What are the different ways, physiological and pharmacological, in
which blood vessel diameter can be modified?
Detection of changes in blood pressure (leading to resultant changes in blood vessel diameter):
Baroreceptors at the carotid sinus (the bifurcation of internal and external carotid arteries in the neck) and the aortic arch detect changes in blood pressure. These non-encapsulated nerve endings, located in the adventitial layer of arteries, are technically mechanoreceptors; they respond to arterial distension occurring due to a change in blood pressure. Afferents from the carotid region form the fine carotid sinus nerve which ascends into the glossopharyngeal nerve (9th cranial nerve). Afferents from the aortic region form the aortic (depressor) nerve before ascending into the vagus (10th cranial nerve). These cranial nerves terminate in the nucleus tractus solitarius.
Blood pressure increase detected by Baroreceptors nucleus tractus solitarius in brainstem depressor response reduced vasoconstriction lowering of peripheral resistance fall in blood pressure
Changing of peripheral vascular resistance
The changing of peripheral resistance involves vasodilation/vasoconstriction.
Vasodilation is an increase in the diameter of a blood vessel, whilst vasoconstriction is the decrease of the diameter. Vasodilation is a passive process, resulting from the recoil of elastic elements in the vessel walls as the smooth muscle walls relax. Vascular resistance is owed largely to the arterioles of the systemic circulation. Arteriole resistance regulates blood flow to the tissues downstream. In addition to this arteriole resistance (in combination with the cardiac output) regulates the systemic arterial pressure.
Intrinsic (local) mechanisms:
In tissues with low tolerance of ischaemia (inadequate blood flow), such as the brain and heart, intrinsic flow adjustment mechanisms dominate. These include: Local temperature, myogenic effects, local metabolites, autocoids and NO.
The efferent limb of the extrinsic system comprises the autonomic vasomotor nerves: sympathetic vasodilator fibres, sympathetic vasoconstrictor fibres...