Ch 14 part 2 apk.docx

Ch 14 - Blood Vessels and Pressure Recall that two measurements of arterial blood pressure are as follows: Systolic blood pressure (SBP) is the maximum pressure in the aorta, which occurs during systole. Diastolic blood pressure (DBP) is the minimum pressure in the aorta, which occurs during diastol...

Ch 14 - Blood Vessels and Pressure Recall that two measurements of arterial blood pressure are as follows: Systolic blood pressure (SBP) is the maximum pressure in the aorta, which occurs during systole. Diastolic blood pressure (DBP) is the minimum pressure in the aorta, which occurs during diastole. The blood pressure in the aorta never drops to zero, like pressure in the ventricles does, because the arteries are elastic. As we have mentioned, mean arterial pressure (MAP) is the weighted average arterial blood pressure in the aorta during the cardiac cycle. We have also mentioned that mean arterial pressure is closer to the diastolic blood pressure because the heart spends more time in diastole than in systole. Assuming the heart spends twice as much time in diastole as in systole, the mean arterial pressure can be measured as follows: To calculate Mean Arterial Pressure: (Systolic Blood Pressure + 2(Diastolic Blood Pressure)) / 3 Measuring Blood Pressure A sphygmomanometer is a device used to measure blood pressure. The arm is placed through an inflatable cuff that restricts blood flow, usually in the brachiaI artery. Most blood pressure devices automatically measure blood pressure. However, if a doctor is manually measuring your blood pressure, he or she will follow these steps: lnflate the cuff to completely occlude the brachial artery. No sounds should be heard through a stethoscope placed near the brachial artery because no blood flow is occurring. You must Inflate the cuff to a pressure that is higher than what you expect the systolic blood pressure to be. Slowly deflate the cuff, lowering the pressure and eventually allowing blood flow. Record the pressure when the first sound is heard through a stethoscope. This represents the systolic blood pressure, The sound occurs because blood begins turbulent flow through the artery. Continue deflating the cuff until no sound is heard and record the pressure. This is the diastolic blood pressure. The sound disappears because blood does not make a sound when the artery is uncompressed. (This is called laminar flow.) Blood pressure Is reported like a fraction. The top number is the systolic pressure The bottom number is the diastolic pressure. For example, an individual with blood pressure of 120/80 has a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg. Blood pressure measurements are important because they provide an Indication of how hard the heart is working to pump blood. In people who have high blood pressure, the heart has to work very hard to get the blood to its destination. This Is why high blood pressure is a major risk factor for heart attacks. Arterioles and Arteriolar Tone Whereas arteries and veins can be observed without a microscope, arterioles, capillaries, and venules cannot. For this reason, arterioles, capillaries, and venules are considered part of the body's microcirculation. As we have noted, arterioles provide the most significant resistance to blood flow in the vasculature. In fact, they provide about 60% of the vasculature’s overall resistance; this is a protective measure to prevent the capillaries from bursting due to overpressure, The arteriolar walls don't have much arranged smooth muscle that acts to increase resistance. (Circularly arranged smooth muscles are called sphincters. When they contract, the hole that they form gets smaller.) The diagram to the right shows how blood pressure changes as the blood makes its way from the heart, to the tissues, and back to the heart. This diagram shows the pulsatile nature of pressure, as the cardiac cycle alternates between periods of systole and diastole. Again, note that the biggest drop in blood pressure happens at the arterioles. Also, note that blood pressure gets less and less pulsotile as the blood moves away from the heart, and by the time it gets to the capillaries it has completely lost its pulsatile nature. This makes sense because the capillaries have no muscle wall, so they cannot expand to accommodate a higher volume. Blood flow through the capillaries has to be very smooth, or they will burst. Many students believe that capillaries will have the largest resistance to blood flow since they are much smaller than arterioles. Although capillaries do have a smaller radius than arterioles, they have a large total surface area which allows the blood to spread out, thus decreasing the resistance to blood flow. So even though capillaries are smaller, arterioles have greater resistance to blood flow than capillaries. Also, note that the velocity of blood is high as It moves out of the heart into the arteries, but it slows down at the arterioles and gets very low at the capillaries. It then picks up again in the venules and veins. This is important because we need the blood to move through the capillaries slowly to facilitate the exchange of oxygen, carbon dioxide, and nutrients with the body. Note that the arterioles constantly maintain a certain amount of arteriolar tone, even without external stimulation. This arteriolar tone gives the blood vessels the ability to either vasoconstrict (and become smaller in diameter) or vasodilate (and become larger in diameter). Arteriolar tone con be Increased or decreased for two purposes: Control of blood flow - The arterioles con vosoconstrict at one location and