OPTION H: Further human physiology The Cardiac Cycle

The cardiac cycle Repeating sequence of actions in the heart which result in the pumping of blood to the lungs and all other parts of the body. Cardiologists refer to contraction of the heart’s chambers as systole and relaxation as diastole. There are three sequences of contraction:

Atrial systole:

Ventricular systole: the ventricles contract and blood is forced out into the arteries. The bicuspid and tricuspid (atrioventricular) valves prevent blood from flowing back into the atria. the atria relax and start to fill with blood again.

Diastole: both atria and ventricles are relaxing and the heart fills up with blood from the veins. Semilunar valves are closed and prevent blood from passing back into the ventricles from the arteries.

The cardiac cycle includes all the events from the beginning of one heartbeat to the beginning of the next. ION_(OXFORD)/original/Page-343.html (Pearson pg 620)

To start the cycle, the sinoatrial node fires an electrical signal throughout the walls of the atria. This causes the atria to undergo systole. ION_(OXFORD)/original/Page-343.html

The signal then reaches the atrioventricular node, from which the signal spreads throughout the heart via specialized heart muscle tissue called Purkinje fibers. This causes the ventricles to undergo systole. ION_(OXFORD)/original/Page-343.html

After the ventricles are emptied, the semilunar valves close. The ventricles begin diastole, the atrioventricular valves open and ventricles start filling with blood. When the atria are filled and the ventricles are 70 per cent filled, the cycle has ended ION_(OXFORD)/original/Page-343.html

Artherosclerosis and heart attacks.- Coronary heart disease (CHD) refers to damage to the heart as a consequence of reduced blood supply.

Atherosclerosis is hardening of the arteries caused by the formation of plaques, or atheromas, on the inner lining of arteries. Plaques are areas that are swollen and accumulate a diversity of debris composed of lipids, cholesterol, cell debris and calcium. iMc6acnSgqJw9twYVQLY1w__4Y09cswGlhgh5Dgy

The plaques can reduce the speed at which blood moves through vessels. This can trigger a clot, or thrombosis, which can block the blood flow through the artery and deny the tissue access to oxygen.

There are three major coronary arteries which supply the heart muscle with oxygen rich blood. These arteries carry blood that has recently been to the lungs Cardiac muscle is highly oxygen demanding. If one off the three major coronary arteries or their branches is blocked, some portion of the heart muscle is likely to be deprived of its oxygen. Coronary thrombosis occurs when a coronary artery or one of its main branches become blocked. This is also known as a heart attack.

Symptoms of coronary thrombosis include: Pain in the chest area, often radiating out towards the left arm. Constricting sensation in or around the throat. Breathing difficulties Severe dizziness, sometimes fainting. a / grunge-image-of-a-man-having-a-chest-pain-or-heart- attack.jpg

Risk factors affecting coronary heart disease: Old age leads to less flexible blood vessels Heredity, having parents who have experienced heart attacks indicates a genetic precondition Race, some ethnic groups have far higher rates of CHD than others Gender, risk in females increases post- menopause correlated with a fall in estrogen levels; males are at greater risk compared with females correlated with lower levels of estrogen. Hypertension/high blood pressure causes the heart to work harder. rt_disease_s6_lifestyle_risk.jpg

Risk factors affecting coronary heart disease: Smoking raises blood pressure because nicotine causes vasoconstriction High salt diet, excessive amounts of alcohol and stress are also correlated with CHD Eating too much saturated fat and cholesterol promotes plaque formation Obesity strains the heart Sedentary lifestyle, lack of exercise is correlated with obesity. Diabetes when it is not well managed. content/uploads/2011/05/post-28-risk-factors-of-heart- disease.jpg

Sources Allot, A., & Mindorff, D. (2010). IB Biology Course Companion. New York: Oxford University Press. Damon, A., McGonegal, R., Tosto, P., & Ward, W. (2007). Biology Higher Level. London: Pearson Baccalaureate.

Questions: Answer exercises 12, 13, and 14 from the Pearson IB Biology HL book pg 627.