Cardiac output

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Unit 3A Human Form & FunctionCells, metabolism & regulation Regulation of blood pressure

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Study Guide Read: Our Human Species (3rd edtn) Chapter 10, sections 4-12 Complete: Human Biological Science Workbook Topic 7 – Regulation of Blood Pressure

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Blood pressureblood pressure is the pressure of the circulating blood against the walls of the blood vessels. Pressure is highest when the ventricles in the heart contract (systole) and lowest when they relax (diastole).

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systolic and diastolic pressureBlood pressure is recorded as two numbers: the top number is called the systolic pressure and the bottom number is called the diastolic pressure. Adult blood pressure is considered normal at around 120/80.

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Measuring blood pressureBlood pressure is usually recorded from the brachial artery in the arm, just above the elbow. Blood pressure falls as the blood moves further away from the heart. Blood pressure in the veins (venous pressure) is much lower than the arterial blood pressure.

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Blood pressure is measured using a sphygmomanometer. Values are reported in millimeters of mercury (mmHg)A traditional analogue sphygmomanometer (left) and an electronic digital sphygmomanometer (right). Kate Whitley, Wellcome ImagesWellcome Photo Library

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Physiological factors affecting blood pressure

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The SA nodeThe heart contains specialised conductive tissue which regulates the heartbeat. The sinoatrial node (SA node or pacemaker) is a cluster of specialised cardiac cells in the wall of the right atrium which initiates the heartbeat. The atrioventricular node (AV node) is the secondary pacemaker which regulates the beating of the ventricles.

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Conductive tissueSinoatrial (SA) node – the pacemakerAtrioventricular (AV) nodePerkinje fibresThe Sourcebook of Medical Illustration (The Parthenon Publishing Group, P. Cull, ed., 1989)

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Cardiac outputThe cardiac output is the total amount of blood being pumped by the heart over a particular period of time – e.g. the minute volume is the amount of blood pumped by the heart over one minute The cardiac output is affected by the stroke volume (the volume of blood pumped by each ventricle during a cardiac cycle ) and the heart rate (the number of heart beats over a particular period of time). Cardiac output = Stroke volume x Heart rate

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Factors that affect stroke volumeVenous return (Frank-Starling law) Autonomic NS (sympathetic stimulation increases stroke volume) Hormones (thyroxine, adrenaline / noradrenaline, glucagon) Calcium & potassium levels

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The Frank – Starling law of the heartThe greater the volume of blood entering the heart during diastole, the greater the volume of blood ejected during systolic contraction (stroke volume).

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What does this mean?When we exercise more blood is returned to the heart. The more the heart muscle is stretched, the stronger the strength of the contraction (rather like a rubber band). The stronger the contraction of the ventricle, the more blood is ejected from the heart i.e. the stroke volume is increased.

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The heart in diastolic and systolic phasesMariana Ruiz Villarreal

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Control of heart rateStimulus - Change of blood pressure Receptor - Baro (pressure) receptors in right atrium, aorta & carotid artery (sinus) Control centre - Cardiac centre in medulla oblongata Transmission - ANS Effector - SA node (AV node in some cases) Response - Sympathetic stimulation speeds up heart Parasympathetic stimulation slows heart down

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REGULATION OF THE HEART RATE

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REGULATION OF THE HEART RATE

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Other factors that influence heart rateThe heart rate can also be affected by: Hormones (adrenaline) Temperature Drugs Salts (potassium & calcium) Emotions (e.g. fear, anger)

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The renin-angiotensin systemThe renin-angiotensin system is a hormone system that regulates blood pressure and fluid balance.

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ActivationA sustained fall in blood pressure is detected by pressure receptors in the kidneys (the juxtaglomerular apparatus). The kidneys release the hormone renin. Renin activates another hormone, angiotensin.

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The effects of angiotensin - 1Angiotensin (together with aldosterone and antidiuretic hormone) cause the kidneys to reabsorb salt and water and create the sensation of thirst (see Section 5). This increases blood pressure.

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The effects of angiotensin - 2Angiotensin causes arterioles throughout the body to constrict (vasoconstriction). This also results in an increase in blood pressure.ArterioleSmooth muscleIn response to sympathetic stimulation, or hormones, such as angiotensin and adrenaline, smooth muscle coiled around arterioles contracts and squeezes the blood vessels, making them smaller – this is vasoconstriction.

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Last Updated: 8th March 2018

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