gases oxygen and carbon dioxide (exception: insects). Then it transports oxygen-rich blood or hemolymph from the respiratory organs ( lungs , gills or skin ) into the tissues and oxygen-poor, carbon dioxide-rich liquid back to the respiratory organs (see also breathing ). Oxygen transporters can be present to support these processes ; in vertebrates, this is hemoglobin , which is packed in the red blood cells .
Nutrients obtained from digestion such as fats , sugar or proteins are transported from the digestive tract into the individual tissues, where they can be used, processed or stored as required. The resulting metabolic or waste products (for example urea or uric acid ) are transported to other tissues or to the excretory organs (in vertebrates, the kidneys and liver ).
In addition, messenger substances such as hormones , cells of the body's immune system and components of the coagulation system can be transported . The even distribution of the blood or the hemolymph throughout the body also regulates the pH value , the extracellular ion concentration and the osmolarity of the body fluids ( homeostasis ).
In some groups of animals, the circulatory system fulfills other functions in addition to transporting substances. This includes temperature regulation within the body ( thermoregulation ) and very different hydraulic functions. Examples are the stretching of the legs in spiders by hydrostatic pressure and the erection of the genital organs in vertebrates by the increased amount of blood in the erectile tissue as well as the provision of the pressure for the filtration of the urine in the kidneys .
Forms of circulatory systems
Diffusion is very slow over longer distances because the duration increases with the square of the distance. For example, it takes a glucose molecule over eight minutes to diffuse over a millimeter in water at 37 ° C. Therefore, all multicellular animals that are more than just a few cell layers thick have developed ways of generating a flow of liquid that allows transport by convection .
Animals without blood circulation
The animal groups of sponges , cnidarians , roundworms and flatworms can create a flow of fluid even without a circulatory system. With the help of moving flagella, sponges generate a current through the body cavity. Cnidarians achieve this through muscle contractions. Flatworms set their tissue fluid in motion with the help of cilia . In the cnidarians and flatworms, there are widely ramified intestinal systems, which are referred to as gastrovascular systems ("gastric vascular system"), and which can partially compensate for the lack of a blood vessel system. Roundworms set their internal fluids in motion through muscle contractions.
The animals that do not have a circulatory system also include a few poly-bristles and flukes (both groups of annelids), many copepods , barnacles and arrowworms , all of which set the fluid in the secondary body cavity ( coelom ) in motion, like the roundworms, by contracting the muscles of the body wall . Hopperlings, on the other hand, use the movement of their limbs for this, the multi- bristle Tomopteris (see Phyllodocida ) does this by flickering. In these cases one speaks of a Coelom cycle.
Differentiation between open and closed circulatory systems
There are two types of circulatory systems in other multicellular animal groups. In closed circulatory systems, blood flows from the heart via arteries , capillaries and veins back to the heart in blood vessels. In open circulatory systems, the fluid, which is then called hemolymph rather than blood, leaves the vessels in order to flow back to the heart over different distances through systems of gaps between the tissues.
The exact delimitation is inconsistent in the literature. A textbook from 2009 stated that in closed circulatory systems the blood flows exclusively through blood vessels lined with endothelium . Examples of closed circuits include cordworms, some annelids and all vertebrates. In cephalopods, the cycle is "almost" closed. In a review from 2012, however, it is said that real endothelium only occurs in vertebrates and that the invertebrate vessels are lined with an extracellular matrix . Cells within the basement membrane are also found in some species , but the lining is incomplete and these cells have no connecting channels ( gap junctions ) to one another, so there is no real endothelium.
In this work, too, all vertebrates are assigned a closed circulatory system, but also some invertebrates, namely annelids and cephalopods. It is expressly mentioned that the demarcation between open and closed circulatory systems is not always clear and is too much of a simplification. Because in some animals with an open circuit, such as the lobster (see below ) or the marine abalone ( Haliotis ), in which the abdominal arteries branch into capillary-like vessels, there are also features of a closed circuit. And even in the case of closed circuits in vertebrates, there would be vascular areas where there would be direct contact of the blood with the interstitium (see also below). Another textbook from 2008 states that a group of vertebrates, namely the jawless ( hagfish and lampreys ), in contrast to the other vertebrates, have a partially open circulatory system in which the blood in some areas of the body empties into open lacunae. The higher cephalopods (squid and octopus) are assigned a closed circulatory system.
