The general discussion about auto-regulation with its probable mechanisms have already been made (chap- 6.

1. Perfusion pressure; As the mean arterial BP rises, one would enpect that the brain blood flow should rise, and vice versa. In actual practice, this is not so; between 60 to about 150 mm Hg of BP (the mean BP) the blood flow is not affected. This is autoregulation of brain blood flow (cf. coronary circulation autoregulation, fig 5.10. 2). The general discussion about auto-regulation with its probable mechanisms have already been made (chap- 6. sec V). It is suspected that the principal, factor for autoregulation (so far as brain blood flow is concerned) is the accumulation (or washing out) of CO2 (vide infra), although the myogenic response to increased distension may also play some role (for details, see mechanism of autoregulation, chap 6, sec V). However, if the systemic BP falls very severely (below 60 mm Hg. as in severe cardiovascular shock) the patient develops the risk of cerebral anoxia greatly, as in such severe cases, the autoregulation fails to protect 2. C02 concentration When the CO2 concentration of the local brain tissue nses. there is vasodilatation leading to. *To a beginner, this may be puzzling, the mean BP in the big vessels of the pulmonary circulation is about 12 mm Hg (cf. systemic, 100 mm Hg) but the capillary BP, at the beginning of the capillary, is as high as 8 mm Hg (cf. systemic, 32 mm Hg), how? Answer is, proper arterioles virtually do not exist in this circulation. Therefore. BP drop does not occur in appreciable degree increased blood flow. Conversely, if C02 is washed out from the body (say by voluntary hyperventilation) there may be cerebral vasoconstriction leading to cerebral anoxia and visual blackouts. 3. 02 tension, High 02 tension in the inspired air produces cerebral vasoconstriction so that the brain is protected from the dangers of '02 poisoning' (for details of 02 poisoning see chap. 6 sec IV). 4 Ana esthetics Majority of the aneasthetics reduce the brain blood flow. The popular anaesthetic halothane is a particular example. Fortunately they decrease the cerebral metabolism also. 5. Sympathetic stimulation. The traditional teaching is that, when the sympathetic system is stimulated, although the pial vessels are constricted the vessels supplying the grey and white matter are not affected. In recent years there have been claims that sympathetic stimulation does produce cerebral vasoconslnction. It has also been claimed that there are cholnergic sympathetic vasodilator libers. But all these are controversial VALUES, MEASUREMENT Average blood flow rate in the brain is about 55 ml/min/ 100 gm of tissue. (This means the 02 consumption is about 3.5 ml/100 gm/min). Assuming the brain weighs about 1. 4 kg in the adult, this represents a total blood flow of 770 ml/min. Cerebral (brain) blood flow in man is usually measured by nitrous oxide method. Circulation in small circumscribed regions of the brain also can now-a-days be measured by radioactive xenon CONDITIONS AFFECTING THE FLOW Apparently, mental hard work or sleep does not produce a change in cerebral blood flow, But this does not mean that there is actually no change. What happens perhaps, is that the active part becomes more vascular and readjustment of brain blood flow occurs in other parts of the brain, so that the total flow does not change. Thus, repeated exposure to light in animals, causes a rise in blood flew in the visual cortex; or when hand movements are (mentally) conceived by a man. the frontal regions of brain show a marked rise in blood supply IVTHE MICRO CIRCULATION 1. Introduction, 2, Functional anatomy. 3. Transcapillary exchange 4. Factors governing the flow. 5. Capillary fragility, Introduction. The term "microcirculation" means circulation in those vessels which cannot be seen by the naked eye Such vessels are: (1) very small arteriales (also called metarterioles), (ii) the precapillary sphincter region (iii) the capillanes (the main ingredients) and (iv) the finest venules. It will be interesting to note, that the great William Harvey who first established the fad that the blood circulates was troubled because he could not directly demonstrate the micro circulation (as microscope was not available in his time)' With the introduction of microscope, the "missing portion in the circulatory tree " viz. the microcirculations, (mainly the capillaries) could be anatomically demonstrated. For our present day knowledge of the microcirculation we are indebted to such illustrious persons like. Zweifach, Landis and others Functional anatomy (fig. 5. 10. 4). The arteroles break up into finer artenoles called metarterioles, the metarterioles give rise to still finer vessels which contain, at their origin the sphincters, called;precapiliary sphincters' These vessels continue and eventually become capillanes. The capillanes open into finest venules which in turn, open into comparatively bigger or muscular venules. From the melarterioles arise, another kind of vessel, called' thoroughfare channels' (also called 'preferential channel) which bypasses the capillanes (fig. 5 10. 4) The walls of the metarterioles contain some smooth muscles. The precapillary sphincters also contain smooth muscles. But the capillaries do not contain any such muscle. Because exchange between the blood and the tissues takes place at the capillaries, the absence of muscles is essential (as otherwise, capillary walls would have been too thick), smooth muscle again reappears on the walls of muscular (ie.. comparatively biqqer venules Sympathetic motor supply is present to the smooth muscles of metarferioles, precapillary sphi