CARDIOV L3

Cardiac Cycle and Heart Anatomy: Lecture Notes

  • Context and teaching approach

    • The instructor has observed a drop in student performance when lecture recordings are released, noting many students use recordings as the first exposure rather than a revision tool.

    • Emphasis on attending live lectures for engagement and retention.

    • This lecture picks up from where the previous one left off, focusing on heart anatomy and the cardiac cycle, with an upcoming ECG-focused session by Doctor McBride.

  • Overview of the cardiac cycle

    • The cycle consists of sequential phases that repeat each heartbeat: ventricular filling (diastole), atrial systole, isovolumetric ventricular contraction (systole), ventricular ejection, and isovolumetric ventricular relaxation.

    • The left ventricle ejects blood into the aorta, supplying the arterial tree and systemic tissues.

    • Timing: the cycle lasts about one second in a resting heart, with isovolumetric phases lasting very short times (about 0.05 s each).

    • At a high level, phases are characterized by valve status (inlet/mitral and outlet/aortic valves), ventricular/arterial pressures, and ventricular volumes.

  • Key numerical references (typical values used in the lecture)

    • Systemic arterial pressure range (as a representative example): P<em>sys ≈120 mmHg,P</em>dia≈80 mmHgP<em>{sys} \,\approx 120\ \mathrm{mmHg},\quad P</em>{dia} \approx 80\ \mathrm{mmHg}

    • End-diastolic volume (EDV) of one ventricle: VED≈120 mLV_{ED} \approx 120\ \mathrm{mL}

    • End-systolic volume (ESV) of one ventricle: VES≈60 mLV_{ES} \approx 60\ \mathrm{mL}

    • Stroke volume (SV): SV=V<em>ED−V</em>ES≈60 mLSV = V<em>{ED} - V</em>{ES} \approx 60\ \mathrm{mL}

    • Cardiac cycle duration: ~T≈1 sT \approx 1\ \mathrm{s} per beat (variable with fitness and heart rate)

    • Isovolumetric phases duration: ~tiso≈0.05 st_{iso} \approx 0.05\ \mathrm{s} each (two per cycle)

    • Ventricle volume at end of filling: roughly 80% full just before atrial contraction

    • Blood volume in systemic veins: ~64%64\% of total blood volume (venous reservoir)

    • Typical RBC diameter: DRBC≈9 μmD_{RBC} \approx 9\ \mu\mathrm{m}

    • Blood donation volume per session: about 0.5 L0.5\ \mathrm{L} (half a liter)

    • Blood volume drop before hypovolemic shock risk: ~20%20\% of total blood volume

    • Approximate capillary diameter: ~the size of a red blood cell, i.e., around 9 μm9\ \mu\mathrm{m}, illustrating tight capillary passages

  • The phases in detail (ventricular cycle)

    • Ventricular filling (early diastole)

    • Arterial pressure in the systemic arteries is falling as blood flows downstream.

    • Atrial pressure remains low; the inlet valve (mitral) is open, the outlet valve (aortic) is closed.

    • The ventricle fills passively; by the end of this phase the ventricle is ~80% full.

    • Atrial systole follows (SA node depolarization) leading to a small rise in atrial pressure and a final top-up of the ventricle (adds ~20% of filling).

    • Atrial contraction has little immediate effect on arterial pressure because the arteries are not connected directly to the atria.

    • The SA node impulse triggers atrial contraction; there is a small delay before ventricular contraction begins (AV node delay ~100 ms), ensuring atrial filling precedes ventricular contraction.

    • Isovolumetric ventricular contraction (early systole)

    • Ventricular pressure rises rapidly; mitral valve closes as soon as ventricular pressure exceeds atrial pressure, causing the first heart sound (lubb).

    • Both inlet (mitral) and outlet (aortic) valves are closed, so ventricular volume remains constant during this phase (isovolumetric).

    • This phase lasts only briefly (the ~0.05 s mentioned for the isovolumetric interval).

    • Ventricular ejection (systole)

    • When ventricular pressure exceeds arterial pressure, the aortic valve opens and blood is ejected into the aorta.

    • Ventricular and arterial pressures become similar during most of this phase; ventricular pressure may be slightly higher to drive flow into the aorta.

    • Ventricular volume falls as blood is expelled; the rate of ejection changes over the course of the phase due to the myocardium’s shortening limit.

    • Pressure peaks early in this phase and then declines as contraction eases, even though ejection continues.

