[PHYSIO]LEC_206_CIRCULATION-BIOPHYSICS-OF-PRESSURE-FLOW-AND-RESISTANCE.pdf

(006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCIO...

(006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 OUTLINE I. FUNCTION OF THE CIRCULATION II. HEART AND BLOOD VESSELS III. PHYSICAL CHARACTERISTIC OF THE CIRCULATION IV. FUNCTIONAL PARTS OF THE CIRCULATION V. VOLUMES OF BLOOD IN THE CIRCULATION VI. CROSS-SECTIONAL AREAS AND VELOCITIES OF BLOOD FLOW VII. BASIC PRINCIPLES OF CIRCULATORY FUNCTION VIII. INTERRELATIONSHIPS OF PRESSURE, FLOW, AND RESISTANCE IX. BLOOD FLOW X. BLOOD PRESSURE XI. RESISTANCE TO BLOOD FLOW XII. BLOOD FLOW IN A PARALLEL CIRCUIT XIII. EFFECT OF HEMATOCRIT AND BLOOD VISCOSITY ON VASCULAR RESISTANCE AND BLOOD FLOW XIV. EFFECTS OF PRESSURE ON VASCULAR RESISTANCE AND TISSUE BLOOD FLOW I. FUNCTION OF THE CIRCULATION Transport nutrients to the body tissues Transport waste products away Transport hormones from one part of the body to another Maintain an appropriate environment and proper milieu in all the tissue fluids of the body for survival and optimal Figure 1. Distribution of blood in the Circulatory System function of cells IV. FUNCTIONAL PARTS OF THE II. HEART AND BLOOD VESSELS CIRCULATION  Controlled to provide the necessary cardiac output and Arteries arterial pressure to cause the needed tissue blood flow, - transport blood under high pressure to the there are several factors that control the heart and the tissues blood, one is the nervous system others would be the - Have strong vascular walls, developed to hormones secreted by the other parts of the body receive high pressure and high velocity - Blood flows at a high velocity in the arteries Questions to answer: - from the aortic orifice from the left ventricle What are the mechanisms for controlling blood volume and ascends to aortic arch to the and blood flow? descending strong walls How does this process relate to the other functions of Arterioles the circulation? - last small branches of the arterial system - they are strong walled blood vessels like arteries III. PHYSICAL CHARACTERISTIC OF THE - Act as control conduits through which blood is CIRCULATION released into the capillaries Systemic circulation - Strong blood walls - aka greater circulation or peripheral circulation Capillaries - Supplies blood flow to all the tissues of the - Exchange fluid, nutrients, electrolytes, body except the lungs hormones, and other substances between the - It inserts to the right atrium; Either arterial blood and the interstitial fluid segment or venous segment - Walls are thin and have many minute capillary Pulmonary circulation pores, thin walled compared to arterioles and - lung circulation arteries - Unoxygenated blood goes to the PC Page 1 of 10 CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 - Minute capillary pores, serve as "exchanging Velocity across the valve is directly proportional to blood area" of the body flow but inversely proportional to the area, meaning if - Very permeable because of their porosity the area is smaller the velocity would be higher. Venules ○ Aorta has smaller cross sectional area, hence - collect blood from the capillaries and gradually will have higher velocity to maintain flow. coalesce into progressively larger veins Velocity of Blood flow (v) is inversely proportional to Veins vascular cross-sectional area (A) - conduits for transport of blood from the venules v = F/A back to the heart If you want to have a good blood flow, there should be a - Pressure is very low —> venous walls are thin high velocity and larger area - They are muscular enough to contract or expand - serve as a controllable reservoir for the extra blood depending on the needs of the circulation V. VOLUMES OF BLOOD IN THE CIRCULATION 16% in the heart 84% in the systemic circulation; SC will be responsible for the large amount of fluid, where in the heart has lesser amount of blood Capillaries ○ 7% of total volume - Capillaries only receive low blood volume - Most important function of the circulation occurs Table 1. Cross-Sectional Area (cm2) - Diffusion of substances between the blood and the tissues A. TO MAINTAIN BLOOD FLOW - Small capacity or reservoir compared to Resting conditions venous segments, having highest blood - velocity averages about 33 cm/sec in the aorta, volume compared to arteries convert to cm - Though they only have small amount, must - Capillaries average velocity about 0.3 mm/sec have normal or almost total diffusion capacity; - Velocity of