Describe the cardiac cycle, including its main phases and key events.

7. (a) (i) The cardiac cycle describes the sequence of events that occurs during one complete heartbeat. It consists of three main phases: 1. Atrial Systole (Atrial Contraction): • The sino-atrial (SA) node initiates an electrical impulse, causing the atria to contract. • This pushes the remaining blood from the atria into the ventricles, which are already mostly filled. • The atrioventricular (AV) valves (tricuspid and bicuspid) are open, and the semilunar valves are closed. 2. Ventricular Systole (Ventricular Contraction): • The impulse passes through the atrioventricular (AV) node and Purkinje fibers, causing the ventricles to contract. • As ventricular pressure rises, the AV valves close, preventing backflow into the atria (producing the "lub" sound). • When ventricular pressure exceeds arterial pressure, the semilunar valves (aortic and pulmonary) open, and blood is ejected into the aorta and pulmonary artery. 3. Diastole (Relaxation Phase): • The ventricles relax, causing ventricular pressure to drop. • The semilunar valves close, preventing backflow from the arteries into the ventricles (producing the "dub" sound). • As ventricular pressure falls below atrial pressure, the AV valves open, and blood passively flows from the atria into the ventricles, refilling them for the next cycle. The atria also relax and fill with blood during this phase. 7. (a) (ii) The sino-atrial (SA) node and atrio-ventricular (AV) node are crucial components of the heart's electrical conduction system. • The sino-atrial (SA) node is located in the wall of the right atrium, near the opening of the superior vena cava. It is the natural pacemaker of the heart, initiating the electrical impulses that cause atrial contraction. • The atrio-ventricular (AV) node is located in the interatrial septum, near the junction of the atria and ventricles. It receives the impulse from the SA node, delays it briefly, and then transmits it to the ventricles via the Bundle of His and Purkinje fibers. (A labelled diagram of the heart would typically show the SA node in the upper right atrium and the AV node in the lower part of the interatrial septum, just above the ventricles.) 7. (b) During exercise, the body's demand for oxygen and nutrients increases, requiring a higher heart rate and stronger contractions. Both the nervous and hormonal systems coordinate this response: Nervous System Coordination: • Sensory input: During exercise, chemoreceptors in the carotid arteries and aorta detect changes in blood chemistry (e.g., increased CO_2, decreased O_2, decreased pH). Proprioceptors in muscles and joints detect movement. • Cardiac Control Centre: These signals are sent to the cardiac control centre in the medulla oblongata of the brain. • Sympathetic Activation: The medulla oblongata increases the activity of the sympathetic nervous system and decreases the activity of the parasympathetic nervous system. • Neurotransmitter Release: Sympathetic nerves release norepinephrine (noradrenaline) directly onto the SA node and ventricular muscle cells. Norepinephrine binds to receptors, increasing the rate of depolarization of the SA node, thus increasing the heart rate. It also increases the force of ventricular contraction. Hormonal System Coordination: • Adrenal Gland Stimulation: The sympathetic nervous system also stimulates the adrenal medulla (part of the adrenal gland). • Hormone Release: The adrenal medulla releases adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream. • Systemic Effects: These hormones travel through the blood to the heart. They bind to the same receptors on the SA node and cardiac muscle cells as the sympathetic neurotransmitters. • Increased Heart Rate and Force: Adrenaline and noradrenaline further increase the firing rate of the SA node, leading to a sustained increase in heart rate, and enhance the force of myocardial contraction, ensuring more blood is pumped per beat. 7. (c) Hypertension (high blood pressure) is a significant health concern, with rising cases in urban areas often linked to lifestyle factors. 1. Lifestyle Factor: High-sodium diet • Explanation: Urban diets often include a high intake of processed foods, fast food, and restaurant meals, which are typically rich in sodium. High sodium intake leads to increased water retention in the body. The kidneys respond by reabsorbing more water to dilute the excess sodium, which increases the total blood volume. • Effect on Cardiac Cycle: An increased blood volume means the heart has to pump a larger quantity of blood. This increases the workload on the ventricles, particularly the left ventricle, as it must generate greater pressure to eject blood into the systemic circulation. Over time, this sustained higher pressure and increased workload can lead to the thickening of the ventricular walls (ventricular hypertrophy) and reduced elasticity of blood vessels, making the heart less efficient and contributing to chronic hypertension. 2. Lifestyle Factor: Lack of physical activity (Sedentary lifestyle) • Explanation: Urban lifestyles often involve less physical exertion due to desk jobs, reliance on vehicles, and limited opportunities for active recreation. A sedentary lifestyle contributes to weight gain (obesity), reduced cardiovascular fitness, and can lead to insulin resistance and dyslipidemia. • Effect on Cardiac Cycle: Lack of regular exercise weakens the heart muscle and reduces the elasticity of blood vessels. Obesity, often associated with inactivity, increases the total peripheral resistance in the circulatory system, meaning the heart has to pump against greater resistance to circulate blood. This increased resistance forces the heart to work harder and generate higher pressure during ventricular systole. Over time, this constant strain can lead to a less efficient cardiac cycle, with the heart struggling to maintain adequate blood flow, and can result in sustained high blood pressure. 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