Cardiac Cycle

The cardiac cycle encompasses the series of events that occur with each heartbeat. It consists of two primary phases: diastole and systole, which are commonly discussed in the context of blood pressure.

1. Diastole - Ventricles are relaxed.
2. Systole - Ventricles contract.

1.0Cardiac Cycle Systole and Diastole

• One complete rhythmic relaxation and contraction of all the parts of the heart which result in receiving and pumping of the blood.
• Contraction → Systole
• Relaxation → Diastole

2.0Cardiac Cycle Mechanism

• Initially, all four heart chambers are in a relaxed state, referred to as joint diastole. 
• During this phase, the tricuspid and bicuspid valves are open, allowing blood from the pulmonary veins and vena cava to flow into the left and right ventricles through the respective atria. 
• The semilunar valves remain closed.
• The sinoatrial node (SAN) generates an action potential that triggers a simultaneous contraction of both atria, known as atrial systole, which increases blood flow into the ventricles by approximately 30%. 
• The action potential is then conducted to the ventricles via the atrioventricular node (AVN) and the AV bundle, and transmitted through the bundle of His to the ventricular musculature, leading to ventricular systole. During this contraction, the atria relax (diastole), which coincides with the ventricular contraction.
• As the ventricles contract, the pressure within them rises, causing the tricuspid and bicuspid valves to close and preventing backflow into the atria. 
• The increased ventricular pressure also forces open the semilunar valves, allowing blood to flow from the ventricles into the pulmonary artery and aorta. 
• Following this, the ventricles enter the diastole and relax, leading to a decrease in pressure that causes the semilunar valves to close and prevent blood from flowing back into the ventricles. 
• As the pressure in the ventricles continues to drop, the tricuspid and bicuspid valves open again, allowing blood to flow from the atria into the ventricles, thus beginning the cycle once more.

Outer circle- Atria       Inner circle - Ventricles

• On average, an adult experiences approximately 72 cardiac cycles per minute.
• Duration of cardiac cycle = 60/72 = 0.8 sec.
• End Diastolic Volume (EDV) = 120 ml. 
• End Systolic Volume (ESV) = 50 ml.  
• Stroke Volume (SV) = EDV – ESV = 120 - 50 = 70 ml.  
• It is the amount of blood pumped by each ventricle in one cardiac cycle, generally 70 ml. It is also known as beat volume. 
• Cardiac Output = SV × Number of beats/min. = 70 ml × 72 beats /min. = 5 L/min at rest 
• The formula for calculating the time of cardiac cycle = 1 / heart beat per minute  
• It refers to the volume of blood ejected by each ventricle every minute, averaging about 5 liters (or 5040 mL) in a healthy individual. The end-diastolic volume is the amount of blood present in each ventricle at the conclusion of the ventricular diastole, while the end-systolic volume is the amount remaining in each ventricle at the end of the ventricular systole.
• Blood enters into the heart because the muscles of the atria relax.  
• Blood leaves out of the heart because the muscles of the ventricles contract. 
• The body can adjust stroke volume, heart rate, and consequently cardiac output. For instance, an athlete typically has a much higher cardiac output compared to an average person.

Heart Beat 

• Rhythmic pulsation of the musculature of the heart is called heartbeat.  
• Human Adults: 72 
• New Born Baby : >120

Pulse 

• Pulse is tactile arterial palpation that occurs due to the cardiac cycle. 
• Pulse is easily felt in the wrist's radial artery and the neck's carotid artery. 
• Number of pulses = Number of cardiac cycles.

Heart Sounds 

• Throughout each cardiac cycle, a stethoscope can detect two distinct and easily audible sounds. 
• The first heart sound, known as 'Lub,' occurs with the closure of the tricuspid and bicuspid valves. The second heart sound, 'Dub,' corresponds to the closure of the semilunar valves. 
• These sounds hold significant diagnostic value in clinical assessments.

3.0ECG and Cardiac events 

The major complexes in an ECG are as follows:

1. P Wave: Represents atrial depolarization lead to atrial systole.

2. QRS Complex: Reflects the initial ventricular depolarization leading to ventricular systole.

3. T Wave: Indicates the final phase of ventricular repolarization leads to end of ventricular systole.

4. QRS Complex: Represents the complete ventricular complex.

These components collectively provide insights into the heart's electrical activity.

4.0Regulation of Cardiac Activity

• The heart's normal functions are primarily regulated internally by specialized muscle tissue known as nodal tissue, making the heart myogenic. 
• Additionally, a neural center in the medulla oblongata can adjust cardiac function through the autonomic nervous system (ANS). 
• Sympathetic nerve signals, a part of the ANS, can enhance heart rate, increase the strength of ventricular contractions, and boost cardiac output. 
• Conversely, parasympathetic nerve signals, another component of the ANS, can slow the heart rate, reduce the speed of action potential conduction, and decrease cardiac output. 
• Hormones from the adrenal medulla can also contribute to an increase in cardiac output.