Assessment Statements H.5.1 Explain the events of the cardiac cycle, including atrial and ventricular systole and diastole, and heart sounds. H.5.2 Analyse data showing pressure and volume changes in the left atrium, left ventricle and the aorta, during the cardiac cycle. H.5.3 Outline the mechanisms that control the heartbeat, including the roles of the SA (sinoatrial) node, AV (atrioventricular) node and conducting fibres in the ventricular walls. H.5.4 Outline atherosclerosis and the causes of coronary thrombosis. H.5.5 Discuss factors that affect the incidence of coronary heart disease.

Events of the cardiac cycle Sinoatrial (SA) node fires electrical signal throughout walls of atria to begin the cardiac cycle the SA signal causes atria to undergo systole - atria contract & blood flows to the ventricles SA signal reaches atrioventricular (AV) node, which spreads signal throughout the Purkinje fibres causing ventricles to undergo systole, when ventricular pressure rise above atrial pressure, atrioventricular valves slap shut, causing “lub” sound semi-lunar valves open, blood is pumped into arteries as ventricles relax, ventricular pressure falls, pressure in artery exceeds pressure in ventricle, semilunar valves close, causing “dub” sound atrioventricular valves open, ventricles begin diastole and start filling with blood all four chambers are in diastole and filling with blood when atria are completely filled with blood and ventricles are 70 % full, the cycle ends & the cycle repeats

Higher human biology animation: Cardiac cycle

Pressure & volume changes in the left atrium, left ventricle & the aorta, during the cardiac cycle

Mechanisms that control the heartbeat heartbeat is myogenic i.e. initiated in heart muscle itself SA node (pacemaker) sends waves of excitation to atria stimulus is then passed to the AV node conducting fibres (Purkinje fibres) conducts impulses to lower ventricles heartbeat is moderated by ANS i.e. parasympathetic nervous system

Control of the heartbeat

Role of the (SA) sinoatrial node in the cardiac cycle SA node is located in the wall of right atrium of heart muscle SA has characteristics of both nerve and muscle tissue SA node initiates each impulse & acts as pacemaker of the heart no nerve impulses needed for contraction i.e. SA is myogenic SA is connected to nerves which slow or accelerate heart rate; impulses spread out in all directions through walls of atria stimulating atrial systole (contraction) fibres in walls of atria prevent impulses from reaching ventricles impulses only reach AV node after atrial contraction

Role of the AV (atrioventricular) in the cardiac cycle AV node situated at the base of the right atrium, it receives impulse (wave of excitation) from atrial walls AV node causes time delay before the impulse is passed to the ventricular tissue AV node then passes the impulse to modified muscle fibres called Purkinje fibres (bundle of His) in the ventricular wall

Role of conducting fibres in the cardiac cycle AV bundles (bundle of His) originates from AV node, run along interventricular septum & branch into Purkinje fibres AV bundles conducts nerve impulse from AV node to Purkinje fibres Purkinje fibres are specialized muscle fibres found in ventricular muscles Purkinje fibres are insulated from the muscle and do not cause contraction their function is to relay impulses from the AV bundle to the ventricle muscles causing a contraction the impulse emerges into the muscle at the apex of the heart so that the ventricular contraction begins at the apex spreading upwards

Atherosclerosis and coronary thrombosis Atherosclerosis – is a progressive degeneration of artery walls lipids (cholesterol) are deposited on endothelium narrowing the artery lumen fibrous tissue may also be laid down impeding blood flow, causing platelets to stick together clotting factors may then be released, a blood clot within the vessel or thrombus may then form if atherosclerosis occurs in coronary artery, coronary thrombosis, flow of blood to part of heart muscle is reduced or stopped leading to lack of glucose & oxygen myocardial infarction, heart attack, cardiac arrest, heart failure may result

Causes of coronary thrombosis atheroma, fatty deposits in arteries occurs atheroma causes hardening of arteries i.e. atherosclerosis (arteriosclerosis) rough surface in artery lumen causes rupture of platelets blood clots form in coronary artery - coronary thrombosis

Incidences of coronary heart disease events Incidences of coronary heart disease events was significantly associated with: with age; gender (men); diabetes & hyperlipidemia i.e. high lipid levels usually cholesterol Is there significant difference between events of gender (men), diabetes & hyperlipidemia? Support your answer. * BNP = brain natriuretic peptide

Factors that affect the incidence of coronary heart disease genetic factors – some people are predisposed for high cholesterol levels & high blood pressure age – older people are at greater risk due to less elasticity in arteries sex – males are at greater risk of heart disease than pre- menopausal women because they have less estrogen, as estrogen protects against heart disease smoking – nicotine causes vasoconstriction of blood vessels, increases blood pressure, heart-rate & decreases oxygenation of heart muscle diet – eating too much saturated fat, high cholesterol & LDL in blood leads to plaque formation in arteries - coronary thrombosis exercise – helps reduce high blood pressure, reduces the rate of fatty deposits building up in the inner lining of arteries & thickens the heart muscle walls so they pump blood more efficiently obesity – lead to increase in blood pressure & leads to plaque formation in arteries high salt diet, excessive amounts of alcohol & stress can also affect the incidences of coronary heart disease

Revision Questions Explain the events of the cardiac cycle. [7] Describe the mechanisms that control the heartbeat. [4] Outline how the contraction of the atria and the ventricles is controlled. [4] Explain the role of the SA (sinoatrial) node in the cardiac cycle. [6] Explain the role of the AV (atriolventricular) node in the cardiac cycle. [4] Explain the role of the conducting fibres in the cardiac cycle. [4] Outline the condition atherosclerosis and how it may cause coronary thrombosis. [5] Outline how coronary thrombosis can be caused.[3] Discuss the factors which that affect the incidence of coronary heart disease. [7]

The graph above shows pressure changes in the left atrium, left ventricle and the aorta, during the cardiac cycle. Explain the changes in pressure.

The graph above shows volume changes in the left ventricle during the cardiac cycle. Explain the changes in volume.