Cardiovascular System: The Heart as a Pump

Cardiovascular System - The Heart as a Pump

• Curriculum: Phase 1/Semester 2/CVS/Session 2/L 2 (2018/2019)
• Lecturer: Dr. Shahlaa Kh. Chabuk
• Qualifications: MSc, PhD, Physiology
• Institution: Hammurabi Medical College/Babylon University

Cardiac Cycle Valve Function

• Describe when each heart valve opens and closes during the cardiac cycle.
• Explain the flow pattern through each valve.

Heart Sounds

• Explain the origin of the first and second heart sounds.

Pressure Profile and Tasks

• Analyze a diagram illustrating pressure profiles in the left atrium, left ventricle, and aorta during a single cardiac cycle in a healthy adult.
• Label the pressure axes.
• Label the time base (assuming a heart rate of 60 bpm).
• Indicate valve opening and closing points (mitral and aortic valves).
• Mark the position of the first and second heart sounds on the diagram.

The Heart

• The heart is composed of two pumps in series.
• Each side comprises a thin-walled atrium and a muscular ventricle.
• Blood flows into and out of ventricles via valves.
• Atrioventricular valves (mitral and tricuspid) regulate blood flow.
• Outflow valves (aortic and pulmonary) control blood flow.

Heart Muscle

• Heart muscle is a specialized form.
• Heart muscle cells are electrically connected.
• These cells contract when an action potential occurs within the membrane.
• Action potentials cause a rise in intracellular calcium.
• Action potentials are long duration, causing a single contraction (systole), lasting approximately 280 milliseconds.
• Action potentials spread from cell to cell.

Pacemakers

• Action potentials originate in a specialized group of cells (pacemakers).
• These signals spread throughout the heart, coordinating contractions.
• Pacemakers generate one action potential at regular intervals.

Phases of the Cardiac Cycle

• Each action potential triggers one heart beat (systole).
• The interval between beats is called diastole.

Spread of Excitation - 1

• The pacemaker (sino-atrial node) initiates the heartbeat in the right atrium.
• Excitation spreads across the atria, leading to atrial systole.
• The signal reaches the atrioventricular node, where it is delayed for about 120 milliseconds.

Spread of Excitation - 2

• The signal then spreads through the ventricular myocardium, from inner (endocardial) to outer (epicardial) surfaces.
• Ventricular contraction begins at the apex and forces blood towards the outflow valves.

The Cardiac Cycle

• At rest, the sinoatrial (SA) node generates an action potential once per second, producing one heartbeat.
• A short atrial systole follows, then a longer ventricular systole.
• Ventricular systole lasts about 280 milliseconds.
• Ventricular relaxation (diastole) lasts about 700 milliseconds before the next systole.

Ventricular Pumping

• The alternating systole and diastole, together with inflow and outflow valves, creates a reciprocating pumping mechanism for the heart.
• Blood fills the ventricles from the veins during diastole.
• During systole, ventricles pump blood into arteries.

The Left Ventricle

• The aortic valve is the outflow valve.
• It allows blood to flow from the ventricle to the aorta but not vice-versa.
• The valve opens when intraventricular pressure exceeds aortic pressure.
• The valve closes when aortic pressure exceeds ventricular pressure.

Cardiac Cycle, Graphs and Charts

• Diagrams and graphs of pressure/volume dynamics are given and must be analyzed.

Ventricular Filling

• During systole, blood gathers in the atria.
• At the end of systole, high atrial pressure opens the atrioventricular (AV) valves, causing rapid ventricular filling.
• This process lasts approximately 200-300 milliseconds.
• Minimal flow occurs during the middle third of ventricular filling.
• Atrial contraction contributes up to 20% of ventricular filling volume.

Isovolumic Contraction

• At the start of systole, intraventricular pressure rises, closing the AV valves.
• For ~20-30 ms, pressure increases but not enough to open the semilunar valves.
• This is isovolumetric contraction because ventricular volume remains unchanged.

Ejection Period

• Once the semilunar valves open, the ejection phase begins.
• Approximately 70% of blood ejection occurs in the first third of the ejection period.
• The remaining 30% is ejected during the next two-thirds, in the slow ejection period.

Isovolumic Relaxation

• During late systole, ventricular relaxation reduces intraventricular pressure.
• Once this pressure is lower than the aortic pressure, the semilunar valves close.
• The AV valves remain closed, preventing filling, for approximately 30-60 ms.
• This period is called isovolumic relaxation.

Normal Volume of Blood in Ventricles

• After atrial contraction, ventricles hold approximately 110-120 ml of blood (end-diastolic volume).
• Ventricular contraction ejects about 70 ml of blood (stroke volume).
• 40-50 ml of blood remains in each ventricle (end-systolic volume).

Preload and Afterload

• Preload is the tension on the heart muscle at the start of systole, related to end-diastolic volume/pressure.
• Afterload is the tension the heart muscle works against during systole.

Cardiac Output and Venous Return

• Cardiac output is the blood pumped into the aorta per minute (stroke volume x heart rate).
• Venous return is the blood flow from veins to the right atrium.
• Cardiac output typically equals venous return, except during transient periods.

Normal Cardiac Output

• Normal resting cardiac output is about 5 liters/minute.
• Stroke volume is approximately 70 ml.
• Heart rate is around 72 beats per minute.
• Cardiac output during exercise can increase to over 20 liters/minute.

Cardiac Output Calculation

• Stroke Volume (SV) = End Diastolic Volume (EDV) - End Systolic Volume (ESV)
• Cardiac Output (CO) = SV × Heart Rate (HR)

Factors Affecting Stroke Volume

• End-diastolic volume (EDV) is affected by venous return and preload.
• End-systolic volume (ESV) is affected by contractility and afterload.

Heart Sounds

• Two main sounds are associated with valve closures.
• The first sound ("lub") is the closure of the atrioventricular valves.
• The second sound ("dub") is the closure of the semilunar valves.
• These sounds occur at ventricular systole onset and end.
• Normal interval from first to second sound is about 280 milliseconds.
• The interval from the second sound to the next first sound is approximately 700 milliseconds.

Extra Heart Sounds

• Occasionally, extra sounds are heard; a third sound is heard during early diastole, and a fourth sound during atrial systole.

Heart Murmurs

• Turbulent blood flow produces murmurs.
• Murmurs occur due to narrowed valves (stenosis) or valves not closing properly (incompetence)
• Murmurs occur when blood flow is highest, so their presence can pinpoint their origin in the cardiac cycle.