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Circulation and its significance
The circulatory system is present in all living organisms. It helps in carrying the circulatory fluid throughout the body. The circulatory system carries nutrients, oxygen, and hormones to the cells and removes carbon dioxide and other waste products. Circulation can be classified into intracellular and extracellular circulation. The circulation of fluid within the cells is called intracellular circulation. It is seen in unicellular organisms like amoeba and paramecium. The circulation of fluid outside the cell within the body is called extracellular circulation. William Harvey discovered the circulation of blood in the body.
Types of circulation and differences in characteristics
The process of circulation and the organs involved increase with an increase in the body complexity. Simple organisms like hydra and sponges do not have a proper circulatory system as the water in which they are present performs the functions of circulatory fluid by bringing in food and oxygen and removing wastes and carbon dioxide. In higher organisms, circulation within a body is broadly classified into open and closed circulatory systems.
Characteristics | Open circulatory system | Closed circulatory system |
Organs for blood flow | Through open channels and spaces called sinuses and lacunae | Through heart and blood vessels |
Contact with blood | Direct | Indirect |
Blood flow | Slow | Rapid |
Blood pressure | Low | High |
Exchange of nutrients and gases between blood and tissues | Direct | Through the capillary wall |
Efficiency | Low; blood flow cannot be controlled | High; blood flow is regulated by the smooth muscles of the blood vessels |
Respiratory pigment | If present, dissolved in plasma | Present, usually held in the RBCs |
Examples | Higher invertebrates – arthropods (insects, prawns) and a few molluscs (clams, snails, oysters) | Echinoderms, a few molluscs (squids), annelids, and all vertebrates |
In a closed circulatory system, a dynamic organ, a distribution system made of arteries and capillaries, and a reservoir and return system made of veins are present. The heart is a dynamic, chambered muscular organ that pumps blood throughout the body. Different types of circulation are seen based on the number of chambers in the heart.
- In fish, the heart is two-chambered (one atrium and one ventricle). The deoxygenated blood from the ventricle of the heart is pumped to the gills for oxygenation, which flows into different organs.
- In amphibians and reptiles (except crocodiles), the heart is three-chambered (two atria and one ventricle). The oxygenated blood is received from the lungs, gills, or skin into the left atrium, whereas the deoxygenated blood from the body parts flows into the right atrium. Both get mixed in the ventricle, and this is termed as incomplete double circulation.
- In crocodiles, mammals, and birds, the heart is four-chambered (two atria and two ventricles). Two circulatory pathways are present due to the presence of separate chambers for oxygenated and deoxygenated blood, and the mixing of blood is prevented. This is termed double circulation.
Double circulation in animals with a four-chambered heart
In double circulation, the blood passes through the heart twice in different pathways to complete one cycle. It includes systemic circulation, which carries the blood between the heart, the tissues, and back to the heart, and pulmonary circulation, which carries the blood between the heart, the lungs, and back to the heart.
In systemic circulation, the oxygenated blood entering the left atrium through the pulmonary veins is pumped into the left ventricle via the mitral valve. This is further pumped into the aorta via the aortic valve. The aorta branches into major arteries, smaller arteries, arterioles, and capillaries. The exchange of nutrients, gases, and wastes takes place in the capillaries with the tissues. The deoxygenated blood from the capillaries moves into the venules, veins, and vena cavae, which drains into the right atrium of the heart via the superior and inferior vena cava.
In pulmonary circulation, the deoxygenated blood moves from the right atrium to the right ventricle through the tricuspid valve. The blood is pumped to the pulmonary artery through the pulmonary valve. The pulmonary artery splits into the left and right pulmonary arteries that enter the left and right lungs. In the lungs, the gaseous exchange occurs in the alveoli. Oxygen is added, and carbon dioxide is removed from the blood. The oxygenated blood leaves the lungs via the pulmonary veins, which return the blood to the left atrium.
Other significant circulation systems
In coronary circulation, the coronary arteries that branch from the aorta supply oxygenated blood to the heart muscles, and the coronary veins drain the deoxygenated blood.
The portal venous system is seen when one capillary bed drains into the next via the veins. This type of circulation is seen in the network of blood vessels between the alimentary canal and the liver. The hepatic portal system helps in the distribution of nutrients absorbed from the small intestine to the liver before being sent to other tissues.
Conclusion
The process of circulation and the organs involved increase with an increase in the body complexity. Simple organisms like hydra and sponges do not have a proper circulatory system as the water in which they are present performs the functions of circulatory fluid by bringing in food and oxygen and removing wastes and carbon dioxide. In higher organisms, circulation within a body is broadly classified into open and closed circulatory systems.
Circulation helps in the distribution of useful substances like oxygen, nutrients, and hormones and the removal of waste substances like carbon dioxide.Circulation within the body can be of the open and closed circulatory system.Double circulation comprises systemic and pulmonary circulation.Coronary circulation provides the heart with blood supply.
