March 08, 2008

Pumping Iron To Stay Healthy

fat lady jogging, aerobicsWho doesn't know the importance of exercise? Your skeletal muscle bulk increases, performance of the lung is increased, the metabolic profile gets better and most importantly the cardiovascular health is improved.

Exercise may be aerobic or anaerobic, depending on oxygen requirements. A sprinter running at top speed spends energy very fast. The amount of energy produced by oxidative burning alone is insufficient to meet the demand. The muscles have to make energies without the use of oxygen, in addition. Thus activities such as these would require anaerobic glycolysis as well: anaerobic exercise.

On the other hand, a marathoner who is jogging at a much leisurely pace, does not burn energy at such a humongous pace and the amount of energy that comes via aerobic glycolysis is the major source to meet his energy requirement: aerobic exercise.

They can also be classified as isotonic and isometric. When someone is trying to lift a heavy load but can not really accomplish it, is said to be doing isometric exercise; Iso meaning 'same' and metric refers to length 'meter'. Conversely, when a guy is lifting weights or doing push-ups, is contracting his muscles, and this is known as isotonic exercise; signifying that the tone remains the same but the length of the muscle changes. Physics hardly gives any credit to the one doing isometric exercise; dismissing it since they are doing no work (work= force x distance; the displacement here is zero).

Whether isometric or isotonic, the heart rate rises in both occasions. It is effected by increased sympathetic discharge in response to stimulation of mechanoreceptors and chemoreceptors in the muscle. The mere thought of exercise stimulates the motor cortex as well as the sympathetic centers in the brain.Blood flow in the skeletal muscular circulation increases 15-30 fold. Increased sympathetic cholinergic vasodilator discharge, raised temperature, pCO2,K+,H+, and decreased pO2 are responsible for the vasodilation. This vasodilation opens up dormant capillaries by dilating them, increasing the total surface area available for gas exchange. Vasodilation also raises the hydrostatic pressure leading to possible development of edema due increased exudation in the interstitium. Raised osmotic overload from accumulation of osmotically active particles like lactic acid also contribute to build up of fluid in the intercellular spaces. Increased levels of lactic acid, H+, 2,3 DPG and raised temperature all shift oxyhemoglobin dissociation curve to the right, implying that more O2 is delivered to the tissues. In fact, oxygen consumption is increased about 100 fold!

The picture is a bit different in the systemic circulation. Both systolic and diastolic blood pressure raises (due to rise in the peripheral resistance) in isometric exercise while the stroke volume remains almost unchanged. In isotonic exercises, stroke volume increases. Rise in blood pressure stretches the vessel walls, by shear stress. This causes elaboration of nitric oxide (NO), a chemical that relaxes the smooth muscles. Previously called EDRF (endothelium derived relaxing factor), this chemical acts as a vasodilator and cellular messenger. The diastolic BP remains normal, may even fall due to widespread vasodilation leading to diminished peripheral resistance. Increase in venous return is aided by increased activity of the thoracic pump (increased depth of respiration or hyperpnea and increased rate or tachypnea), increased activity of the muscle pump, increased venoconstriction from sympathetic stimulation, pressure on the veins by the distended arteries and enhanced blood flow from the viscera from splanchnic vasoconstriction.

animated heart symbolizing exerciseThis increase in venous return is essential to maintain the end diastolic volume (EDV), when we realize that the diastole has shortened considerably due to increased heart rate. However, the size of the heart decreases rather than increases (as would be predicted by Frank-Starling's law). Regular exercise enables one to burn fatty acids more efficiently, as an energy source. This has its own advantage of not driving the pH too low, delimiting enzymatic reactions and curbing the production of lactate, the chemical that causes muscle cramps. Above all, scientists from University of Bonn, at Germany have conclusively proven that exercising gives us the runners' high, an euphoric effect due to increase in the level of endorphins, in the central nervous system (CNS). Endorphins are analogous to endogenous morphines (opioids) that are responsible for the elevation of mood.

Decide now: Are you going to give yourself a go today?
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