Mammalian FEB 18

Mammalian FEB 18 - Thursday, February 18th VI. Regulation...

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Thursday, February 18th VI. Regulation of Cardiac Output A. Factors influencing Cardiac Output Heart Rate Stroke Volume HR= 70 bmp/70 mL = stroke volume, which will calculate to 4900 mLs or 5 liters per minute (CO). Although, there are definitely things that will increase cardiac output. Can increase up to 6 or 7 fold during exercise. Cardiac Reserve - the difference between the maximum cardiac output and the cardiac output you'd have at rest. There are two mechanisms we talk about when regulating cardiac output, mechanisms in the heart and outside. B. Intrinsic mechanisms - mechanisms in the heart itself. 1. Homeometric / or homeometric autoregulation - If the atria (particularly the right atrium) is stretched, there is an enhanced irritability of the SA node, or makes the SA node depolarize more frequently than it normally does and therefore increase the HR. What would increase the stretching of the right atrium? The vena cava drains into the right atrium. (Heart Rate) 2. Heterometric -- Starling's Law - Preload - stretching the ventricles. If venous return is increased, we stretch the ventricles. This is Starling's Law as applied to cardiac muscle, instead of skeletal muscle. This will result in the actin and myosin having more interaction. This increases the force of contraction, which will cause the stroke volume to increase. (Stroke volume) C. Extrinsic mechanisms - both the sympathetic and the parasympathetic nervous system innervate the heart. 1. Parasympathetic - via the vagi, the effect of vagal stimulation is to decrease heart rate. Decrease the slope of the pre-potential, and the resting membrane potential is more hyperpolarized, and the threshold is more hypopolarized. The parasympathetic response is fast acting- very sensitive and can act on a beat to beat basis. a. Innervation - b. Effect and ionic explanation 2. Sympathetic a. Innervation - via T1-T4, and innervates all parts of the heart. b. Effect and ionic explanation - increase heart rate by increasing the rate at which potassium permeability progressively declines. Sympathetic activity increases heart rate this way. Sympathetic activity also increases the force of contraction by increasing the amount of trigger calcium that goes into the ventricular muscle fibers. Due to increased venous return, we get increased preload (end diastolic volume), which can increase heart rate via homeometric mechanism. Because of the heterometric law, we also increase stroke volume (stretching of ventricles). Also external effects - parasympathetic decreases heart rate, and sympathetic increases heart rate and increases force of contraction. The sympathetic nervous system can also mobilize blood from the veins and increase venous return. We haven't talked about afterload yet.
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Mammalian FEB 18 - Thursday, February 18th VI. Regulation...

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