Activation of P1 receptors in the heart increases both the force of contraction and the rate of beating; thus, the rate of delivery of blood to the rest of the body (the cardiac output) is increased. This is probably an effect of noradrenaline released from sympathetic nerve endings rather than of adrenaline, except at very high adrenaline concentrations (e.g. in severe stress).
In the blood vessels, the resistance of particular blood vessels (the diameter of the lumen) is regulated by smooth muscle in the walls. These smooth muscles are regulated by the sympathetic nervous system. For the most part, this is achieved through a1 and a 2 receptors, which bring about contraction of the muscle and vasoconstriction (narrowing of the vessels). This has two effects. In the body as a whole, blood pressure will be increased since the heart is pumping blood through narrower channels. In specific organs and tissues, this is a means of selectively increasing or decreasing blood flow under different conditions. In fact, in most tissues there is continuous vasomotor tone; the sympathetic fibres are active continuously, under the influence of the medulla oblongata in the brainstem (the very primitive part of the brain); variations in flow are brought about by relaxation of this tone, or further constriction.
In skeletal muscle, it was mentioned earlier that the smooth muscle of the blood vessels is innervated by cholinergic sympathetic fibres. Activation of these fibres leads to vasodilatation (opening up of the vessels with a consequent increase in blood flow). This is certainly true in some animals although its significance in humans has been questioned. It will be discussed again in connection with the increased skeletal muscle blood flow that is seen during exercise (Section 8.4.5).
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