Total peripheral resistance to blood flow is primarily decreased by vasodilation, which increases the radius of small arteries and arterioles. Vasodilation reduces resistance dramatically because resistance is inversely proportional to the fourth power of the vessel radius. Other factors that can decrease total peripheral resistance include a decrease in blood viscosity or a shorter effective vessel length, but changes in vessel diameter have the most significant impact. Key explanations:
- Vasodilation (widening of blood vessels), especially arterioles, decreases resistance by increasing vessel radius.
- Blood viscosity (thickness) influences resistance; lower viscosity decreases resistance.
- The length of blood vessels influences resistance, with longer vessels increasing it; thus, shorter or fewer vessels reduce resistance.
- Certain conditions like exercise cause vasodilation in working muscles, which decreases total peripheral resistance despite increased cardiac output.
- Vasodilatory substances include nitric oxide, prostacyclin, and beta-2 adrenergic receptor activation by epinephrine.
- Conversely, vasoconstriction and increased blood viscosity increase resistance.
In summary, an increase in the radius of arterioles (vasodilation) is the most effective way to decrease total peripheral resistance to blood flow.