Open circulatory systems
Hemolymph as a uniform body fluid
In different groups of invertebrates one finds an open circulatory system in which the hemolymph is pumped from the heart into more or less short arteries and from there into the body cavities. Capillaries are usually missing, instead the hemolymph flows around the tissue until it finally flows into open veins and through them back to the heart or returns directly from the body cavity to the heart. The hemolymph flows slowly and with low pressure. The volume of fluid moved is relatively large, as the extracellular fluid is almost completely involved in the circulation. In snails it corresponds to about 50% of the body volume.
In the species of snail known as the Californian sea hare ( Aplysia californica ), the proportion of hemolymph in the body weight is even 79.3%, in the common beach crab ( Carcinus maenas ) 37% and in the American cockroach ( Periplaneta americana ) 19.5% . In general, it makes up 15–40% of the weight of insects. It must be taken into account that this amount of fluid contains all of the extracellular fluid. In animals with a closed circuit, the comparison quantity therefore includes not only the blood volume, but also tissue fluid and the lymph.
Consequences and advantages of an open circulatory system
The slow circulation of the hemolymph is believed to be a reason why the body size of open circulatory animals is limited compared to vertebrates.
A closed circulatory system requires a higher blood pressure and thus a stronger heart muscle than an open system, in which a thin-walled heart is sufficient for the slower blood flow with lower pressure.
The heart's activity has little effect on the very variable pressure. Instead, it depends on the movement of other muscles, posture, and the position of the internal organs. It can be increased, for example, by filling the intestine with air or water, which can play a role in molting and in the development of wings or other body parts after the adult animals ( imago ) have hatched from the pupa or the last larval stage . In insects, in addition, the respiratory movements of the abdomen (have abdomen ) have an effect on the pressure.
Closed circulatory systems
Closed means not closed: mass transfer
As mentioned, a closed circulatory system is a system in which blood flows from the heart to the organs and back through blood vessels. “Closed” in this sense does not mean that the blood in the vascular system is hermetically sealed from the rest of the body. Substance exchange takes place particularly in the capillaries , but not only there.
Some of the places where there is an intensive exchange or transfer of substances in vertebrates should be mentioned as examples. Capillaries with discontinuous endothelium ( sinusoids ) are found in their liver , bone marrow and lymph nodes . Openings up to a micrometer in size between the endothelial cells of the vessel wall facilitate the exchange of substances and the passage of blood cells. There are large gaps in the lining in the liver sinusoids . In this way, sinusoidal and extravascular volumes can be differentiated in the dog's liver. Vertebrates also have one or more open passages to the lymphatic system (see thoracic duct ). In the hemochorionic placenta (e.g. in primates) the maternal spiral arteries open and the blood is released into a placental labyrinth, where it washes around the chorionic villi and is drained by the spiral veins.
Benefits of a closed circulatory system
Despite these transitions to other systems, such a circulatory system is referred to as closed because, unlike open circulatory systems, the blood does not flow back to the heart through the body cavity or gaps between the tissues and does not mix completely with the tissue fluid as in open circulatory systems.
Due to the separation of blood and tissue fluid, the volume of fluid to be transported is comparatively small. For example, it corresponds to about 15% of the body volume in squids , but only 6–8% in humans. In the giant earthworm Glossoscolex giganteus (an annelid) it is 6.1%, in the octopus Octopus honkongensis 5.8% and in the pond frog ( Rana esculenta ) 5.6%.
Closed circulatory systems have developed independently of one another in several animal phyla. They mostly occur in very active animals or animals living in low-oxygen environments. The development took place parallel to oxygen transporters in the blood. This means that only small amounts of blood need to be moved to meet the need for oxygen.
Important advantages of closed systems for animals with an active lifestyle are the effective distribution of blood in all body regions and the possibility of targeted regulation of the blood flow control of the organs in order to be able to meet particularly high requirements in one place without unnecessarily increasing the total amount of blood moved. A parallel blood flow to the organs (instead of serial blood flow) and thus blood flow to all organs with oxygen-rich blood is only possible with a closed system.
