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.
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. During exhalation, when air pressure increases within the thoracic cavity, pressure in the thoracic veins increases, speeding blood flow into the heart while valves in the veins prevent blood from flowing backward from the thoracic and abdominal veins. The individual veins are larger in diameter than the venules, but their total number is much lower, so their total cross-sectional area is also lower.
Also notice that, as blood moves from venules to veins, the average blood pressure drops, but the blood velocity actually increases. This pressure gradient drives blood back toward the heart. Again, the presence of one-way valves and the skeletal muscle and respiratory pumps contribute to this increased flow. Since approximately 64 percent of the total blood volume resides in systemic veins, any action that increases the flow of blood through the veins will increase venous return to the heart.
Maintaining vascular tone within the veins prevents the veins from merely distending, dampening the flow of blood, and as you will see, vasoconstriction actually enhances the flow. As previously discussed, vasoconstriction of an artery or arteriole decreases the radius, increasing resistance and pressure, but decreasing flow. Venoconstriction, on the other hand, has a very different outcome. The walls of veins are thin but irregular; thus, when the smooth muscle in those walls constricts, the lumen becomes more rounded.
The more rounded the lumen, the less surface area the blood encounters, and the less resistance the vessel offers. Vasoconstriction increases pressure within a vein as it does in an artery, but in veins, the increased pressure increases flow. Recall that the pressure in the atria, into which the venous blood will flow, is very low, approaching zero for at least part of the relaxation phase of the cardiac cycle.
Thus, venoconstriction increases the return of blood to the heart. Another way of stating this is that venoconstriction increases the preload or stretch of the cardiac muscle and increases contraction.
Blood flow is the movement of blood through a vessel, tissue, or organ. The slowing or blocking of blood flow is called resistance. Blood pressure is the force that blood exerts upon the walls of the blood vessels or chambers of the heart. The components of blood pressure include systolic pressure, which results from ventricular contraction, and diastolic pressure, which results from ventricular relaxation.
Pulse, the expansion and recoiling of an artery, reflects the heartbeat. The variables affecting blood flow and blood pressure in the systemic circulation are cardiac output, compliance, blood volume, blood viscosity, and the length and diameter of the blood vessels. In the arterial system, vasodilation and vasoconstriction of the arterioles is a significant factor in systemic blood pressure: Slight vasodilation greatly decreases resistance and increases flow, whereas slight vasoconstriction greatly increases resistance and decreases flow.
In the arterial system, as resistance increases, blood pressure increases and flow decreases. Cremer A, et al. Journal of the American Heart Association. Williams B, et al. Journal of Hypertension. Fuster V, et al. Epidemiology of hypertension. In: Hurst's the Heart. McGraw-Hill Education; Mancusi C, et al.
Higher pulse pressure and risk for cardiovascular events in patients with essential hypertension: The Campania Salute Network. European Journal of Preventive Cardiology. See also Medication-free hypertension control 6 surprising signs you may have obstructive sleep apnea After a flood, are food and medicines safe to use? Alcohol: Does it affect blood pressure? Beta blockers: How do they affect exercise? Blood pressure chart Blood pressure cuff: Does size matter?
Blood pressure: Does it have a daily pattern? Blood pressure: Is it affected by cold weather? Blood pressure medication: Still necessary if I lose weight? Blood pressure medications: Can they raise my triglycerides? Blood pressure readings: Why higher at home? Blood pressure test Blood pressure tip: Get more potassium Blood pressure tip: Get off the couch Blood pressure tip: Know alcohol limits Blood pressure tip: Stress out no more Blood pressure tip: Watch the caffeine Blood pressure tip: Watch your weight Blood sugar levels can fluctuate for many reasons Blood sugar testing: Why, when and how Bone and joint problems associated with diabetes How kidneys work Build resilience to better handle diabetes Bump on the head: When is it a serious head injury?
Caffeine and hypertension Calcium channel blockers Calcium supplements: Do they interfere with blood pressure drugs? Can whole-grain foods lower blood pressure? Diabetes and foot care Diabetes and Heat Diabetes and menopause Diabetes and summer: How to beat the heat Diabetes and travel: Planning is key Diabetes and electric blankets 10 ways to avoid diabetes complications Diabetes diet: Should I avoid sweet fruits?
Diabetes diet: Create your healthy-eating plan Diabetes foods: Can I substitute honey for sugar? Diabetes and liver Diabetes management: Does aspirin therapy prevent heart problems? Diabetes management: How lifestyle, daily routine affect blood sugar Diabetes: Eating out Diabetes nutrition: Sweets Diabetes symptoms Diabetes treatment: Can cinnamon lower blood sugar? Using insulin Diuretics Diuretics: A cause of low potassium? Diuretics: Cause of gout?
Dizziness Do infrared saunas have any health benefits? Do you know your blood pressure? Does obstructive sleep apnea increase my risk for Alzheimer's disease? Drug addiction substance use disorder High blood pressure and exercise Fibromuscular dysplasia Free blood pressure machines: Are they accurate?
Home blood pressure monitoring Glomerulonephritis Glycemic index: A helpful tool for diabetes? The maximal aortic pressure following ejection is termed the systolic pressure P systolic. As the left ventricle is relaxing and refilling, the pressure in the aorta falls. The lowest pressure in the aorta, which occurs just before the ventricle ejects blood into the aorta, is termed the diastolic pressure P diastolic.
When blood pressure is measured using a sphygmomanometer, the upper value is the systolic pressure and the lower value is the diastolic pressure. The difference between the systolic and diastolic pressures is the aortic pulse pressure , which typically ranges between 40 and 50 mmHg. The mean aortic pressure P mean is the average pressure geometric mean during the aortic pulse cycle.
0コメント