Blood Vessels Part 2: Exam 2 Flashcards
Blood Flow
Volume of blood flowing through vessel, organ, or entire circulation in given period
- measured in ml/min, it is equivalent to cardiac output (CO) for entire vascular system
- overall, relatively constant when at rest, at any given moment, varies at individual organ level, based on needs
Blood Pressure (BP)
Force per unit area exerted on wall of blood vessel by blood
- expressed in mm Hg
- measured as systemic arterial BP in large arteries near heart
- pressure gradient provides driving force that keeps blood moving from higher to lower-pressure areas
Resistance (Peripheral Resistancef)
Opposition to flow
- measurement of amount of friction blood encounters with vessel walls, generally in peripheral (systemic) circulation
3 Important Sources of Resistance
1) Blood Viscosity
- thickness or “stickiness” of blood due to formed elements and plasma proteins
- greater the viscosity, less easily molecules can slide past each other
- increased viscosity equals increased resistance
2) Total Blood Vessel Length
- longer the vessel, greater the resistance encountered
3) Blood Vessel Diameter
- greatest influence on resistance
- frequent changes after peripheral resistance
- viscosity and blood vessel length are constant
- fluid close to walls move more slowly than in middle of tube (laminar flow)
- resistance varies inversely with fourth power of vessel radius
- radius increases, resistance decreases and vice versa
- example: if radius is doubled, resistance drops 1/16 as much
- smaller diameter arterioles are determinants of peripheral resistance
- radius changes frequently, in contrast to larger arterioles that do not change often
- abrupt changes in vessel diamater or abstacles such as fatty plaques from atherosclerosis dramatically increase resistance
- laminar flow is disrupted and becomes turbulent flow, irregular flow that causes increases resistance
Relationship Between Flow, Pressure and Resistance
- blood flow (F) is proportional to BP gradient
- BP gradient increases, blood flow speeds up
- blood flow is inversely proportional to peripheral resistance (R)
- R increases, blood flow decreases so:
F=BP/R
- R increases, blood flow decreases so:
- R is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter
Systemic Blood Pressure
- pumping action of heart generates blood flow
- pressure results when flow is opposed by resistance
- systemic pressure is highest in aorta and declines throughout pathway
- steepest drop occurs in arterioles
Arterial Blood Pressure
- determined by 2 factors:
1) elasticity (compliance or distensibility) of arteries close to heart
2) volume of blood forced into them at any time - blood pressure near heart is pulsatile: rises and falls with each heartbeat
Systolic Pressure
pressure exerted in aorta during ventricular contraction
- top number of BP
- left ventricle pumps blood into aorta, impacting kinetic energy that stretches aorta
- averages 120 mm Hg in normal adult
Diastolic Pressure
lowest level of aortic pressure when heart is at rest
- bottom number of BP
Pulse Pressure
different between systolic and diastolic pressure
Pulse
throbbing of arteries due to difference in pulse pressures, which can be felt under skin
Mean Arterial Pressure (MAP)
It is pressure that propels blood to tissues
- pulse pressure phases out near end of arterial tree
- flow is non pulsatile with a steady MAP pressure
- heart spends more time in diastole, not just an average of diastole and systole
- MAP is calculated by adding diastolic pressure + 1/3 pulse pressure
Example:
- BP = 120/80
- Pulse Pressure = 120 - 80 = 40
- So MAP = 80 + (1/3)*40 = 80 + about 13 = 93 mm Hg
- pulse pressure and MAP decline with increasing distance from heart
Clinical Monitoring of Circulatory Efficency
- Vital Signs: pulse and blood pressure, along with respiratory rate and body temp
- Taking a Pulse
- Radial Pulse (wrist): most used, but there are other clinically important pulse points
- Pressure Points: areas where arteries are close to body surface (can compress to stop blood flow in event of hemorrhaging)
Measuring Blood Pressure
- systemic arterial BP is measured indirectly by auscultatory methods using a sphygmomanometer
1) wrap cuff around arm superior to elbow
2) increase pressure in cuff until it exceeds systolic pressure in brachial artery
3) pressure is released slowly, examiner listens for sounds of Korotkoff with a stethoscope - systolic pressure: normally less than 120 mm Hg (pressure when sounds first occur as blood start to spurt through artery)
- diastolic pressure: normally less than 80 mm Hg (pressure when sounds disappear because artery no longer constricted, blow flowing freely
Capillary Blood Pressure
- ranges from 35 mm Hg at beginning of capillary bed to about 17 mm Hg at the end of the bed
- low capillary pressure is desirable because:
1) high BP would rupture fragile, thin-walled capillaries
2) most capillaries are permeable, low pressure forces infiltrate into interstitial spaces
