Physiological actions of histamine::
smooth muscles::
The smooth muscles are stimulated by histamine it causes marked contraction of the bronchial musculature giving rise to asthma and asphyxia (decrease in gaseous exchange ) the edematous swelling of bronchioles is associated with bronchial asthma the action of histamine on smooth muscles of intestine uterus and gall bladder are relatively of lesser intensity giving rise to colicky pain all the contractile effects of histamine are mediated through histamine H1- receptors histamine causes relaxation of rat uterus and this is mediated through histamine H2 receptors
Cardiovascular system::
Histamine produces increase in force of contraction and rate of heart histamine causes fall in blood pressure which may be followed by a rise due to the subsequent release of adrenaline there is dilatation of the capillaries and arterioles which is chiefly responsible for the fall in BP. in addition the increased permeability of the capillary walls results in loss of fluid from circulation and consequently decrease in blood volume this phenomenon together with capillary dilatation causes circulatory collapse this has been observed especially when histamine liberators are administered or in anaphylactic and peptone shocks the hepatic veins and pulmonary veins constrict due to contraction of the muscles of the veins which causes congestion in these organs
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