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Lecture 8

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

1    Lecture 8: Blood Pressure Regulation 1. The Baroreflex and Baroreceptors To date, almost all of the cardiovascular system concepts and control systems that we have discussed will affect blood pressure. If blood pressure is changing, then nervous, hormonal and local effects will all converge to affect either cardiac output (by altering stroke volume or heart rate) or total peripheral resistance. Much of blood pressure regulation is extrinsic to blood vessels, occurring as a result of nervous or hormonal input; conversely, some of the regulation (as detailed in the previous lecture – local control of blood flow) is intrinsic to vessels blood vessels or the organs which they supply. There are also differences in the regulation of blood pressure over the short term (that is, over seconds or minutes) and its regulation over the long-term (from minutes to days). The latter have more to do with fluid balance and excretion of fluid from the kidneys or retention of fluid at the kidneys. Over the short term, acute regulation of blood pressure is accomplished with a reflex circuit termed the baroreflex. The baroreflex is a set of reflex responses with occur in response to stimulation (or inhibition or a reduction of stimulation) of baroreceptors (blood pressure sensor cells, which are located in blood vessels throughout the circulatory system). A very common and important example of the baroreflex response occurs in response to a decrease in mean arterial pressure. A decrease in MAP is sensed by the baroreceptors and they, in turn, cause an increase in sympathetic output and a decrease in parasympathetic output. This leads to increases in HR, SV and TPR and therefore an increase in MAP. If blood pressure were to increase, then the baroreceptors would cause the opposite to occur; a decrease in sympathetic activity and an increase in parasympathetic activity. Keep in mind that baroreceptor activity increases in response to high blood pressure and decreases in response to low blood pressure (see below). 2    2. Aortic Arch and Carotid Sinus Baroreceptors The two main populations of baroreceptors in our circulation are located within the arterial side of the circulation, positioned to monitor blood flow to the systemic circuit and to the brain. The first of these baroreceptor populations is located in the aortic arch just as the aorta leaves the left ventricle and begins to curve downward. These baroreceptors are well positioned to sense the pressure of blood flowing to the entire body (systemic circuit). Further upward, along the common carotid artery, is a little bulge (or sinus) in the blood vessel called the carotid sinus. The baroreceptors in the carotid sinus are well positioned to sense the pressure of blood flowing to the brain. The common carotid artery bifurcates into external and internal vessels downstream from the carotid sinus, and just before that bifurcation lies a small organ called the carotid body, which is the primary location of arterial oxygen chemoreceptors. On a per-weight basis, the carotid body receives more blood flow than any other organ in the body, including the brain. 3    Both types of baroreceptors, carotid sinus and aortic arch, send signals to the brain stem, specifically a region called the nucleus tractus solitarius (NTS). This is an important integrative relay centre at the border of the medulla and the spinal cord. The input from these baroreceptors can then cause changes in parasympathetic and sympathetic output to the heart and blood vessels. The forebrain and higher brain functions are less directly connected to blood pressure regulation, but there is some integration: for instance, in the cortex incorporation of emotion and pain sensing can affect blood pressure. The hypothalamus (which is the body's thermostat) can affect blood pressure due to changes in body temperature. 4    There are other receptors (pressure-sensitive and otherwise) that are important for blood pressure regulation. There are receptors in the venous system, called venous baroreceptors, and ones in the heart, called cardiac baroreceptors. They monitor pressure in the venous system and the heart, respectively. Arterial chemoreceptors measure levels of oxygen and carbon dioxide in the blood. They are very important for control of breathing but have effects on blood pressure as well. Proprioceptors are position sensors in the joints. They relay sensory information regarding movement and the initiation of exercise which likewise affects blood pressure. 5    3. Baroreceptor Activity There is always a tonic level of baroreceptor activity when blood pressure is normal. In other words, the baroreceptors are always sending action potentials to the brain. As blood pressure increases, the depolarization of the baroreceptors caused by the stretching of blood vessels causes the action potentials sent to the brain to increase in frequency, and the opposite situation occurs when blood pressure is lowered. Remember, the primary purpose of blood pressure regulation is to prevent blood pressure form falling too low (which would cause shock and death). High blood pressure can also kill, but only over many years. It can be particularly dangerous over a long time, however, because it resets the pressure baseline for the baroreceptors, desensitizing them. At first, high blood pressure may cause the baroreceptors to initiate the baroreflex, but if blood pressure remains elevated over time, the baroreceptors will gradually adjust to a new normal. Over time, this creeping normalcy can cause major cardiovascular damage. 6    4. Cardiovascular Changes Initiated by the Baroreceptor If blood pressure decreases due to bleeding or hemorrhage, then the baroreflex will cause an increase in cardiac output and total peripheral resistance, thereby partially (but not entirely) compensating for the reduction in pressure. In the exam
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