    • Isovolumetric ventricular relaxation (early diastole)

    • The ventricle relaxes and pressure falls; the aortic valve closes as the arterial pressure exceeds ventricular pressure, producing the second heart sound (dup).

    • The outflow valve closes before the pulmonary valve in the two ventricles, which can cause a slight split of the second heart sound depending on the relative pressures between left and right sides.

    • Both valves are closed again during this brief period, so ventricular volume remains constant (another isovolumetric interval).

    • Ventricular filling completes the cycle and the process begins again

    • As ventricular pressure falls below atrial pressure, the mitral valve opens and the ventricle begins filling again.

    • Blood returns from the venous system to the atria, increasing atrial pressure slightly; the cycle restarts with another ventricular filling phase.

  • The Wiggles diagram (graphical representation)

    • Also called a standard diagram for cardiac cycles; widely used in physiology to illustrate timing relationships.

    • Key features:

    • Time axis (horizontal) around ~1 s per cycle; one second per beat is a general reference but varies with individuals.

    • Phases labeled across the top: filling, atrial systole, isovolumetric contraction, ejection, isovolumetric relaxation, etc.

    • Pressure traces (left ventricle/arteries) from 0 to ~120 mmHg120\ \mathrm{mmHg} on the arterial side.

    • States of valves (mitral/aortic) and heart sounds (lubb and dup).

    • Volume traces for the left ventricle showing EDV (~120 mL) and the minimal volume during ejection (~60 mL).

    • Historical note: Diagram originated about a century ago by Doctor Carl Wiggles; not a new concept.

    • Practical use: Helps memorize the sequence and the relationships between pressure, volume, and valve states.

  • Blood pressure: measurement and interpretation

    • What is measured: pressure in systemic arteries (brachial artery in the arm is typical for cuff measurements).

    • Method: cuff inflation and auscultation (stethoscope) or electronic devices detect flow and turbulence to determine systolic and diastolic pressures.

    • Typical values:

    • Common healthy range around 120/80 mmHg120/80\ \text{mmHg}, sometimes cited as ~115/75 mmHg115/75\ \mathrm{mmHg} depending on age and physiology.

    • Why this matters: mean arterial pressure (MAP) is driven by this arterial pressure range and is a key determinant of tissue perfusion.

    • The cuff method captures the maximum arterial pressure (systolic) and the minimum arterial pressure (diastolic).

  • Heart sounds and valve mechanics

    • First heart sound (lubb): occurs when the mitral valve closes at the onset of isovolumetric contraction.

    • Second heart sound (dup): occurs when the aortic valve closes at the onset of isovolumetric relaxation.

    • The second heart sound can sometimes be heard as a split (lub-dub) due to different closing times of the aortic and pulmonary valves, often subtle in healthy individuals.

    • The relative loudness/frequency difference between inlet and outlet sounds is attributed to valve size and cusps: the mitral valve (inlet) has a larger opening and more tissue, producing a deeper tone, whereas the aortic outlet has smaller cusps and produces a higher-pitched sound.

  • Blood vessels: six classes and their general roles

    • Elastic arteries

    • Size roughly analogous to a finger; receive the pulsatile output from the ventricle.

    • Role: store some of the pulsatile energy as elastic energy via elastin in the tunica media to maintain continuous flow during diastole and absorb pressure spikes.

    • Important to prevent high pressure from reaching delicate downstream capillaries.

    • Muscular arteries

    • Range from pencil-thick to very small; the most common arteries in the body.

    • Also called distributing arteries due to their role in directing blood flow to tissues.

    • Structure: tunica media rich in smooth muscle cells; thick walls relative to lumen enable vasoconstriction and vasodilation under autonomic control, thereby altering blood flow distribution.

    • Arterioles

    • Diameter small; wall relatively thick for size; contains several concentric smooth muscle layers (often 3 or more).

    • Critical role in regulating blood flow into capillaries and contributing the largest drop in pressure (greatest resistance) per unit vessel in the systemic circuit.

    • Function: major site for flow and pressure regulation into capillaries; significant determinant of mean arterial pressure via total peripheral resistance (TPR).

    • Capillaries

    • Smallest vessels; diameter roughly the size of a red blood cell; single-cell-thick walls (endothelium with a thin basement membrane).

    • Primary site of exchange: nutrients and oxygen diffuse out into tissue fluid; CO2 and waste products diffuse into blood.

    • Fluid exchange is driven by hydrostatic pressure and osmotic forces; involves formation of tissue fluid (interstitial fluid).

    • Venules

    • Small vessels just larger than capillaries; walls mainly endothelial with some connective tissue and occasional smooth muscle.