aorta is 1/1000 as rapid as what must come in to the heart must come out capillaries and the total volume must be similar. - In capillaries, to maintain blood flow since they - HOWEVER, blood flow is still the same in the have higher cross-sectional area therefore aorta, capillaries and venous segments needs lower velocity because cross sectional area of the capillaries - Higher velocity, higher flow but lesser the cross has the highest x-sectional area, compared to sectional area the other vessels. Blood remains in the capillaries for only 1 to 3 sec Thus, all diffusion of nutrient food substances and VI. CROSS-SECTIONAL AREAS AND VELOCITIES OF electrolytes that occurs through the capillary walls must BLOOD FLOW be performed in this short time; must be performed at a cross-sectional areas of the veins are much larger than short time to maintain blood flow that is normal in each those of the arteries segment of the circulatory system Large blood storage capacity of the venous system in comparison with the arterial system B. PRESSURES IN THE DIFFERENT PORTIONS OF Because the same volume of blood flow (F) must pass THE CIRCULATION through each segment of the circulation each minute 1. Systemic Circulation Aorta MP is high (~100 Hg) Page 2 of 10 CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 Heart pumping is pulsatile (120/80mmHg); 120  For blood flow to occur, there must be a pressure highest pressure and 80 is diastolic at relaxation difference at higher to lower pressure, left atrium and Mean pressure falls progressively to about O aorta having high pressure mmHg by the time it reaches the termination of the  Lowest is at the vena cavae, as they enter there will be superior and inferior vena cavae, lower pressure is an increased pressure BUT not high enough to needed to occur a blood flow from highest pressure overcome the venous system. to lower pressure 2. Systemic Capillaries Pressure VII. BASIC PRINCIPLES OF CIRCULATORY o High as 35 mmHg near the arteriolar ends FUNCTION o Low as 10 mmHg near the venous ends PRINCIPLE 1: Blood flow to most tissues is controlled according average “functional” pressure in most vascular beds to the tissue need o 17mmHg pressure when tissues are active: need a greatly increased o Pressure low enough that little of the plasma supply of nutrients thus much more blood flow than leaks through the minute pores of the capillary when at rest (20-30x resting level) walls When we eat, blood flow goes to stomach, sleepy and 3. Pulmonary Circulation very sleepy because majority of blood goes to stomach  The pressure is also pulsatile to help in the churning of those material; thus, brain  Pressure is far less: slightly has less blood flow o PASP and PADP: ~25/8 mmHg Heart normally can’t increase its cardiac output >4-7x o MPAP 16 mmHg than resting levels o Mean pulmonary capillary pressure: ~ 7 mmHg Not possible simply to increase blood flow everywhere o In the case where pulmonary pressure is in the body when a particular tissue demands increased higher than systemic circulation; like in venous flow thrombosis which was dislodged in the lower o Microvessel, nervous control and hormones extremity obstructing pulmonary trunk creating continue to monitor tissue needs a pressure, that blood flow going to the systemic circulation is lesser because of higher PRINCIPLE 2: Cardiac output is the sum of local tissue flows pressure in the PC when blood flows through a tissue, it immediately o Another is congenital heart disease, right to left returns by way of the veins to the heart, to maintain shunting (left atrium to right atrium) of blue milieu, blood flow to aorta and venous segment is the babies where they have higher pulmonary same pressure, pulmonary circulatory bed is Heart acts as an automaton responding to the demands destroyed creating a higher pressure and of the tissues reverse shunting Heart needs help in the form of special nerve signals to make it pump the required amounts of blood flow NOTE: total blood flow through the lungs each minute is the same through the systemic circulation. Heart needs secretion of hormones to pump adequately PRINCIPLE 3: Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control the circulatory system is provided with an extensive system for controlling the arterial blood pressure o Example: BP decrease to below 100 mmHg Nervous reflexes elicits a series of circulatory changes to raise the pressure back toward normal Nervous signals: o increase force of heart pumping o