Snails and clams
Mussels and snails have a strong heart, which usually consists of two sections, atrium and chamber. The pressure that can be generated and thus the flow rate is comparatively high for open circulation systems. In the Roman snail it can reach 19 mmHg (2.5 kPa) during contraction. With it, the blood is transported over long vessels before it escapes into the body cavity (haemocoel). Some mussels use the high pressure to move the foot and thus to move. Breathing gases are also transported with the blood.
Most cephalopods, namely squid , octopus and cuttlefish , have a closed circulatory system. This probably developed from an open one, as it occurs with Nautilus . The closed cycle of the squid and octopus has three chambered hearts. Oxygen-rich blood flows from the body's heart into the organs, from there the oxygen-poor blood flows to the two gill hearts, which direct it to the gills. From there the blood returns to the heart of the body.
Additional peristaltic contractions in the vessels of the gills, arms, or vena cava have been observed in some species . According to their more agile way of life, cephalopods have significantly higher circulatory pressures than snails, and these in turn have higher pressures than mussels. In the octopus Octopus dofleini (see Octopus ) a blood pressure of 45 to 30 mmHg (6 and 4 kPa) was measured in the aorta . In the common octopus ( Octopus vulgaris ) it has been shown that the gill heart beats synchronously shortly after the body heart. It has also been observed that breathing movements additionally stimulate the circulation.
Earthworms and other little bristles
Little bristles like the earthworm have a closed circuit with a main abdominal vessel and a main dorsal vessel, both of which run along the length of the animal. They are connected by smaller vessels. The dorsal vessel can contract rhythmically and peristaltically and thereby move the blood towards the front end. There are several ring vessels with lateral hearts that create a flow to the abdominal vessel, in which there is a flow to the rear end. The blood returns to the dorsal vessel through the smaller connecting vessels mentioned above.
As a special feature of annelid worms, there is no central heart, but rather several contracting sections in the blood vessel system. Gas exchange with the air takes place through the skin. Oxygen-poor blood is conducted through smaller vessels from the abdominal vessel to the well-perfused skin and from there on to the dorsal vessel.
The giant earthworm Glossoscolex giganteus (see Glossoscolecidae ), which has a diameter of 2–3 cm, a length of 1.2 meters and a weight of 0.5 to 0.6 kilograms, has been well studied. The peristaltic contractions of the dorsal artery can build up a pressure of 10-18 mmHg (1.4-2.4 kPa). The 5 pairs of lateral hearts, which usually beat synchronously, can build up a pressure of 80 mmHg or over 10 kPa in the abdominal vessel, a value that comes close to the arterial blood pressure of vertebrates. This high pressure is used to supply blood to the skin muscle tube and the metanephridia . The blood pressure changes dramatically during the movement of the animal. The highest values are reached when it stretches to drop to about half values while the animal is shortened.
The number of contraction waves in the dorsal vessel of Glossoscolex giganteus was around eight per minute at rest, and around 11 per minute in the common earthworm ( Lumbricus terrestris ). The rate increases with physical exertion and is influenced by the volume of the vessel. The filling quantity also influences the development of force during the contraction. In electron microscopic images of the common earthworm's blood vessels, you can see them filled with hemoglobin particles floating freely in the blood.
Many bristles , mostly marine animals, have a similar circulatory system as the few bristles, but the blood vessels often have a thinner wall. The vessels are not always lined with endothelium (or, depending on the definition, with endothelial-like cells), so according to one possible definition it is an open circulatory system. In some species, additional accessory hearts appear in the periphery of the animal, especially on the gills.
In the third group of annelids, the leeches , a real blood vessel system occurs only in the trunk leeches . With the other leeches it is reduced, the transport and supply tasks are taken over by a closed coelom system . Its lateral and abdominal vessels can contract rhythmically. Hemoglobin is dissolved in the liquid . In the well-studied leeches ( Hirudo medicinalis ), the side vessels act as peristaltically contracting hearts, which generate a blood flow towards the front end and a pressure of up to 100 mmHg. Valves and sphincters at the ends prevent reflux. In the back and abdominal vessels there is flow towards the rear end.
Cordworms (Nemertea or Nemertini)
The cordworms , a group of marine species, have a vascular system that is often referred to as the blood vessel system, but is probably a modification of the coelom , i.e. the formation of the secondary body cavity. It is therefore not homologous to the primary blood vessel system of the Bilateria . The main vessels are laterally lengthways in the animal, which are connected at the front end and near the anus on the abdomen. Some species also have a dorsal vessel or a split of the main vessels. The larger vessels can contract, and the body muscles also contribute to the movement of fluids.