    • Important for leukocyte (white blood cell) migration into tissues during immune responses; leukocytes can exit through gaps in endothelium where flow is slow.

    • Veins

    • Large, thin-walled, low-pressure vessels; function as a reservoir for blood (
      about 64%64\% of total blood volume resides in systemic veins).

    • Blood can be drawn from veins for transfusions (donor donations involve deliberate removal of blood while veins constrict to maintain volume).

    • Venous return to the heart is aided by muscle pumps in the legs: contraction of leg muscles compresses deep veins with valves that allow one-way flow toward the heart.

    • When standing for long periods, contracting muscles helps venous return and reduces risk of fainting due to impaired cerebral perfusion.

  • Key physiological concepts tied to vessels

    • Flow and radius relationship

    • Flow through a vessel is proportional to the fourth power of its radius: F∝r4F \propto r^4

    • Small changes in radius cause large changes in flow; e.g., halving the radius reduces flow to 1/16 of its previous value; doubling the radius increases flow dramatically.

    • Mean arterial pressure and total peripheral resistance (TPR)

    • TPR, combined with cardiac output, determines MAP; arterioles contribute most to the pressure drop across the circulation.

  • The coronary arteries: anatomy and clinical significance

    • Coronary arteries are muscular arteries that supply the heart wall (myocardium). They run on the outside of the heart, within the coronary sulcus.

    • Major branches include a left and a right coronary artery; ostia (entrances) are located behind the aortic valve cusp openings.

    • The left coronary system includes the left anterior descending (LAD) artery, which supplies a large portion of the left ventricle and is a common source of clinically significant ischemia when blocked.

    • Coronary artery pathology

    • Atherosclerosis can reduce coronary blood flow; when flow is reduced to about 20% of normal, ischemia can occur, leading to angina and potential infarction if blood flow is severely compromised.

    • Angina is chest pain due to transient ischemia; infarction is cell death (myocardial necrosis) due to prolonged ischemia.

    • Clinical considerations

    • Blockages may vary in size and location; the impact depends on the area supplied (e.g., conduction system or ventricular walls) and the extent of collateral supply.

    • Coronary artery disease is a major clinical concern; management aims to restore adequate blood flow and prevent recurrent ischemia.

  • Lymphatics and tissue-fluid balance (brief note)

    • Capillaries leak fluid into tissue (interstitial) fluid driven by hydrostatic pressure; cells exchange nutrients and waste with this fluid.

    • Lymphatics collect excess interstitial fluid to prevent edema and return it to the venous system, maintaining fluid balance.

    • Edema occurs if hydrostatic pressure is too high or if lymphatic drainage is insufficient.

  • Practical takeaways and connections

    • The cardiac cycle is a coordinated sequence of pressure changes, valve movements, and blood volume shifts designed to maintain continuous blood flow to tissues.

    • Understanding the cycle is foundational for diagnosing and interpreting ECGs and other cardiac physiology in later lectures.

    • Blood pressure is a clinical proxy for systemic perfusion and is tightly regulated by arterial properties, venous return, cardiac output, and neurohumoral control.

    • The six classes of blood vessels reflect a functional continuum from pulsatile flow reception (elastic arteries) to pressure regulation and exchange (capillaries) to venous return (veins).

    • Anatomical details (e.g., tunica intima, tunica media, tunica externa) underlie vascular function and susceptibility to disease (e.g., atherosclerosis in muscular arteries).

  • Quick recap of terminology and concepts to review

    • Cardiac cycle phases: ventricular filling, atrial systole, isovolumetric ventricular contraction, ventricular ejection, isovolumetric ventricular relaxation.

    • Heart sounds: lubb (mitral valve closure) and dup (aortic valve closure).

    • EDV, ESV, SV, and the basic relationship SV = EDV − ESV.

    • Waveforms in the Wiggles diagram: pressure traces, volume traces, valve states, and heart sounds across the cycle.

    • Vessel wall layers: tunica intima (in contact with blood), tunica media (smooth muscle, contraction/relaxation control flow), tunica externa (adventitia).

    • Flow-versus-radius principle: F∝r4F \propto r^4.

    • Edema and lymphatics: balance of hydrostatic pressure, osmotic forces, and lymphatic return.

  • Final note

    • The course will continue with physiologic integration (ECG and cardiac cycle interpretation) in forthcoming lectures, building on the anatomy covered here.

    • Additional labs (e.g., sheep heart lab) will reinforce the external/internal anatomy and vascular relationships discussed.