USMLE Cardiovascular 7: Blood Pressure, Pulse Pressure, and Baroreceptors 

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Welcome to LY Med, where I go over everything you need to know for the USMLE STEP 1, with new videos every day.
Follow along with First Aid, or with my notes which can be found here:
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This video is on the physiology of blood pressure, pulse pressure and baroreceptors! Blood pressure measures the pressure your blood exerts on your blood vessels. During systole, the ventricles contract and push that blood into your vessels, which we can measure via the systolic blood pressure. In diastole, the heart relaxes, and the pressure drops and this is measured via the diastolic blood pressure. Now your BP is highly regulated and depends on your cardiac output. Do you recall what makes up our CO? This includes heart rate and stroke volume (which can further be divided into preload, afterload, and contractility). Preload is filling, and increased filling/fluid increases blood pressure. This is the basis behind why we give IV fluids in hypotensive patients! In fluid overloaded hypertensive patients, we may give anti-hypertensives to get rid of some of this fluid in the form of diuretics. Afterload directly relates to resistance and pressure. The largest way to increase afterload is by decreasing the diameter, or lumen size of your blood vessels. THis increases blood pressure. Meanwhile, in vasodilation, you increase the lumen and decrease pressure. Contractility plays a role in BP as you can pump more blood out. Sympathetic and parasympathetic control really plays a role here, in particular beta 1 receptors.
Your body modulates these factors automatically, making sure your blood pressure is appropriate to the situation. Some pathways include the RAAS system and your heart releasing ANP and BNP. Another mechanism is through baroreceptors. These are mechanoreceptors that sense pressure. There are two:
Carotid baroreceptors: located in the carotid sinus, these link up to the glossopharyngeal nerve (CN IX)
Aortic arch baroreceptor: seen in the aorta, these link up to the vagus nerve (CN X). Both ultimately transmit to the solitary nucleus of the medulla. What is the physio behind these receptors? Well, when there is increased blood pressure, that stretches these muscles and receptors and causes depolarization. The more they stretch, the more they depolarize and fire. Your brain senses this as increased BP and will try to lower the sympathetic fight or flight response. You can stimulate this with a carotid massage! Conversely, if you have low BP, there will be less stretch and firing. Your body will notice this as low pressure and will try to compensate by increasing sympathetic firing! That is how your baro receptors work!
Our last topic will be on pulse pressure. Pulse pressure is the difference between the systolic pressure and diastolic. Pulse pressure is proportionate to cardiac output, stroke volume, and ventricular compliance. The more blood you're able to pump out, the more your pulse pressure. Increased pulse pressure is seen in exercise, hyperthyroidism, aortic regurgitation, and aortic stiffening. Decreased pulse pressure is seen in low systolic pressure and volume. This is seen in aortic stenosis, cardiogenic shock, cardiac tamponade, and heart failure.
Done with this video. In our next video, we will discuss hypertension and related pathology and pharmacology. See you then!