18. BODY FLUIDS AND CIRCULATION
Circulation is the transport of nutrients, oxygen, CO2 and excretory products to the concerned tissues or organs. For circulation, simple organisms (sponges, coelenterates etc.) use water from their surroundings. Complex organisms use body fluids (blood & lymph) for circulation.
Circulatory system is 2 types- Open and Closed.
- Open circulatory system: Here, the blood pumped by the heart passes through large vessels into open spaces or cavities called sinuses. E.g. Arthropods and molluscs.
- Closed circulatory system: Here, the blood pumped by the heart is always circulated through blood vessels. This is more advantageous as the flow of fluid can be more precisely regulated. E.g. Annelids and chordates.
- Fishes: 2-chambered heart (an atrium + a ventricle).
- Amphibians: 3-chambered heart (2 atria + a ventricle).
- Reptiles (except crocodiles): 3-chambered heart (2 atria + a ventricle). Ventricle is incompletely partitioned.
- Crocodiles, birds & mammals: 4-chambered heart.
Types of circulation
- Single circulation: In fishes. In this, heart receives impure blood only (venous heart).
Deoxygenated blood → to heart → to gills → oxygenated blood → to body parts → deoxygenated blood → to heart.
- Incomplete double circulation: In amphibians & reptiles. In this, left atrium gets oxygenated blood from gills/ lungs/skin and right atrium gets deoxygenated blood from other body parts. However, they get mixed up in the single ventricle. It pumps out mixed blood.
- Double circulation: In birds & mammals. Right atrium gets deoxygenated blood and passes to right ventricle and left atrium gets oxygenated blood and passes to left ventricle. The ventricles pump it out separately without any mixing up.