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Introduction to the Cardiovascular System - Part 7

Chảy máu não là chịu ảnh hưởng mạnh mẽ bởi áp suất riêng phần của khí carbon dioxide, một mức độ ít hơn, oxy trong máu động mạch (Hình 7-12). Lưu lượng máu não là rất nhạy cảm với thay đổi nhỏ trong động mạch áp suất riêng phần của CO2 | ORGAN BLOOD FLOW 157 Cerebral blood flow is strongly influenced by the partial pressure of carbon dioxide and to a lesser extent oxygen in the arterial blood Fig. 7-12 . Cerebral blood flow is highly sensitive to small changes in arterial partial pressure of CO2 pCO2 from its normal value of about 40 mm Hg with increased pCO2 hy-percapnea causing pronounced vasodilation and decreased pCO2 hypocapnea causing vasoconstriction. Hydrogen ion appears to be responsible for the changes in vascular resistance when changes occur in arterial pCO2. The importance of CO2 in regulating cerebral blood flow can be demonstrated when a person hyperventilates which decreases arterial pCO2. When this occurs a person becomes light headed as the reduced pCO2 causes cerebral blood flow to decrease. Severe arterial hypoxia hypoxemia increases cerebral blood flow. Arterial pO2 is normally about 95-100 mm Hg. If the pO2 falls below 50 mm Hg severe arterial hypoxia it elicits a strong vasodilator response in the brain which helps to maintain oxygen delivery despite the reduction in arterial oxygen content. As described in Chapter 6 decreased arterial pO2 and increased pCO2 stimulate chemoreceptors which activate sympathetic efferents to the systemic vasculature to cause vasoconstriction however the direct effects of hypoxia and hypercapnea override the weak effects of sympathetic activation in the brain so that cerebral vasodilation occurs and oxygen delivery is enhanced. Although sympathetic nerves innervate larger cerebral vessels activation of these nerves has relatively little influence on cerebral blood flow. Maximal sympathetic activation increases cerebral vascular resistance by no more than 20 to 30 in contrast to an approximately 500 increase occurring in skeletal muscle. The reason in part for the weak sympathetic response by the cerebral vasculature is that metabolic mechanisms are dominant in regulating flow therefore functional sympatholysis occurs during sympathetic .