H5 THE TRANSPORT SYSTEM H5.1 Explain the events of the cardiac cycle including atrial and ventricular systole and diastole, and heart sounds The heart undergoes dramatic movements as it alternately contracts (systole), forcing blood out of its chambers, and then relaxes (diastole), allowing its chambers to refill with blood.

The cardiac cycle includes all events associated with the flow of blood through the heart during one complete heartbeat, that is, atrial systole and diastole followed by ventricular systole and diastole

The cardiac cycle is marked by pressure and volume changes within the heart with pressure changes being 5 times greater in the left side of the heart   Each ventricle pumps the same volume of blood per beat and the overall relationships are the same for both chambers

One cardiac cycle involves the following phases:   Blood enters the atria; the bicuspid and tricuspid valves open when atrial pressure exceeds ventricular pressure. This resting period is called diastole (atrial filling) Then the 2 atria contract simultaneously (atrial systole) causing the blood to be pushed into the ventricles Almost immediately the ventricles contract (ventricular systole); this increases the pressure in the ventricle which closes the tricuspid and bicuspid valves and opens the semilunar valves, pushing the blood into the aorta and pulmonary artery. The atria relax. When the ventricles relax (ventricular diastole), some of the blood in the aorta and pulmonary artery will try to flow back and will close the semilunar valves

The sound that the heart makes is usually considered to be a “lub-dup” sound. The “lub” is caused by the closing of the bicuspid and tricuspid valves The “dup” is caused by the closing of the semilunar valves in the arteries Therefore: Ventricular systole = “lub” Ventricular diastole = “dup”

Systole: Contraction of the heart muscle Diastole: Relaxation of heart muscle A). First the atria go through systole and diastole together B). Then the ventricles go through systole and diastole together

H5.2 Analyze data showing pressure and volume changes in the left atrium, left ventricle and the aorta, during the cardiac cycle In the data below, you can see that the volume and pressure inside the chambers of the heart changes as it goes through the cardiac cycle

The atria fill up with blood during ventricular systole and diastole The atria fill up with blood during ventricular systole and diastole. As they fill up, both volume and pressure increase. When atrial systole starts, the volume of the atria decreases and the pressure goes up. After the blood flows into the ventricle, the atria relax and the volume increases while the pressure decreases. All these changes are small compared to the changes which occur in the ventricles and the changes in the left ventricle are much greater than in the right ventricle

The left ventricle quickly fills with blood and then ventricular systole starts. The pressure in the ventricle increases in 0.1s. The pressure remains high for the next 0.2s and the volume of the ventricle goes from 100% to almost 0%. During diastole, the ventricle also fills up with blood so its volume increases. The pressure remains low until atrial systole forces blood into the ventricle which also sharply increases the volume of the ventricles

H5.3 Outline the mechanisms that control the heartbeat including the SA (sinoatrial) node, AV (atrioventricular) node and conducting fibers in the ventricular walls The ability of the heart muscle to depolarize and contract is not dependent on the nervous system The heart contains specialized autogenic (self-depolarizing) cells which can begin depolarization without neural activation

The SA node is a group of autogenic cells located in the wall of the right atrium just below the entrance of the superior vena cava The SA node acts as the pacemaker of the heart by producing spontaneous depolarization at a faster rate than other autogenic cardiac cells (generates impulses about 75-100 times/minute) Its characteristic rhythm, called sinus rhythm, determines heart rate From the SA node, the depolarization wave spreads by way of gap junctions throughout the atria via the internodal pathway to the AV node

The AV node is located in the inferior portion of the right atrium just above the tricuspid valve The AV node conducts impulses more slowly because the fibers are of a smaller diameter and there are fewer gap junctions here thus allowing the atria to respond and contract fully before the ventricles contract From the AV node the impulse sweeps to the bundle of His (also referred to the AV bundle) in the inferior part of the interatrial septum

The bundle of His splits into two pathways, the right and left bundle branches which run along the interventricular septum toward the apex of the heart The Purkinje fibers complete the pathway through the interventricular septum and penetrate into the apex of the heart and turn superiorly to stimulate the myocardial cells of the left and right ventricles, causing their almost simultaneous contraction

H5.4 Outline atherosclerosis and the causes of coronary thrombosis Atherosclerosis is the result of an accumulation of fatty materials within the walls of the arteries causing blood flow to be reduced   The lumen of the artery may be further reduced in size by a clot (thrombus) that forms as a result of the atherosclerosis; in severe cases, the artery may become completely blocked

The most serious consequences to the heart are hypertensive heart disease and coronary artery occlusion which in turn, increase the risk of heart attack (coronary thrombosis) and stroke

H5.5 Discuss factors which affect the occurrence of coronary heart disease Atherosclerosis, as well as other conditions, can lead to coronary heart disease.   The following risk factors can lead to coronary heart disease: Genetic predisposition (having parents who have experienced a heart attack) Smoking Hypertension High blood cholesterol levels Aging; 10% of individuals with CHD are over 65 Diets high in saturated fat

Being male (higher risk than being female) Lack of exercise Obesity Race – seen in more ethnic groups than others Stress and diabetes