Total Peripheral Resistance & Blood Flow Regulation - Video & Lesson Transcript | gtfd.info
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CV Physiology: Hemodynamics (Pressure, Flow, and Resistance)
The lowest pressure is in the venae cavae, just before they empty into the right atrium. In this section we review the law of physics that explain the interaction of pressure, volume, flow, and resistance in the cardiovascular system. Many of these principles apply broadly to the flow of all types of liquids and gases, including the flow of air in the respiratory system. However, in this chapter we will focus on the flow of blood and its relevance to the function of the heart.
The Pressure of Fluid in Motion Decreases over Distance Pressure in a fluid is the force exerted by the fluid on its container.
Arterial Blood Pressure
If the fluid is not moving, the pressure it exerts is called hydrostatic pressure Fig. For example, a column of fluid in a tube exerts hydrostatic pressure on the floor and sides of the tube. In the heart and blood vessels, pressure is commonly measured in millimeters of mercury mm Hg.
One millimeter of mercury is equivalent to the hydrostatic pressure exerted by a 1-mm-high column of mercury on an area of 1 cm2.
CV Physiology | Determinants of Resistance to Flow (Poiseuille's Equation)
In a system in which fluid is flowing, pressure falls over distance as energy is lost because of friction. In addition, the pressure exerted by moving fluid has two components: Pressure within our cardiovascular system is usually called hydrostatic pressure even though it is a system in which fluid is in motion.Blood Vessels, part 2: Crash Course A&P #28
Some textbooks are beginning to replace the term hydrostatic pressure with the term hydraulic pressure. Hydraulics is the study of fluid in motion. Pressure Changes in Liquids Without a Change in Volume In the walls of a fluid-filled container contract, the pressure exerted on the fluid in the container increases.
You can demonstrate this principle by filling a balloon with water and squeezing the water balloon in your hand. Water is minimally compressible, and so the pressure you apply to the balloon is transmitted throughout the fluid. As you squeeze, higher pressure in the fluid causes part of the balloon to bulge. If the pressure becomes high enough, the stress on the balloon will cause it to pop.
The water volume inside the balloon did not change, but the pressure in the fluid increased.
In the human heart, contraction of the blood-filled ventricles is similar to squeezing a water balloon: This high-pressure blood then flows out of the ventricle and into the blood vessels, displacing lower-pressure blood already in the vessels. The pressure created in the ventricles is called the driving pressure because it is the force that drives blood through the blood vessels. When the walls of a fluid-filled container expand, the pressure exerted on the fluid decreases.
Thus, when the heart relaxes and expands, pressure in the fluid-filled chambers falls. Pressure changes can also take place in the blood vessels. If blood vessels dilate, blood pressure inside them falls. If blood vessels constrict, blood pressure increases. Volume changes of the blood vessels and heart are major factors that influence blood pressure in the cardiovascular system. Blood Flows from an Area of Higher Pressure to One of Lower Pressure As stated earlier, blood flow through the cardiovascular system requires a pressure gradient.
This pressure gradient is analogous to the difference in pressure between two ends of a tube through which fluid flows Fig. This relationship says that the higher the pressure gradient, the greater the fluid flow. A pressure gradient is not the same thing as the absolute pressure in the system. Fore example, the tube in Figure b has an absolute pressure of mm Hg at each end. This pressure difference is important, because later, we will see that blood flows from high to low pressure.
However, one of the most important factors influencing blood flow is the size or radius of the blood vessel the blood is passing through. The artery constricts during vasoconstriction, decreasing blood flow. Blood vessels - and in particular, the more muscular arteries - are often the source of resistance.
One way an artery can actively resist blood flow is by contracting the smooth muscle in its wall, causing the artery to constrict.
When an artery constricts, we call it vasoconstriction. This is an easy term to recall if you remember that 'vaso' refers to 'blood vessel. This causes a decreased blood flow through its lumen, or hollow center. If, on the other hand, the smooth muscle in the wall of the artery relaxes, the blood vessel moves into a state of vasodilation, and its lumen dilates, or gets bigger. This offers decreased resistance and causes an increased blood flow. This is much like a nozzle at the end of a hose.
The same equation also applies to engineering studies of the flow of fluids. Although understanding the math behind the relationships among the factors affecting blood flow is not necessary to understand blood flow, it can help solidify an understanding of their relationships.
Please note that even if the equation looks intimidating, breaking it down into its components and following the relationships will make these relationships clearer, even if you are weak in math. Focus on the three critical variables: It may commonly be represented as 3.
One of several things this equation allows us to do is calculate the resistance in the vascular system. Normally this value is extremely difficult to measure, but it can be calculated from this known relationship: The important thing to remember is this: Two of these variables, viscosity and vessel length, will change slowly in the body.
Only one of these factors, the radius, can be changed rapidly by vasoconstriction and vasodilation, thus dramatically impacting resistance and flow.
Further, small changes in the radius will greatly affect flow, since it is raised to the fourth power in the equation. The Roles of Vessel Diameter and Total Area in Blood Flow and Blood Pressure 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? 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.
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. 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.
Disorders of the…Cardiovascular System: Arteriosclerosis 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.
- Hemodynamics (Pressure, Flow, and Resistance)
- Determinants of Resistance to Flow (Poiseuille's Equation)
- Total Peripheral Resistance & Blood Flow Regulation
Eventually, this buildup, called plaque, can narrow arteries enough to impair blood flow. 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.
Venous System 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. Skeletal Muscle Pump 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 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. 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. Respiratory Pump The respiratory pump aids blood flow through the veins of the thorax and abdomen. During inhalation, the volume of the thorax increases, largely through the contraction of the diaphragm, which moves downward and compresses the abdominal cavity. The elevation of the chest caused by the contraction of the external intercostal muscles also contributes to the increased volume of the thorax.
The volume increase causes air pressure within the thorax to decrease, allowing us to inhale. Additionally, as air pressure within the thorax drops, blood pressure in the thoracic veins also decreases, falling below the pressure in the abdominal veins. This causes blood to flow along its pressure gradient from veins outside the thorax, where pressure is higher, into the thoracic region, where pressure is now lower.
This in turn promotes the return of blood from the thoracic veins to the atria.