Contraction of the large venous reservoirs providing more blood to the heart Figure 2. Normal Blood Pressure in the diff. portions of o Generalized constriction of the arterioles so that Circulatory System when a person is lying in the horizontal more blood accumulates in the large arteries to position increase the arterial pressure Page 3 of 10 CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 Kidneys: additional major role in pressure control both means the quantity of blood that passes a given point in by secreting pressure-controlling hormones and by the circulation in a given period of time regulating the blood volume, RAAS NV: 5000ml/min o For example, if blood pressure is decreased to Laminar and turbulent flow 100mmHg, nervous system will stimulate the pump, if this mechanism isn’t sustained the kidney will have also a major rile like in RAAS VIII. INTERRELATIONSHIPS OF PRESSURE, FLOW, AND RESISTANCE Blood flow through a blood vessel is caused by: 1. Pressure Gradient - pressure difference of the blood between the two ends of the vessel which pushes the blood through the vessel Figure 4. Types of Blood flow o Ex: aorta has the highest pressure compared to capillaries at only 17mmHg A. TYPES OF BLOOD FLOW 1. Laminar Blood Flow 2. Vascular Resistance - the impediment to blood flow  aka STREAMLINE FLOW through the vessel  Steady rate through a long smooth blood o Ex: Imagine a tube with water, for example hose. Water adherent to the side of the hose will have a vessel higher resistance (due to friction) compared to  Flows in streamlines water inside the hose, at the center of the hose.  Each layer of blood remaining the same distance from the vessel wall IX. BLOOD FLOW  Center has the highest velocity meanwhile the  Flowmeters – device that can be inserted in series sides have friction and resistance. Thus, lower with blood vessels or applied to the outside of the velocity vessel to measure blood flow.  parabolic blood flow  Ohm’s law: F = ∆ P/R  F= blood flow  ∆ P = pressure difference from 1 point to another  R = friction between the flowing blood and the intravascular endothelium all along the inside of the vessel Figure 5. Laminar Blood flow Figure 3. Interrelationships of pressure, resistance, and blood Parabolic Velocity Profile during Laminar Flow flow o velocity of flow in the center of the vessel is far greater than that toward the outer edges NOTE: Difference in pressure between the two ends of the - Fluid molecules touching the wall vessel, it is not the absolute pressure in the vessel that move slowly because of adherence to determines rate of flow. Higher pressure difference means the vessel wall increased flow. Higher friction like an artery with plaque, the - Next layer of molecules slips over blood flow will be decreased. these, the third layer over the second, Page 4 of 10 CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 the fourth layer over the third, and so Directly proportional to velocity, density and diameter and forth density. Inversely proportional to viscosity. HIGHER VISCOSITY, o The fluid in the middle of the vessel can move LOWER TURBULENCE. rapidly because many layers of slipping molecules exist X. BLOOD PRESSURE o In echocardiogram, we compute blood flow Force exerted by the blood against any unit area of the indirectly. For us to know the flow of blood, vessel wall e.g., aortic valve, if there is an obstruction - Commonly measured in millimeters of mercury there is higher velocity (mm Hg) o 50 mmHg pressure: means that the force 2. Turbulent Flow exerted is sufficient to push a column of  Blood flowing in all directions in the vessel mercury against gravity up to a level 50  Continually mixing within the vessel millimeters high  Causes: Obstruction, sharp turn, rough surface - BP varies depending on the gravity  Eddy current and the height it is being measured o Whorls in the blood hence, always put the brachial arm at o Blood flows greater friction/resistance the level of the heart o it means there is turbulent blood flow o Can also be measured in centimeters of water (cm H2O) o 1 mm Hg = 1.36 cm H2O o Pressure of 10 cm H2O: means a pressure sufficient to raise a column of water against gravity to a height of 10 centimeters invention in 1846 by Poiseuille Figure 6. Turbulent Flow XI. RESISTANCE TO BLOOD FLOW Impediment to blood flow in a vessel It cannot be measured by any direct means B. REYNOLD’S NUMBER Calculated from measurements of blood flow and pressure difference between two points