Blood Flow Flashcards
Systolic blood pressure
(TOP) Peak arterial pressure reached during ejection of blood by the heart
Diastolic blood pressure
(BOTTOM) Lowest arterial pressure reached during diastole (when the heart is relaxed and filling with blood)
Compare blood pressures throughout the circulatory system
Arterial circulation: high pulsatile pressure, high flow velocity Veins: low pressure and velocity, no pulse in veins Largest drop in MAP occurs in arteriole region (artery -> capillary) - site of highest vascular resistance
Blood flow (Q) through tubes
Q (L/min) is CONSTANT even though diameter changes because tubes are in SERIES Blood flows from aorta -> arteries -> arterioles -> capillaries -> venules -> vena cava
Define blood flow (Q)
Q (L/min) is the quantity of blood passing a particular observation point in a given time interval
Analogy: “volume” of cars that pass a checkpoint (cars/hour)
Velocity would be the speed of each individual car
Principles of blood flow
Q is the same for branches in series and additive for branches in parallel
- Flow in a region equals flow out if there are no leaks (conservation of mass) (i.e. flow in aorta is the same as flow in all capillaries together)
- Flow across any total cross-sectional area is constant (Q of A1 = Q of A2)
What controls the rate of blood flow to each tissue?
Q to each tissue is determined by metabolic NEED. Distriubtion of flow is controlled by adjustment of resistances located right before the capillary beds (arterioles and precapillary sphincters)
During exercise: Q increases to skin, muscles and heart, maintained to brain
Note: CO is controlled mainly by sum of all local tissue flows
Continuity Equation and Use
In a closed system, Q1 = Q2 past any two total cross-sectional areas must be the same st any instant. So:
V1 x A1 = V2 x A2
Ratio of velocities depends ONLY on total cross-sectional area (πr2)
Velocity of blood in aorta is a lot faster than v in capillaries, which have a larger total A (slow for gas exchange)
How long does it take for the blood to have one pass through the entire pumonary and systemic circulation? Transit time of capillaries?
About 1 minute
In capillaries: v = 500 microns/sec and length = 0.5mm, so transit time is 1 second (just long enough for gas exchange)
Forces that effect blood flow
Pressures need to be different for blood to move: it flows from areas of high pressure to low pressure.
Resistance comes from the walls of vessels and from viscosity of blood (frictional forces)
Equation is like Ohm’s Law (V =IR)
Poiseuille’s Law
Gives the pressure drop across various segments of the cardiovasculra system
Drops gradually along long arteries and veins
Drops more rapidly along arterioles and capillaries (greatest drop in long vessel with small radius)
Can also calculate resistance in each segment
What is the major physiological variable that determines the resistance to blood flow?
RADIUS of vessels, primarily the arterioles.
This is because the length of blood vessels and viscosity of blood do not normally change
Temperature is inversely correlated to viscosity. I.e. cold temp increases viscosity of blood which contributes to decreased blood flow (fingers get cold when shoveling
**Vasoconstriction **counters the effect of temp - reduces blood flow to conserve heat
Resistance of blood vessels in series and parallel
Total flow through vessel network determied by the overall resistance
In Series: R = R1+R2+R3… (ADD)
In Parallel: 1/R = 1/R1 + 1/R2 + 1/R3
*More branches = lower total resistance*
(Overall resistance can be low in capillary network even though it’s high in each individual capillary)
