Blood Flow

  • The cardiovascular system consists of
    • two pumps – left and right heart ventricles
    • two circuits – pulmonary and systemic
  • Because the two circuits are connected in series, flow (mL/min) must beequal in both
  • The systemic circuit
    • begins as a large vessel, the aorta, and branches intosmaller vessels until capillaries are reached within organs.
    • Vascular components include arteries, arterioles, and capillaries.
    • Arterioles
      • They have the highest resistance in the cardiovascular system (MCQ)
      • regulated by the autonomic nervous system.
      • Arteriolar smooth muscle tone depends on sympathetic input, local metabolites, hormones, and other mediators.
    • Capillaries
      • have the largest total cross-sectional and surface areas(MCQ)
      • are the site of exchanges of nutrients, water, and gases.
  • The venous circuit
    • Veins
      • thin-walled vessels under low pressure (MCQ)
      • containmost of theblood in the cardiovascular system.
    • Venules
      • most permeable components of the microcirculation.
  • Hemodynamics – Velocity and Blood Flow
  • Velocity
    • inversely related to the total cross-sectional area of all vessels of a particular segment of the cardiovascular system.
    • The cross-sectional area of the aorta is approximately 2.8 cm2, whereas the area of the combined capillaries is approximately 1357 cm2.
    • The aorta, therefore, has the highest velocity, and the capillaries the lowest.(MCQ)
  • Blood flow
    • it has the dimensions of volume per unit time, for example, cubic centimeters per second.
    • the flow through the aorta per minute (ie, cardiac output) is equivalent to the flow to the right atrium per minute (ie, venous return)is equivalent to the flow through the combined capillaries per minute.
  • Hemodynamic Equivalent of Ohm’s Law
    • The equivalent relationship for a liquid in motion is

                        Cardiac output  = mean arterial pressure – right arterial pressure

                                                                        Total peripheral resistance



  • CO = cardiac output
  • P = pressure difference (mm Hg) Q = volume flow (L/min)
  • R = resistance (mm Hg/L/min)


  • Poiseuille’s equation gives the relationship of flow, pressure, and resistance

                                                            Q = P1 P2 ,


          • Q = blood flow (L/min)
          • P1 = upstream pressure for segment
          • P2 = pressure at end of segment
          • R = resistance of vessels between P1 and P2
  • The equation states that flow (Q) is directly proportional to the driving pressure (P) and inversely proportional to the resistance (R).
  • Resistance is directly proportional to the length (ë) of the vessel and to the viscosity of blood (f):


        • r4 = radius of the blood vessel to the fourth power.
      • The greater the vessel length, the greater the resistance(MCQ)
      • greater the viscosity, the greater the resistance.
      • The most important factor determining resistance is the radius of the vessel
      • The equation emphasizes that if the vessel radius doubles (ie, resistance decreases), then flow will increase 16-fold, if other factors remain constant.(MCQ)
      • thesystemic and pulmonary circulations have approximately the
        • same number of total capillaries
        • with the same total cross-sectional area (1357 cm2)
        • blood viscosities and flows are both equal
      • lower pressure difference across the pulmonary circuit must be due to the difference in vessel length between the pulmonary and systemic circuits.

Reynolds Number and Turbulence

        • Laminar flow does not generate an audible sound
        • Turbulent flow involves random pressure fluctuations, and sounds are heard.
        • The Reynolds number (a dimensionless variable relating viscous and inertial forces) serves as a useful indicator for the transition of laminar flow to turbu- lent flow. (MCQ)
        • The Reynolds number is calculated from the following equation:


            1. NR = Reynolds number
            2. V = mean velocity (cm/s) D = tube diameter (cm)
            3. p = fluid density
            4. n = fluid viscosity (Poises)
          • Turbulent flow usually occurs when the Reynolds number exceeds a critical value of 3000 (MCQ)
          • Because the viscosity of blood is relatively high, the Reynolds number for turbulent flow is not exceeded in most parts of the circulation.


