Classification and causes
In pathophysiology, two types of circulatory hypoxia are distinguished:
- Local: restricted to a specific part of the body. It occurs during local blood flow disorders: ischemia, venous hyperemia, or stasis. Another cause can be impaired oxygen diffusion at the microcirculatory level.
- Systemic: affects the entire body. The main cause is general circulatory system failure.
Mechanisms of systemic hypoxia
The systemic form develops as a result of three key factors:
- Hypovolemia: a critical reduction in circulating blood volume (CBV).
- Heart failure: the heart is unable to provide adequate cardiac output, even if venous return is normal.
- Vascular failure: generalized decrease in arterial and venous tone, which increases the capacity of the vascular bed and slows down blood return to the heart.
Changes in blood gas composition
In systemic circulatory hypoxia, arterial blood parameters remain within normal limits because the lungs function properly. However, tissues "extract" more oxygen from the passing blood, leading to the following changes:
| Parameter | Parameter |
|---|---|
| $P_vO_2$ and $S_vO_2$ | Decreased |
| $C_vO_2$ | Reduced |
| Arteriovenous difference | Enlarged |
| Blood pH | Reduced (acidosis) |
Briefly: answers to topical questions
Which pathological conditions or types of shock lead to vascular failure in hypoxia?
Acute circulatory failure or vascular failure includes the conditions and types of shock described in the sources:
- Vascular (distributional) shock — includes the neurogenic form (traumatic, pain, spinal cord injury, anesthesia complication) and the anaphylactic form (generalized hypersensitivity reaction). Its pathogenesis: pronounced vasodilation, increased capillary permeability, and arteriovenous shunting; hemodynamically — redistribution of intravascular blood volume and a significant decrease in SVR.
- Septic (toxic-infectious) shock — associated with an infectious process, most often Gram-negative flora (E. coli, Proteus, Klebsiella), less frequently Gram-positive; accompanied by the release of predominantly endotoxins, activation of complement, coagulation, fibrinolysis, platelet, and neutrophil systems, and stimulation of nitric oxide, TNF-α, and interleukin production. The outcome is acute circulatory failure.
- Collapse — acute vascular failure; can be associated with a rapid decrease in CBV, blood pooling, or an acute drop in total peripheral vascular resistance.
What compensatory mechanisms are activated in the body during systemic circulatory hypoxia?
In response to systemic circulatory failure and circulating blood volume (CBV) deficit, the following mechanisms are activated:
- With a CBV deficit of up to 10%, venous vessels adapt to the reduced volume (venomotor mechanism).
- Centralization of circulation — an adaptive reaction aimed at maintaining core blood flow in the brain and heart through pronounced peripheral vasoconstriction.
- Tissues maximally extract oxygen due to a drop in volumetric blood flow velocity, which leads to a decrease in venous blood oxygen content from 14–15 vol.% to 4–5 vol.% and an increase in the arteriovenous oxygen difference.
What specific causes lead to hypovolemia resulting in circulatory hypoxia?
Hypovolemia (reduction in circulating blood volume) develops as a result of absolute or relative loss of fluid and plasma.
- Acute massive blood loss.
- Loss of a large volume of blood plasma (e.g., in extensive burns).
- Acute and significant dehydration: severe vomiting, profuse diarrhea, increased sweating, massive losses of isotonic fluid (as in cholera).
- Blood redistribution with pooling in venous vessels, sinuses, and capillaries (in shock, intoxications).
Remember the formula "System — Heart — Vessels": if one of these links fails (insufficiency), blood does not reach the tissues, and circulatory hypoxia develops.
Class and exam questions
Why does arterial blood remain normal in circulatory hypoxia?
Because in this form of hypoxia, lung function and the process of hemoglobin oxygen saturation are not impaired. The problem lies entirely in the delivery of already saturated blood to the tissues.
What is the arteriovenous difference and why does it increase?
This is the difference between the oxygen content in arterial and venous blood. It increases because oxygen-deprived tissues extract more oxygen from the blood than usual due to slowed blood flow.
What is the difference between local and systemic forms?
The local form is limited to a specific organ or tissue area (e.g., in ischemia), whereas the systemic form encompasses the entire body due to dysfunctions of the heart or total blood volume.
What else to study on this topic
- Mechanisms of ischemia and venous hyperemia development.
- The influence of humoral factors on vascular tone.
- Compensatory body reactions in acute heart failure.
- Differential diagnosis of circulatory and hemic hypoxia.
- Pathophysiological consequences of metabolic acidosis for the myocardium.
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