this was a long introduction to the more intricate details of blood flow dynamics. the beginning of the lecture introduced the larger structures of the circulatory system: the major arteries and aortas: the aortic arch, which branches into the braciocephalic, left common carotid, and left subclavian, with the braciocephalic branching off into the right subclavian and right common carotid. the thoracic aorta, which has intercostal, brachial, and esophageal branches. finally, the abdominal aorta, which has a visceral branch that is divided into the renal, suprarenal, and the GI branch, as well as a common iliac and femoral artery. other details of larger architecture: the right pulmonary artery goes underneath the aortic arch. the ligamentum arteriosum is the vestigial remains of the ductus arteriosum in the developing heart, which shunted blood from the pulmonary artery directly to the aorta, past the non-functioning lungs. the superior vena cava is a combination of the right and left brachiocephalic veins, which are each a convergence of the right and left internal jugular and subclavian veins.
we then zoom in and look at the smaller architecture of arteries, arterioles, capillaries, veins and venules. arteries have three layers, a tunica intima which has endothelium, sub endothelial CT, and an internal elastic lamina. tunica media is the middle layer with smooth muscle and external elastic lamina. tunica adventitia is the outermost layer, with fibrocollagen. arterioles have extensive smooth muscle (which, as it is explained later, helps create a large resistance which causes the largest pressure drop in the circulatory system), running 1-5 layers deep in the tunica media with less fibrous tissue in the tunica intima and adventitia. capillaries are described as the place for nutrient exchange, either through the membrane for lipid soluble solutes such as gas, or through the extensive pore network for water soluble solutes. veins and venules have thinner walls and less elastin, allowing them to function as a reservoir for the blood, containing up to 64% of the circulating blood.
then we shift to blood flow dynamics and begin with a simple equation describing blood flow: Q=P/R. blood flow equals pressure gradient divided by vascular resistance. blood flow is essentially cardiac output, which is stroke volume times heart rate, and is regulated by neural and endocrine systems. resistance is related to vascular resistance and is regulated by metabolic and neurohumoral systems. pressure is described as an "emergent property" which arises from the interaction of flow and resistance. which seems like just a fancy way of saying P=Q*R. pressure difference is then defined as Paorta-Pvenacava, but since Pvenacava is negligible, P=Paorta. we later find out that Paorta is essentially mean arterial pressure (the formal definition / estimation of which is diastolic pressure plus 1/3 of the difference between systolic and diastolic pressure). thus the most useful, applicable form of this flow equation seems to be (cardiac output) = (mean arterial pressure) / (vascular resistance)
several more terms are introduced in the discussion of blood flow. resistance is elucidated in the poiseuille equation as being proportional to the viscosity of the blood and length of the vessel while being inversely proportional to the radius^4. velocity is the measure of flow taking into account surface area; it can also be described as the speed at which the blood flows along the length of the vessel (as opposed to the flow, which does not account for this). capillaries have the greatest total surface area, making the velocity of blood drop considerably, allowing time for nutrient exchange. viscosity is used as a segue into the idea of laminar flow, which occurs as a result of blood/blood friction and also blood/vessel wall friction, causing the velocity to be greatest in the center of the vessel. finally, turbulence is described as proportional to the reynold's number, which is proportional to diameter of the vessel, density of blood, and velocity, and inversely proportional to the viscosity.
there are a few different aspects of blood pressure that are looked at in this last section. the first is the act of taking blood pressures via a sphygmanometer, which can measure systolic and diastolic pressure by listening for the pressure at which the korotkow sound appears, which represents the turbulence caused by the systolic pressure briefly opening up the occluded brachial artery. the diastolic pressure can then be determined when the korotkow sounds disappear, since the artery will be continuously open when diastolic pressure is slightly greater than the pressure of the cuff. the second is the idea of the two different types of blood pressure on the microscopic level: pressure that arises from blood/blood interaction, going along the length of the vessel, is described as perfusion pressure and is related to kinetic energy. the blood pressure that arises from blood/vessel wall interaction is called transmural pressure and is related to potential energy (in this case stored as pressure in the arteries).
the next aspect of pressure that is looked at is the pulse pressure, which is defined as the difference between the systolic and diastolic pressures, and as such is directly related to both stroke volume and compliance. we learn that in healthy circulation, there is a certain level of compliance in the aortas which allows for some of the stroke volume to be effectively "stored" in the arteries during systole, and the pressure created from this storage allows blood to flow in the capillaries even during diastole. in contrast, in arterial dysfunction involving low compliance, the stroke volume translates directly into the capillaries, not allowing for any additional flow during diastole. this is displayed graphically on the pressure vs. stroke volume graph, in which compliance is a line with a positive slope; where decreased compliance increases the slope of the compliance line, thereby increasing systolic pressure and reducing diastolic pressure. finally, three pathologies relating to pulse pressure are described: artherioscerlosis is a disease in which the arteries have lower compliance, causing higher systolic pressure and therefore larger pulse pressure. hypothyroidism and aortic stenosis are both diseases in which the stroke volume is reduced, which also leads to an reduced pulse pressure.
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