Blood Circulation: Review Questions Flashcards
hemodynamics?
• The term hemodynamics refers to the principles that govern blood flow in the cardiovascular system.
what is stressed volume?
The volume of blood contained in the arteries is called the stressed volume
(meaning the blood volume under high pressure).
Passage of lipid soluable sybstances vs water soluble substances through capillaries
◦ Lipid-soluble substances (e.g., O2 and CO2) cross the capillary wall by dissolving in and diffusing across the endothelial cell membranes.
◦ In contrast, water-soluble substances (e.g., ions) cross the capillary wall either through water-filled clefts (spaces) between the endothelial cells or through large pores in the walls of some capillaries (e.g., fenestrated capillaries).
Velocity of Blood Flow
• The velocity of blood flow is the rate of displacement of blood per unit time. (Linear velocity)
how changes in diameter alter the velocity of flow through a vessel:
Relationships Between Blood Flow, Pressure, and Resistance
Poiseuille equation
◦ The relationship between resistance, blood vessel diameter (or radius), and blood viscosity is described by the Poiseuille equation.
◦ The most important concepts expressed in the Poiseuille equation are as follows:
‣ First, resistance to flow is directly proportional to viscosity (η) of the blood;
• for example, as viscosity increases (e.g., if the hematocrit increases), the resistance to flow also increases.
‣ Second, resistance to flow is directly proportional to the length (l) of the blood vessel.
‣ Third, and most important, resistance to flow is inversely proportional to the fourth power of the radius (r4) of the blood vessel.
Arrangment of blood vessels in series
◦ illustrated by the arrangement of blood vessels within a given organ
◦ The total resistance of the system arranged in series is equal to the sum of the individual resistances
‣ Of the various resistances in series, arteriolar resistance is by far the greatest.
• The total resistance of a vascular bed is determined, therefore, in large part by the arteriolar resistance.
◦ When resistances are arranged in series, the total flow at each level of the system is the same.
◦ Although total flow is constant at each level in the series, the pressure decreases progressively as blood flows through each sequential component
‣ The greatest decrease in pressure occurs in the arterioles because they contribute the largest portion of the resistance.
Arrangment of blood vessels in parallel
◦ illustrated by the distribution of blood flow among the various major arteries branching off the aorta
◦ the total resistance in a parallel arrangement is less than any of the individual resistances
◦ When blood flow is distributed through a set of parallel resistances, the flow through each organ is a fraction of the total blood flow.
‣ The effects of this arrangement are that there is no loss of pressure in the major arteries and that mean pressure in each major artery will be the same and be approximately the same as mean pressure in the aorta.
◦ Affect of changing the resistance of a single component:
‣ adding a resistance to the circuit causes total resistance to decrease, not to increase.
‣ On the other hand, if the resistance of one of the individual vessels in a parallel arrangement increases, then total resistance increases.
• If one of the four blood vessels is completely occluded, its individual resistance becomes infinite. The total resistance of the parallel arrangement then increases
Laminar flow vs turbulent flow
◦ In laminar flow, there is a smooth parabolic profile of velocity within a blood vessel, with the velocity of blood flow highest in the center of the vessel and lowest toward the vessel walls
the velocity of flow at the vessel wall is zero, and the velocity at the center of the stream is maximal.
◦ In turbulent flow (see Fig. 4.6), the fluid streams do not remain in the parabolic profile; instead, the streams mix radially and axially
‣ Because kinetic energy is wasted in propelling blood radially and axially, more energy (pressure) is required to drive turbulent blood flow than laminar blood flow.
‣ Laminar flow is silent, while turbulent flow is audible.
• Reynolds number
◦ dimensionless number that is used to predict whether blood flow will be laminar or turbulent.
◦ It considers a number of factors including diameter of the blood vessel, mean velocity of flow, and viscosity of the blood
◦ Values:
‣ If Reynolds number (NR) is less than 2000, blood flow will be laminar.
‣ If Reynolds number is greater than 2000, there is increasing likelihood that blood flow will be turbulent.
‣ Values greater than 3000 always predict turbulent flow.
