Blood Pressure

Blood Pressure

Blood pressure is the pressure exerted by the blood’scirculating volume on the walls of the arteries, veins, and heart chambers. It is regulated by the body’s homeostatic mechanisms, involving blood volume, the lumens of arteries and arterioles, and the force of cardiac contraction. The systemic blood pressure is at its highest level in the aorta, declining along the blood pathways, until it is at 0 mm Hg in the right atrium. The largest drop in blood pressure occurs in the arteri-oles, which have the most resistance to blood flow. The pressure gradient continues, even though it is small, allowing blood to flow all the way back to the heart.

Arterial Blood Pressure

Arterial blood pressure rises and falls accord-ing to cardiac cycle phases. It is important in the ­maintenance of blood flow through the capillary beds. Arterial blood pressure must always be ­sufficient in order to overcome peripheral resistance. Arterial blood pressure is equivalent to the term “blood pres-sure.” The maximum pressure during ventricular contraction is called the systolic pressure, averag-ing 120 mm Hg in a healthy adult. FIGURE 19- 7 shows changes in blood pressure as distance from the left ventricle increases. The lowest pressure that remains in the arteries before the next ventricular contrac-tion is called the diastolic pressure, which averages between 70 and 80 mm Hg in a healthy adult. There-fore, systole refers to periods of contraction, whereas diastole refers to periods of relaxation. The cardiaccycle includes atrial systole and diastole, followed by ventricular systole and diastole. An electrocar-diogram illustrates all these mechanical events. The cardiac cycle is signified by continual pressure and blood volume changes within the heart.

In diastole, the aortic valve closes. Blood cannot flow back into the heart. There is a recoiling of the walls of the aorta and other elastic arteries. There is enough pressure maintained so the blood can flow into the smaller vessels. Aortic pressure drops to its lowest level at this time, which is described as the dia-stolic pressure. The difference between the systolic and diastolic pressures is known as pulse pressure. During systole, it is felt in an artery as a throbbing pul-sation. This is due to ventricular contraction, whichforces blood into the elastic arteries, expanding them. Pulse pressure is temporarily raised by increased stroke volume and quicker blood ejection because ofincreased contractility from the heart. Pulse pressure­ is chronically increased by atherosclerosis. This is because of the loss of elasticity in the elastic arteries. A single blood pressure value is reported by using meanarterial pressure (MAP).This is calculated by addingone-third of the pulse pressure to the diastolic pres-sure, as follows:

MAP = Diastolic pressure + (Pulse pressure/3)

If the systolic pressure is 120 mm Hg and the diastolic pressure is 90 mm Hg, the MAP would be calculated as follows:

MAP = 90 + [(120 – 90)/3] = 90 + 10 = 100 mm Hg

Healthy individuals have a normal range of sys-tolic and diastolic pressures. When these pressures become abnormal, clinical problems develop. Hyper-tension describes abnormally high blood pressure, and hypotension­ describes abnormally low blood pressure.Hypertension is much more common than hypotension. However, many cases of hypotension are caused by use of antihypertensive drugs that is excessive. According to the American Heart Association, adult hypertension exists when the blood pressure reaches 140/90. Blood pressure of 120/80 or less is normal. Blood ­pressure between 121/81 and 139/89 signifies pre-hypertension. For pre-hypertensive patients, it is recommended that dietary changes and drug therapy are used in order to prevent hypertension from ­developing.

Hypertension greatly increases the heart’s work-load, resulting in gradual enlargement of the left ventricle. With more muscle mass, the body has a greater demand for oxygen. When coronary cir-culation is not sufficient, there will be signs and symptoms of coronary ischemia. Increased arterial pressure puts physical stress upon the body’s blood vessel walls. The increases or encourages devel-opment of arteriosclerosis, as well as risk of heart attack, stroke, and aneurysms.

Arterial blood pressure is measured with a device called a sphygmomanometer or blood pressure cuff. Its results are reported as a fraction of the sys-tolic pressure over the diastolic pressure. The upper or first number indicates the arterial systolic pressure in mm Hg, and the lower or second number indi-cates the arterial diastolic pressure, also in mm Hg. A millimeter of mercury is a unit of pressure equal to 0.001316 of normal atmospheric pressure. This means a blood pressure of 120/80 displaces 120 mm of Hg on a sphygmomanometer, showing the systolic pressure, and also displaces 80 mm of Hg on the same device, showing diastolic pressure.

The artery walls are distended as blood surges into them from the ventricles, but they recoil almost immediately. This expansion and recoiling can be felt as a pulse in an artery near the surface of the skin. Most commonly, the radial artery is used to take a ­person’s pulse, although the carotid, brachial, and ­femoral arteries also can be used. Arterial blood pressure depends on heart rate, stroke volume, blood volume, peripheral resistance, and blood viscosity. The recoil-ing of arteries to their original dimensions is known as elastic rebound.

Capillary Blood Pressure

In the capillaries blood pressure drops off to only approximately 35 mm Hg, with the ends of capil-lary beds having only 17 mm Hg of pressure. This is important because the capillaries are fragile and eas-ily ruptured. They are also extremely permeable, and low capillary pressures can cause filtrate to be forced out of the bloodstream into the interstitial space.

