The length of a vessel is directly proportional to its resistance: the longer the vessel, the greater the resistance and the lower the flow. As with blood volume, this makes intuitive sense, since the increased surface area of the vessel will impede the flow of blood.
Likewise, if the vessel is shortened, the resistance will decrease and flow will increase. The length of our blood vessels increases throughout childhood as we grow, of course, but is unchanging in adults under normal physiological circumstances. Further, the distribution of vessels is not the same in all tissues. Adipose tissue does not have an extensive vascular supply. One pound of adipose tissue contains approximately miles of vessels, whereas skeletal muscle contains more than twice that.
Overall, vessels decrease in length only during loss of mass or amputation. An individual weighing pounds has approximately 60, miles of vessels in the body.
Gaining about 10 pounds adds from to miles of vessels, depending upon the nature of the gained tissue. One of the great benefits of weight reduction is the reduced stress to the heart, which does not have to overcome the resistance of as many miles of vessels. In contrast to length, the diameter of blood vessels changes throughout the body, according to the type of vessel, as we discussed earlier. The diameter of any given vessel may also change frequently throughout the day in response to neural and chemical signals that trigger vasodilation and vasoconstriction.
The vascular tone of the vessel is the contractile state of the smooth muscle and the primary determinant of diameter, and thus of resistance and flow. The effect of vessel diameter on resistance is inverse: Given the same volume of blood, an increased diameter means there is less blood contacting the vessel wall, thus lower friction and lower resistance, subsequently increasing flow. A decreased diameter means more of the blood contacts the vessel wall, and resistance increases, subsequently decreasing flow.
The influence of lumen diameter on resistance is dramatic: A slight increase or decrease in diameter causes a huge decrease or increase in resistance. This means, for example, that if an artery or arteriole constricts to one-half of its original radius, the resistance to flow will increase 16 times. Recall that we classified arterioles as resistance vessels, because given their small lumen, they dramatically slow the flow of blood from arteries. In fact, arterioles are the site of greatest resistance in the entire vascular network.
This may seem surprising, given that capillaries have a smaller size. How can this phenomenon be explained? Figure 4 compares vessel diameter, total cross-sectional area, average blood pressure, and blood velocity through the systemic vessels. Although the diameter of an individual capillary is significantly smaller than the diameter of an arteriole, there are vastly more capillaries in the body than there are other types of blood vessels.
Part c shows that blood pressure drops unevenly as blood travels from arteries to arterioles, capillaries, venules, and veins, and encounters greater resistance. However, the site of the most precipitous drop, and the site of greatest resistance, is the arterioles.
This explains why vasodilation and vasoconstriction of arterioles play more significant roles in regulating blood pressure than do the vasodilation and vasoconstriction of other vessels.
Figure 4. The relationships among blood vessels that can be compared include a vessel diameter, b total cross-sectional area, c average blood pressure, and d velocity of blood flow. Part d shows that the velocity speed of blood flow decreases dramatically as the blood moves from arteries to arterioles to capillaries. This slow flow rate allows more time for exchange processes to occur.
As blood flows through the veins, the rate of velocity increases, as blood is returned to the heart. Compliance allows an artery to expand when blood is pumped through it from the heart, and then to recoil after the surge has passed. This helps promote blood flow. In arteriosclerosis, compliance is reduced, and pressure and resistance within the vessel increase. This is a leading cause of hypertension and coronary heart disease, as it causes the heart to work harder to generate a pressure great enough to overcome the resistance.
Arteriosclerosis begins with injury to the endothelium of an artery, which may be caused by irritation from high blood glucose, infection, tobacco use, excessive blood lipids, and other factors. Artery walls that are constantly stressed by blood flowing at high pressure are also more likely to be injured—which means that hypertension can promote arteriosclerosis, as well as result from it. Recall that tissue injury causes inflammation.
