I was going to discuss Vitamin K2 before, because I had cheese this morning, and cheese has the highest amount of K2 which is responsible for calcium distribution into the bones and teeth. Vitamin K2 and Vitamin D work hand in hand. While Vitamin D helps the body absorb calcium in the intestines into the blood while Vitamin K2 takes the calcium from the blood and transports them into the bones and teeth. That is that on vitamin K2 for now, I will do well to take that topic someday and explain the vitamin properly while explaining its relationship with Vitamin D. So today, I will be discussing the Circulatory system in fish. I decided to change my mind because I was discussing with a friend, and she was mistaking opened circulation for closed circulation, only for her to end up saying that fishes exhibit open circulation. So I decided to do this very fast, but before then, did you know that the study of fish is called ichthyology?
Let me first correct the type of circulatory system that she talked about. Fish has a closed circulatory system and a single circulatory system. With a closed circulatory system, the circulatory fluids are enclosed within the heart and the vessel and are never free-flowing. In closed circulation, the circulatory fluid doesn't mix with the interstitial fluid directly. With an open circulatory system which is present in insects, the circulating fluids come out in the body cavities of the insects. This type of circulatory system is not enclosed within vessels, rather the blood mixes with interstitial fluid and tissues. With fish, the circulatory fluid of the fish remains within the vessels of the fish and the heart of the fish. That discussed, let me explain the single circulatory system which I said the fish also have. With a single circulatory system, the heart pumps blood to the vessels which return it back to the heart once. The heart pumps the blood to the body through the vessel and then back to the heart. This is different from the double circulatory system where the circulatory fluid goes to the heart then the heart sends it to respiratory tissues such as the lungs which then return the blood back to the heart and then to the body. I guess I have been able to explain that again. I said that the circulatory fluid in a single circulatory system reaches the heart once in circulation, while in a double circulatory system, the circulatory system reaches the heart twice before completing the cycle.
In fish, blood circulation is limited within the lumen of the vessels and the heart chambers and in the complete circulation of the blood, it goes to the heart once. When the blood passes through the capillaries of the fish, it gives oxygen and nutrients to the cells and tissues and takes up carbon dioxide and metabolic waste away from the cells and tissues where it is moved to the veins which are a combination of the capillary to form the venous system. The veins take the blood back to the heart which is a double-chamber heart, with two accessory structures. The blood from the vein goes to the sinus venosus where blood is collected from the veins. The blood then goes to the Atrium which has a weak contraction then pumps the blood to the ventricle which is a chamber of the heart that has a stronger contraction. When the Atrium pumps the blood to the ventricle, a backward movement is prevented by the sinoatrial valve. In the Ventricle, the Atrioventricular valve prevents the backward movement of blood to the Atrium when the Ventricle contracts and push blood to the Conus arteriosus and then from the Conus arteriosus to the ventral aorta. This valve and organs are located in the ventral area of the fish which is around the belly side of the fish. The opposite side of the fish is the dorsal side of the fish. It is important to know that the large vessels around the fish that supplies the kidney is known as the dorsal aorta. From the ventral aorta, blood goes to the respiratory organ of the fish which is the gills through the vessel known as the afferent branchial vessels. From the gills, gaseous exchange occurs where carbon dioxide is taken out of the blood and oxygen is absorbed into the blood. To state clearly, until the blood reaches the gills, they are deoxygenated blood, and when it reaches the capillaries of the gills, they become oxygenated. The oxygenated blood from the capillaries of the gills goes to the efferent vessels which exit the gills. In the capillaries of the gills, the blood pressure becomes low. Different efferent vessels combine to form the dorsal aorta which is on the opposite side of the ventral side. The blood from the dorsal aorta takes the blood to the tissues via tissue capillaries.
In summary, fishes have a single-closed circulatory system where the blood passes through the heart once. The heart of a fish has two chambers and it is made up of the atrium, ventricle, sinus venosus, and bulbus arterios/Conus arteriosus. Blood that goes into the heart is deoxygenated and when pumped out of the heart, they are pumped to the gills through the ventricle which pumps the blood to the gill capillaries. The gill capillaries carry oxygenated blood to the dorsal aorta than the systemic capillaries, where the blood is distributed and then collected by the veins (deoxygenated blood) and back to the heart.
Citation
https://www.necropsymanual.net/en/teleosts-anatomy/circulatory-system/
https://byjus.com/question-answer/how-does-blood-circulation-occur-in-fishes/