The valve between the left atrium and left ventricle is the: MCQ with Answer and Explanation

Explanation:
The Sinoatrial (SA) node, located in the right atrium, is the natural pacemaker because its cells are autorhythmic; they can spontaneously generate action potentials without external neural input. Crucially, the SA node has the highest intrinsic rate of depolarization (about 70-80 beats per minute) compared to other autorhythmic tissues like the AV node or Purkinje fibers. Therefore, it sets the pace for the entire heart, overriding slower pacemakers.

Explanation:
Norman Myers introduced the concept of biodiversity hotspots in 1988—regions with high endemic species and significant habitat loss (≥70% original vegetation lost). Examples: Western Ghats, Indo-Burma. Conservation efforts prioritize these areas. Darwin contributed evolution, Humboldt to biogeography, E.O. Wilson is known for sociobiology and biodiversity studies. Hotspots cover only 2.4% of Earth's land but hold >50% of plant species.

Explanation:
Basophils, and similarly mast cells in tissues, release histamine and other inflammatory mediators during allergic and hypersensitivity reactions. Neutrophils are first responders to infection, lymphocytes are adaptive immune cells, monocytes become macrophages. Histamine causes vasodilation and bronchoconstriction.