Venous Blood Pressure

The venous blood pressure is steady and regular. It does not pulsate with the ventricular contractions like the arterial blood pressure. In the veins, the pres-sure gradient is only approximately 15 mm Hg. Con-sider that from the aorta to the ends of the arterioles, the pressure is approximately 60 mm Hg. Venous blood pressure is usually too low to cause venous return to be adequate. Therefore, the muscular pump, respiratory pump, and sympathetic venoconstriction are used.

The muscular pump uses skeletal muscle activity to contract and relax around the veins, moving blood toward the heart. Each vein valve keeps blood that has passed from flowing backward. As pressure changes in the body’s ventral cavity during breathing, the respi-ratory pump moves blood toward the heart. Inha-lation increases abdominal pressure, squeezing local veins and forcing blood to the heart. Simultaneously, the chest pressure decreases. The internal and external thoracic veins then expand and increase blood entry into the right atrium. The volume of blood in the veins is then reduced by sympathetic venoconstriction. Sympathetic control causes the smooth muscle layer around the veins to constrict, reducing venous vol-ume. Blood is therefore pushed toward the heart. Together, the muscular pump, respiratory pump, and sympathetic venoconstriction increase venous return and stroke volume.

Total Peripheral Resistance

The difference in pressure over the entire systemic circuit is sometimes called circulatory pressure. This is approximately 100 mm Hg. Total peripheral resistance is defined as the resistance of the entire cardiovascular system. For circulation to occur, the circulatory pres-sure must overcome the total peripheral resistance. The relatively high pressure of the arterioles is mostly reflected by the large pressure gradient of the arterial network, which is about 65 mm Hg. Total peripheral resistance combines vascular resistance, blood viscos-ity, and turbulence. Vascular resistance is the most important component of total peripheral resistance and involves vessel length and diameter. Viscosity, or the resistance to blood flow caused by interactions among molecules and suspended materials, is the second component. Turbulence is defined as changes that increase resistance and slow down the blood flow, including irregular surfaces, high flow rates, and sud-den changes in the diameters of blood vessels.

Net Filtration Pressure

The net filtration pressureis the difference between the net osmotic pressure and the net hydrostatic pres-sure. At arterial ends of capillaries, this is usually 10 mm Hg. This positive value shows that fluid usu-ally moves out of capillaries, into the interstitial fluid. This means that filtration is occurring. However, at the venous ends of capillaries, the net filtration pressure is usually –7 mm Hg. This negative value shows that fluid usually moves into the capillaries, meaning that reabsorption is occurring. Whenever net filtration pressure is zero, hydrostatic and osmotic forces are equal. Therefore, the transition between filtration and reabsorption occurs where capillary hydrostatic pres-sure is 25 mm Hg.

Tissue Perfusion

Blood flow through tissues is also known as tissueperfusion. This occurs by homeostatic regulationof cardiovascular activities so that needs for oxygen and nutrients are met. The factors affecting tissue per-fusion are cardiac output, peripheral resistance, and blood pressure. Cardiovascular regulation ensures that blood flow changes occur at appropriate times in areas of the body that require it without significantly changing blood pressure and flow to the vital organs.

The three mechanisms that are involved include autoregulation, neural mechanisms, and endocrine mechanisms. Autoregulation involves local factors that alter blood flow inside capillary beds, with pre-capillary sphincters opening and closing because of chemical changes in interstitial fluids. Neural mecha-nisms occur in response to arterial pressure changesor blood gas level changes in certain areas. Endocrinemechanisms involve hormones that enhance short-term changes and that also balance long-term changes in cardiovascular activities.

Dilation of precapillary sphincters are promoted by vasodilators. At the tissue level, local vasodilators help to speed up blood flow through their tissues of origin. Local vasodilators include acids from tissue cells, such as lactic acid; increased carbon dioxide or decreased tissue oxygen levels; increased concen-trations of hydrogen or potassium ions in interstitial fluid; endothelial cells releasing nitric oxide; elevations in local temperature; and release of chemicals, such as nitric oxide or histamine during local inflammation. Also, localvasoconstrictors, such as thromboxanes, prostaglandins, and endothelins, stimulate precapil-lary sphincters to constrict. Together, local vasodila-tors and vasoconstrictors balance blood flow in single capillary beds. Higher concentrations of these factors affect arterioles as well.

Blood Volume

Blood volume is defined as the sum of formed ele-ments and plasma volumes in the vascular system. Blood volume varies with age, body size, and gen-der. Most adults have approximately 5 liters of blood, which makes up 8% of the body’s weight in kilograms. This is only slightly more than 1 gallon of blood. Blood pressure and volume are usually directly proportional. Any changes in volume can initially alter pressure. When measures are taken to restore normal blood volume, normal blood pressure can be reestablished. Fluid balance fluctuations may also affect blood vol-ume. The entire blood supply pumps through each side of the heart about once per minute.

1. Explain blood pressure and how it is calculated.

2. Contrast systolic and diastolic blood pressure.

3. Describe the differences between arterial and venous blood pressure.

4. What is net filtration pressure?