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Regulation of Blood Pressure: Short Term Regulation & Baroreceptors

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  • 0:07 Blood Pressure
  • 0:36 Mean Arterial Pressure
  • 2:48 Baroreceptors
  • 3:24 Blood Pressure Regulation
  • 5:35 Baroreceptor Reflex…
  • 6:56 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Blood pressure in your blood vessels is closely monitored by baroreceptors; they send messages to the cardio regulatory center of your medulla oblongata to regulate your blood pressure minute-by-minute. In this lesson, you will learn how baroreceptors regulate your short-term blood pressure.

Blood Pressure

Your cardiovascular system must maintain an adequate blood pressure in order for blood to be delivered to all of your organs and tissues. If the pressure drops too low, the organs will not receive an adequate perfusion of nourishing blood. If the pressure rises too high, it could damage the delicate inner lining of your blood vessels and eventually lead to heart disease or a stroke. In this lesson, you will learn how the body regulates the blood pressure to keep it from going too low or too high.

Mean Arterial Pressure

Pressure fluctuates with each beat of your heart, what we call one cardiac cycle. We remember that a cardiac cycle has two phases: diastole, which is the phase where the heart is filling with blood but not pumping, and systole, which is the phase when the ventricles contract and pump blood. Blood pressure is defined as the pressure exerted by the blood against the walls of the blood vessels, and it's at its lowest point during diastole and reaches a peak at systole.

Blood pressure is recorded in millimeters of mercury (mmHg) with systolic pressure written first, followed by diastolic pressure. Therefore, a normal blood pressure would be written like this: 120/80. Instead of trying to consider the constant fluctuations of blood pressure, we will look at blood pressure in terms of mean arterial pressure (MAP). Mean arterial pressure is defined as the average arterial blood pressure during a single cardiac cycle.

There are three important factors that affect mean arterial pressure: cardiac output, total peripheral resistance, and blood volume. If not compensated by a decrease in any other variables, we can say that when these three factors increase, so does the mean arterial pressure. We previously learned that cardiac output is the amount of blood pumped per minute by each ventricle. The higher the cardiac output, the higher the mean arterial pressure, because there is more blood being pumped out of the heart and flowing into the arterial system.

We also learned that total peripheral resistance is the total resistance to flow of blood in the systemic circulation. We see that as resistance increases, so does the pressure within the blood vessels. For example, if an arteriole constricts, its lumen will decrease in size, but the blood will pass through the arterial with more force or pressure. Just like a hose nozzle - if you make it smaller, it's going to cause the water to shoot out under higher pressure. Blood volume is also directly related to blood pressure. We know that the circulatory system is a closed system. The more fluid a closed system holds, the greater the pressure.

Baroreceptors

Baroreceptors transmit changes in blood pressure to the brain.
Baroreceptor Transmits to Brain

Blood pressure is constantly monitored by your body and adjusted constantly to meet the needs of your body. This monitoring is performed by baroreceptors. Baroreceptors are special receptors that detect changes in your blood pressure. Baroreceptors are found within the walls of your blood vessels. The aorta and the carotid sinus contain important baroreceptors which constantly monitor blood pressure fluctuations. These baroreceptors transmit their data to the central nervous system, and more specifically, to the cardio regulatory center of the medulla oblongata.

Blood Pressure Regulation

If blood pressure within the aorta or the carotid sinus increases, the walls of these arteries stretch and stimulate increased activity within the baroreceptor. This information is then sent via nerves to the cardio regulatory center within the medulla, which responds by initiating mechanisms that decrease the blood pressure to a normal level. Let's take a look at what happens to bring your blood pressure back down to a normal level when it gets too high.

To lower blood pressure, we first see a decrease of sympathetic input and an increase in parasympathetic input to the heart. We previously learned that the sympathetic nervous system can increase heart rate and stimulate the heart muscle to pump with more force. We also learned that the parasympathetic nervous system can decrease the heart rate. Therefore, by shutting off the sympathetic stimulation and boosting the parasympathetic stimulation, we decrease the heart rate and stroke volume, which decreases the cardiac output and decreases blood pressure. Second, if the baroreceptors are detecting that blood pressure is too high, the cardio regulatory center of the medulla will also decrease sympathetic input to the blood vessels. This causes vasodilation, which decreases total peripheral resistance and decreases blood pressure.

The opposite happens when the baroreceptors of the aorta or carotid sinus detect a drop in blood pressure. A decrease in blood pressure causes a decrease in action potentials sent to the cardio regulatory center of the medulla. Therefore, to raise blood pressure, the body will first cause an increase in sympathetic nerve activity to the SA node, causing it to fire more frequently, which increases the heart rate. The heart muscle is also stimulated to pump with more force, and this increases the stroke volume. When heart rate and stroke volume increase, we see an increase in cardiac output. As we learned, an increase in cardiac output causes an increased blood pressure, restoring blood pressure back to a normal level. Second, this causes an increased sympathetic input to the blood vessels, which stimulate the smooth muscle to contract, causing vasoconstriction, which increases total peripheral resistance and increases blood pressure.

Explanation of how the body lowers blood pressure
Lowering Blood Pressure

Baroreceptor Reflex and Orthostatic Hypotension

You experience these shifts in blood pressure many times throughout your day. For example, when you go from a lying to a standing position, you experience a fall in blood pressure. This drop in blood pressure is almost instantly compensated for by a baroreceptor reflex, which is a homeostatic mechanism to maintain blood pressure. The baroreceptor information is transmitted to the medulla in this baroreceptor reflex, and this stimulates the sympathetic nervous system and inhibits the parasympathetic nervous system, resulting in an increased heart rate and increased stroke volume and increased vasoconstriction.

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