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Cardiac Output (CO) (Facts of CO (CO = Heart Rate x Stroke Volume (Heart…

- - - - Stroke volume depends on: preload, contractility, and afterload.
        
        Preload: degree to which cardiac muscles are stretched just before they relax
        
        Contractility: contractile strength achieved at a given muscle length
        
        Positive Inotropic agents = increase contractility
        
        Negative Inotropic agents = decrease contractility
        
        Afterload: the pressure that the ventricles must overcome to eject blood
- - - - Positive factors -> increase HR
      - Negative factors -> decrease HR
    - - Decreases HR
        
        PNS initiated cardiac responses are mediated by Ach, which hyperpolarizes the membranes of its effector cells by opening K+ channels.
    - - Increases HR
        
        Sympathetic stimulation enhances contractility and speeds relaxation by enhancing Ca+ movements in the contractile cells.
        
        When fright, anxiety, or exercise activates the SNS, sympathetic nerve fibers release NE at their cardiac synapses. NE binds to binds to β1-adrenergic receptors in the heart. This causes threshold to be reached more quickly, and thus the SA node fires more rapidly. The heart then responds by beating faster.
    - - Ages: HR decrease by older ages
      - Gender: HR of women are faster than that of men
      - Exercise: increasing HR while exercise. Regular exercise decreases HR
      - Body temperature: Heat increases HR by enhancing the metabolic rate of cardiac cells. Cold decreases HR.
    - - Hormones
        
        Epinephrine enhances HR and contractility. Thyroxine causes a sustained increase in HR when released in large quantities.
      - Ions
        
        Normal heart function depends on having normal levels of intercellular and extracellular ions. Plasma electrolyte imbalances can endanger the heart.
    - - Both divisions of the ANS continuously send impulses to the SA node, but the dominant influence is inhibitory. Because of this, the heart exhibits vagal tone, and HR is slower than it would be if the vagus nerves where not innervating it.