          • Compliance describes the distensibilityof blood vessels.
          • Vascular compliance (C) is the slope of the relationship between a rise in volume in the vessel and the rise in pressure produced by that rise; hence,


          • The compliance of combined veins is about 19 timesgreater than the compliance found in the combined arteries.
          • Systolic pressure is a function of the stroke volume (and compliance).(MCQ)
          • Diastolic pressure is a function of the heart rate and the arteriolar resis-tance, which determines run-off into the veins.(MCQ)

Pressure Profile

          • As blood flows through the systemic circulation, pressure decreases progressively from the aorta, where it is highest, to the vena cava, where it is lowest
          • Because the greatest resistance to flow occurs in the arterioles, the largest decrease in pressure occurs across the arterioles.
          • Local arteriolar dilation in an organ decreases arteriolar resistance, which increases blood flow and pressure downstream
          • Local arteriolar constrictionincreases arteriolar resistance and decreases flow and pressure down- stream.
          • Atrial pressure is lower than venous pressure
            • pressure is 5–10 mm Hg in the left atrium
            • 15 mm Hg in peripheral venules.

Arterial Pressures

          • Systolic arterial pressure
            • highest arterial pressure during the cardiac cycle.
            • It represents the pressure developed when the heart contracts most forcibly.
            • As blood flows from the aorta to the peripheral arteries.(MCQ)
              • Arterial peak systolic pressure increases
              • minimum diastolic pressure falls
          • Diastolic pressure
            • lowest arterial pressure during the cardiac cycle
            • represents the pressure when the heart is relaxed and not contracting.
          • Pulse pressure
            • difference between systolic and diastolic pressures(MCQ)
            • determined primarily by stroke volume and arterial compliance.
            • Pulse pressure and both arterial pressuresincrease with aging due to decreased compliance of vessels.(MCQ)
            • The pulse pressure also increases as blood moves out along the arterial tree.(MCQ)
          • Mean arterial pressure
            • average arterial pressure over time
            • calculated by adding diastolic pressure plus one third of pulse pressure.(MCQ)
            • Mean pressure, the driving force for flow, decreases as one moves out along the arterial tree.(MCQ)
            • The fall in mean pressure across the arteriolar bed means that capillary pressure is normally nonpulsatile

