Unit 4-Lecture 2.pptx - Unit 4-Systemic Circulation Lecture...

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Unit 4-Systemic Circulation Lecture 2-Blood Pressure
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Blood Pressure (BP) Force per unit area exerted on the wall of a blood vessel by its contained blood Expressed in millimeters of mercury (mm Hg) Measured in reference to systemic arterial BP in large arteries near the heart The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas
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Resistance Resistance – opposition to flow Measure of the amount of friction blood encounters Generally encountered in the systemic circulation Referred to as peripheral resistance (PR) The three important sources of resistance are blood viscosity, total blood vessel length, and blood vessel diameter
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Resistance Factors: Viscosity and Vessel Length Resistance factors that remain relatively constant are: Blood viscosity – “stickiness” of the blood Blood vessel length – the longer the vessel, the greater the resistance encountered
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Resistance Factors: Blood Vessel Diameter Changes in vessel diameter are frequent and significantly alter peripheral resistance Larger diameter vessels provide less resistance to the contained blood Small-diameter arterioles are the major determinants of peripheral resistance Fatty plaques from atherosclerosis: Cause turbulent blood flow Dramatically increase resistance due to turbulence
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Systemic Blood Pressure The pumping action of the heart generates blood flow through the vessels along a pressure gradient, always moving from higher- to lower-pressure areas Pressure results when flow is opposed by resistance Systemic pressure: Is highest in the aorta Declines throughout the length of the pathway Is 0 mm Hg in the right atrium
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Systemic Blood Pressure Figure 19.5
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Arterial Blood Pressure Systolic pressure – pressure exerted on arterial walls during ventricular contraction Diastolic pressure – lowest level of arterial pressure during a ventricular cycle Pulse pressure – the difference between systolic and diastolic pressure Mean arterial pressure (MAP) – pressure that propels the blood to the tissues MAP = diastolic pressure + 1/3 pulse pressure
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Capillary Blood Pressure Capillary BP ranges from 20 to 40 mm Hg Low capillary pressure is desirable because high BP would rupture fragile, thin-walled capillaries Low BP is sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues
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Venous Blood Pressure Venous BP is steady and changes little during the cardiac cycle The pressure gradient in the venous system is only about 20 mm Hg A cut vein has even blood flow; a lacerated artery flows in spurts
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Factors Aiding Venous Return Venous BP alone is too low to promote adequate blood return and is aided by the: Respiratory “pump” – pressure changes created
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