Architectonics and hemodynamic features
The coronary bed originates directly from the aorta, which ensures high perfusion pressure—the main physical factor linearly determining the volume of blood flow. Drained venous blood collects into the coronary sinus, which empties into the right atrium. Outflow also occurs through small thebesian veins that open directly into the heart chambers.
The myocardial vascular network is very dense, and capillaries have an initially high basal tone. Arteriovenous anastomoses and arteriolar-sinusoidal shunts are present in the bed. An important clinical feature is the presence of numerous end-artery type vessels. Because of this, when patency is impaired, there is a high risk of developing ischemia or infarction, which is especially relevant for elderly patients.
Phasic nature of blood supply
Due to continuous muscle contractions, the lumen of the coronary arteries constantly changes. Blood supply fundamentally depends on the current phase of the cardiac cycle:
- Systole. Blood flow drops sharply (accounting for about 40.8% of the total volume). The minimum value is recorded during the phase of isometric contraction. This happens for two reasons: the tense myocardium physically compresses the capillaries from the outside, and the open cusps of the aortic semilunar valves block the ostia of the coronary arteries.
- Diastole. As soon as heart relaxation begins, blood inflow increases rapidly. It reaches its peak during the phase of isometric relaxation.
To prevent contracting muscle cells from suffering from hypoxia during moments of blood flow interruption, they contain the intracellular protein myoglobin. During diastole, it actively binds oxygen, and during systole, it releases it to the tissues.
Nervous regulation
The regulation of vascular lumen is carried out through spinal centers and vagus nerve nuclei, which, in turn, are controlled by the hypothalamus and cerebral cortex.
- Parasympathetic system: Stimulation of the vagus nerves causes a slight decrease in coronary blood flow. This effect can be masked by the general influence on the heart: slowing of the rhythm and a subsequent drop in blood pressure.
- Sympathetic system: Its effect is biphasic in nature. Initially, nerve impulses cause constriction of resistance vessels. However, this spasm is soon replaced by powerful vasodilation (dilation), which is associated with the active accumulation of intense metabolic products. The purely vasoconstrictor effect remains dominant only under super-strong stimulation.
Influence of heart rate and humoral control
Although hemodynamic and neural mechanisms are important, humoral regulation is primary in determining the final lumen of the coronary vessels.
Changes in heart rate (HR) affect blood supply in an interesting way. During tachycardia, two factors compete:
- Limiting factor: rhythm acceleration shortens diastole (the time of optimal inflow) and can reduce blood pressure.
- Stimulating factor: increased heart work accelerates metabolism, which causes active vessel dilation.
Ultimately, an increase in HR leads to an increase in total blood flow. Bradycardia, conversely, lengthens diastole, physically improving conditions for blood inflow while simultaneously reducing the energy demands of the myocardium.
Briefly: answers to topical questions
What energy substrates does the myocardium consume?
The myocardium consumes fatty acids, ketone bodies, lactate, and glucose. At rest, the primary source of energy is fatty acids and their breakdown products supplied from the liver (e.g., acetoacetate). During intense physical exertion, the heart muscle switches to utilizing lactate delivered from working skeletal muscles, as well as glucose. In general, additional energy sources such as glucose and lactate provide 30–40% of the total demand of the constantly working heart.
What specific metabolites cause humoral vasodilation of the coronary vessels?
Humoral (metabolic) vasodilation of coronary vessels is caused by metabolite substances: adenosine, bradykinin, prostaglandins, lactic acid (lactate), and NO (nitric oxide). During insufficient blood flow, ATP breakdown products accumulate in the myocardium, especially in the ischemic zone, and dilate coronary vessels by stimulating purine receptors; ADP, adenosine monophosphate, and adenosine are also indicated in this mechanism in the study guide.
What are end-type vessels in the myocardium?
End-type vessels in the myocardium are described in sources as a feature of the coronary network: despite a dense capillary network, there are many such vessels, so their occlusion poses a high risk of infarction, especially in old age. In the heart, anastomoses exist between small coronary vessels; they are developed in the posterior wall of the left ventricle and the septum. Collaterals are fewest in the region of the descending branch of the left coronary artery, which is why infarctions of this localization occur most frequently.
Systole — the heart "holds its breath" (vessels compressed, valves closed). Diastole — the heart "breathes" (vessels open, blood flows in). Myoglobin works as an "oxygen tank" for systole.
Class and exam questions
Why does coronary blood flow drop during systole?
The contracting myocardium mechanically compresses capillaries, and open aortic semilunar valves block the ostia of the coronary arteries.
What is the role of myoglobin in the heart?
It stores oxygen during the relaxation phase and releases it to myocardial cells during contraction, when blood flow is temporarily halted.
How does the sympathetic nervous system affect cardiac vessels?
The effect is biphasic: short-term constriction is replaced by marked dilation due to the active release of metabolites by the working muscle.
What else to study on this topic
- Humoral regulation (key substances and mechanisms)
- Anatomy and physiology of the thebesian veins
- Mechanisms of extravascular compression
- Influence of the hypothalamus and cortex on cardiac blood flow
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