Cardiac Physiology Notes: Conduction, Valves, and Cardiac Cycle
Pacemaker/Conduction system
Conductive (pacemaker) cells are the ones that can initiate their action potentials on their own.
They start at approximately .
They slowly have sodium leaking into them all the time; as the leak continues, the charge depolarizes (sodium coming into the cell) until threshold is reached.
Depolarization is followed by repolarization (potassium leaving the cell), which brings the charge back down again. The speaker says “catastrophe leaving the cell rapidly,” which is the rapid repolarization phase.
If there is nothing to trigger the signal, the impulse can still propagate down into the ventricles and cause depolarization there, leading to ventricular contraction.
The impulse moves rapidly through all the ventricular cells, telling them to depolarize and contract.
Lab prep and study habits
A worksheet will be provided this week with a lot of content for the lab.
Students should keep an untouched copy of the worksheet and practice filling it in by themselves with no notes before the exam.
Practicing with no notes is emphasized as beneficial for performance on the exam.
Valves, mechanics, and disease implications
The discussion focuses on the aortic valve and the AV valves (mitral and tricuspid). Scar tissue around these valves can form and restrict their movement.
Analogy: a door with rusty hinges — if hinges are rusty, opening the door requires more force and may not open fully.
When valves don’t move easily, less blood can enter the chamber and the heart must generate higher pressure to open the valve.
If the heart has to work too hard without adequate recovery, tissues can start to degrade; this can lead to hypertrophy of the heart muscle (hypertrophic cardiomyopathy).
If the ventricle contracts but cannot open a valve properly, some blood can be pushed back into the atria (backflow).
This scenario contributes to higher workload and potential progression toward heart failure.
The speaker notes that these issues predominantly affect older patients.
Aortic stenosis is given as an example: more work is required to open the aorta, especially on the left side of the heart.
The left ventricle may become weaker if it has to work harder to overcome stenosis, compared with the right side.
If right side pressure and output remain higher than left’s, blood will back up through the left heart and into the systemic circulation, eventually affecting capillaries and causing fluid buildup.
The cardiac cycle: ventricle mechanics and sounds
When the ventricles contract, the AV valves are essentially closed (the openings are squeezed shut).
Ventricles initially pressurize with all valves closed (isovolumetric contraction).
As pressure rises, the semilunar valves (aortic and pulmonary) open, allowing blood to be ejected into the aorta and pulmonary trunk.
Once the valves open and blood is ejected, the cork-like opening behavior of the ventricle is described with the turkey baster analogy: squeezing causes no suction until the valve opens; when opened, blood is drawn out (ejection).
After ejection, semilunar valves close and the ventricle begins relaxation (isovolumetric relaxation) for a brief period until ventricular pressure falls enough for AV valves to open again and filling to resume.
The student is prompted to recall heart sounds in relation to valve events during the cardiac cycle.
Heart sounds and valve events
Heart sound #1 (S1, often called the lubb): caused by the closing of the AV valves at the start of ventricular systole.
The instructor refers to it in a humorous way as the sound of the AV valves closing during ventricular contraction.
The timing of S1 overlaps with the beginning of isovolumetric contraction when AV valves close but semilunar valves have not yet opened.
Heart sounds in the notes reference a sequence with terms like “sound one” and “sound three”; the canonical mapping (in standard physiology) is:
S1: AV valve closure (mitral and tricuspid) at the start of ventricular systole.
S2: Semilunar valve closure (aortic and pulmonary) at the end of ventricular systole.
S3/S4: Additional heart sounds that may appear under certain conditions; not elaborated in this transcript.
The lecture uses a narrative approach to relate valve events to sounds and to the phases of the cardiac cycle (isovolumetric contraction, ejection, isovolumetric relaxation, and ventricular filling).
Ventricular filling, atrial contribution, and valve status during phases
Ventricular filling occurs during diastole and is aided by atrial systole (atrial contraction) to push the last bit of blood into the ventricles.
