- The circulatory patterns are of two types
- Open circulatory system is the one in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. Example- arthropods and molluscs.
- Closed circulatory system in which the blood pumped by the heart is always circulated through a closed network of blood vessels. Example- Annelids and chordates
- All vertebrates possess a muscular chambered heart.
- Fishes have a 2-chambered heart with an atrium and a ventricle.
- Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle.
- Crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles.
- In fishes, the heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart (single circulation).
- In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from other body parts, however, they get mixed up in the single ventricle which pumps out mixed blood, which is called as incomplete double circulation.
- In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides, and the ventricles pump it out without any mixing up, hence, these animals have double circulation.
Fig. Open circulatory system