In crustaceans, the circulatory systems range from very simple to very complex. The Kiemenfußkrebs Branchinecta has a tubular heart, which almost runs through the whole body, and relatively few vessels.
A peculiarity occurs with the decapods . From an anatomical point of view, these also have an open circulatory system, but functionally many aspects of a closed system: small blood vessels correspond to capillaries and the hemolymph flows through defined channels through the tissue back to the heart, which in some species are so small that they functionally correspond to blood vessels. Numerous blood vessels have muscular valves with the help of which the blood flow to the downstream organs is regulated. The heart is a muscular, contractile chamber and lies in a sac called the pericardial sinus or pericardium. The hemolymph flows from there via several arteries into numerous parts of the body and finally exits the vessels deep in the tissue. A sinus on the ventral side collects the hemolymph and guides it to the gills. After oxygen uptake, it reaches the pericardial sinus via veins, where it returns to the heart through small openings, the ostia .
The heart of these crabs can generate pressures of 8–15 mmHg (1–2 kPa), in the case of the lobster up to 20 mmHg, and when the heart muscle relaxes, the pressure in the aorta does not drop to zero, but remains due to the wind kettle effect of the large vessels at about 1.5 mmHg (0.2 kPa) and thus higher than in the open body cavity (haemocoel), so that a continuous flow of fluid is guaranteed. This one reached Hummer a cardiac output of 10 to 30 ml per minute and an orbital period of its hemolymph of 2-8 minutes.
The heartbeat is controlled by nerve cells (neurogenic heart) and not by specialized muscle cells as in vertebrates. The cardiac ganglion lies on the back of the heart and is in turn controlled by the central nervous system.
Heart rates between 61 and 83 beats per minute have been observed in lobsters. Dissolved in its hemolymph is the copper-containing oxygen transporter hemocyanin and there were three to eleven morphologically distinguishable hemocytes found their tasks probably in the innate immune response and hemostasis are.
Within the crustaceans, apart from the decapods, flow in vessels was observed over long distances only in the woodlice . The heart of the Daphnia is similar to that of the decapod, but there are no vessels. The tubular heart of gillworts , to which Artemia belongs, is more similar to that of insects. The barnacles , including barnacles and barnacles , there is no heart in the real sense. The muscles of the thorax take over the pumping function.
Despite a high metabolic rate, insects have a rather simple, open circulatory system. This is possible because, unlike most circulatory animals, insects do not use them to transport respiratory gas, but only to transport nutrients, immune cells, signaling molecules and the like. The gas exchange takes place via a tracheal system , hollow tubes that lead directly to the tissue from the surface of the body.
In many insects the vascular system consists only of a long heart lying along the back, which is closed at the back and merges into the aorta at the front. Waves of contraction propel the hemolymph towards the head, where it exits into one sinus and seeps to another sinus to flow back through the body. Body movements keep the flow going, so that, as with other arthropods, a transition occurs via ostia back into the heart. When the heart relaxes, it is stretched by the elastic fibers of the suspension apparatus, so that the hemolymph flows in through negative pressure. During contraction, the ostia are closed by flaps made of connective tissue. A maximum pressure of only about 7 mmHg (0.9 kPa) was measured in the locust Locusta migratoria .
Often there are additional, simple hearts in the antennae, wings and legs, in some species there can be several dozen. They serve to supply tissues that are particularly active in the metabolism and are also called accessory hearts or pulsatile organs.
In order to be able to flow through the ends of the long, thin limbs, they are divided lengthwise into two areas by septa. Only at the end of the extremity is there an opening so that the hemolymph flows in on one side and back on the other.
In some species of butterflies , the blowfly Calliphora , the Goliath beetle and the rhinoceros beetle , it has been observed that the direction of the heart's beat can be reversed. In these species, the heart has one or two openings at the back through which the hemolymph can then exit. Most of the time, more blood is conveyed to the front than to the back. The alternating direction allows the hemolymph to oscillate between the front and rear, which also leads to increased ventilation of the trachea.