During ventricular filling, AV valves are open, allowing blood to flow from the atria to the ventricles.
The speaker asks about valve status during various phases (e.g., “Are the AV valves open or closed during isovolumetric contraction?”) to emphasize that during isovolumetric contraction all valves are closed.
When the ventricles are relaxing, the ventricles open up to allow filling as blood returns from the atria; this relaxation phase is when suction-like effects can occur (turkey baster analogy).
Stroke volume, end-systolic volume, and cardiac output
Stroke Volume (SV): the amount of blood ejected by the ventricle with one heartbeat.
A key relationship described: the amount ejected from the left ventricle must equal the amount ejected from the right ventricle (to maintain circulatory balance).
The speaker provides a value: the average stroke volume is roughly (as stated in the lecture; this value is notably smaller than typical physiologic numbers and may be context-specific for teaching).
End-Systolic Volume (ESV): the amount of blood remaining in the ventricle after systole; the speaker refers to it as the “ending systolic volume.”
End-Diastolic Volume (EDV): the amount of blood in the ventricle at the end of diastole, just before contraction.
Relationship:
If stroke volume can be increased (e.g., with stronger contraction and better valve opening), the heart’s circulation improves; the speaker notes that the relationship between EDV, ESV, and SV determines pumping efficiency.
Cardiac Output (CO): the volume of blood pumped by the heart per unit time. The speaker previews that CO will be covered in the next class and warns not to confuse it with SV.
Canonical relationship (for reference): where HR is heart rate.
Left vs right heart: pathophysiology and clinical implications
The left ventricle (LV) is responsible for pumping blood into the aorta; the right ventricle (RV) pumps into the pulmonary trunk.
If the left side develops a stenosis or becomes weaker, the LV must work harder to generate the pressure needed to push blood into the aorta.
Over time, left-sided stress can lead to hypertrophy and potential failure; the right side may initially compensate by maintaining its stroke volume, but unequal ejection (e.g., LV around 50 mL vs RV around 70 mL) leads to a backlog.
When the left heart cannot keep pace with the right, blood backs up into the pulmonary circuit, whereas right-sided failure can cause systemic venous congestion.
This imbalance can cause fluid buildup in capillaries, leading to edema and other complications.
The discussion frames congestive heart failure as a consequence of unequal ventricular performance and valve disease.
Practical and exam-oriented notes
Expect questions on: pacemaker cell action potentials, resting and threshold potentials, and the direction of ion flow during depolarization and repolarization.
Be able to explain the valvular disease analogies (rusty hinges vs. healthy hinges) and their hemodynamic consequences.
Understand how pressure generation relates to valve opening/closing, and how hypertrophy arises from chronic high afterload (e.g., aortic stenosis).
Be comfortable with the cardiac cycle phases: diastole (filling) and systole (ejection), including isovolumetric contraction/relaxation when all valves are closed.
Know the standard heart sounds and their anatomical correlates (S1 = AV valve closure; S2 = semilunar valve closure; S3/S4 as extra sounds in certain conditions).
Master the core formulas:
Stroke volume:
Cardiac output (preview): .
Remember the clinical flow: pressure must be adequate to overcome afterload; insufficient recovery time can lead to tissue fatigue and heart failure.
Conceptual takeaway: the heart is a double pump that must operate in balance; disease in the left or right side disrupts this balance and leads to systemic issues.
connections to broader physiology and real-world relevance
The conduction-delivery of impulse sets the rhythm and coordinate timing of atrial and ventricular contractions; dysfunction can cause arrhythmias or inefficient pumping.
Valve integrity is essential for unidirectional flow; scar tissue and stenosis illustrate how structural changes translate to functional limitations and higher afterload.
The interplay between EDV/ESV and SV impacts overall cardiac output and tissue perfusion, illustrating why both preload and afterload matter in clinical scenarios.
The discussion ties basic concepts to real-world disease states: hypertrophic cardiomyopathy, valvular stenosis, and congestive heart failure are common clinical conditions with clear hemodynamic explanations.