1.6 EN: Circulatory System of Fish - YouTube

Schematic of circulatory system in the South American lungfish, Lepidosiren paradoxa
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The mammalian lymphatic system consists of blind-ending lymphatic vessels, lymph nodes and other tissues, including the spleen, that collect, filter and return fluid that drains from the tissues to the venous system and transport fats and proteins from the gut into the blood system (; ). In teleost fishes, some circulatory vessels contain clear fluid and have been described as lymphatic vessels (). Yaniv et al. () and Isogai et al. () have also demonstrated that these vessels share some morphological and molecular characteristics with the lymphatic vessels found in other vertebrates, and suggested that these clear vessels in zebrafish could be used as a model for the mammalian lymphatic system. However, several decades ago these same vessels were hypothesized as part of a secondary vascular system (SVS), receiving inflow from the arterial system via arterio-arterial anastomoses and emptying into the venous circulation (; ; ; ; ; ). These vessels are not blind-ending and sometimes include red blood cells (RBCs) (). Furthermore, there is no evidence that the SVS vessels are draining edematous tissues or are playing a role in lipid transport. If not lymphatic, what then is the role of this secondary vascular system?
Microscopic view of circulation (blood flow) in the tail of a goldfish
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The fish heart is remarkably well suited to the gill respiratory apparatus and represents one of the major evolutionary innovations in the vertebrates. Perhaps its greatest advantage is that the blood it delivers to the tissues of the body is fully oxygenated because it is pumped through the gills first, as shown in the circulation cycle in figure 23.11b. Blood is pumped first through the gills, toward the right side of the cycle, where it becomes oxygenated; from the gills, it flows through a network of arteries and capillaries to the rest of the body; then it returns to the heart through the veins. This arrangement has one great limitation, however. Recall from the discussion in section 23.2 that blood loses pressure when it passes through capillaries, and so in fish, the blood loses much of the pressure developed by the contraction of the heart as it passes through the capillaries in the gills. Because of this, the circulation from the gills through the rest of the body is sluggish. This feature limits the rate of oxygen delivery to the rest of the body. It seems that lungfish display the beginnings of a double circulatory system:
Photo provided by FlickrIt seems that lungfish display the beginnings of a double circulatory system:
Photo provided by FlickrEverything You Need to Know About the Circulatory System of Fish
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As in other vertebrates the circulatory system of fish is comprised of both static and dynamic components. The dynamic part is the blood with all its constituent parts that flows continuously around the fish's body. The static parts are the heart, the veins and arteries leading to and from it and the capillaries that connect them. Fish have less blood per gram of body weight than mammals, normally between 3% and 8% of a fish's body weight is blood, however in the hagfish and lampreys it is greater, for them between 8% and 20% of the animal's body weight. Fish have a simple circulatory system, which consists of a two-chambered heart, blood, and blood vessels. Unlike humans, they have a single circulatory pattern.At the centre of the static, comparatively, parts of the circulatory system is the heart which is normally situated below the pharynx and immediately behind the gills. A fish's heart has four chambers, but unlike us the chambers of their heart are not all muscular, and hey are not so built into a single organ, rather they are located one behind the other, this is particularly evident in the sharks and rays.Not all th plasma of the blood returns through the capillaries, some of it mixes in with the fluid that surrounds the cells of every tissue and is eventually drained away into the lymphatic vessels. This secondary circulation of lymph vessels eventually empties into the main blood veins. However because it left the main circulatory system it is unaffected by the beating of the heart and its flow is slow. In some fish it is helped along by small lymph hearts, muscular valved tubes which help express the blood into the veins. Lymphatic hearts can be found in various places in those species that have them, in the Eel () has one lymphatic heart situated in its tail.