Metabolic rate is usually determined by measuring oxygen consumption. Therefore it is natural that muscular exertion would be a major factor affecting metabolic rate. Another factor to consider is environmental temperature. If it falls below body temperature, heat conserving mechanisms kick in. This causes the metabolic rate to rise.
When environmental temperature increases to a point where it is high enough to raise body temperature, metabolic enzyme action accelerates. Therefore metabolic rate rises with an increase in body temperature. Other factors affecting metabolic rate include emotional state, height, weight and circulating levels of hormones such as adrenaline and thyroxine, but not insulin.
BMR is a measure of the energy requirement for the maintenance of metabolic integrity, nerve and muscle tone, circulation and respiration in the human body under controlled conditions of thermal neutrality. The BMR of an average sized man is about 2000 kcal/day. A normal BMR in an adult male is around 40 kcal/m2/h. The BMR falls during prolonged starvation due to loss of skeletal muscle bulk. A high fever increases BMR.
In comparison to males BMR is generally lower in females and higher in children. BMR is measured 12–14 hours after a meal, by direct or indirect calorimetry. Feeding increases BMR because of the necessary energy expenditure that occurs during the assimilation of nutrients into the body. BMR is also very closely related to body surface area, since this is where the majority of heat exchange takes place.
Glucose is the favoured fuel source of all tissues. Physiological concentration of glucose in the blood is tightly regulated to stay within the normal range of 3.9–6.7 mmol/L. If blood glucose drops to below 2.7 mmol/L hypoglycaemia reduces the energy available to the vital organs, which can lead to coma and death.
The main advantages of glucose as a metabolic fuel are that it is water soluble, it can cross the blood–brain barrier and it can be oxidized anaerobically. However, it also yields a relatively small amount of ATP compared to other fuels, and is osmotically active, thus it can damage cells. Glucose is the only fuel for RBCs, which have no mitochondria.
Glucose transport across membranes occurs via a transporter protein, i.e. GLUT transporters. These transmembrane proteins bind glucose on one face of the membrane, then undergo conformational change to translocate glucose across the membrane.
Glycolysis is the process by which glucose is converted to pyruvate to enter the TCA cycle. Two ATP molecules are produced per molecule of glucose that enters the pathway. There are three irreversible steps in glycolysis, catalysed by the following enzymes: hexokinase/glucokinase, phosphofructokinase and pyruvate kinase.
Glycolysis that occurs in the presence of oxygen produces pyruvate which can then enter the TCA cycle to produce CO2. It is performed by all tissues. Glycolysis can occur in the absence of oxygen to produce lactate. Anaerobic glycolysis allows cells that lack mitochondria to produce ATP, for example RBCs. Remember, pyruvate/lactate formation takes place in the cytosol not the mitochondria.
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