in the vessel o pressure difference between two points is 1 mmHg and the flow is 1ml/sec, the resistance is said to be 1 peripheral resistance unit o Resistance to blood flow is expressed in peripheral resistance unit (PRU) o Measure of the tendency for turbulence to occur  Resistance can also be expressed using CGS o tendency of the vessel to have turbulence (centimeters, grams, seconds) unit o v is the velocity of blood flow (in o A basic physical unit centimeters/second) o CGS = dyne sec/cm5 o d is the vessel diameter (in centimeters) o Calculated using the formula o ρ is density o η is the viscosity (in poise) o Causes of turbulence in the proximal aorta and pulmonary artery: - High velocity of blood flow - Pulsatile nature of flow - Sudden change in vessel diameter - Large vessel diameter A. TOTAL PERIPHERAL VASCULAR RESISTANCE Rate of blood flow through the entire circulatory system NOTE: small vessels: Reynold’s number is almost never high is equal to the rate of blood pumping by the heart > enough to cause turbulence CARDIAC OUTPUT (is the stroke volume times heart rate per minute) Page 5 of 10 CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 ○ volume of ejected during each systole is the stroke volume ADULT:  Rate of blood flow approximately 100 ml/sec  Pressure difference from the systemic arteries to the systemic veins is about 100mmHg  Total peripheral resistance: - Resistance of the entire systemic circulation - About 100/100, or 1 PRU - In conditions that causes all the blood vessels in the body to become extremely constricted the resistance can be as high as 4 PRU while in contrast to over-dilation of blood vessels throughout the body, the resistance can be as low as 0.2 PRU Figure 7. Demonstration of the effect of vessel diameter on blood flow B. TOTAL PULMONARY VASCULAR RESISTANCE if mean pulmonary arterial pressure averages 16 mmHg D. POISEUILLE’S LAW and mean left atrial pressure averages 2 mmHg  In the cross section of blood vessels, it shows - net pressure difference of 14 MM concentric rings with varying diameter calculates is 0.14 PRU when the cardiac output is o indicates difference in velocity of flow in each normal at about 100 ML/sec adjacent ring in accordance to the laminar flow About one seventh that in the systemic circulation o Poiseuille’s law is the integration of velocity of Total systemic circulatory resistance > pulmonary blood flow in all concentric rings multiplied to resistance the areas of the rings Poiseuille’s law: F—> π∆ Pr4/8ηl C. CONDUCTANCE OF BLOOD IN A VESSEL F is the rate of blood flow ∆P is the pressure difference between the ends of the  Measure of blood flow through a vessel for a given vessel pressure difference r is the radius of the vessel  Conductance can be expressed in unit of blood flow and l is the length of the vessel pressure such as; η is the viscosity of the blood o milliliters per second per millimeter of mercury Rate of blood flow is directly proportional to the (ml/sec mm Hg) fourth power of the radius of the vessel, which o liters per second per millimeter of mercury demonstrates once again that the diameter of a blood (L/sec mm Hg) vessel (which is equal to twice the radius) plays by far small changes in vessel diameter markedly change its the greatest role of all factors in determining the rate conductance of blood flow through a vessel. Reciprocal to resistance 1/resistance Longer length is increased flow. Increases in proportion to the 4th power of the diameter, More viscous blood, slower flow. in accordance with the following: Diameter increased, the higher flow. Smaller diameter to a bigger diameter of vessel wall, ○ Ex: In anaphylactic shock, blood vessels will there will be a higher conductance. dilate. For the flow to remain constant the ○ Conductance ∝ Diameter4 blood flow should increase to maintain normal blood pressure. ○ Ex: Obstruction to flow in aortic stenosis. But in calcific aortic stenosis there is post stenotic dilation of the vessels, therefore the flow should be higher to maintain a normal blood pressure Page 6 of 10 CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 - Can be expressed in the formula E. ARTERIOLAR RESISTANCE  2/3 of total systemic blood flow resistance come from arteriolar resistance of small arterioles  With internal diameter of 4µm to 25 µm but due to - Aorta to the arterioles at the lower extremity has lower strong vascular walls it can further increase up to amount of blood compared to the branching points, fourfold compared to the unidirectional series circuit.  