As inflammation spreads into the artery wall, it weakens and scars it, leaving it stiff sclerotic. As a result, compliance is reduced. Moreover, circulating triglycerides and cholesterol can seep between the damaged lining cells and become trapped within the artery wall, where they are frequently joined by leukocytes, calcium, and cellular debris. Eventually, this buildup, called plaque, can narrow arteries enough to impair blood flow. Figure 5. Sometimes a plaque can rupture, causing microscopic tears in the artery wall that allow blood to leak into the tissue on the other side.
When this happens, platelets rush to the site to clot the blood. This clot can further obstruct the artery and—if it occurs in a coronary or cerebral artery—cause a sudden heart attack or stroke. Alternatively, plaque can break off and travel through the bloodstream as an embolus until it blocks a more distant, smaller artery.
Ischemia in turn leads to hypoxia—decreased supply of oxygen to the tissues. Hypoxia involving cardiac muscle or brain tissue can lead to cell death and severe impairment of brain or heart function. A major risk factor for both arteriosclerosis and atherosclerosis is advanced age, as the conditions tend to progress over time.
However, obesity, poor nutrition, lack of physical activity, and tobacco use all are major risk factors. Treatment includes lifestyle changes, such as weight loss, smoking cessation, regular exercise, and adoption of a diet low in sodium and saturated fats. Medications to reduce cholesterol and blood pressure may be prescribed. For blocked coronary arteries, surgery is warranted. In angioplasty, a catheter is inserted into the vessel at the point of narrowing, and a second catheter with a balloon-like tip is inflated to widen the opening.
To prevent subsequent collapse of the vessel, a small mesh tube called a stent is often inserted. In an endarterectomy, plaque is surgically removed from the walls of a vessel. This operation is typically performed on the carotid arteries of the neck, which are a prime source of oxygenated blood for the brain. In a coronary bypass procedure, a non-vital superficial vessel from another part of the body often the great saphenous vein or a synthetic vessel is inserted to create a path around the blocked area of a coronary artery.
The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart.
Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during diastole is very low, often approaching zero when the atria are relaxed atrial diastole.
These physiological pumps are less obvious. In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle. This mechanism, known as the skeletal muscle pump Figure 6 , helps the lower-pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart.
As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods.
Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart. Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness. Figure 6. The contraction of skeletal muscles surrounding a vein compresses the blood and increases the pressure in that area.
This action forces blood closer to the heart where venous pressure is lower. Note the importance of the one-way valves to assure that blood flows only in the proper direction. The respiratory pump aids blood flow through the veins of the thorax and abdomen. The flow of blood around the circulatory system is modulated by numerous interacting factors.
The science dedicated to understanding this flow is called hemodynamics. At its simplest, imagine a perfect, rigid tube with no resistance and with a homogeneous liquid flowing through in a perpendicular manner. Flow can be calculated using the following formula:.
While the above example is a simple calculation, in reality there are numerous factors that influence velocity and flow. Movement of blood throughout the circulatory system is created by differences in pressure generated by the pumping of the heart. Pressure is greatest immediately after exiting the heart and drops as it circulates around the body, particularly through the arterioles and capillary networks.
A greater difference in pressure results in a greater velocity assuming all else remains equal, so when increased blood flow is required the heart can pump more quickly and also in larger volume.
Resistance is the force that must be overcome by pressure in order for flow to occur, and is a factor of vessel length, diameter, surface composition, and the viscosity of the liquid flowing through. As resistance increases the difference in pressure which influences velocity decreases, which in turn reduces flow.
The following is a brief description of the parameters that govern flow through a given vessel. Because friction develops between moving blood and the stationary vessels walls, this fluid movement has a given resistance vascular , which is the measure of how difficult it is to move blood through a vessel. One can then describe a relative relationship between vascular flow, the pressure difference, and resistance i.
This equation may be applied not only to a single vessel, but can also be used to describe flow through a network of vessels i. It is known that the resistance to flow through a cylindrical tube or vessel depends on several factors described by Poiseuille including: 1 radius, 2 length, 3 viscosity of the fluid blood , and 4 inherent resistance to flow, as follows:. It is important to note that a small change in vessel radius will have a very large influence 4th power on its resistance to flow; e.
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