Denne viser hvordan CO2-responsen er litt forskjellige i forskjellige blodkar. Den er sterkere i blodkar inni hjernen enn i blodkar i kraniet, ansiktet og ryggraden. Blodkar i ryggraden har større respons enn blodkar i ansiktet, men mindre respons enn blodkar i hjernen.

http://jp.physoc.org/content/590/14/3277.long

Because of methodological limitations, almost all previous studies have evaluated the response of mean blood flow velocity (V_mean) in the middle cerebral artery (MCA) to changes in CO₂ as a measure of CO₂ reactivity across the whole brain (Aaslid et al. 1989; Ainslie & Duffin, 2009; Ainslie & Ogoh, 2009).

ICA, VA and BA CO₂ reactivity was significantly higher during hypercapnia than during hypocapnia (ICA, P < 0.01; VA, P < 0.05; BA, P < 0.05), but ECA and MCA were not significantly different.

The major finding from the present study was that cerebral CO₂ reactivity was significantly lower in the VA and its distal artery (BA) than in the ICA and its distal artery (MCA). These findings indicate that vertebro-basilar circulation has lower CO₂ reactivity than internal carotid circulation. Our second major finding was that ECA blood flow was unresponsive to hypocapnia and hypercapnia, suggesting that CO₂ reactivity of the external carotid circulation is markedly diminished compared to that of the cerebral circulation. These findings suggest that different CO₂ reactivity may explain differences in CBF responses to physiological conditions (i.e. dynamic exercise and orthostatic stress) across areas in the brain and/or head.

Hypercapnic cerebral CO₂ reactivity in global CBF was greater than the hypocapnic reactivity (Ide et al. 2003) (Table 3). The mechanisms underlying this greater reactivity to hypercapnia compared with hypocapnia may be related to a greater influence of vasodilator mediators on intracranial vascular tone compared with vasoconstrictive mediators (Toda & Okamura, 1998; Ainslie & Duffin, 2009). In humans, Peebles et al.(2008) recently reported that, during hypercapnia, there is a large release of nitric oxide (NO) from the brain, whereas this response was absent during hypocapnia.

The difference in CO₂ reactivity between vertebro-basilar territories (VA and BA) and the cerebral cortex (ICA and MCA) may be due to diverse characteristics of vasculature, e.g. regional microvascular density (Sato et al. 1984), basal vascular tone (Ackerman, 1973; Haubrich et al. 2004; Reinhard et al. 2008), autonomic innervation (Edvinsson et al. 1976; Hamel et al. 1988) and regional heterogeneity in ion channels or production of NO (Iadecola & Zhang, 1994; Gotoh et al. 2001).

Interestingly, the response of the ECA to changes in CO₂ may be similar to other peripheral arteries. It has long been appreciated that the vasodilatory effect of hypercapnia is much more profound in cerebral than in peripheral vasculature, particularly leg (Lennox & Gibbs, 1932; Ainslie et al. 2005) and brachial arteries (Miyazaki, 1973). These findings suggest that control of CO₂ is particularly important in the cerebral circulation. The high resting metabolic requirements of the brain, compared with that of other vasculature, might be one reason why this circulatory arrangement is desirable (Ainslie et al. 2005). Specifically, high CO₂ reactivity may be a way for the brain to match metabolism with flow (Ainslie et al. 2005).

Lower CO₂reactivity in the vertebro-basilar system may be important for maintaining central respiratory function because  in central chemoreceptors is regulated by  and blood flow to maintain breathing stability.

In summary, our study shows that cerebral CO₂ reactivity in the vertebro-basilar circulation is lower than that in the internal carotid circulation, while CO₂ reactivity in the external carotid circulation is much lower compared with two other cerebral arteries. These findings indicate a difference in cerebral CO₂ reactivity between different circulatory areas in the brain and head, which may explain different CBF responses to physiological stress. Lower CO₂ reactivity in the vertebro-basilar system may be beneficial for preserving blood flow to the medulla oblongata to maintain vital systemic functions, while higher CO₂ reactivity in the internal carotid system may imply a larger tolerance for varied blood flow in the cerebral cortex.