Arterioles responding with only small changes in Ex: If the other kidney is removed, the total diameter to nervous signals or local tissue chemical peripheral resistance will be increased, adding signals. a pathway will reduce the total vascular  Arterioles can either cut-off or increase the blood resistance. flow to a tissue with maximum arteriolar constriction or dilation. F. RESISTANCE TO BLOOD FLOW IN SERIES AND PARALLEL VASCULAR CIRCUITS  Blood pumped by the heart flows from vessels with high pressure (e.g., aorta) to vessels with lower Figure 9. Vascular Resistance in Parallel pressure (e.g., Superior vena cava)  Blood vessels are arrange in either XII. BLOOD FLOW IN A PARALLEL CIRCUIT o series vascular circuits Adding more blood vessels to a circuit reduces the total o parallel vascular circuits vascular resistance Many parallel blood vessels make it easier for blood to 1. Series Vascular Circuits flow through the circuit  arteries, arterioles, capillaries, venules, and veins - each parallel vessel provides another pathway  1 direction, no exit points or conductance for blood flow o Example aorta to the abdominal descending blood flow through each tissue is a fraction of the total aorta blood flow (cardiac output)  Flow through each blood vessel is the same and the - determined by the resistance (the reciprocal of total resistance to blood flow (Rtotal) is equal to the sum conductance) for blood flow in the tissue and of the resistances of each vessel pressure gradient  Resistance IN SERIES is the summation of all - Ex: Nephrectomy - dec TVC and TBF but inc resistances TPVR  Expressed using the formula - Ex: Amputation: decreased TVC, total blood flow but increased resistance XIII. EFFECT OF HEMATOCRIT AND BLOOD VISCOSITY ON VASCULAR RESISTANCE AND BLOOD FLOW  Viscosity of blood can affect the Poiseuille’s equation  Greater viscosity = lower flow in blood vessels (only if Figure 8. Vascular Resistance in Series other factors are constant)  Blood is 3x more viscous than water 2. Parallel Vascular Circuits - branched vessels - Permits each tissue to regulate its own blood flow, to a HEMATOCRIT great extent, independently of flow to other tissues  Hematocrit (percent of cells in blood volume) - Greater amounts of blood will flow through this parallel  Contributes in blood viscosity in which larger number of system than through any of the individual blood vessels suspended blood cells in the plasma creates frictional > the total resistance is far less than the resistance of drag on nearby cells and walls of blood vessels any single blood vessel Page 7 of 10 CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020  Increase in blood viscosity directly proportional to hematocrit Figure 11. Effects of Arterial Pressure  Blood flow or changes in arterial pressure are usually short-lived because autoregulatory mechanisms on the peripheral tissues maintains optimal blood flow Figure 10. Effect of haematocrit on blood viscosity  The viscosity of blood at normal hematocrit ranges from A. PRESSURE – FLOW RELATIONSHIP IN PASSIVE 3 to 4 thus the same amount of pressure is required to VASCULAR BEDS force whole blood to the same blood vessel compared  Isolated blood vessels or tissues that lack autoregulation to water depends on arterial pressure to increase blood flow and  Normal hematocrit push the blood through its vessels o Adult men 42  Increased blood flow and pressure also distends the o Adult women 38 elastic vessel and reduce its vascular resistance o May show decreased level in cases of anemia  if the distending pressure is reduced, the passive blood and increase in person with polycythemia vessels start to collapse and as it reaches critical closing which ranges to 60 to 70 exhibiting a viscosity pressure, blood flow ceases as the passive blood 10x greater than water. vessels completely shuts off o Very high hematocrit greatly slows blood flow in vessels  Minor factors that may contribute to blood viscosity (mostly not significant in hemodynamic studies) o Plasma protein concentration o Types of protein in the plasma XIV. EFFECTS OF PRESSURE ON VASCULAR RESISTANCE AND TISSUE BLOOD FLOW  The increase in arterial pressure and force of blood flow Figure 12. Effects of Arterial Pressure initiates activation of compensatory mechanisms (e.g. increased vascular resistance)  Sympathetic stimulation and vasoconstrictors alter the  Blood flow autoregulation is a response of tissues to passive pressure – flow relationship (refer to the graph) maintain normal blood flow during event of changes in o during inhibition of sympathetic stimulation arterial pressure (approx. 