The system circulation in fish it is a closed circulatory system similar to that of other vertebrates. However, the blood makes a single circuit in the heart of the fish, for this reason, it is known as a simple closed circulatory system or "single cycle circulation".
Humans and terrestrial vertebrates have a dual circulation. The right side of the heart is responsible for receiving the blood that returns from the body in a "deoxygenated" way. This blood enters the right atrium, then the right ventricle and is pumped to the lungs to be oxygenated.
The oxygenated returning blood from the lungs enters the left ventricle through the left atrium and is then pumped along all branches of the arteries through the circulatory system of the tissues. This is a double closed circulatory system.
In fish, the heart only has one atrium and one ventricle, therefore deoxygenated blood returning from the body enters the atrium and ventricle to be pumped to the gills of the fish, where it is oxygenated..
That is, the oxygenated blood circulates through the body of the fish and, finally, it reaches the heart "deoxygenated" again..
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Three different types of circulatory system can be found in fish, which vary from other vertebrates in many respects. These three types are:
- The typical circulatory system of aquatic breathing teleosts.
- The circulatory system of air-breathing teleosts.
- The circulatory system of lungfish.
All three types of system are "simple closed" circulatory systems and share the following characteristics.
The heart is made up of four continuous chambers, arranged in series. These chambers are contractile, except for the elastic bulb in teleost fish. This type of heart maintains a unidirectional flow of blood through it..
The four chambers are the venous sinus, the atrium, the ventricle, and the arterial bulb. All these are connected one after the other, as if it were a series circuit. Deoxygenated blood enters the venous sinus and exits the arterial bulb.
This arrangement of the main organs of the circulatory system of fish contrasts sharply with the circulatory system of most vertebrates, since the latter have their components arranged in parallel..
Since it is in series, blood enters the heart continuously in "deoxygenated" form, travels through the four chambers of the heart, is pumped to the gills, oxygenated, and is subsequently pumped throughout the body..
In general, fish use their gills as a kind of “kidneys” for detoxification of their body. Through these they excrete carbon dioxide and carry out ionic and acid-base regulation..
Unidirectionality within the heart is produced and maintained by three valves. Blood always enters through one place, passes through the chambers of the heart, and exits through a different place towards the gills..
The three valves that allow this are the valve at the sinoatrial connection, the valve at the atrioventricular connection, and the valve at the outlet of the ventricle..
All valves, except the one furthest (distal) from the ventricle, communicate with each other, but a closed valve at the outlet of the arterial bulb maintains a pressure difference between the cone and the central aorta.
When the pressure in the ventricle and the arterial bulb rises and exceeds the pressure in the central aorta, the folds of the distal valve open and expel blood into the aorta. During ventricular systole (contraction) the proximal valve folds close.
This closure prevents backflow of blood into the ventricle as it relaxes. This contraction of the arterial bulb proceeds relatively slowly. From the heart to the aorta, each group of valves closes to prevent backflow of blood..
On an evolutionary scale, the circulatory system of terrestrial vertebrate animals is thought to have specialized from organisms with a circulatory system similar to that of lungfish..
However, none of the three systems is considered more evolved than the others. All three are successful adaptations to the environment they inhabit and the lifestyle of the organisms that possess them..
Fish with purely aquatic respiration oxygenate their blood by exchanging gases through the flow of blood through their gills. The respiratory circulation through the gills and systemic of the body is in series, typical of fish.
The heart is not divided, that is, the four chambers that compose it are connected in series, and the pacemaker is in the first chamber, the venous sinus. The ventricle expels blood into a small aorta through the arterial bulb.
The blood that leaves the aorta is directed towards the gill to carry out the exchange of gases with the water and to be oxygenated. Traverses the gills to a very long and rigid dorsal aorta.
From the dorsal aorta, blood is directed to the tissues of the rest of the body and a small portion, representing about 7%, is directed to the heart to carry out primary circulation and oxygenate the heart muscles. Once the tissues are oxygenated, the blood returns to the heart to start the cycle again.
Fish with air respiration live in the water, but rise to the surface to take in air bubbles that complement their supply of necessary oxygen. These fish do not use the gill filaments to take advantage of oxygen from the air.
Instead, these types of fish use the oral cavity, portions of the intestine, the swim bladder, or their skin tissue to capture oxygen from the air. Generally, in fish that have air respiration, the gills are reduced in size to avoid the loss of oxygen from the blood to the water..
Fish that have air respiration as the main oxygen contributor, have developed a variety of circulatory shunts to allow changes in the flow of blood distribution to the gills and the organ that allows air respiration.
In fish with air respiration, the oxygenated and deoxygenated blood flows are moderately separated. The deoxygenated blood is conducted through the first two gill arches and by the organ that performs air respiration..
Oxygenated blood flows, in most cases, through the posterior branchial arches to the dorsal aorta. The fourth branchial arch is modified so that the afferent and efferent arteries connect and allow oxygenation of the blood.
This system that connects the afferent and efferent arteries is specialized to allow effective gas exchange through the gills, despite the fact that oxygenation of the blood occurs to a greater degree through air respiration..
The most complete division of the heart is found within lungfish, they have gills and defined "lungs". There is only one species alive today with this type of circulatory system, it is an African fish of the genus Protopterus.
The heart in this type of fish is divided into three chambers instead of four like other fish. It has an atrium, a ventricle and an arterial bulb.
This has a partial septum between the atrium and the ventricle, it has spiral folds in the heart bulb. Due to these partitions and folds, a clear separation is maintained between oxygenated and deoxygenated blood within the heart..
The anterior gill arches of these fish lack lamellae and oxygenated blood can flow from the left side of the heart directly into the tissues, while in the lamellae present in the posterior gill arches there is an arterial connection that allows blood flow to be derived..
This connection prevents the passage of blood through the lamellae when the fish breathes solely and exclusively through the lung. Blood circulates from the posterior branchial arches to the lungs or enters the dorsal aorta through a specialized duct known as the "ductus.".
The ductus is directly involved in the control of blood flow between the pulmonary artery and the systemic circulation of the fish body. The vasomotor portion and the "ductus" act reciprocally, that is, when one contracts the other dilates. The "ductus" is analogous to the "ductus arteriosus" of mammalian fetuses.
The absence of lamellae in the anterior gill arches of these fish allows blood to flow directly into the systemic circulation through the dorsal aorta..
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