◦ major influences on Reynolds number in the cardiovascular system:
‣ changes in blood viscosity
• decreases in viscosity (e.g., decreased hematocrit) cause an increase in Reynolds number.
‣ changes in the velocity of blood flow
• narrowing of a blood vessel, which produces an increase in velocity of blood flow, causes an increase in Reynolds number.
• Thus velocity (also in the numerator of the equation for Reynolds number) increases as radius decreases, raised to the second power. Hence, the dependence of Reynolds number on velocity is more powerful than the dependence on dia
◦ major influences on Reynolds number in the cardiovascular system:
‣ changes in blood viscosity
• decreases in viscosity (e.g., decreased hematocrit) cause an increase in Reynolds number.
‣ changes in the velocity of blood flow
• narrowing of a blood vessel, which produces an increase in velocity of blood flow, causes an increase in Reynolds number.
• Thus velocity (also in the numerator of the equation for Reynolds number) increases as radius decreases, raised to the second power. Hence, the dependence of Reynolds number on velocity is more powerful than the dependence on diameter.
Factors that effect the viscosity of blood
what cardiac output?
Th e output of the heart per unit of time
Effect of various conditions on cardiac output:
FACTORS CONTROLLING CARDIAC OUTPUT
ohm law connection to blood flow
The relationship between mean fl ow, mean pressure, and resistance
in the blood vessels is analogous in a general way to the
relationship between the current, electromotive force (voltage),
and resistance in an electrical circuit as expressed in
Ohm’s law:
Doppler flow meters
Various noninvasive devices have therefore been developed to measure fl ow.
Most commonly, blood velocity can be measured with Doppler fl ow meters.
Ultrasonic waves are sent into a vessel diagonally, and the waves refl ected from the red and white blood cells are picked up by a downstream sensor.
Th e frequency of the refl ected waves is higher by an amount that is proportional to the rate of fl ow toward the sensor because of the Doppler eff ect.
velocity vs flow
LAW OF LAPLACE
ARTERIAL PRESSURE
Th e pressure in the aorta and in the brachial and other large arteries in a young adult human rises to a peak value (systolic
pressure) of about 120 mm Hg during each heart cycle and falls to a minimum (diastolic pressure) of about 70 mm Hg.
The arterial pressure is conventionally written as systolic pressure over diastolic pressure, for example, 120/70 mm Hg.
One millimeter of mercury equals 0.133 kPa, so in SI units this value is 16.0/9.3 kPa.
pulse pressure
the diff erence between the systolic and diastolic pressures, is normally about 50 mm Hg
mean pressure
average pressure throughout the cardiac cycle.
EFFECT OF GRAVITY on blood pressure
Mean arterial pressure and pulse pressure
Blood pressure simple definition:
the pressure the blood exerts on the surface area of the blood vessles
Factors affecting vascular resistance
Factors predicting turbulance
how does anemia affect blood flow: turbulance/laminar?
how does thrombus affect blood flow: turbulance/laminar?
Shear
Diffrence in relative velocities of adjacent layers in blood flow
Compliance
The volume of blood a vessel can store at a certain pressure
tendancy of blood vessels to stretch out with pressure
reverse: tendancy to recoil back to its origiional shape, elastance
What is the dicrotic notch (incisura)?
why is the PP in the large arteries larger than the PP in the aorta?
* but in the large blood pressure the DBP is still lower, and what matters is that the DBP drives bloos flow (2/3 of the total MAP)
Cardiac output
Cardiac output with fick principle
Stroke volume
Frank starling relationship
Ejection fraction is % of end-diastolic volume, that is ejected with each stroke usually:
- Differs between L ventricle diastolic & systolic volumes
- Stroke volume ÷ end-diastolic volume ratio
- Stroke volume ÷ end-systolic volume ratio
- About 40-65%
- Stroke volume ÷ end-diastolic volume ratio
- About 40-65%
Note: END-DIASTOLIC VOLUME = volume of blood within a ventricle immediately before a contraction. END-SYSTOLIC VOLUME = volume after a contraction. STROKE VOLUME = the difference between them.