In contrast to all other invertebrates, insects have developed an effective blood-brain barrier. The diffusion of water-soluble molecules from the hemolymph into the immediate vicinity of the nerve cells is therefore severely restricted.
In contrast to insects, arachnids (including spiders , mites , scorpions ) like crustaceans have to transport not only nutrients but also respiratory gases with the hemolymph. The tubular heart lying on the back in the abdomen (opisthosoma) is similar to that of insects. Vessels allow targeted blood flow to the book lung . A special feature is that spiders do not use muscle power to stretch their legs, but rather hydraulically via a relatively high pressure of the hemolymph, which reaches there via its own blood vessels.
Echinoderms are the only originally bilateral animals that have developed five-fold symmetry. However, the larvae are bilateral. All related groups live exclusively in the sea, including sea stars , sea urchins , brittle stars and sea cucumbers . The echinoderms have two vascular systems. A key feature is their ambulacral system , sometimes referred to as the water vascular system. However, it does not contain sea water, but a body fluid. It has an annular central canal, from which a radial canal branches off into the five body rays. This vascular system is a new formation of the echinoderms and does not correspond to the blood vessel system of the other animal groups.
There is also the blood vessel system, which in echinoderms is also known as the blood capillary system or the hemal capillary system. There is no endothelium, and some vessel walls contain myofilaments. The vessels contain a high concentration of glycoprotein particles and blood cells, so-called amebocytes and coelomocytes. However, the greater part of these cells is located in the coelomic spaces and in the connective tissue. Some blood cells are phagocytic , others carry oxygen. In the sea cucumber there are disc-like cells containing hemoglobin. It is unclear whether, how strongly, in which direction and how regularly the blood flows in the vessels. At least in part, there is probably no or little flow.
Essential parts of the blood vessel system are an "oral vascular ring" that runs around the mouth (in the animal below or in front) and an "aboral vascular ring", the equivalent at the other end of the body (see illustration). Both are connected by the axial organ, a structure that contains many blood vessels and blood cells and is often darkly colored by the latter. The dorsal bladder is attached to it, which pulsates and presumably takes over the function of a heart. It is also connected to the vessels of the intestines and the gonads . The axial organ also has excretory functions and is probably homologous to the glomeruli of the vertebrate kidney.
Primal choir dates
The tunicates (tunicates) and the acrania ( Lanzettfischchen ) which, like the vertebrates to the chordates belong, have also an open circulatory system. A simple, tubular heart merges into numerous well-defined channels, which, however, have no vessel walls and are therefore not regarded as blood vessels. In some tunicates, for example Ciona , the direction of the peristaltic contractions of the heart can reverse, and thus the direction of flow. In the lancet fish the cycle is largely closed, the vessels only open into a few lacunae.
The lancetfish Branchiostoma lanceolatum has no blood cells and no hemoglobin or other oxygen transporters. There is no central heart; the blood is driven by several contractile blood vessels. It flows forward on the stomach and back on the back.
Round-mouthed fish (Cyclostomata, Agnatha)
A group of vertebrates also has a partially open circulatory system, namely the round- mouthed fish, to which the jawless fish belong, lampreys and hagfish . Coming from the systemic heart, the blood in some tissues remains in blood vessels as in other vertebrates, but in other tissues it passes into open blood gaps. The hagfish have additional, accessory hearts in some areas of the body. As with the other vertebrates, the blood vessels are lined with endothelium .
Of all vertebrates, hagfish have the lowest arterial blood pressure (6-10 mmHg) and the highest relative blood volume (18% of body mass). Hagfish have multiple sinuses that communicate with blood vessels. The largest lies on the back and extends from the snout to the caudal fin. It lies between the skeletal muscles and the skin. The sinus system can hold up to 30% of the total blood volume. Presumably, an active passage into the blood vessels can be generated by contraction of skeletal muscles.
The heart of the hagfish has an atrium and a chamber, but no coronary vessels for its own oxygen supply. Instead, the blood in the heart flows through channels and lacunae in the heart wall. The heart rate is 20-30 beats per minute. The cardiac output achieved with 8-9 ml per minute per kg of body weight values that make up the genuine bony fish closer. The heart has a Frank Starling mechanism , but is not influenced by regulatory nerves, as is the case with other vertebrates. A second chambered heart, which is also made up of myocardial cells , transports blood from the intestine to the portal vein of the liver. It beats in a different rhythm than the systemic heart.
