Essentials of Human Evolution

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If imbalance in volume between LV and RV → volume of blood in pulmonary circuit increases → pulmonary congestion – build-up of fluid → fluid may enter interstitial fluids

Altering stroke volume – changes in calcium release

Changes in calcium release alter contractility

contractility = change in contractile energy not due to changes in muscle length (EDV)

Amount of Ca2+ released depend on

- Ca2+ stored in the SR

- Ca2+ influx through the (cell membrane) voltage-dependent Ca2+ channels

- Sympathetic activation → increase contractility

- Increase in cardiac contractility (increase amount of Ca2+ available for contraction) with sympathetic stimulation of ventricular muscle cells is caused by noradrenaline and circulating hormonal adrenaline

- act on β-receptors on cardiac muscle cells to increase Ca2+ entry and Ca2+ release from SR

Ventricles of heart have deep sympathetic innervation

isolated ventricular Muscle cells not capable of excitation and hence requires electrical stimulus

- sympathetic activation of ventricular muscle cells → increases contractility by increase amount of Ca2+ available for contraction

Sympathetic activation

Increase Ca2+ entry and SR release

Norepinephrine (neuronal noradrenaline) Epinephrine (hormonal adrenaline) interact with β-adrenergic receptors → activates adenylyl cyclase (membrane bound) → production of cAMP → activates protein kinases → act on voltage dependent calcium channels, ryanodine receptors and SR calcium ATPase by phosphorylating them → increases probability of them opening → increase Ca2+ influx → increase Ca2+ release from SR → increase force and velocity of contraction

Summary – altering stroke volume

- Force of contraction varies with muscle length (length-force relationship)

- Longer length → increase no. of cross bridges

- Changes in Ca2+ release

- Increase Ca2+ → stronger contraction

- Decrease Ca2+ → weaker contraction

- During exercise → increase EDV → increase venous return to heart

- increase sympathetic activation of veins → decreases diameter → constriction → increase pressure → increase blood flow back to heart → increase EDV