What direct effect do histamines have on capillaries? a) they decrease the diameter of capillaries. b) they prevent phagocytes from sticking to the walls of capillaries. c) they allow capillary walls to open and become leaky.

c) They allow capillary walls to open and become leaky.

Explanation:

Histamines are biogenic amines involved in immune responses in the human body. Its action is directly related to the chemical mediation of the inflammatory and allergic response. Its effect on the vascular system is vasodilatory, ie, it increases the diameter of the smaller vessels and decreases the blood pressure. As a result, blood vessels become more permeable to white blood cells and defense proteins, allowing a more efficient immune response.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.