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University of Toronto Scarborough
Biological Sciences
Stephen Reid

1Lecture 5 Regulation of Cardiac Output and Heart Rate continued Stroke Volume Regulation Blood Flow and Blood Pressure1 Sympathetic and Parasympathetic Innervation of the HeartWe finished previously looking at the effects of sympathetic and parasympathetic innervation of the heart and how we can quantify the amount of sympathetic and parasympathetic tone by manipulation with various agonists Atropine was used to block muscarinic receptors and dampen parasympathetic tone therefore increasing heart rate and sotalol to block beta receptors in the heart and reduce sympathetic tone thereby lowering heart rate By measuring the effects of atropine and sotalol against a normal heart rate we can see the amount of sympatheticparasympathetic tone2 Other CardiovascularRelated Output from the BrainThere are other related outputs from the brain to the cardiovascular system and vice versa As well as the sympathetic and parasympathetic nerves going from the brainstem to the heart there is sympathetic innervation of blood vessels arteries and veins Norepinephrine is the neurotransmitter and it causes a constriction of the blood vessels vasoconstriction which raises total peripheral resistance see lecture 8 and therefore increases blood pressure Sympathetic stimulation of the adrenal gland causes the release of epinephrine and norepinephrine into the circulation When we look at the effects of epinephrine on blood flow and pressure well see that its effects change from organ to organ depending on the proportion of alpha to beta adrenoceptors Generally alpha receptors cause vasoconstriction whilst beta receptors cause vasodilationWhat we havent looked at yet is input into the cardiovascular control centres The most important input comes from baroreceptors or pressure sensors There are two major populations of baroreceptors in the circulatory system one in the aorta and one in the carotid arteries in an area called the carotid sinus These sense arterial pressure and are very important for regulating overall blood pressure in the body As well there are feedback receptors in skeletal muscle that are also important in cardiac regulation and are thought to play an important role in the control of breathing during exercise As an aside there is no known respiratory control system that can account for the increase in breathing that occurs during exercise 3 Stroke Volume SV RegulationSo weve looked at sympathetic and parasympathetic influence on the regulation of heart rate but heart rate is only half of cardiac output How is stroke volume ml of blood pumped per heart beat the second part of the equation COHR x SV regulated Whilst regulation of heart rate was fairly simple regulation of stroke volume is considerably more complex with many more factors involvedUltimately three major factors will influence the regulation of stroke volume 1 the force of ventricular contraction 2 the end diastolic volume and 3 the afterload blood pressure The first is relatively easy to modify as we have just discussed epinephrine and norepinephrine in
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