vosodilote in another to direct blood flow from one individual capillary bed to another. This is primarily achieved through intrinsic control mechanisms, such as local fact ors (chemicals) like nitric oxide. Regulation of mean arterial pressure - The vasoconstriction and vasodilation of arterioles affects total peripheral resistance and thus regulates mean arterial pressure. This Is primarily achieved through extrinsic control mechanisms, such as the activity of the autonomic nervous system and hormones. Three different types of capillaries Include continuous capillaries, fenestrated capillaries, and discontinuous capillaries. Dr. Nguyen will only focus on continuous capillaries and fenestrated capillaries in this class. Continuous capillaries have narrow lntercellular gaps (or clefts) that can be described as leaky tight Junctions because they allow for the exchange of some fluid. Fenestrated capillaries have wider lntercellular gaps which allow for larger structures like cells to be exchanged as well as for the exchange to occur more quickly. Capillary beds form an intertwining network. Blood flows into a capillary bed from a terminal arteriole. It then enters a metarteriole, a transitional type of vessel that gives off true capillaries. Gas exchange occurs in the true capillaries, and then the capillaries converge bock and drain Into a postcapillary venule. The path of blood through o capillary might look something like this: Note that smooth muscle rings called precapillary sphincters wrap around the branches of capillaries and control blood flow to certain capillary beds. These sphincters ore largely controlled by Intrinsic factors, especially local regulators such as nitric oxide (which we discuss later in these notes). Contraction of precapillary sphincters causes vasoconstriction and redirects blood away from the capillary bed, and directly into the venules. Earlier, we discussed how arteries served as pressure reservoirs. Veins, on the other hand, serve as volume reservoirs. While arteries store pressure because of their low compliance, veins con store a large volume of blood because of their high compliance and their relatively thin walls. At rest, 60% of the body's blood Is stored in veins. (When you're engaged in a high amount of activity, you con redistribute some of this blood to the arterial side of circulation.) The relatively high compliance of veins (compared ta arteries) is shown above. A given change In pressure produces a much larger change in volume in veins than in arteries. This is why veins are considered volume reservoirs. The movement of blood from the capillaries bock to the right atrium through the veins is driven by the pressure gradient between the peripheral veins and the right atrium. The venous return-the rote of blood flow bock to the heart-con be increased by the following: Skeletal muscle pump - When a person moves, the skeletal muscles surrounding his or her veins contract, compressing the veins and squeezing blood through the veins just like toothpaste Is squeezed through a tube. The blood flows in one direction because veins hove one-way valves that prevent backflow. Note that venous valves are only present in the peripheral veins. Respiratory pump - When you inhale, your thoracic volume increases. This increases the pressure difference between the abdominal and thoracic veins, facilitating increased venous return. Exhaling forcefully decreases the volume of the thoracic cavity, which puts pressure on the veins entering the heart and squishes the blood inside the organ. This inhalation and exhalation both promote pulling of the blood from the veins into the heart. This is one of the most significant methods of increasing venous return, especially during exercise. Blood volume - Venous pressure increases when the blood volume increases, and venous pressure decreases when blood volume decreases. Changes in blood volume occur relatively slowly, however. Venous return decreases when blood pools. For example, people commonly faint when they have to stand for a long time, such as at a wedding ceremony. Blood pooled in his legs, reducing venous return to the heart. Orthostatic hypotension ls the temporary signal of low blood volume, which causes you to lose a bit of consciousness. This occurs when you stand up too quickly after lying down. Venomotor tone - Recoil that a vein is made out of an outer wall of connective tissue, a middle wall comprised of smooth muscle, and an inner endothelium. A vein has less smooth muscle than an artery, but it does have some, so the sympathetic nervous system can still produce venoconstriction (constriction of the veins). This increases the pressure inside the vein, pushing blood toward the heart. In addition, venoconstriction makes veins less compliant, which increases pressure and facilitates venous return to the heart. Increased venous return to the heart results in increased atrial pressure, which increases end-diastolic pressure, which increases end- diastolic volume, which increases stroke volume and cardiac output. The final result ls an increase in mean arterial pressure. Intrinsic Regulation of Blood Flow Recall that extrinsic control mechanisms control the MAP and ensure that all organs have an ample supply of blood. It Is the Intrinsic control mechanisms, however, that determine the distribution of blood to organs and the amount of blood directed to individual capillary beds. If a tissue's metabolic need is low, intrinsic mechanisms will decrease the flow to that tissue. If the metabolic need is high, they will Increase the flow to that tissue. The flow of blood to any particular