Back To CourseBiology 105: Anatomy & Physiology
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Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.
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.
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.
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.
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.
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.
If the baroreceptors' sensitivity is diminished due to disease or chronic high blood pressure, the blood pressure may not be restored as quickly, resulting in low blood pressure when you stand up. This condition is called postural or orthostatic hypotension and can leave a person feeling dizzy and light-headed, and it could even cause a person to faint. You can recall these terms by remembering that posture has to do with lying or standing and that hypo means low and tension refers to blood pressure. You can also recall the term orthostatic by remembering that 'orthos' is the Greek word for 'upright' and 'statos' is Greek for 'standing.' So, we see that it is literally low blood pressure caused by standing.
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. The average arterial blood pressure during a single cardiac cycle is called the mean arterial pressure (MAP). Mean arterial pressure is influenced by three main factors, including cardiac output, total peripheral resistance, and blood volume. If not compensated by a decrease in other variables, we can say that when these three factors increase, so does the mean arterial pressure.
Blood pressure is constantly monitored by baroreceptors. Baroreceptors are special receptors that detect changes in your blood pressure. Important baroreceptors are found in the aorta and the carotid sinus. If the blood pressure within the aorta or carotid sinus increases, the walls of the arteries stretch and stimulate increased activity within the baroreceptors. This information is relayed to the cardio regulatory center of the medulla, which responds to the high blood pressure by causing decreased heart rate and causing vasodilation. The opposite happens when the baroreceptors of the aorta and the carotid sinus detect a drop in blood pressure. In this case, the cardio regulatory center responds to the low blood pressure by causing an increased heart rate and causing vasoconstriction.
The baroreceptor reflex is a homeostatic mechanism that quickly responds to the drop in blood pressure that results from actions such as moving from a lying to a standing position. If the baroreceptors' sensitivity is diminished due to disease or chronic high blood pressure, the blood pressure may not be restored as quickly, resulting in low blood pressure when standing. This condition is called postural or orthostatic hypotension and can leave a person feeling dizzy, light-headed, or could even cause the person to faint.
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Back To CourseBiology 105: Anatomy & Physiology
16 chapters | 179 lessons | 15 flashcard sets