Hemodynamics Flashcards
special features of arteries
highly elastic walls, large radii. Serve as pressure reservoir
special features of arterioles
highly muscular, well-innervated walls, small radii. primary resistance vessels; determine distribution of cardiac output.
features of capillaries
thin-walled, large total cross-sectional area, site of exchange; determine distribution of extracellular fluid between plasma and interstitial fluid
features of veins
thin walled, highly distensible, large radii. serve as blood reservoir
Blood volume
Systemic > Pulmonary Circulation
Veins > All other segments
Veins = “Volume Reservoirs”
(2nd reserve = pulmonary circulation)
** Significance:
Reserve for immediate blood loss (ex: hemorrhage)
Blood pressure
Aorta > Rest of circulation
Segment across which there is greatest drop in pressure: Arterioles = “Resistance vessels”
Arteries = “Pressure Reservoirs”
** Significance: Arteries maintain MAP, the driving force for blood flow, throughout cardiac cycle
What does MAP stand for
mean arterial pressure
driving force for blood flow
flow = Pressure gradient = Mean Arterial Pressure (MAP)
MAP must be maintained to ensure adequate blood supply to brain & heart
Indirect methods to determine BP
auscultatory (SBP and DBP) and palpatory (SBP only)
Equation for MAP at rest and during exercise
at rest: MAP = DBP + 1/3 (SBP – DBP)
during exercise: MAP = DBP + 1/2 (SBP-DBP)
hydrostatic pressure difference
effects of gravity (delta h from heart)
Venous (more compliant) blood pooling during standing ↓ VR, ↓ CO, ↓ MAP–> compensation to restore MAP
Role of muscle pump & one-way venous valves
Cross-sectional area
Aorta:
Greatest individual diameter
Smallest segment CSA
Capillaries:
Smallest individual diameter
Greatest segment CSA
Blood Flow (another word for it)
Cardiac Output (CO). Equal for all segments of circulatory system. Also called Q.
Fick Principle
Indirect method to determine blood flow (CO, Q). Based on total O2 consumption & difference between arterial & venous O2 content.
Q = (VO2)/ A-VO2 Difference (arterial-venous difference)
Cardiac Output is Oxygen Consumption over the difference between arterial and venous oxygen levels.
Blood flow velocity
Aorta: Highest
Capillaries: Lowest
Suited for segment function
Indirectly related to total
CSA of vessels in segment
Doppler Ultrasound: Non-invasive
measure of blood flow velocity
Flow is constant through each segment, what varies is the cross-sectional area (capillaries high vs. aorta low) and velocity (high in the aorta, low in the capillaries)
Flow velocity (v) = Flow (Q) / Cross-sectional Area (A)
Reynolds Number
higher Reynolds number, more likely turbulence:
Reynolds number = (diameter x velocity x density)/ viscosity
turbulent flow produces murmurs
factors that contribute to laminar vs. turbulent flow
Increased velocity (can be as a result of decreased local diameter, atherosclerosis, cardiac valve lesions, –>application to sphygmomanometry)
increased diameter decreased viscosity (anemia)
Compliance
index of vessel distensibility: (garden hose vs. balloon, eg)
What is the change in volume for a given change in pressure?
What is the volume of blood stored for a given pressure?
C = delta V / delta P
Systemic veins are ~ 20 x more compliant vs. systemic arteries –> veins are major blood reservoir and provide compensation during hemorrhage
Effect of aging on pulse pressure
A compliant artery has a smaller pulse pressure (PP) vs. a stiffer artery
PP increases with age:
1. Arteriosclerotic changes decrease vessel compliance due to changes in elasticity
2. General increase in BP results in operating on the flatter part of P-V curve
Aneurysms
At a given BP: artery with 2x radius must withstand 2x wall tension
Aorta: vessel with greatest wall tension (greatest radius & pressure)
T (wall stress) is related to Pressure (P) x radius (r)/ 2 x wall thickness (h)
** Significance:
Damage or reduction of elastic fibers → vessel enlarges (aneurysm)
If r > LaPlace equilibrium: ↑ Wall tension → blowout of vessel
Hemodynamics
physical factors that govern blood flow (relating flow, pressure, and resistance)
Ohm’s law works with blood flow
I = delta V/R (current, voltage difference, resistance)
F (blood flow, = CO) = delta P (pressure gradient)/ R (resistance)
= MAP/ TPR (mean arterial pressure/ total peripheral resistance or systemic vascular resistance)
what is the driving force for blood flow?
pressure gradient
relationship between flow and resistance
inverse
Series resistance
Flow is equal at all points (segments of the circulatory system), adding resistance in series increases overall resistance in the system, and total resistance of a series circuit is greater vs. any one individual resistance.
R total = sum of all Rs
parallel resistance
Flow can be independently regulated (supply to different organs). Adding resistance in parallel to a system decreases overall resistance in the system. Total resistance of a circuit in parallel is less vs any one individual resistance.
1/r total = 1/r1 + 1/r2….
** Significance:
Allows for regulation of flow distribution to meet tissue demands while maintaining MAP
Obesity: adds in parallel, decreasing TPR, necessitating increased CO to maintain MAP
How is resistance regulated?
Radius is key determinant of resistance (Poiseuille’s Law)
↑ Radius –> ↓ Resistance to Flow –> ↑ Flow
blood flow relationships
Directly proportional to Pressure Gradient (Driving Force)
Directly proportional to vessel Radius to the 4th power
Inversely proportional to vessel length & blood viscosity
viscosity factors
anemia decreases viscosity
polycythemia increases viscosity
Poiseuille’s Law: radius
Radius is the key (vasoconstriction vs vasodilation)
Radius is taken to the fourth power in the equation
Intrinsic control of resistance
Mediated by local factors on arteriolar smooth muscle
Likely metabolic, myogenic, and endothelial mechanisms
Important for matching blood flow to tissues’ needs
Autoregulation
Flow is generally independent of blood pressure
Flow is generally proportional to tissue metabolism
Flow is independent of nervous reflexes
Examples of tissues mainly regulated by intrinsic factors: Cerebral circulation Coronary circulation Skeletal muscle during exercise Renal circulation
impact of vasodilation
decreased contraction of circular smooth m. in arteriolar wall –> decreased resistance & increased flow through the vessel
Caused by decreased myogenic activity, oxygen, pH, sympathetic stimulation, ATP
OR
increased CO2, lactic acid, nitric oxide, ADP, heat
Vasoconstriction
increased contraction of circular smooth m. in arteriolar wall –> increased resistance & decreased flow through vessel
caused by increased myogenic activity (response to stretch), increase O2, decrrease CO2, increased endothelin, increased sympathetic stimulation, cold
active hyperemia
increased tissue metabolism leads to increased vasodilators, dilation of arterioles, decreased resistance and increased blood flow –> more O2 and nutrient supply to tissue as long as metabolism is increased
reactive hyperemia
decreased tissue blood flow due to occlusion–> metabolic vasodilators accumulate, dilation of arterioles, occlusion prevents blood flow –> remove occlusion –> decreased resistance creates increased blood flow. Vasodilators wash away, arterioles constrict and blood flow returns to normal.
