Question

How does blood circulation occur in fishes?

Answer 1

A fish's heart is also referred to as a venous heart. They are usually two chambered only: a single auricle for transportation of oxygenated blood and a single ventricle that passes de - oxygenated blood. It is mainly a type of close circulation of blood.

Complete answer:
A Vertebrate heart is a hollow, muscular organ composed of cardiac muscular fibres. It acts as a pumping organ of the blood vascular system. It receives blood from and pumps blood to the various organs and tissues of the body.
The heart is divided internally into intercommunicating chambers, the number of which varies in different vertebrate groups. The reason for this variation is that during the evolution of the heart certain chambers have been incorporated into others and some have divided into separate chambers. The heart in all vertebrates has 1 or 2 auricles and 1 or 2 ventricles. These chambers are called true chambers. The heart of lower vertebrates has additional chambers,namely,sinus venosus and conus arteriosus,or bulbus arteriosus, or truncus arteriosus. These are termed accessory chambers.
A fish heart handles only deoxygenated blood, and is often called a venous heart. It is two-chambered, having a single auricle and a single ventricle. The accessory chambers,sinus venosus and conus arterious or bulbus arterious, are present. The heart receives the deoxygenated blood from all over the body, except the gills, into the sinus venosus, and pumps in via the auricle, ventricle and conus/bulbus into the short artery, the ventral aorta.
The latter gives off 4 or 5 pairs of afferent branchial arteries,which distributes the blood from gills flow via efferent branchial arteries, epibranchial arteries, lateral dorsal aortae into the dorsal aorta,whose branches distributes it to the rest of the body by splitting into capillaries.The blood returns to the heart through vein.Since the blood passes through the heart only once in a complete circuit around the body, the fishes are said to have a single circulation is that the entire body receives oxygenated blood.
A disadvantage is that the narrow gill capillaries slow down the blood flow and the body receives blood at a low pressure. This slows the rate of oxygen supply to the cells and limits the metabolic rate that fish can attain.

Note:
An essential requirement of most animals is the transport of material from the body where it is formed to the place where it is needed. Main materials that are transported through this include nutrients, gases, hormones and nitrogenous wastes. As the animal increases in size and complexity, the amount of substance to be moved into and out of the body increases.