Control Mechanisms and Special Circulations

          • Autoregulation
            • Autoregulation is the maintenance of constant blood flow over a wide range of blood pressures.
            • Constant flow is due to
              • metabolic theory of autoregulation
                • Increases or decreases in local metabolites
              • myogenic theory of autoregulation
                • Smooth muscle contraction in response to increases or decreases in pressure
          • Active Hyperemia
            • Active hyperemia is defined as increased blood flow to an organ caused by increased tissue metabolic activity and accumulation of vasodilator metabolites.
            • In exercise, blood flow will increase to skeletal muscles involved to meet increased metabolic demand.
          • Reactive Hyperemia
            • If arterial inflow to a vascular bed is stopped for a few minutes, the blood flow, on release of the occlusion, immediately exceeds the flow before the occlusion, producing reactive hyperemia.
            • A number of metabolites may mediate the metabolic vasodilation that occurs during the interval of occlusion, such as CO2, H+, K+, lactic acid, and adenosine(MCQ)
            • The increase in flow is proportional to the length of the occlusion.
          • Coronary Blood Flow
            • The principal factor responsible for perfusion of the myocardium is aortic pressure.
            • Changes in coronary blood flow are caused mainly by caliber changes of the coronary resistance vessels in response to metabolic demands of the heart.
            • A decrease in O2 supply or an increase in O2 demand apparently causes
            • the release of a vasodilator (adenosine) that decreases coronary resistance and increases coronary flow proportionally.
          • Cutaneous Circulation and Temperature
            • The skin contains two types of resistance vessels
              • Arterioles
              • arteriovenous anastomoses.
            • Arteriovenous anastomoses
              • shunt blood from the arterioles to venulesand venous plexuses, bypassing the capillary bed.
              • found primarily in fingertips, palms ofthe hand, soles of the feet, ears, nose, and lips (ie, exposed regions).
              • These vessels are almost exclusively under sympathetic neural control by temperature receptors from higher centers and become maximally dilated when their nerve supply is interrupted.
              • They do not appear to be under metabolic contro
              • theyfail to exhibit reactive hyperemia or autoregulation (MCQ)
          • Fetal Circulation at Birth
            • In the fetus, blood returning to the right heart is divided into two streams by the edge of the interatrial septum (crista dividens).
            • The larger stream is shunted to the left atrium through the foramen ovale.
            • The other stream passes into the right atrium, where it is joined by superiorvena cava blood returning from the upper parts of the body.
            • Because of the large pulmonary resistance, due to the low fetal partial pressure of O2 in alveolar gas, only one tenth of the right ventricular outputgoes through the lungs.
            • The remainder right ventricular output passes through the ductusarteriosus from the pulmonary artery to the descending aorta. (MCQ)
            • Blood flows from the pulmonary artery to the aorta because the pulmonary resistance is high and the diameter of the ductusarteriosus is as large as the descending aorta.
            • At birth, the asphyxia that starts with clamping of the umbilical vesselsactivates the infant’s respiratory center.
            • As the lungs fill with air, pulmonary vascular resistance decreases to about one-tenth of the value existing before lung expansion.
            • The left atrial pressure is raised above the pressure in the inferior vena cava and right atrium
            • thisreversal of the pressure gradient across the atria abruptly closes the valve over the foramen ovale.
            • With the decrease in pulmonary vascular resistance, the pressure in the pul- monary artery falls, causing a reversed blood flow through the ductusarteriosus.
            • Closure of the ductusarteriosus appears to be initiated by the high O2 tension of the arterial blood passing through it.(MCQ)
            • The presence of vasodilator prostaglandins is thought to be the reason for failure of the ductusarteriosus to close. (MCQ)
            • The administration of indomethacin, which blocks prostaglandin synthesis, often leads to closure of the ductus in infants in whom it fails to close.
            • Generally,cerebralbloodflowtothebrainisconstant.
            • Cerebralmetabolicproducts(diminished O2,elevated CO2 and H+) contribute to the control of cerebral blood flow locally in accordance with local metabolism.
            • Cerebralcirculationismaintainedinhypertensionbyacombinationofsympatheticvasoconstriction, hormonal vasoconstriction, and homeostatic autoregulation.
            • Cushing’s phenomenon
              • patients with brain tumors who had cerebral ischemia also have increased systemic blood pressure with a simultaneous decrease in heart rate.(MCQ)
              • Thisresponse,called,iscausedbycerebralischemicstimulationof vasomotor regions in the medulla that help maintain cerebral blood flow in the face of increased resistance caused by expanding intracranial tumors.
          • Pulmonary Blood Flow
            • Pressures Within the Pulmonary Circuit
              • The most important difference between the pulmonary and systemic circu- lations is the low blood pressure in the pulmonary arteries.
                • The pulmonary arterial systolic pressure is approximately 22 mm Hg(MCQ)
                • leftventricular systolic pressure is around 120 mm Hg.
              • The pulmonary circulation is a low-resistance circuit that must accommodate the entire cardiac output at rest and during exercise.
              • When pulmonary arterial pressure increases, vascular resistance decreases for two reasons:
                • Increased pressure increases the caliber (distention) of the arteries.
                • Increased pressure causes more capillaries to open (recruitment).
            • Effects of Gravity on Blood Flow
              • Because of the low blood pressures in the pulmonary circulation, gravity has a large effect on blood flowto different parts of the lung
              • In an upright subject, the effect of gravity causes
                • blood flow to be larger at the base than at the apex
                • Ventilation is also larger at the base than at the apex.
              • Although the base receives the greatest ventilation, it does not match the very high blood flow.
                • Thus, the base is an underventilated region, inwhich the V/Q  ratio is less than 0.8.(MCQ)
                • anunderventilated lung unit acts like a pulmonary shunt.
              • Even though the apex receives the lowest ventilation, it is too high for the low blood flow.
                • Therefore, the apex can be considered an overventlated region, in which theV/Q  ratiois greater than 0.8.(MCQ)
                • An overventilated lung unit acts like dead space
            • Hypoxic Vasoconstriction
              • A decrease in alveolar PO2 produces a local vasoconstriction of pulmonary arterioles, thereby lowering blood flow to that part of the lung.
              • In other systemic organs, hypoxia results in vasodilation of arterioles.
            • Pulmonary Edema
              • The two causes of pulmonary edema are
                • Increased capillary permeability
                • Increased pulmonary blood pressure due to hypoxic vasoconstriction, leftheart failure, or loss of surfactant
            • Circulation divisions in a vertical lung.
              • In zone 1
                • no blood flow occurs
                • alveolar pressure is higher than pulmonary arterial pressure
                • In normal persons with adequate cardiac output, there is no zone 1 be- cause pulmonary arterial pressure is greater than alveolar pressure
              • In zone 2
                • pulmonary arterial pressureis greater thanalveolar pressure
                • blood flow occurs but alveolar pressure is greater than pulmonary venous pressure.
              • In zone 3,
                • pulmonary arterial and venous pressures are both greater than alveolar pressure
                • blood flow is maximal
            • Shunts
              • In an absolute right-to-left shunt
                • venous blood is delivered to the left side of the heart without contacting ventilated alveoli
                • this shunt produces hypoxemia
                • The shunt results in a
                  • decrease in arterial PO2
                  • wideningof alveolar-arterial (A-a) difference.
              • With a significant pulmonary shunt
                • e.g  in regional atelectasis
                • breathing 100% O2 does not result in a significant increase in systemic arterial PO2, leading to a diagnosis of a pulmonary right-to-left shunt.


