The sugar content of blood is not a passive constituent. Blood sugar level is found to exert enormous influence in regulating the functions of various tissues in different ways.
The following is a brief summary:
i. On Stomach:
Low blood sugar stimulates gastric secretion and gastric movements. Hunger contractions are always stimulated by low sugar (as also by insulin injections). They stop when the blood sugar rises.
ii. On Liver:
Glycogenesis in liver is stimulated when blood sugar level is high as contrast to the hypoglycaemic condition when instead of glycogenesis, glycogenolysis occurs.
iii. On Muscles:
Muscles glycogen is formed when its glycogen content is low whereas it does not follow this principle when its glycogen content is high in spite of high blood sugar level. Muscle glycogen has got no glycostatic action.
iv. On Heart:
Fall of blood sugar level results lactic acid in muscle but these are converted into glycogen in cardiac muscle.
v. On Central Nervous System:
Since sugars are main fuel of central nervous system activity, the hypoglycaemia results interference, i.e., hypoactivity of central nervous systems, e.g., feeling of fatigue, abnormal behaviour, i.e., anxiety and irritability, appearance of tremor and vasomotor disturbances, etc.
vi. On Autonomic Nervous System:
Low blood sugar stimulates the sympathetic and raises blood sugar level.
It acts in two ways:
a. By stimulating adrenaline-secretion.
b. By acting directly on the liver, through its sympathetic nerves and thus mobilising its glycogen content. Hyperglycaemia stimulates the parasympathetic which increases insulin secretion and reduces blood sugar,
vii. On Adipose Tissue:
When sugar supplied is more than can be utilised, blood sugar tends to rise. This extra amount is converted into fat by the adipose tissue and is deposited.
viii. On the Utilisation of Glucose by other Tissues:
Hypoglycaemia reduces the rate of glucose oxidation in the tissues. Hyperglycaemia, on the other hand, stimulates glucose utilisation in the tissues. When viewed from this standpoint it seems that high blood sugar in diabetes mellitus is a protective phenomenon which tries to maintain the rate of glucose utilisation by the tissues.
ix. On Endocrines:
a. Adrenaline (Epinephrine):
Low blood sugar stimulates adrenaline secretion (probably not a direct action but through the sympathetic).
Hypoglycaemia depresses and hyperglycaemia stimulates insulin secretion by directly acting upon the islets of Langerhans.As we know that these factors also act through the parasympathetic.
x. On Neoglucogenesis:
When blood sugar tends to fall, neoglucogenesis is stimulated in the liver. As blood sugar rises neoglucogenesis is inhibited in a normal subject. Thus in a normal subject the rate of neoglucogenesis is controlled by blood sugar level.
xi. On Kidney Threshold:
In a chronic diabetic case, with persistently high blood sugar level, the renal threshold for glucose rises. Under such conditions blood sugar may go much above the renal threshold (180 mgm per 100 ml) and yet the urine may be free from sugar.
Problems of Carbohydrate Metabolism:
Problems of carbohydrate metabolism are the following:
Above problems are physiologically satisfied by a regulating mechanism existed in the body what is known as regulation of normal blood sugar level. These problems have been dealt with in their respective places.