Cardiac Output, Blood Flow, and Blood Pressure Regulation

Cardiac Output (CO)

• The volume of blood ejected by the ventricles per 1 minute~5L at rest

• ↑ with:

  • Sympathetic stimulation and exercise

  • Vigorous exercise ↑ CO to 21 L/min for fit person and up to 35 L/min for world class athlete

• CO= HR X SV 

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Heart Rate (HR)

• Heart rate is the number of heart beats per minute (BPM)

• Normal HR at rest:

  • Adult= 60-100 BPM (average ~70 BPM)

  • Athlete= 40-60 BPM

  • Infant= ~120 BPM

• HR increase with:

  • increase metabolic demands

  • Sympathetic NS activity (NE and/or EPI)

• HR decrease with:

  • Parasympathetic NS activity (Acetylcholine) 

Stroke Volume (SV)

• Volume of blood pumped by ventricle/beat (or per systole)

• Determined by extent of venous return and by sympathetic activity

• SV= EDV- ESV

  • EDV= end diastolic volume

  • ESV= end systolic volume

Frank-Starling Law of the Heart

• Normal heart pumps during systole most of the blood it receives in diastole

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Cardiac Reserve

• The maximum percentage that the cardiac output can increase above the resting level.

• Cardiac reserve= COmax- COrest 

Factors Affecting Stroke Volume

• Inotropy= contractility

  • The ability of the heart muscle to generate force during myocardial ventricular contraction

  • Positive inotropic agents:↑contractility

    • Include: Hypercalcemia, Catecholamines, and sympathetic stimulation 

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  • Negative inotropic agents:↓contractility

    • Include:Hypocalcemia, Hyperkalemia, Parasympathetic stimulation 

• Preload

  • Amount of tension in the L ventricle immediately before it contracts

    • ↑ preload = ↑force of contraction

    • Closely related to venous return (VR) → ↑VR → ↑preload 

  • Frank-Starling law of heart - SV∝ EDV

    • ventricles eject as much blood as they receive

    • the more they are stretched, the harder they contract 

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• Afterload 

  • The “load” the heart has to push against in order to eject the blood from the ventricles

  • Blood pressure in the aorta and pulmonary trunk 

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Anrep Effect

• Sudden increase in afterload results in an increase in myocardial contractility to compensate → due to increased myocardial stretch

• Due to increases Ca2+ entering the cytoplasm through the Na+/Ca2+ exchanger in the T-tubules

• A very important consequence of these events is that the cardiac output of the left ventricle must equal the output of the right ventricle so that the blood flow through both the pulmonary and systemic circulations are equal. 

Factors Affecting Heart Rate

• Chronotropy= affecting time or rate

• Sympathetic system:↑HR → Positive chronotropic effect

• Parasympathetic system:↓HR → Negative chronotropic effect 

Blood Flow and Perfusion

• Perfusion= Amount of blood flowing through an organ, tissue, or blood vessel / time (ml/min)

• Blood is constantly reconditioned so composition remains relatively constant

• Reconditioning organs receive more blood than needed for metabolic needs:

  • Digestive organs, kidneys, skin

• At rest, blood flow is constant and = CO 

• Perfusion to other organs is adjusted according to metabolic needs

  • Ex: during exercise, fight or flight, etc.

• Brain CANNOT tolerate disrupted blood supply:

  • Completely dependent on oxygen for energy 

  • ↓O2 → brain damage in ~5 min

Hemodynamics

• 2 main variables that govern hemodynamics:

  • Pressure gradient: Fluid flows down its pressure gradient

    • Blood will always flow from an area of high pressure to an area of low pressure

    • Pressure gradient can be adjusted by:

      • ↑ Pressure to overcome resistance → ↑ force of heart contraction

      • ↓ Pressure in the tissue or organ → ↓ Resistance 

  • Resistance: opposes fluid blood flow 

    • Opposition of blood flow through a vessel

    • Referred to as peripheral resistance (PR) or (TPR)

    • Depends on 3 things:

      • Blood viscosity

      • Vessel length: the longer the vessel→ ↑the resistance

      • Vessel radius or diameter 

      • Major determinant= vessel’s radius

      • Slight change in radius → ↑ change in blood flow 

      • R is proportional to 1/ r4

        ↓radius or diameter of blood vessel → ↑friction→ ↑↑ R 

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Regulation of Blood Flow

• Local Blood Flow Regulation

  • Blood vessels undergo autoregulation→ vasodilation or vasoconstriction

    • No involvement from CNS or hormones

    • Respond to local chemical and physical changes (ex. stretch= Myogenic response) 

    • Short-term regulation

    • Long-term regulation→ changes in blood volume:Kidneys 

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  • • Endothelial cell control:

      • Secrete vasoactive substances:

        • Nitric Oxide (NO)