organ, like all blood flows, is determined by the pressure gradient and the resistance to blood in that organ: Organ Blood Flow =The Pressure gradient/Organ resistance Mean arterial pressure provides the same driving pressure to all organs in the systemic circuit that receive blood in parallel. Therefore, any difference in blood flow to these organs must be due to differences in resistance. Organs can increase or decrease the amount of blood they receive by decreasing or increasing resistance. If blood flow to one area of the body is decreased, that blood is still in the system and it must be directed elsewhere. This commonly occurs with different organ systems; as we exercise, we increase blood flow to skeletal muscles. However, that blood has to come from somewhere. It is usually token from on organ which does not currently have a very high metabolic need, such as the gut or kidneys. (Note that cardiac output increases by as much as a factor of five during heavy exercise, reflecting the body's heightened metabolic needs.) This mechanism of intrinsically distributing blood occurs within organs as well. A good example of this would be the brain- as you use different parts of your brain, blood flow is directed to that port from other brain areas. For example, you use different parts of your brain for studying and for running. When you engage in these activities, blood is directed toward the appropriate areas and away from other areas. Types of Intrinsic Regulation As we have discussed, the key determinant of o vessel's resistance is its diameter. When the smooth muscle in a vessel contracts, vasoconstriction occurs, the vessel's resistance increases, and less blood flows through the vessel. When the smooth muscle in a vessel relaxes, vasodilation occurs, the vessel's resistance decreases, and more blood flows through the vessel. Four types of intrinsic regulation act on blood vessels to change vessel diameter: 1 Metabolic activity (active hyperemia) - Hyperemia refers to a state of higher- than normal blood flow. Active hyperemia occurs when blood flow is higher than normal because of an increase in the metabolic rate in the tissue. An Increase in oxygen consumption and carbon dioxide expulsion causes vasodilation in the affected tissue. This decreases resistance, increases blood flow, and thus increases oxygen delivered to those tissues. This is an example of a negative feedback loop-once oxygen and carbon dioxide levels ore normalized, the vessel returns to its normal arteriolar tone. 2 Changes in blood flow (reactive hyperemia) - Reactive hyperemia occurs when blood flow is higher than normal in response to a situation in which blood flow temporarily falls. For example, if you cut off circulation to your brachial artery with a pressure cuff (such as when you take your blood pressure), the distal ports of your arm will not receive oxygen and they will be unable to get rid of carbon dioxide. This will trigger vasodilation, which will reduce resistance, increase blood flow, and ultimately Increase the delivery of oxygen and the removal of carbon dioxide from the affected tissues. EXAM TIP: Note that both active hyperemia and reactive hyperemia both result in the same type of negative feedback to counteract low levels of oxygen. The difference is what causes each type of regulation. 3 Smooth muscle stretch (myogenic response) - Some tissues have arterioles with stretch-sensitive muscle fibers that produce vasoconstriction when they ore stimulated and vasodilation when they are under less- than-normal stretch. This is o myogenic response because it occurs automatically, and it is triggered by the muscle itself. For example, when perfusion pressure (the blood pressure of the organ) increases and blood flow increases in these tissues, stretch sensors will sense on Increase in stretch of the smooth muscles in the arterioles. This triggers constriction of the arterioles, which increases resistance and decreases blood flow through the tissue. This Is also a negative feedback loop because this mechanism is responding to vasodilation (an increase in stretch) with vasoconstriction (constriction of the arteriole). 4 Locally produced chemical messengers - Multiple chemicals made by tissues in or around vessels can cause the vascular smooth muscle to either contract or relax. Many chemicals are created by the endothelium, and it is these chemicals that cause constriction or dilation. A very common chemical is nitric oxide (NO), which is also called endothelium derived relaxing factor (EDRF). Nitric oxide Is produced In the endothelium, and It leads to vasodilation. Its release is stimulated by histamine, and It serves to Increase the concentration of cGMP, a second messenger whose function Is to cause vessel relaxation. The relaxation of the smooth muscle forming the arteriole walls increases the radius (thus decreasing resistance), so blood flow increases. Nitric oxide has many clinical uses. Two examples are as follows: o Nitroglycerine - Nitroglycerine acts as a nitric oxide donor. Cardiac patients often carry nitroglycerine with them to toke in the case of emergency heart pain (to relax their vessels). People who suffer from angina (heart pain) often carry nitroglycerine pills with them and put them under their tongues when they experience significant pain. o Viagra - Viagra does not contain nitric oxide, but it has the same effects. Viagra inhibits PDES, on enzyme that facilitates the breakdown of cGMP to GMP. It effectively maintains vasodilation in the penis, thus increasing blood flow to the penis and allowing males to maintain an erection for a longer period of time.