Chapter 19 - Section 19.5 - Study Guide - Assess Your Learning Outcomes - Page 740: 4

The **cardiac cycle** refers to the sequence of events that occur during one complete heartbeat, from the beginning of one heartbeat to the beginning of the next. It includes all the mechanical and electrical events that take place as the heart contracts and relaxes to pump blood through the circulatory system. The cardiac cycle consists of four main phases: 1. **Atrial Systole (Atrial Contraction):** - This phase marks the contraction of the atria. - It occurs after the P wave on the electrocardiogram (ECG). - During atrial systole, the atria contract, pushing blood into the ventricles to complete ventricular filling. 2. **Ventricular Systole (Ventricular Contraction):** - This phase corresponds to the contraction of the ventricles. - It begins after the QRS complex on the ECG. - During ventricular systole, the pressure in the ventricles rises, causing the atrioventricular (AV) valves (tricuspid and mitral valves) to close to prevent backflow. The semilunar valves (pulmonary and aortic valves) open, allowing blood to be ejected into the pulmonary artery and aorta. 3. **Isovolumetric Relaxation:** - This phase occurs immediately after ventricular systole. - Both sets of heart valves are closed (AV and semilunar), and the ventricles are momentarily not contracting. - The ventricles start to relax, and blood from the atria begins to flow into the ventricles, preparing for the next cardiac cycle. 4. **Ventricular Diastole (Ventricular Relaxation):** - This phase represents the relaxation of the ventricles. - The ventricles are in diastole, allowing them to fill with blood. - The atria are also in diastole, and blood from the veins continues to flow into the atria, providing the blood that will be pumped into the ventricles during the next cardiac cycle. These phases together make up the cardiac cycle, and they ensure the efficient and coordinated pumping of blood through the heart and into the circulatory system. The cardiac cycle is a continuous process that repeats with each heartbeat, maintaining blood circulation to meet the body's oxygen and nutrient needs.

The **cardiac cycle** refers to the sequence of events that occur during one complete heartbeat, from the beginning of one heartbeat to the beginning of the next. It includes all the mechanical and electrical events that take place as the heart contracts and relaxes to pump blood through the circulatory system. The cardiac cycle consists of four main phases: 1. **Atrial Systole (Atrial Contraction):** - This phase marks the contraction of the atria. - It occurs after the P wave on the electrocardiogram (ECG). - During atrial systole, the atria contract, pushing blood into the ventricles to complete ventricular filling. 2. **Ventricular Systole (Ventricular Contraction):** - This phase corresponds to the contraction of the ventricles. - It begins after the QRS complex on the ECG. - During ventricular systole, the pressure in the ventricles rises, causing the atrioventricular (AV) valves (tricuspid and mitral valves) to close to prevent backflow. The semilunar valves (pulmonary and aortic valves) open, allowing blood to be ejected into the pulmonary artery and aorta. 3. **Isovolumetric Relaxation:** - This phase occurs immediately after ventricular systole. - Both sets of heart valves are closed (AV and semilunar), and the ventricles are momentarily not contracting. - The ventricles start to relax, and blood from the atria begins to flow into the ventricles, preparing for the next cardiac cycle. 4. **Ventricular Diastole (Ventricular Relaxation):** - This phase represents the relaxation of the ventricles. - The ventricles are in diastole, allowing them to fill with blood. - The atria are also in diastole, and blood from the veins continues to flow into the atria, providing the blood that will be pumped into the ventricles during the next cardiac cycle. These phases together make up the cardiac cycle, and they ensure the efficient and coordinated pumping of blood through the heart and into the circulatory system. The cardiac cycle is a continuous process that repeats with each heartbeat, maintaining blood circulation to meet the body's oxygen and nutrient needs.