The lamprey's circulatory system is similar to that of jaw-bearing fish: blood flows from the heart ventricle into the ventral aorta, from there to the gill arteries, and on to the dorsal aorta, where it flows towards the tail. From this arterioles and finally capillaries branch off, the return flow to the heart occurs through veins.
Common features of the jaw-bearing vertebrates (Gnathostomata)
Jaw-bearing vertebrates ( jaw mouths ) have a closed cycle. Here the blood flows through a continuous network of blood vessels that reaches all organs . The vertebrate capillary endothelia can be continuous, fenestrated, or discontinuous. These cycles are described in the following sections.
The cardiovascular system of the fish is the simplest of the jaw-bearing vertebrates. The heart consists of four spaces, two introductory thin-walled ones, sinus venosus and atrium, one thick-walled, muscular chamber and the final globe or conus arteriosus . There is a valve between the atrium and the ventricle that prevents the blood from flowing back. Like the heart, the bloodstream itself is relatively simple in structure. The deoxygenated blood is pumped from the heart into the gills , where it is enriched with oxygen from the water. The oxygen-rich blood is then transported further into the body. In the capillaries it releases the oxygen and in return it absorbs carbon dioxide. In addition to the heart, the muscles of the gills also take part in the pumping process. The disadvantage of this construction is that the blood pressure in the capillary network of the gill circulation drops sharply, so the blood flow through the body is relatively slow. The blood volume is less than a tenth of the body weight. The oxygen content in the blood of the fish is far below that of the human blood.
In most fish, the heart and gills are connected in series as described. A mixing of oxygen-poor with oxygen-rich blood does not take place. The Australian lungfish has as the land vertebrates a separate pulmonary circulation .
In amphibians (amphibians) the heart consists of a chamber and two atria. Gas exchange takes place both in the lungs and in the skin . The two cycles of amphibians are therefore referred to as the lung-skin cycle and the body cycle . Since, unlike fish, they are not connected in series, one speaks of a double cycle .
The left atrium receives oxygenated blood from the lungs, and the right atrium receives a mixture of deoxygenated blood from the body and oxygenated blood from the skin. Both atria pump blood into the single chamber. This chamber has an outflow tract ( trunk or conus arteriosus ), which divides into a trunk for each of the two circuits. A ridge-like elevation in the ventricle and in the lumen of the outflow tract ensures that the blood flows through the heart relatively “ single-origin ”, so that the blood from the two atria does not mix much. Most of the oxygenated blood is pumped into the carotid arteries and the aorta, while the less oxygenated blood is directed into the pulmonary skin artery. Like reptiles and birds, amphibians already have a renal portal circulation .
Amphibians originally have four paired gill arch arteries that arise from the aorta on either side. In adult amphibians, the first develops into the carotid artery , which supplies the head. The arteries of the second sheet combine to descending aorta , the descending aorta. The third branchial arch artery regresses, and the fourth the paired aortic arch develops .
Most of the taxa grouped as reptiles have a heart that, like that of amphibians, consists of two atria and a chamber. However, this is almost completely divided into two halves by a partition. Deoxygenated blood flows from the body into the right atrium, and oxygenated blood from the lungs flows into the left atrium . Both atria pump the blood into the heart chamber, from which three arteries branch off. In the right, oxygen-poor blood flows to the lungs, in the left, oxygen-rich blood flows to the head and into the body. However, since the separation of the heart chamber is not complete, mixed blood is formed (around 10 to 40 percent). This flows through the middle artery into the body.
A special feature is the crocodile , in which two heart chambers are completely separated. But there is a connecting window between the left and right arteries with the Panizza foramen . The left aorta arises from the right ventricle, the right from the left. The oxygen-rich blood in the right ventricle mixes through the window with the oxygen-poor blood in the left ventricle in the area of the right aorta, so that mixed blood is led into the body's circulation (the peripheral circulation) and mainly reaches the peripheral areas of the body. At the same time, the left aorta transports oxygen-rich blood into the body and especially into the head of the animal. During the diving process, the Panizzae foramen closes completely, so that the right aorta is only supplied with oxygen-poor blood, but the head continues to receive oxygen-rich blood.
