Explain the Factors Affecting Peripheral Resistance

i) If more than 30% of the blood volume of a person is lost through bleeding (haemorrhage), the compensatory mechanism may fail. In that case, there may be a reduction in the vasomotor center activity and a fall in peripheral resistance, which may turn out to be irreversible. The perfusion to the tissues is decreased. This condition is known as shock.

ii) Some emotionally disturbing events, such as sight of blood or frightening objects may lead to fainting due to generalized vasodilation. Although this is called vaso- vagal attack, it seems to be mediated by sympathetic cholinergic vasodilatation fibers, which supply skeletal muscles. It is generally short-lasting. The treatment is to lie down so that the blood flow, especially to the brain, is maintained. If this treatment is not promptly instituted, the person may faint and fall due to a reduction in blood flow to the brain - a condition known as syncope. Syncope is actually a spontaneous loss of consciousness caused by insufficient blood to the brain. It may be considered nature's way of instituting the treatment. Although rude and potentially traumatic, the fall can be life-saving.

iii) Anaphylactic shock, trauma, peritonitis (inflammation of the peritoneum), crush syndrome or an allergy following a bee bite. In all these conditions, there is production of some toxic substances, which lead to systemic vasodilation and increased capillary permeability. Increased vascular capacity due to vasodilation and reduced volume due to increased capillary permeability is a dangerous combination. It frequently leads to circulatory shock.

iv) Stimulation of myelinated pain fibers may produce generalized vasodilation. Besides, the factors discussed above, there are a few other factors affecting the viscosity of blood which also impacts on blood pressure.