Cardiac Cycle: Diastole and Systole

Cardiac Cycle

Diastole

• Period of cardiac cycle when heart muscle relaxes and chambers fill with blood
• Lasts approximately 2/3 of the cardiac cycle
• Atria and ventricles relax, and atrioventricular valves open
• Blood flows from atria to ventricles through open valves
• Ventricular pressure decreases, and volume increases

Systole

• Period of cardiac cycle when heart muscle contracts and pumps blood out of the heart
• Lasts approximately 1/3 of the cardiac cycle
• Atria and ventricles contract, and atrioventricular valves close
• Blood is pumped out of the heart through semilunar valves
• Ventricular pressure increases, and volume decreases

Heart Rate Regulation

• Heart rate regulated by autonomic nervous system (ANS)
• Sympathetic nervous system (SNS) increases heart rate
• Parasympathetic nervous system (PNS) decreases heart rate
• Baroreceptors in blood vessels detect changes in blood pressure and send signals to the brain
• Brain sends signals to the heart to adjust heart rate accordingly

Cardiac Output

• Amount of blood pumped by the heart per minute
• Calculated by multiplying heart rate and stroke volume
• Factors affecting cardiac output:
  • Heart rate
  • Stroke volume
  • Preload (venous return)
  • Afterload (blood pressure)
  • Contractility (heart muscle strength)

Electrical Conduction

• Electrical signals control the heartbeat
• Sinoatrial (SA) node: natural pacemaker, generates electrical impulses at a rate of 60-100 bpm
• Atrioventricular (AV) node: relays impulses from SA node to ventricles
• Bundle of His: group of specialized fibers that carry impulses from AV node to ventricles
• Purkinje fibers: branch off from bundle of His, stimulate ventricular contraction

Cardiac Cycle

Diastole

• Heart muscle relaxes, and chambers fill with blood during diastole
• Diastole lasts approximately 2/3 of the cardiac cycle
• Atria and ventricles relax, and atrioventricular valves open, allowing blood to flow from atria to ventricles
• Ventricular pressure decreases, and volume increases during diastole

Systole

• Heart muscle contracts, and pumps blood out of the heart during systole
• Systole lasts approximately 1/3 of the cardiac cycle
• Atria and ventricles contract, and atrioventricular valves close, pumping blood out of the heart through semilunar valves
• Ventricular pressure increases, and volume decreases during systole

Heart Rate Regulation

• Autonomic nervous system (ANS) regulates heart rate
• Sympathetic nervous system (SNS) increases heart rate
• Parasympathetic nervous system (PNS) decreases heart rate
• Baroreceptors in blood vessels detect changes in blood pressure, sending signals to the brain to adjust heart rate accordingly

Cardiac Output

• Cardiac output is the amount of blood pumped by the heart per minute
• Calculated by multiplying heart rate and stroke volume
• Factors affecting cardiac output include heart rate, stroke volume, preload, afterload, and contractility

Electrical Conduction

• Electrical signals control the heartbeat
• Sinoatrial (SA) node generates electrical impulses at a rate of 60-100 bpm
• Atrioventricular (AV) node relays impulses from SA node to ventricles
• Bundle of His and Purkinje fibers carry impulses from AV node to ventricles, stimulating ventricular contraction

Cardiac Cycle

Atrial Contraction

• Atrial contraction lasts for approximately 0.1 seconds
• Triggered by the sinoatrial (SA) node, which depolarizes the atria
• Atria contract, pumping blood into the ventricles, contributing to 20-30% of total ventricular filling

Ventricular Contraction

• Ventricular contraction lasts for approximately 0.3 seconds
• Triggered by the atrioventricular (AV) node, which depolarizes the ventricles
• Ventricles contract, pumping blood out of the heart and into the circulatory system
• Responsible for the majority of cardiac output

Cardiac Output Regulation

• Regulated by the autonomic nervous system (ANS) and hormone signals
• ANS regulates heart rate and contractility through sympathetic and parasympathetic nerves
• Hormones such as epinephrine and norepinephrine increase cardiac output during stress or exercise
• Baroreceptors in the aorta and carotid sinus help regulate blood pressure and cardiac output

Systole

• The period of ventricular contraction, lasting approximately 0.3 seconds
• Characterized by:
  • Increased blood pressure
  • Increased ventricular pressure
  • Closed atrioventricular valves
  • Open semilunar valves

Diastole

• The period of ventricular relaxation, lasting approximately 0.4 seconds
• Characterized by:
  • Decreased blood pressure
  • Decreased ventricular pressure
  • Open atrioventricular valves
  • Closed semilunar valves