            • Ventilation-Perfusion Differences
              • In the normal lung, the V/Q ratio is approximately 0.8.
              • Physiologic dead space is defined as anatomic dead space plus the volume of all airways that behave as if they have received no blood flow.
                • In health, anatomic dead space and physiologic dead space are essentiallyequal.
                • In ventilation-perfusion mismatch, the amount of physiologic dead space ismuch greater than the amount of anatomic dead space.
                • Some regions of the lung may have a hig V/Q  – PO2 in these alveoli is below average.
                • Bohr method measures physiologic dead space
                  • measuresvolume of all airways in which no CO2has been added from the blood
                • In many pulmonary diseases, the physiologic shunt and the physiologic dead space will be increased.
                • The consequence of increased physiologic dead space is wasted ventilation.
              • Hypoventilation
                • associated with equal decreases in PO2 in the alveolar, pulmonary end capillary, and systemic arterial compartments
                • Supplemental oxygen or increased alveolar ventilation will return arterial PO2 to normal.
              • Diffusion impairment
                • refers to a lung structural problem (eg, increased thickness of lung membrane).
                • With significant diffusion impairment, the A-a gradient widens.
                • Supplemental oxygen will increase the gradient across the alveolar membranes and return arterial PO2 toward normal.
              • Exercise increases ventilation and pulmonary blood flow
                • During exercise, the V/Q ratio is greater than 0.8,(MCQ)
                  • ventilationincreases more than cardiac output
                  • base-to-apex flows become more equal.
            • High Altitude
            • At high altitude, atmospheric pressure is reduced from 760 mm Hg, resulting in decreased alveolar and arterial PO2 (hypoxemia).
            • Low PO2 stimulates peripheral chemoreceptors, inducing hyperventilation, a decrease in alveolar and arterial PCO2, and respiratory alkalosis.
            • Hypoxemia stimulates erythropoietin,
              • The increased Hb production increases O2 content of the blood.
            • 2,3-DPG levels increase
              • shifts the oxyhemoglobin dissociation curve to the right and facilitating O2 extraction by the tissues.(MCQ)
            • Hypoxemia also results in hypoxic vasoconstriction (ie, pulmonary vaso- constriction), resulting eventually in hypertrophy of the right ventricle due to increased work of the right heart.
            • Hyperbaric Chamber
              • Breathing room air (21% O2; 79% N2) in a hyperbaric environment(MCQ)
              • increases the partial pressure of O2 and N2 in alveoli and arterial blood
                • Elevated PO2 can produce oxygen toxicity
                • high PN2 can lead to the bends (also known as caisson disease).
              • Sudden decompression causes bubbles of nitrogen to accumulate in the blood and tissues.
              • Treatment is recompression and gradual decompression.

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