Blood flow is directly affected by the pressure gradient and inversely affected by the peripheral resistance. The muscular arterioles are the primary sites where peripheral resistance (PR), which affects blood pressure, is regulated.
Flow, driven by a pressure gradient, is depicted as the labeled block arrow between a large 'BP' and a smaller 'BP'. If the downstream blood pressure increases the gradient will be reduced and flow will decrease. The dashed arrow pointing back to the flow arrow shows this inverse relationship. Decreased flow into this vessel will 'back up' blood in feeder vessels and increase 'upstream' pressure. This inverse relationship is shown by the dashed arrow pointing from the flow arrow to the larger 'BP' acronym.
The diameter of a vessel is the determining factor in how rapidly the pressure will drop as well as how easily blood will flow. As blood is flowing through a vessel some of the blood is in direct contact with the vessel's wall and loses energy through friction. This concept is referred to as peripheral resistance (PR). The energy loss decreases the blood pressure as flow continues through the vessel.
In a small diameter vessel, a large proportion of the blood would experience this resistance and the pressure drop would be great; in a large diameter vessel, a small proportion of blood would experience resistance and the pressure drop would be small. In other words, small diameter vessels offer high resistance and large diameter vessels offer low resistance. This causes an inverse relationship between peripheral resistance and the downstream blood pressure shown by the dashed arrow in the map.
Arterioles are the primary type of vessels whose diameter is highly variable. This is accomplished by changing the degree of contraction of the abundant circular smooth muscles in their walls. The greater the degree of vasoconstriction (VCN) --the smaller their diameter--the greater the peripheral resistance encountered by blood flowing through them. This is shown as the solid arrow (direct relationship) between VCN and PR.
The sympathetic division of the autonomic nervous system can exert generalized control of most arterioles in the body. This is accomplished by wide-ranging vasomotor nerves (VM) that secrete norepinephrine and cause vasoconstriction. These stimulatory nerves are shown as a solid line.
Individual arterioles are also responsive to 'local controls' such as the concentration of oxygen (O2) and carbon dioxide (CO2) in the interstitial fluid surrounding them. The relationship between oxygen concentration and the extent of vasoconstriction is direct (solid arrow)--the greater the oxygen (O2)concentration the greater the extent of vasoconstriction (VCN). In other words, if there is sufficient oxygen in the tissue then the blood flow--which would bring in more oxygen--is reduced. Also, the relationship between carbon dioxide (a waste product of cellular activity) concentration and vasoconstriction is inverse (dashed arrow); the greater the carbon dioxide (CO2) concentration the less the extent of vasoconstriction (VCN). In other words, if there is a lot of carbon dioxide in the tissue a greater blood flow is needed to allow its diffusion into, and removal by, the blood.
Last updated: 7/14/2005