Metabolism and Sensitivity to Hypoxia
The brain exhibits massive metabolic activity. Despite its modest mass (about 2% of body weight), it consumes one-fifth of all energy generated by the body at rest. Maintaining brain function requires about 15% of the total cardiac output, which translates to approximately 750 mL/min.
Nervous tissue is critically dependent on a continuous supply of oxygen and substrates. The brain is extremely sensitive to hypoxia and ischemia:
- 5–15 seconds of complete blood flow cessation is enough to cause functional failure, leading to loss of consciousness.
- 5–7 minutes of absent perfusion leads to fatal consequences, triggering irreversible structural changes in the cerebral cortex.
Therefore, maintaining stable and sufficient cerebral blood flow is a vital requirement for the body.
Unique Features of the Arterial Bed and Microcirculation
The angioarchitecture and operational principles of cerebral vessels differ significantly from other organs. The principle of functional hyperemia applies here: maximum blood flow is always directed to the brain regions actively engaged in current activity.
Microcirculatory features:
- Absence of arteriovenous anastomoses — blood cannot bypass the capillary network.
- No "reserve" capillaries — unlike skeletal muscle, where some vessels can remain in reserve, absolutely all capillaries in the brain function continuously.
Another key characteristic is the absence of pulsation in intracranial cerebral arteries. The pulse wave coming from the carotid arteries is effectively damped. This occurs for two reasons: first, cerebral arteries are markedly tortuous; second, intracranial volume and intracranial pressure remain constant.
Relationship Between Intracranial and Systemic Pressure
The skull is a rigid, sealed structure. This dictates a fundamental physiological rule: the total volume of its contents, including circulating blood, cerebrospinal fluid (CSF), and brain tissue, is always constant.
Since the total volume of blood within the skull cannot change, the adaptation of regional blood supply during functional loads is achieved not by influxes of extra blood from outside, but through internal mechanisms:
- Redistribution of blood between various cerebral vascular beds.
- Alteration of linear blood flow velocity in specific areas.
A direct relationship exists between intracranial pressure (ICP) and systemic arterial pressure in the systemic circulation. If ICP rises pathologically (e.g., due to a growing tumor), there is a risk of vessel compression and decreased cerebral perfusion. In response, a compensatory mechanism is triggered: a marked increase in systemic arterial pressure occurs. This is necessary to force blood into the cranial vault and preserve an adequate level of metabolism.
Specifics of the Cerebral Venous System
The cerebral venous system also lacks many characteristics typical of the systemic circulation:
- Absence of a capacitance function. Cerebral veins cannot pool (store) blood; they function strictly as drainage pathways.
- Negative pressure. When a person assumes an upright posture, negative pressure develops in large intracranial veins and dural venous sinuses.
In typical tissues, negative pressure would cause vessel collapse, blocking blood flow. However, the anatomical feature of intracranial veins and sinuses is that their walls are rigidly fixed to the skull bones. Consequently, the veins do not collapse even under negative pressure, ensuring uninterrupted venous outflow in any body position.