2017. augusztus 6., vasárnap

Az agynak: (szőlő)cukrot és oxigént, bármi áron !

The metabolism of the brain is remarkable in several respects. First, the brain of adult mammals normally uses only glucose as fuel (Fig. 22-8). Second, the brain has a very active respiratory metabolism; it uses almost 20% of the total O2 consumed by a resting human adult. The use of O2 by the brain is fairly constant in rate and does not change significantly during active thought or sleep. Because the brain contains very little glycogen, it is continuously dependent on incoming glucose from the blood. If the blood glucose should fall significantly below a certain critical level for even a short period of time, severe and sometimes irreversible changes in brain function may occur.
Although the brain cannot directly use free fatty acids or lipids from the blood as fuels, it can, when necessary, use D-β-hydroxybutyrate (a ketone body) formed from fatty acids in hepatocytes. The capacity of the brain to oxidize β-hydroxybutyrate via acetyl-CoA becomes important during prolonged fasting or starvation, after essentially all the liver glycogen has been depleted, because it allows the brain to use body fat as a source of energy. The use of β-hydroxybutyrate by the brain during severe starvation also spares muscle proteins, which become the ultimate source of glucose for the brain (via gluconeogenesis) during severe starvation.
The concentration of glucose dissolved in the plasma is also subject to tight regulation. We have noted the requirement of the brain for glucose and the role of the liver in maintaining the glucose concentration near the normal level of 80 mg/100 mL of blood (about 4.5 mM). When blood glucose in a human drops to half this value (the hypoglycemic condition), the person experiences discomfort and mental confusion (Fig. 22-10); further reductions lead to coma, convulsions, and in extreme hypoglycemia, death. Maintaining the normal concentration of glucose in the blood is therefore a very high priority of the organism, and a variety of regulatory mechanisms have evolved to achieve that end. Among the most important regulators of blood glucose are the hormones insulin, glucagon, and epinephrine. 

Lehninger-Nelson-Cox: Principles of Biochemistry, 744.o.

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Szószedet: homeostasis (önbévület*)

homeostasis : The maintenance of a dynamic steady state by regulatory mechanisms that compensate for changes in external circumstances. ___...