Hypercapnia slightly raises blood volume and sizably elevates flow velocity in brain microvessels

Abstract

The increase in local cerebral blood flow (LCBF) caused by hypercapnia may be mainly accomplished by raising the velocity of plasma and/or red blood cell (RBC) flow through the microvessels and not by perfusing more capillaries. This suggestion was tested in awake rats exposed to 8% CO2 and in control rats. LCBF was measured by the 14C-labeled iodoantipyrine method. The volume of blood in small parenchymal microvessels was estimated from the distribution spaces of 125I-labeled serum albumin (RISA) and 55Fe-labeled RBCs. Hypercapnia elevated LCBF 2.0- to 3.5-fold in the 40 brain areas studied, marginally raised the RBC spaces, and significantly increased the RISA and whole blood distribution spaces (approximately 25 and 19%, respectively). These changes in microvessel distribution volumes could be the result of perfusing a slightly larger fraction of capillaries (recruitment), increasing microvessel diameter somewhat, or both. With hypercapnia, the mean transit times fell to approximately 45% of control, which indicated that LCBF was mainly increased by raising the velocity of RBC and plasma flow through already perfused microvessels. Overall, few, if any, capillaries or other microvessels were recruited by hypercapnia.

Single‐coil arterial spin‐tagging for estimating cerebral blood flow as viewed from the capillary: Relative contributions of intra‐ and extravascular signal

“…In the second case, ␤ was set to 1.0, so that the microvascular bed was not distensible, and all flow changes were matched by velocity changes in the microvascular bed. As we shall briefly discuss below, the case for rat microvascular bed almost certainly lies between these two extrema (33)(34)(35). The quantity ␣ was adjusted so that, for an assumed value of ␤, the protons of water in a 0.5 mm-long exchange microvessel with flow equivalent to 100 ml/100 g-min would traverse the exchange microvessel in 0.6 sec.…”

“…The quantity ␣ was adjusted so that, for an assumed value of ␤, the protons of water in a 0.5 mm-long exchange microvessel with flow equivalent to 100 ml/100 g-min would traverse the exchange microvessel in 0.6 sec. This typical transit time was derived from a consideration of previous work, which demonstrated that typical mean transit times for water were on the order of 600 ms for flows of 100 ml/100 g-min (34,36).…”

“…In contrast to the resting state, PS/F is not constant when flow changes in the cerebral vasculature (34,35). While there are some changes in surface area, and thus in PS product, the major adjustment in cerebral vasculature appears to be in T mean , indicating that velocity changes are the major means of accommodating changes in flow (34,45).…”

Changes in the Arterial Fraction of Human Cerebral Blood Volume during Hypercapnia and Hypocapnia Measured by Positron Emission Tomography

“…The increase in cerebral blood volume (CBV) during hypercapnia has been shown in animals to be less than the increase in CBF (Bereczki et al, 1993;Grubb et al, 1974;Keyeux et al, 1995;Lee et al, 2001). The degree of decrease in CBV during hypocapnia has also been shown in animals to be less than that in CBF (Grubb et al, 1974).…”

Nicotine Raises the Influx of Permeable Solutes across the Rat Blood—Brain Barrier with Little or No Capillary Recruitment

“…This "recruitment" could, however, be due partly to some dilation of not only arterioles and venules but also capillaries, as was recently reported by Atkin son et al (1990) and Duelli and Kuschinsky (1993). The volume of a blood vessel is proportional to its radius squared (?-), whereas the surface area is pro portional to r. Microvessel dilation would thus have less effect on S (and the PS product) (Chen et al , 1994b) than on microvessel blood volume (Shockley and LaManna, 1988;Bereczki et al, 1993b). Ac cordingly, the PS product appears to be a better indicator of capillary recruitment than does the mi crovessel blood volume, and the findings of little or no capillary recruitment by Chen et al seem to be more reliable than the reports of some capillary re cruitment by Shockley and LaManna and Bereczki et al To summarize for hypoxia, the capillary-labeling studies of Weiss and colleagues all show a near dou bling of the number of perfused capillaries in a va riety of brain regions, but these data have been se verely questioned on technical grounds by Kuschin sky and Paulson.…”