In the dinosaurs, too, there was presumably a complete separation of the ventricles. This is due to their position in the family tree between the crocodiles and the birds, both of which have a continuous partition in the heart.
Bloodstream of Birds and Mammals
The heart of birds and mammals, including that of humans , is completely divided into two halves, although it is an organ . Each of these halves consists of an atrium and a chamber, which each work as a unit. So there are four rooms in total. While the right half of the heart pumps the blood through the pulmonary circulation , which enriches the blood with oxygen, the left half of the heart pumps the blood through the body's circulation to supply the organs with nutrients and oxygen.
These two circuits are connected in series so that all of the blood has to flow through the lungs. The organs in the body's circulation are perfused in parallel.
An important advantage of having your own separate pulmonary circulation is that there can be significantly less pressure in it than in the body's circulation. If the pressure was the same as in the body's circulation, more fluid would pass from the blood into the air in the lungs and thereby hinder the gas exchange . In addition, the lungs with their capillaries act as a filter against blood clots ( thrombi ) and the like, before the blood is pumped from the left side of the heart to the brain, among other things. The lungs also have thrombus-dissolving properties.
In the pulmonary circulation, the blood leaves the right ventricle of the heart via the pulmonary trunk (lat. Truncus pulmonalis ) in the direction of the lungs , where it is enriched with oxygen. Then it is pumped from the pulmonary vein (Latin pulmonary vein ) into the left atrium . From the left atrium, it enters the left ventricle, from where it enters the circulatory system through the aorta . While in mammals the aorta runs on the left side of the body, in birds it lies on the right. After supplying the organs, the blood , which is now enriched with carbon dioxide, returns through the anterior (in humans: upper ) and posterior (or lower ) vena cava into the right atrium. When the blood enters the right ventricle from the right atrium, the cycle begins again.
The portal vein system of the liver is a specialty . Blood that comes from the organs of the digestive tract is collected in the portal vein and reaches the liver, where the absorbed nutrients are used. The blood flows through two capillary systems one after the other before it comes back to the heart. The pituitary gland also has a portal vein system . Like reptiles, birds also have a renal portal vein .
Development of the circulatory systems
Phylogenesis: common evolutionary origin of all circulatory systems
Circulatory systems can be found in evolution for the first time in bilateria , i.e. animals that have two clearly definable more or less mirror-image halves (right and left). The Bilateria include all animal groups from worms to insects and vertebrates , but not sponges and cnidarians .
The last common ancestor of all these groups lived about 600 to 700 million years ago and it is believed to be a segmented , bilateral animal. A first blood vessel system may have already developed at this time, possibly to overcome the dividing walls between the segments. The flow would have been set in motion by peristaltically contracting vessels, perhaps similar to the dorsal vessels of today's annelid worms . Blood would presumably have seeped through crevices in the extracellular matrix, similar to a primitive closed circulatory system. This model would mean the homology of all circulatory systems in the animal kingdom, which would all have developed from this original type and therefore have a common phylogenetic origin. In some animal phyla, the circulatory system would have completely regressed, for example in the case of flatworms and roundworms. In other lineages it would have evolved into an open circulatory system. In an early cephalopod, on the other hand, the open circulatory system changed back to a closed one. Energy losses in the flow through reflux and the like led to a selection pressure in the direction of the development of chambered hearts (for example in molluscs and vertebrates), in which this problem is avoided.
The finding that many genes and signaling pathways play a role in the development of the cardiovascular system in both the fruit fly Drosophila melanogaster and vertebrates has also supported speculations that there is a common origin for all circulatory systems that go back to a common bilateral ancestor . Common signaling molecules include bone morphogenetic proteins , fibroblast growth factor , Wnt and Notch . For example, the Notch signaling pathway in hemangioblasts in vertebrates and in Drosophila triggers the formation of blood stem cells .
Ontogenesis: Circulatory systems are formed by the mesoderm
Most coelomata (animals with coelom) have a vascular system. Its walls arise during the development of the individual, the ontogenesis , from the mesothelium, sometimes also called the peritoneum . The mesothelium is the mesodermal lining of the body cavity (the coelom ), i.e. an epithelial cell layer. Contractile myofibrils are found in vessel walls, which in some cases trigger blood flow. In vertebrates and some invertebrates, the blood vessels are lined with non-muscular endothelium, which is surrounded by a layer of muscle cells. Although the arthropods and some other primordial mouths (protostomia) do not have a closed coelom in the adult animals, parts of the embryonic mesothelium survive and form epithelially lined vascular spaces. Compared to the situation described above in the original bilateral animal, a continuous mesothelium was lost during the development of the arthropods in the adult animals. However, mesothelial cells continued to form the tubular heart on the back and the adjoining aorta.