        • Endothelin

      • Stimulate angiogenesis

        • Collateral circulation

      • Modulates capillary permeability

        • Changing pore size 

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• vasoconstriction

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• vasodilation

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Blood Pressure (BP)

• Is highest in the aorta

• Declines throughout the length of the circulatory system

  • The steepest change in blood pressure occurs in the arterioles 

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• Blood Pressure vs Vessel Type

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• Measured at brachial artery of arm using sphygmomanometer

  • Systolic pressure:

    • Peak arterial BP 

    • Taken during ventricular contraction (ventricular systole) 

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    • Pulse Pressure

  • Diastolic pressure: Minimum arterial BP 

    • taken during diastole

    • http://www.youtube.com/watch?v=ElCbQMiBC6A&feature=related

    • Pulse pressure: difference between systolic and diastolic pressures 

• “Taking the pulse” is a measure of heart rate. 

• What the health professional feels is increased blood pressure in that artery at systole.

  • The difference between blood pressure at systole and at diastole is the pulse pressure.

  • If your blood pressure is 120/80, your pulse pressure is 40 mmHg. 

  • Pulse pressure is a reflection of stroke volume 

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Mean Arterial Pressure (MAP)

• Average pressure driving blood forward into tissues throughout cardiac cycle 

  • MAP (or BP) = CO x TPR

  • Numerically calculated as: diastolic pressure + 1/3 pulse pressure 

• Arterial Blood Pressure

  • Dependent on two factors:

    • Elasticity of blood vessels closest to the heart → compliance

      • A vein is 24 times more compliant than its corresponding artery.

    • Volume of blood forced into them/time 

• Arterioles = Major Determinants of TPR (resistance)

  • Arterioles are the major determinants of TPR

    • Mechanisms involved in adjusting arteriolar resistance:

      • Vasoconstriction: ↓vessel lumen size = ↑resistance

      • Vasodilation: ↑ vessel lumen size = ↓ resistance

Capillary Blood Pressure

• Capillary BP ranges from 20 to 40 mm Hg

• Desirable because ↑ BP would rupture fragile, thin-walled capillaries

• Sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues 

Regulation of Blood Pressure

• Essential for homeostasis

• Requires:

  • Cooperation of the heart, blood vessels, and kidneys

  • Supervision of the brain 

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Vasoactive Chemicals: Vasoconstrictors

1. Epinephrine

2. Norepinephrine

3. Angiotensin I and angiotensin II***

4. Serotonin

5. Vasopressin (ADH) 

B-type natriuretic peptide (BNP) 

• Released by the ventricles in response to volume expansion and pressure overload

• Also, NT-proB-type Natriuretic Peptide 

• Helps the body compensate in congestive heart failure 

• Blood test is diagnostic for heart failure 

• Effects of Natriuretic Peptides

  • Downregulate the effects of the Sympathetic NS and the RAAS

  • Sodium and water excretion via the kidneys

  • decreasing peripheral vascular resistance by causing smooth muscle relaxation

  • May inhibit cardiac growth and hypertrophy 

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  Vasoactive Chemicals: Vasodilators

  1. Histamine

  2. Bradykinin

  3. Prostacyclin (PGI2)

  4. NO 

Control Centers for Cardiovascular Function and Blood Pressure Regulation

• ANS Control Centers in Medulla Oblongata= Cardiovascular Center 

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  • Vasomotor Center

    • The integrating center for 3 reflexes:

      • Baroreceptor reflex

        • Stretch-sensitive receptors

        • Monitor blood pressure 

        • Important in short-term regulation of BP

          • Postural changes 

        • Baroreceptors adapt to chronic high or low BP → NOT useful for long-term BP regulation

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        • Reflex Tachycardia

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      • Chemoreceptor reflex

        • Located in:

          • Carotid artery

          • Aortic artery

        • Sensitive to:

          • ↓O2 or ↑CO2

          • ↑H+ 

        • Main function: ↑respiration

          • Also ↑BP via activation of vasomotor center

      • Medullary ischemic reflex 

        • Automatic response to a drop in perfusion of the brain

          • Medulla oblongata monitors its own blood supply

            • ↑Sympathetic stimulation to the heart and blood vessels →

              • ↑ HR and force of contraction

              • Widespread vasoconstriction

              • ↑BP and restores normal perfusion to the brain 

• Other Control Centers

  • Other brain centers can affect vasomotor center

    • Hypothalamus: Temperature regulation

    • Cortical regions: Stress, anger, arousal can also ↑BP 

Cardiovascular Effects of Exercise

• ↑Perfusion to working muscle

• Accomplished by:

  • Heart beats faster and stronger

  • ↑BP

  • ↓TPR

  • Blood is removed from venous reservoirs

• Blood is redistributed so that more goes to active tissues

  • Cortex → Medulla → Vasomotor center → Activates sympathetic stimulation 

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