This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
Changes in the caliber of the arterioles may be local, affecting the blood-supply of only one or two organs, or may be general, affecting general arterial pressure. The caliber is determined by the activity of the arterial muscles, which, by their contraction narrow the lumen of the artery, and by their relaxation widen it.
These muscles may act as the result of direct stimulation or depression, or in response to impulses received through the vasomotor nerves. Of these vasomotor nerves there are two sets, the vasoconstrictors and the vasodilators, each set consisting of center, nerves, ganglia, and the nerve-endings in the arterial muscles. The vasoconstrictor centers are masses of cells situated on both sides of the middle line in the medulla oblongata; the vasodilator centers are scattered masses of cells in various parts of the central nervous system. The arterial muscles are in a constant state of contraction or tone, which enables them to resist the bursting pressure of the fluid within; and this resistance tone, though insured to a slight extent by the inherent nature of muscle which makes it contract in response to a demand put upon it, is due in very large measure to the continuous reception of subminimal impulses from the vasoconstrictor center. Thus there is a certain amount of contraction or tone normally present in the arteries, and when the vasoconstrictor centers, ganglia, or nerve-endings are depressed by drugs, this tone is lowered and the arteries dilate.
The vasodilators differ from the vasoconstrictors, for, in the first place, they do not act continuously, but only under special circumstances; and, secondly, they produce dilatation only by inhibiting the contractile impulses, for there are no dilating muscles in the arteries.
Both the vasoconstrictor and the vasodilator nerves belong to the sympathetic system. When both sets are stimulated together, the vasoconstrictor effect prevails; but under excessive or prolonged stimulation the vasoconstrictor is the first to show exhaustion, so that the constriction may be followed by wide dilatation, even the intrinsic tone of the muscle-fibers being probably somewhat inhibited.
Like the vagus and accelerator mechanisms, the vasomotor may be affected by remedies acting directly upon any part of the vasomotor system, viz., center, nerves, ganglia, or nerve-endings; and they may also be affected reflexly by afferent impulses coming to the centers from other parts of the body.
The arteries may be contracted by:
1. Direct stimulation of their muscle-fibers.
2. Direct or reflex stimulation of the vasoconstrictor nervous mechanism, or the neuromuscular junction. The arteries may be dilated by:
1. Direct depression of their muscle-fibers.
2. Direct or reflex depression of the vasoconstrictor nervous mechanism.
3. Direct or reflex stimulation of the vasodilator mechanism. Some of the arteries do not have vasoconstrictor nerves. At least, nerves connected with the vasoconstrictor center have not been demonstrated in the coronary arteries, those of the brain, and those of the lungs. (See Howell.) These arteries, however, maintain their intrinsic tone.
The blood-supply of the heart is somewhat intermittent, and is dependent upon a proper diastolic pause, for during the greater part of systole the blood is squeezed out of the coronaries, while during the diastolic pause the coronaries refill from the aorta and make an active circulation in the relaxed heart. Dilatation of the coronaries is frequently brought about by drugs that constrict other arteries. In the brain the supply of the blood is largely determined by the rise and fall of general arterial pressure plus the influence of gravity. Of the pulmonary circulation we shall speak later.
The caliber of the cutaneous arterioles is under a sensitive control mechanism different from that of the other arterioles of the body, so that their dilatation and contraction frequently take place independently of the general arteriole system, as in blushing. They are weak arteries, however, and regularly tend to be somewhat dilated when general arterial pressure is high.
The veins also contain muscles, but their contraction and dilatation seem to be of little moment in pharmacology. The large veins, even the portal vein, as demonstrated by Burton-Opitz, are scarcely if at all influenced through vasomotor nerves. The venous system, however, forms an enormous reservoir for blood, so that by the accumulation of blood in the veins the arterial system may be readily depleted. Venous pressure varies considerably, that in the superior cava being alternately negative and positive, and that in the inferior cava constantly positive and sometimes as high as 50 or 60 mm. of mercury. It must be remembered that the period of filling of the ventricle is shortened if the venous pressure is high, that during the period of diastasis the venous onflow in the large veins is stopped, and that during auricular systole there is some reflux into the great veins.
The capillaries have no muscles, and dilate or contract mechanically as more or less blood is forced into them. It is their function to serve as a membranous medium of exchange between the blood and the tissue-fluids, in both directions.