70 to 175 mm Hg) by dilates the vessels and increases blood flow adjusting its vascular resistance while in the activity of sympathetic stimulation  Changes in the blood flow may be due to strong show slow increase in blood flow even if the sympathetic stimulation that induces vasoconstriction arterial pressure is high. similarly hormonal vasoconstrictors (norepinephrine, angiotensin II and endothelin) can also briefly reduce blood flow. Page 8 of 10 CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 TEST YOUR KNOWLEDGE a. Blood flow to most tissues is controlled according to the tissue need 1. Increased viscosity of the blood that contributes to b. Heart needs help in the form of special nerve low blood flow can be associated with patients diagnosed with ____ signals to make it pump the required amounts a. Hemolytic anemia of blood b. Thalassemia c. It is possible to increase blood flow c. Polycythemia everywhere in the body when a particular d. Hyperammonemia tissue demands increase flow 2. Which of the following statements best describe the d. Arterial pressure regulation is generally blood flow in parallel vascular circuit? independent of either local flow control or a. Linear and no exit points cardiac output control. b. Blood flow through each vessel is the same 10. Organ that has an additional major role in pressure c. Blood flow in each vessel is a fraction of the control for secretion of pressure-controlling total blood flow hormones and regulation of blood volume d. total resistance to blood flow is equal to the a. Brain sum of the resistances of each vessel 3. Higher conductance indicates _____ b. Kidney a. Retardation on blood flow c. Liver b. Increased arterial pressure d. Heart c. Increase vascular resistance 11. The difference in pressure between the two ends of d. Larger blood vessel diameter the vessel is the absolute pressure in the vessel 4. The force exerted by blood against any unit area of that determines rate of flow. The total blood flow the vessel wall is measured in through the lungs each minute is the same through a. Peripheral resistance unit the systemic circulation. b. Millimeter of mercury a. Both sentences are correct c. Milliliter per minute b. Both sentences are wrong d. Milliliters per second per millimeter of mercury c. First sentence is correct, second sentence is 5. The velocity of blood flow wrong a. Is higher in the capillaries than the arterioles. d. Second sentence is correct, first sentence is b. Is higher in the veins than in the venules. c. Is higher in the veins than the arteries. wrong d. Is reduced in a constricted area of a blood 12. The difference of blood pressure between the two vessel. ends of the vessel which pushes the blood through 6. When the radius of the resistance vessels is the vessel increased, which of the following is increased? a. Pressure gradient a. Diastolic blood pressure b. Stroke volume b. Viscosity of the blood c. Vascular resistance c. Hematocrit d. Velocity d. Capillary blood flow 13. It is the steady flow rate of blood through a long 7. A 30-year-old patient comes to her primary care smooth blood vessel clinician complaining of headaches and vertigo. A a. Laminar flow blood test reveals a hematocrit of 55%, and a b. Streamline flow diagnosis of polycythemia is made. Which of the c. Eddy current following would also be increased? a. Mean blood pressure d. Both a and b b. Radius of the resistance vessels 14. True or false. Reynaud’s number is the measure of c. Radius of the capacitance vessels the tendency for turbulence to occur. d. Central venous pressure 15. Permits each tissue to regulate its own blood flow 8. Which of the following has the highest total cross- to a great extent, independently of flow to other sectional area in the body? tissues. a. Arteries a. Parallel vascular circuits b. Arterioles b. Series vascular circuits c. Capillaries c. Total pulmonary resistance d. Venules d. Conductance if blood in a vessel 9. Which is incorrect? Page 9 of 10 Answers: C C D B B D A C C B D A D False A CMED 1C  (006) CIRCULATION: BIOPHYSICS OF PRESSURE, FLOW, AND RESISTANCE DR. L. ASUNCION-VIADO | 01/18/2020 REFERENCES 1. Guyton, A.C., Hall, J.E. Guyton and Hall Textbook of Medical Physiology. 13th ed., W. B. Saunders, 2015. 2. Doc L. Asuncion-Viado’s PPT and Lecture Page 10 of 10 CMED 1C