Resistance to Blood Flow SLO 3.2

 Resistance is opposition to blood flow due to friction generated as blood slides along the vessel walls.  Most obvious in small arteries and arterioles located at a distance from the heart. Often described as “peripheral resistance”

 Blood vessel diameter – a narrow blood vessel offers more resistance to blood flow  The large vessels such as the carotid arteries and the aorta have sensors called “baroreceptors” in their walls that respond to changes in blood pressure. Resistance in Blood Vessel Causes:

 When these artery walls get stretched by increased blood pressure they transmit signals that result in vasodilation.  At the same time, central controls slow down heart rate and decrease cardiac output.

 With less stretching, the sympathetic nervous system causes the vessels to constrict and causes heart rate to increase

 Blood viscosity or thickness, affects resistance to blood flow  Loss of red blood cells or loss of plasma proteins can cause blood to be thinner – low viscosity, low resistance to blood flow  Increase in red blood cells (polycythemia) or loss of plasma volume (dehydration) can cause blood to be thicker – high viscosity, high resistance to blood flow Blood Viscosity

 The ease with which arteries expand t receive blood  In atherosclerosis the vessels loose their capacity for expansion – like trying to blow up a firm, new balloon Blood Vessel Compliance

 The ability of blood vessels to return to their original size after being stretched.  Arteries close to the heart undergo higher pressure due to the heart forcing blood  Too much added pressure causes these arteries to stretch too wide and over time they lose elasticity Elasticity