Pathomechanisms of Ischemia
The pathogenesis of ischemia is based on two fundamental processes. The first is decreased arterial blood inflow. This variant is mediated by several mechanisms:
- Neurogenic: associated with impaired neural regulation of vascular tone.
- Humoral: occurs due to an excess of vasoconstrictor substances in tissues (angiotensin II, antidiuretic hormone, epinephrine, thromboxane $A_2$, prostaglandin F). It can also develop secondary to increased sensitivity of arteriolar receptors to these agents, for example, against the background of elevated calcium ($Ca^{2+}$) or sodium ($Na^{+}$) ion concentrations.
- Physical (mechanical): creation of an obstruction to blood flow. This can be external compression of a vessel (by a tumor, scar, edema, or tourniquet) or internal obturation (by a thrombus, embolus, or cellular aggregate).
The second process is increased consumption of oxygen and metabolic substrates. In this case, blood flow may actually increase, but the organ's demand rises disproportionately higher. A classic example is myocardial ischemia during emotional stress or a hypertensive crisis. Excess catecholamines force the heart to work harder, and although coronary blood flow increases, oxygen demand grows significantly faster. This leads to an anginal attack and, in severe cases, to infarction.
Manifestations of Ischemia
The clinical and morphological picture consists of macroscopic signs and alterations at the microcirculatory level.
Externally, the ischemic area pales, its temperature drops, and its volume and turgor decrease. Arterial pulsation weakens, while lymph formation and drainage are suppressed. A visual reduction in the number and caliber of arterial vessels is observed.
Under a microscope, the vascular bed demonstrates:
- Reduction in the diameter of capillaries and arterioles.
- Sharp deceleration of blood flow velocity.
- Narrowing of the plasma zone near the vessel walls.
- Widening of the axial "cylinder" (erythrocytes and other formed elements accumulate in the center of the stream).
- Drop in the number of functioning capillaries.
Consequences and Severity Factors
The primary damaging factors in ischemia are hypoxia and the toxic accumulation of metabolic waste products, ions, and bioactive substances. They lead to the suppression of both specific (e.g., renal filtration) and nonspecific organ functions. Structural changes range from reversible dystrophies to hypotrophy, atrophy, hypoplasia, and irreversible necrosis (infarction).
The scale of damage depends on several conditions:
- Rate of occlusion: the faster the blood flow is interrupted, the more severe the consequences, as tissues do not have time to adapt.
- Vessel caliber: involvement of larger arteries causes more extensive ischemia.
- Tissue sensitivity: the brain, heart, and kidneys die the fastest, whereas bones, cartilage, and skeletal muscles exhibit low sensitivity.
- Significance of the organ: ischemia of vital centers threatens the survival of the entire organism, whereas skin ischemia is compatible with life.
- Collateral circulation: the degree of development and speed of recruitment of alternative blood flow pathways.
Role of Collateral Blood Flow
Collateral blood flow is the primary compensatory mechanism that saves tissues from destruction. It is a circulatory system utilizing alternative vessels around and within the lesion zone.
Its activation is triggered by four factors:
- Pressure gradient: blood rushes from the high-pressure zone (before the stenosis) to the low-pressure zone (after the obstruction).
- Chemical factor: potent vasodilators (adenosine, prostaglandins, kinins, acetylcholine) accumulate in the ischemic focus and dilate vessels.
- Neural factor: local parasympathetic influences relieve vascular spasm.
- Anatomical factor: the baseline density of the vascular network. Based on the development of arterial anastomoses, all organs are divided into three groups, which largely determines their survival during vascular catastrophes.