Lect. 8 - PPatterns and Physics of Blood Flow. Arteries. Arterioles (5).pptx
1. Patterns and Physics of Blood Flow
Arteries
Arterioles
Nanuka Tsibadze
Sherwood Physiology – p. 335-350
2.
3. Ohm’s Law
• F is blood flow, ΔP is the pressure
difference (P1 − P2) between the
two ends of the vessel, and R is the
resistance.
• This formula states that the blood flow
is directly proportional to the pressure
difference but inversely proportional to
the resistance.
4. Blood Flow Resistance
The factors that determine the resistance of a blood vessel to
blood flow are expressed by the Poiseuille equation.
Editor's Notes
Blood flow through a blood vessel is determined by two factors: (1) pressure difference of the blood between the two ends of the vessel, also sometimes called “pressure gradient” along the vessel, which pushes the blood through the vessel, and (2) the impediment to blood flow through the vessel, which is called vascular resistance.
Resistance occurs as a result of friction between the flowing blood and the intravascular endothelium all along the inside of the vessel.
Resistance is the impediment to blood flow in a vessel, but it cannot be measured by any direct means.
Note particularly in this equation that the resistance of blood flow is inversely proportional to the fourth power of the radius of the vessel, which demonstrates once again that the
diameter of a blood vessel (which is equal to twice the radius) plays by far the greatest role of all factors in determining the rate of blood flow through a vessel.
For example, if the radius of a blood vessel decreases by one-half, resistance does not simply increase twofold — it increases by 16-fold (24).
Arteries are specialized: 1. to serve as rapid-transit passageways for blood from the heart to the organs (because of their large radius, arteries offer little resistance to blood flow) and 2.
to act as a pressure reservoir to provide the driving force for blood when the [[[ [ heart is relaxing.]]]
How do the arteries act as a pressure reservoir? The heart alternately contracts to pump blood into the arteries and then relaxes to refill with blood from the veins. When the heart is relaxing and refilling, no blood is pumped out. However, capillary flow does not fluctuate between cardiac systole and diastole—that is, blood flow is continuous through the capillaries supplying the organs. The driving force for the continued flow of blood to the organs during cardiac relaxation is provided by elastic recoil of the walls of large arteries.
Arterioles are main resistance vessels in the vascular tree because their radius is small enough to offer considerable resistance to blood flow.
Two basic mechanisms are proposed to explain the phenomena of acute autoregulation of blood flow: the myogenic hypothesis and the metabolic hypothesis.
The myogenic hypothesis states that when vascular smooth muscle is stretched, it contracts. Thus if arterial pressure is suddenly increased, the arterioles are stretched and the vascular smooth muscle in their walls contracts in response to this stretch. Contraction of arteriolar vascular smooth muscle causes constriction (i.e., increased resistance), thereby maintaining a constant flow in the face of increased pressure (recall that F=ΔP/R).
The metabolic hypothesis states that the greater the rate of metabolism or the less the availability of oxygen or some other nutrients to a tissue, the greater the rate of formation of vasodilator substances in the tissue cells. The vasodilator substances then are believed to diffuse through the tissues to the precapillary sphincters, metarterioles, and arterioles to cause dilation. Some of the different vasodilator substances that have been suggested are adenosine, carbon dioxide, adenosine phosphate compounds, histamine, potassium ions, and hydrogen ions.
Over a period of hours, days, and weeks, a long-term type of local blood flow regulation develops in addition to the acute control. A key mechanism for long-term local blood flow regulation is to change the amount of vascularity of the tissues, ie angiogenesis.
Two additional special examples of metabolic control of local blood flow are reactive hyperemia and active hyperemia
Reactive hyperemia is an increase in blood flow in response to or reacting to a prior period of decreased blood flow. For example, reactive hyperemia is the increase in blood flow to an organ that occurs following a period of arterial occlusion.
Active hyperemia illustrates the concept that blood flow to an organ is proportional to its metabolic activity. If metabolic activity in skeletal muscle increases as a result of strenuous exercise, then blood flow to the muscle will increase proportionately to meet the increased metabolic demand. Active hyperemia in skeletal muscle can increase local muscle blood flow as much as 20-fold during intense exercise.