Thus far, we have discussed the function of the brain as if it were independent of its blood flow, its metabolism, and its fluids. However, this is far from true because abnormalities of any of these can profoundly affect brain function. For instance, total cessation of blood flow to the brain causes unconsciousness within 5 to 10 seconds. This occurs because lack of oxygen delivery to the brain cells shuts down most metabolism in these cells. Also, on a longer time scale, abnormalities of the cerebrospinal fluid, either its composition or its fluid pressure, can have equally severe effects on brain function.
Cerebral Blood Flow
Normal blood flow through the brain of the adult person averages 50 to 65 milliliters per 100 grams of brain tissue per minute. For the entire brain, this amounts to 750 to 900 ml/min, or 15 per cent of the resting cardiac output.
As in most other vascular areas of the body, cerebral blood flow is highly related to metabolism of the tissue. At least three metabolic factors have potent effects in controlling cerebral blood flow: (1) carbon dioxide concentration, (2) hydrogen ion concentration, and (3) oxygen concentration.
Increase of Cerebral Blood Flow in Response to Excess Carbon Dioxide or Excess Hydrogen Ion Concentration. An increase in carbon dioxide concentration in the arterial blood perfusing the brain greatly increases cerebral blood flow. This is demonstrated in Figure 61-1, which shows that a 70 per cent increase in arterial Pco2 approximately doubles cerebral blood flow.
Carbon dioxide is believed to increase cerebral blood flow by combining first with water in the body fluids to form carbonic acid, with subsequent dissociation of this acid to form hydrogen ions. The hydrogen ions then cause vasodilation of the cerebral vessels—the dilation being almost directly proportional to the increase in hydrogen ion concentration up to a blood flow limit of about twice normal.
Any other substance that increases the acidity of the brain tissue, and therefore increases hydrogen ion concentration, will likewise increase cerebral blood flow. Such substances include lactic acid, pyruvic acid, and any other acidic material formed during the course of tissue metabolism.
Importance of Cerebral Blood Flow Control by Carbon Dioxide and Hydrogen Ions. Increased hydrogen ion concentration greatly depresses neuronal activity. Therefore, it is fortunate that an increase in hydrogen ion concentration also causes an increase in blood flow, which in turn carries hydrogen ions, carbon dioxide, and other acid-forming substances away from the brain tissues. Loss of carbon dioxide removes carbonic acid from the tissues; this, along with removal of other acids, reduces the hydrogen ion concentration back toward normal. Thus, this mechanism helps maintain a constant hydrogen ion concentration in the cerebral fluids and thereby helps to maintain a normal, constant level of neuronal activity.
Oxygen Deficiency as a Regulator of Cerebral Blood Flow. Except during periods of intense brain activity, the rate of utilization of oxygen by the brain tissue remains within
Relationship between arterial Pco2 and cerebral blood flow.
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