● All vertebrates possess a `color{brown}("muscular chambered heart.")`

● `color{brown}("Fishes")` have a `color{violet}("2-chambered heart")` with an `color{violet}("atrium")` and `color{violet}("a ventricle.")`

● `color{brown}("Amphibians and the reptiles")` (except crocodiles) have a `color{violet}("3-chambered heart")` with two atria and a single ventricle, whereas `color{brown}("crocodiles, birds and mammals")` possess a `color{violet}("4-chambered heart")` with two atria and two ventricles.

● In `color{violet}("fishes")` the `color{violet}("heart pumps")` out `color{violet}("deoxygenated blood")` which is `color{violet}("oxygenated")` by the gills and supplied to the body parts from where `color{violet}("deoxygenated blood")` is returned to the heart (`color{brown}("single circulation")`).

● In amphibians and reptiles, the `color{violet}("left atrium")` receives `color{violet}("oxygenated blood")` from the `color{violet}("gills/lungs/skin")` and the right atrium gets the `color{violet}("deoxygenated blood")` from other body parts.

● However, they get mixed up in the `color{violet}("single ventricle")` which `color{violet}("pumps")` out mixed blood (`color{brown}("incomplete double circulation")`).

● In `color{violet}("birds and mammals, oxygenated and deoxygenated blood")` received by the left and right atria respectively passes on to the ventricles of the same sides.

● The `color{violet}("ventricles pump")` it out without any mixing up, i.e., two `color{violet}("separate circulatory pathways")` are present in these organisms, hence, these animals have `color{brown}("double circulation.")`

● `color{violet}("Human circulatory system,")` also called the `color{violet}("blood vascular system")` consists of a `color{violet}("muscular chambered heart")`, a network of `color{violet}("closed branching blood vessels")` and blood, the fluid which is circulated.

● `color{brown}("Heart, the mesodermally derived organ")`, is situated in the `color{violet}("thoracic cavity,")` in between the two `color{violet}("lungs, slightly tilted to the left.")`

● It has the size of a `color{violet}("clenched fist.")`

● It is protected by a double walled membranous bag, `color{brown}("pericardium")`, enclosing the `color{violet}("pericardial fluid.")`

● Our heart has four chambers, two relatively small upper chambers called `color{brown}("atria")` and two larger lower chambers called `color{brown}("ventricles.")`

● A `color{violet}("thin, muscular wall")` called the `color{brown}("interatrial septum")` separates the right and the left atria, whereas a thick-walled, the `color{brown}("inter-ventricular septum,")` separates the left and the right ventricles.

● The `color{violet}("atrium")` and the `color{violet}("ventricle")` of the same side are also separated by a thick `color{violet}("fibrous tissue")` called the `color{brown}("atrio-ventricular septum.")`