Basal tone and local autoregulation
The lumen of the vascular bed largely depends on the basal tone, which is formed due to the automatism of smooth muscles and the stretching of the elastic vessel wall by the pulse wave. This parameter is directly affected by the high partial pressure of oxygen in arterial blood and the presence of $Ca^{2+}$ ions.
An exceptional role in regulation is played by the endothelium, which secretes biologically active substances:
- Vasodilators (widen the lumen): nitric oxide (NO), prostacyclin, endothelium-dependent hyperpolarizing factor.
- Vasoconstrictors (narrow the lumen): endothelin, superoxide anion, prostanoids, angiotensin II.
Mechanism of action of NO: Nitric oxide diffuses into smooth muscle cells, where it activates guanylyl cyclase (coupled with a G protein). This leads to an increase in intracellular cyclic guanosine monophosphate (cGMP) and relaxation of the wall.
Neural regulation and hemodynamic reflexes
Central nervous regulation is based on the activity of the vasomotor center (VMC) of the medulla oblongata, which has pressor and depressor divisions. When blood pressure changes, systemic reflexes are triggered:
- Depressor (baroreceptor) reflex: In response to an increase in blood pressure, impulse activity from arterial baroreceptors increases. This causes maximal (cessation) inhibition of sympathetic center neurons — they stop sending stimuli to spinal neurons, and the blood vessels dilate.
- Bainbridge reflex: The body's response to an increase in venous pressure.
- Parin's reflex: When pressure in the pulmonary trunk increases, bradycardia, a drop in systemic blood pressure, and dilation of splenic vessels occur.
Features of autonomic innervation: The sympathetic system generally constricts blood vessels. However, there are exceptions: blood vessels of the brain, heart, lungs, and working skeletal muscles dilate during sympathetic excitation. This is due to the presence of cholinergic fibers in the sympathetic trunks that release acetylcholine (ACh). The physiological purpose is the preemptive supply of oxygen to muscles and vital organs during rapid mobilization.
The parasympathetic system (N. vagus) is also ambiguous: its fibers dilate pulmonary vessels (increase blood flow), but at the same time constrict coronary vessels.
Effect of blood gases and metabolic products
Visceral systems have high sensitivity to metabolites and gases, yet respond to them differently depending on the organ.
Effect of oxygen ($O_2$):
- In the myocardium and brain: Hypoxia (a drop in $O_2$) causes active and pronounced vasodilation.
- In the lungs: Alveolar hypoxia (in diseases or inhalation of $O_2$-poor mixtures) leads to simultaneous constriction of pulmonary arterioles. This is a protective mechanism that sharply increases pressure in the pulmonary trunk.
Effect of carbon dioxide ($CO_2$): In coronary vessels, a high concentration of $CO_2$ produces a biphasic reaction: first, an increase in tone due to central influence, followed by a significant drop in pressure (dilation) due to local action. In brain tissue, an increase in $CO_2$ causes dissociation of carbonic acid with the release of $H^+$ ions, leading to vessel dilation ($H^+$ ions themselves do not cross the blood-brain barrier).
Metabolites and ions: Dilation of coronary vessels is promoted by adenosine, bradykinin, prostaglandins, lactate, and NO. In skeletal muscles, local vasodilation during contraction is provided by $K^+$ ions and phosphates. Adenosine is also a key factor in the regulation of cerebral blood flow — its level rises during ischemia, hypocapnia, and hypotension.
Briefly: answers to topical questions
How does the parasympathetic nervous system affect the vascular tone of various organs?
The parasympathetic nervous system normally does not innervate the smooth muscle of most blood vessels.
Sources specifically point out the following vessels and effects:
- Blood vessels of the heart — fibers of the parasympathetic vagus nerve constrict coronary vessels.
- Cerebral arteries — belong to vessels with parasympathetic innervation; stimulation of parasympathetic nerves causes slight dilation of the lumen.
- Vagus nerves to pulmonary vessels — stimulation of cholinergic fibers increases blood flow in the arterioles and capillaries of the lesser circulation; vagotomy yields the opposite result.
- Corpora cavernosa of the penis — parasympathetic fibers S2–S4 cause vasodilation, leading to an erection.
- Kidneys and urinary bladder — parasympathetic innervation is associated with vasodilation.
Parasympathetic innervation is also supplied to the vessels of the genital organs, salivary glands, and mesentery.
Which hormones mediate systemic humoral regulation of vascular tone?
Humoral regulation of vascular tone includes circulating blood substances and local factors; it is subdivided into vasospastic and vasodilating systems.
- Vasoconstrictor (pressor) substances/hormones — adrenaline, serotonin, vasopressin, angiotensin II, as well as certain types of prostaglandins.
- Vasodilator substances/hormones — histamine, kinins, acetylcholine, as well as certain types of prostaglandins.
Prostaglandins of groups A and E cause dilation of small arteries. Prostaglandin E is in a competitive relationship with pressor hormones — vasopressin and adrenaline. Angiotensin II exerts a potent pressor, vasoconstrictive effect with an increase in systolic and diastolic blood pressure. Adrenaline constricts blood vessels in the abdominal organs, lungs, and skin.
Through which types of receptors do catecholamines cause vasoconstriction and vasodilation?
According to sources, vasoconstriction during sympathetic neuronal regulation is associated with the stimulation of postsynaptic $\alpha_1$-adrenergic receptors.
For adrenaline in skeletal muscle arterioles, the effect depends on concentration:
- High concentration — vasoconstriction, $\alpha$-adrenergic effect.
- Low concentration — vasodilation, $\beta$-adrenergic effect.
The sympathetic system constricts everything except the B-H-L-M: Brain, Heart, Lungs, Muscles (working). In these organs, the sympathetic system dilates blood vessels for emergency mobilization of the body.
Class and exam questions
What happens to pulmonary vessels during hypoxia?
Unlike most body tissues, where oxygen deprivation causes dilation, in the lungs alveolar hypoxia leads to generalized constriction of pulmonary arterioles and an increase in pulmonary trunk pressure.
Which ions increase vascular resistance?
Vascular resistance (decreased blood flow) is also caused by ions in solutions of $KCl$, $MnO_4$, and $MgCl_2$. At the same time, $H^+$ and $Ca^{2+}$ ions decrease resistance.
What else to study on this topic
- Humoral regulation of lesser circulation hemodynamics (the role of histamine)
- Detailed mechanism of the baroreceptor (depressor) reflex
- Features of the biphasic effect of carbon dioxide on blood vessels
- Effect of biologically active substances on basal tone
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