Describe double circulation of blood in human beings. Why is it necessary?
Double circulation refers to the process where blood passes through the heart twice during each cycle of passage through the body. In humans, this involves two separate circuits.
In the first circuit, deoxygenated blood from the body enters the thin-walled upper chamber on the right, the right atrium, as it relaxes. When the right atrium contracts, blood transfers to the right ventricle, which then pumps it to the lungs for oxygenation.
In the second circuit, oxygen-rich blood from the lungs enters the upper chamber on the left, the left atrium. When it contracts, blood transfers to the left ventricle, which pumps it out to the body. The ventricles have thicker muscular walls than the atria as they pump blood to various organs. Valves ensure blood does not flow backwards.
Double circulation is necessary because the separation of the right side and the left side of the heart keeps oxygenated and deoxygenated blood from mixing. This separation allows a highly efficient supply of oxygen to the body, which is essential for humans and mammals that have high energy needs and constantly use energy to maintain their body temperature.
Explanation
The answer covers the definition of double circulation, describes both circuits (pulmonary and systemic) with specific chamber names, and explains the necessity based on the textbook context. Key points include: blood passing through heart twice per cycle, the role of separated chambers, thicker ventricle walls, valve function, and the importance for maintaining body temperature in warm-blooded animals.