In vertebrates, ontogenetic development is more complex. The embryonic mesothelium gives rise to several groups of progenitor cells from which the various tissue layers of the cardiovascular system develop, including the endothelium / endocardium , myocardium and blood cells . Based on these considerations, the Drosophila heart would be related to the vertebrate myocardium as well as to the endocardium and the endothelium in blood vessels in general.
Relationship of blood vessels, blood cells and excretory organs
Embryological and molecular evidence suggests that vascular wall cells and blood cells are closely related in developmental biology and that both develop from hemangioblasts . Nephrocytes ( podocytes ) are also closely related to the cell types mentioned. The corresponding systems for blood vessels, blood cells and excretion all go back to the third germinal layer , the mesoderm, which occurs for the first time in the early bilateria or triploblasts .
Some of today's animal phyla of Bilateria have no coelom ( acoelomata , the flatworms) or no real coelom ( pseudocoelomata , roundworms and others) and no vascular system. However, they too have special excretory organs, the protonephridia . These therefore seem to be the oldest epithelial tissues formed from mesodermal parenchyma in the early bilateria . Protonephridia form a branched system of tubes, which is lined on the inside by ciliate epithelium and nephrocytes (also called cyrtocytes here) and which open either to the outside or into the digestive tract. The blink of an eye removes fluid and creates a negative pressure that sucks in more fluid from the body cavity.
Construction of the first circulatory systems
The Coelomata usually have a well-developed circulatory system. The eponymous body cavity, the coelom, is usually divided into several sections (in vertebrates, for example, the peritoneal space , the pleural space and the pericardium .) In some associated invertebrate groups, the vessels are formed in gaps between the mesothelial dividing walls, such as in annelid worms, gill worms and the Lancet fish . Possibly this is the original design, as with the annelid worms, in which the two Coelom spaces, which line the left and right halves of a segment around the intestine, meet on the back and stomach. The walls of the spaces between adjacent segments also meet, but open areas remain between the two mesothelia that connect to form blood vessels. As a result, unlike other tubes or openings in the body, the interior of the primitive blood vessels is surrounded by the basal side of the surrounding epithelium , rather than the apical, "outer" side. In fact, a pronounced basement membrane facing the lumen can be detected in the invertebrate animal groups , the characteristic of the basal side of an epithelium.
The origin of the circulatory system is thus suspected to be in an early bilateral animal, in which the circulatory system resembled current multicorns among the annelid worms. Here a group of mesodermal cells developed into a mesothelium that lined the coelom and formed vessels. Mesothelial cells then differentiated into contractile vascular cells, nephrocytes or blood progenitor cells. Such mesothelial cells would then be a kind of precursor of today's hemangioblasts. The circulatory system of the lancet fish is also similar to that of the assumed original bilateral animal.
The vertebrate circulatory system is different. The vessels are lined with endothelium and surrounded by muscle cells. In contrast to the invertebrate Coelomata, the mesothelium in vertebrates does not contribute to the vessels in the adult animals. However , the relationship can still be observed during embryogenesis . In the early embryo, the lateral mesoderm forms the mesothelium that surrounds the coelom. The inner sheet of the mesothelium, the splanchnopleura , forms the predecessor cells that become endothelium and blood cells (see AGM region ) and also the smooth muscle cells of the blood vessel walls. Directly next to the splanchnopleura is the intermediate mesoderm, from which the excretory system develops. In the case of the zebrafish Danio rerio , both regions even overlap.
The blood vessel precursor cells and blood cells migrate from the splanchnopleura to form the first blood vessels. Endothelial cells begin to form the heart, aorta, large veins, and connections between aorta and veins. Then mesodermal cells accumulate to build up the muscle layer. Blood stem cells separate from the endothelium of the aorta and other vessels in the early embryo. Other blood stem cells, at least in mammals, are probably formed in the yolk sac and placenta .
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