Brain intravascular volumes

Table 2 Regional brain intravascular volumes and uptake data for various substances in 19—21-day-old control rats and rats exposed to lead via the milk throughout suckling (chronic exposure model)... 

Figure 2.4. In vivo measurement of blood-brain barrier (BBB) permeability, (a) Internal carotid artery perfusion technique (i) in the rat. Other branches of the carotid artery are ligated or electrically coagulated (o, occipital artery p, pterygopalatine artery). The external carotid artery (e) is cannulated and the common carotid artery (c) ligated. Perfusion time may range from 15 s to 10 min, depending on the test substance. It is necessary to subtract the intravascular volume, Vo, from (apparent volume of distribution), to obtain true uptake values and this may be achieved by inclusion of a vascular marker in the perfusate, for example labelled albumin. Time-dependent analysis of results in estimates of the unidirectional brain influx constant Ki (pi min which is equivalent within certain constraints to the PS product. BBB permeability surface area product PS can be calculated from the increase in the apparent volume of distribution Vd over time. Capillary depletion, i.e. separation of the vascular elements from the homogenate by density centrifugation, can discriminate capillary uptake from transcytosis. (b) i.v. bolus kinetics. The PS product is calculated from the brain concentration at the sampling time, T, and the area under the plasma concentration-time curve, AUC.

Intravascular volume expansion is a major pathogenic factor in renal parenchymal hypertension. In these patients, excessive renin secretion relative to volume status and heightened sympathetic nerve activity are observed, and those contribute to hypertension in patients with renal injury. Potential mechanisms include afferent stimuli from the injured kidneys to the brain, reduced central dopaminergic tone, reduced baroreceptor sensitivity, abnormal vagal function and endothelial dysfunction in patients with renal dysfunction [45, 46, 49],... 

The essential element, hypoperfusion of vital organs, is present whatever the cause, whether pump failure (myocardial infarction), maldistribution of blood (septic shock) or loss of total intravascular volume (bleeding or increased permeability of vessels damaged by bacterial cell products, bums or anoxia). Fimction of vital organs, brain (consciousness, respiration) and kidney (urine formation) are clinical indicators of adequacy of perfusion of these organs. 

---