The arteries, arterioles, and veins are supplied by vasoconstrictor fibers of the sympathetic nervous system. However, the neural influence on larger vessels is much less compared to arterioles and small arteries. Resistance vessels, also known as capacitance vessels, are more responsive to sympathetic nerve stimulation than veins. They reach maximum constriction at a lower stimulation frequency compared to resistance vessels. Nevertheless, capacitance vessels do not possess β-adrenergic receptors, resulting in a lesser response to vasodilator metabolites. The neurotransmitter released at the sympathetic nerve terminals in blood vessels is norepinephrine. The release of norepinephrine from the nerve terminals is mediated by factors like circulating hormones and locally released substances.

Figure 18.24 illustrates the response of resistance and capacitance vessels to sympathetic fiber stimulation while maintaining constant arterial pressure. Stimulation of sympathetic fibers leads to constriction of resistance vessels, reducing blood flow and decreasing tissue blood volume. This constriction of resistance vessels establishes a new equilibrium of the forces responsible for filtration and absorption across the capillary wall.

In addition to active changes in vessel caliber caused by contraction and relaxation of vascular smooth muscle, passive changes occur due to alterations in intraluminal pressure. An increase in intraluminal pressure causes vessel distention, while a decrease results in a reduction in vessel caliber due to elastic recoil of the vessel walls.

Under basal vascular tone, approximately one-third of the blood volume of a tissue can be mobilized when sympathetic nerves are stimulated at physiological frequencies. Basal tone is significantly low in capacitance vessels. Denervation of these vessels experimentally results in minimal increases in volume when exposed to maximal doses of ACh. Hence, at basal vascular tone, the blood volume closely corresponds to the maximum blood volume of the tissue. Skin capacitance vessels can mobilize more blood compared to muscle capacitance vessels due to their greater sensitivity to sympathetic stimulation and lower basal tone.

Physiological stimuli play a role in mobilizing blood from capacitance vessels. For instance, during physical exercise, sympathetic nerve fiber activation leads to constriction of peripheral veins, increasing cardiac filling pressure. In cases of arterial hypotension such as hemorrhage, the constriction of capacitance vessels helps correct the decreased central venous pressure associated with blood loss.

The efferent fibers of the parasympathetic nervous system's cranial division innervate blood vessels in the head and some viscera. On the other hand, fibers of the sacral division innervate blood vessels in the genitalia and bladder.
Vasoconstrictor fibers of the sympathetic nervous system supply the arteries, arterioles, and veins; the neural influence is much less on larger vessels than on arterioles and small arteries. Capacitance vessels (veins) respond more to sympathetic nerve stimulation than do resistance vessels; the capacitance vessels are maximally constricted at a lower stimulation frequency than the are resistance vessels. However, capacitance vessels lack β-adrenergic receptors, and they respond less to vasodilator metabolites. Norepinephrine is the neurotransmitter released at the sympathetic nerve terminals in blood vessel. Factors such as circulating hormones and particularly locally released substances mediate the release of norepinephrine from the nerve terminals.
The response of the resistance and capacitance vessels to stimulation of sympathetic fibers is illustrated in
Fig.
18.24 . When arterial pressure is held constant, stimulation of sympathetic fibers reduces blood flow (constriction of resistance vessels) and decreases the blood volume of the tissue (constriction of capacitance vessels). Constriction of the resistance vessels establishes a new equilibrium of the forces responsible for filtration and absorption across the capillary wall (see
Eq. 17.20
In addition to active changes (contraction and relaxation of vascular smooth muscle) in vessel caliber, passive changes are also caused by alterations in intraluminal pressure. An increase in intraluminal pressure distends the vessels, and a decrease reduces the caliber of the vessels as a consequence of elastic recoil of the vessel walls.
At basal vascular tone, approximately a third of the blood volume of a tissue can be mobilized when the sympathetic nerves are stimulated at physiological frequencies. Basal tone is very low in capacitance vessels; if these vessels are denervated experimentally, the increases in volume evoked
Change of tissue Arterial pressureBlood flow volume (mL) (mm Hg) •Fig. 18.24 Effect of Sympathetic Nerve Stimulation (2 Hz) on Blood Flow and Tissue Volume of the Lower Limb. Theupward arrow denotesthechangeinslopeofthetissuevolumecurveatthepointatwhichthedecreaseinvolumecausedbyemptyingofcapacitancevesselsceasesandlossofextravascularfluidbecomesevident.Theabruptdecreaseintissuevolumeiscausedbymovementofbloodoutofthecapacitancevesselsandoutofthelowerlimb.Thelate,slow,progressivedeclineinvolume(totherightofthearrow)iscausedbythemovementofextravascularfluidintothecapillariesandhenceawayfromthetissue.Thelossoftissuefluidresultsfromtheloweringofthecapillaryhydrostaticpressuresecondarytoconstrictionoftheresistancevessels.(FromMellanderS.Acta Physiol Scand Suppl. 1960;50[176]:1.) by maximal doses of ACh are small. Therefore, at basal vascular tone, blood volume is close to the maximal blood volume of the tissue. More blood can be mobilized from the capacitance vessels in the skin than from those in the muscle. This disparity depends in part on the greater sensitivity of the skin vessels to sympathetic stimulation, but it also occurs because basal tone is lower in skin vessels than in muscle vessels. Therefore, in the absence of a neural influence, skin capacitance vessels contain more blood than do muscle capacitance vessels.
Physiological stimuli mobilize blood from capacitance vessels. For example, during physical exercise, activation of sympathetic nerve fibers constricts the peripheral veins and hence augments cardiac filling pressure. In arterial hypo-tension (as occurs in hemorrhage), the capacitance vessels constrict and thereby correct the decreased central venous pressure associated with blood loss.
The efferent fibers of the cranial division of the parasympathetic nervous system innervate the blood vessels of the head and some of the viscera, whereas fibers of the sacral division innervate blood vessels of the genitalia, bladder,