Blood Pressure (Advanced)

Blood pressure is expressed as the difference, or change in, pressures between two points along a vessel. As this statement implies, blood pressure is not constant throughout the cardiovascular system. Y-axis = Pressure (mm Hg); values 0-120 X-axis = Circulatory system: Left atrium and left ventricle, which are the chambers of the heart that pump oxygenated blood Aorta, which is the largest artery in the body that receives blood directly from the left ventricle Large arteries, small arteries, and arterioles Capillaries, site of gas exchange Veins Right atrium and ventricle, which receive deoxygenated blood from the body and send it to the lungs via the pulmonary arteries Plots two key principles of blood pressure: BP changes as blood moves through the body BP is pulsatile because of the rhythmic contractions of the heart during the cardiac cycle In the left atrium, blood pressure is relatively low, at about 5 mm Hg. Pressure is much higher in the left ventricle and aorta, where it oscillates between 120 and 80 mm Hg. Arterial pressure rises slightly as it passes through the large arteries, then begins to fall. Pressure drops significantly as blood moves through the arterioles because they are the site of highest resistance to blood flow. Within the capillary networks, blood pressure falls to about 4 mmHg by the time it reaches the venous system. Blood pressure remains low in the right atrium at about 4-6 mm Hg, and rises slightly in the right ventricle. Within the pulmonary arteries, blood pressure is around 2-4 mm Hg. Recall that blood traveling through these vessels travels only a short distance, to the lungs; the low blood pressure is sufficient for lung tissue perfusion, but not so high as to cause damage Blood within the arterial system is the "stressed" volume; it is under high pressure. Blood within the venous system is the "unstressed" volume, as it is under significantly less pressure. Recall that the majority of the blood volume is within the venous, unstressed portion of the circulatory system. Create a new graph to show the arterial pressure changes of a single cardiac cycle: Y-axis = Pressure (mm Hg); values 80 and 120. X-axis = Time This curve shows: Highest arterial pressure, 120 mmHg, is reached during systole, when the left ventricle contracts and blood is ejected to the aorta. Lowest pressure, 80 mmHg, is reached during diastole, the period of ventricular relaxation. Dicrotic notch, (aka, incisura) reflects a temporary drop in pressure after systolic contraction; it is caused by the backflow of blood after the aortic valve closes. Blood pressure is usually reported as systolic pressure over diastolic pressure; 120/80 mm Hg is considered to be a healthy blood pressure. Pulse Pressure: Pulse pressure = systolic pressure – diastolic pressure Stroke volume is the volume of blood ejected from the left ventricle per beat Arterial compliance reflects the ability of the vessel wall to contract or expand to accommodate changes blood flow. Mean arterial pressure (MAP): Mean arterial pressure (MAP) is equal to the diastolic pressure plus 1/3rd of the pulse pressure; it is NOT simply the average of the systolic and diastolic pressures; the equation accounts for the fact that the period of diastole is longer than that of systole. Mean arterial pressure is determined by cardiac output and peripheral arterial resistance (aka, systemic vascular resistance). Refers to the change in pressure between two points along the longitudinal axis of a vessel; P 1 - P 2 Driving pressure can refer to changes in the overall systemic cardiovascular system or simply within a single organ; for example, knowing the driving pressure at the kidneys is important for understanding renal function and pathology. Refers to the change in pressure between two points along the axis of a non-horizontal; for example, the femoral arteries, which deliver blood vertically from the heart to the lower extremities; h 1 – h Refers to change in pressure across the vessel wall; r 1 – r 2. Transmural pressure is important because it influences vessel diameter, and, therefore, resistance to blood flow.