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Cardiac Physiology Lecture Review

Major Blood Vessels & Their Functions

  • Vena Cava

    • Returns systemic venous blood to the right atrium.

    • Two subdivisions:

    • Superior – drains structures above the diaphragm.

    • Inferior – drains structures below the diaphragm ("blood returning from the lower part of the body" = inferior).

  • Pulmonary Artery

    • Only artery that carries deoxygenated blood.

    • Rule: all arteries carry blood away from the heart; thus pulmonary artery sends venous blood to the lungs.

    • If the destination were “whole body,” the vessel would be the aorta.

  • Pulmonary Veins

    • Only veins that carry oxygen-rich blood (answers True to “pulmonary venous blood is O₂-rich”).

  • Carotid Arteries

    • Primary supply of oxygenated blood to the head & brain (external & internal carotids).

  • Aorta

    • Largest elastic artery; distributes to entire systemic circulation.

Valves & Pressure Logic

  • Semilunar (aortic) valve opens when P{LV} > P{aorta} and closes when P{aorta} > P{LV}.

    • Quiz statement “opens when aortic pressure is greater than LV” = False.

  • AV (mitral/tricuspid) valves open when P{atria} > P{ventricle}; close when reverse is true.

Heart Sounds (Phonocardiogram)

Sound

Timing

Mechanical Event

S₁

onset of systole

AV valves close, semilunar open

S₂

onset of diastole

Semilunar close, AV open

  • Interval between S₁–S₂ = systole; S₂–next S₁ = diastole.

Electrical vs. Mechanical Activity

  • Electrical (ECG) precedes mechanical.

    • P-wave = atrial depolarization → immediately triggers atrial contraction (“a-wave”).

    • QRS = ventricular depolarization; masks atrial repolarization; initiates rapid pressure rise.

    • T-wave = ventricular repolarization → ventricular relaxation.

Key Volumes & Derived Variables

Symbol

Definition

Typical Value (rest)

EDV

End-diastolic volume (max fill)

\approx 130\,\text{mL}

ESV

End-systolic volume (residual)

\approx 50\,\text{mL}

SV

Stroke volume

SV = EDV - ESV → \approx 80\,\text{mL}

EF

Ejection fraction

EF = \frac{SV}{EDV}\times100\% (normal \ge 55\%)

Temporal Parameters (HR ≈ 60 b·min⁻¹)

  • One cardiac cycle ≈ 1 s.

    • Systole ≈ 0.33 s (≈⅓).

    • Diastole ≈ 0.67 s (≈⅔).

Graph-Based Insights

Wiggers Diagram Highlights

  1. A-wave – atrial contraction.

  2. C-wave – bulging of closed AV valve as ventricular pressure rises.

  3. V-wave – atrial filling; slight pressure rise just before AV valve opens.

  4. Systolic BP at aorta peak ≈ 120\,\text{mmHg}; Diastolic BP ≈ 80\,\text{mmHg} (varies individually).

Pressure–Volume (PV) Loop

A counter-clockwise box whose corners delineate valve events:

  1. A→B (filling; AV open, PV up).

  2. B→C (iso-volumic contraction; both valves closed; P up, V const).

  3. C→D (ejection; semilunar open; V down).

  4. D→A (iso-volumic relaxation; both valves closed; P down).

  • Area inside loop ≈ ventricular work.

  • Pathology examples:

    • Smaller loop → ↓SV (heart failure).

    • Broad but short loop → pressure defect (aortic stenosis).

Frank–Starling Law

“Increased stretch → increased force.”

  • More venous return ↑ EDV → optimal sarcomere length ↑ → more cross-bridges → ↑SV.

  • Prevents mismatch: if left ventricle ejects ↑ volume, venous return to right heart ↑, restoring balance.

Determinants of Stroke Volume

  1. Preload – degree of myocardial stretch (≈ EDV).

  2. Afterload – arterial resistance LV must overcome (≈ aortic/mean arterial pressure).

  3. Contractility (inotropy) – Ca²⁺-dependent force at given preload; ↑ mainly by sympathetic norepinephrine.

Regulation of Heart Rate

  • Parasympathetic (vagal): dominates at rest; ↓ slope of pacemaker depolarization → lower HR.

  • Sympathetic: catecholamines (NE, EPI) ↑ If & ICa currents → faster SA node firing; also enhances AV conduction.

  • Rapid HR rise at exercise onset = vagal withdrawal; further rise = sympathetic activation.

Cardiac Output ((Q\̇))

  • Core definition: \displaystyle Q\̇ = HR \times SV.

  • Hemodynamic form: \displaystyle Q\̇ = \frac{\Delta P}{R} where \Delta P = P{LV} - P{RA} \approx MAP.

  • Rearranged blood-pressure formula:
    MAP = Q\̇ \times TPR
    (Total Peripheral Resistance ≈ diastolic determinants).

Mean Arterial Pressure (Clinical Calculation)

Two interchangeable expressions:

  1. MAP = Q\̇ \times TPR (physiology models).

  2. MAP = \frac{SBP - DBP}{3} + DBP ("one-third rule").

Factors Influencing Venous Return (and thus Preload)

  1. Skeletal-muscle pump – rhythmic contractions compress deep veins; one-way valves prevent backflow.

  2. Respiratory (ventilatory) pump – inspiration ↓ thoracic pressure, ↑ abdominal pressure → venous suction to RA.

  3. Veno-/vasoconstriction – sympathetically mediated reduction in venous capacitance ↑ VR.

  4. Blood volume & gravity – standing pools blood; moving limbs or lying supine improves VR.

Redistribution of Cardiac Output

  • Exercise:

    • CO may rise from \sim5\,\text{L·min}^{-1} to 20–30\,\text{L·min}^{-1}.

    • Skeletal muscle BF ↑ massively; splanchnic & renal beds constrict.

  • Post-prandial state: GI BF ↑; muscle/skin BF ↓ if at rest.

  • Thermoregulation: Heat ↑ skin BF via vasodilation; cold does opposite.

  • Body position: Supine → ↑ venous return & SV; standing suddenly ↓ VR (orthostatic dip) until reflexes compensate.

Practical / Exam-Style Insights

  • Always anchor valve questions to the phrase “blood flows high → low pressure.”

  • For heart sound questions, name both the valve closing and valve opening.

  • Short-answer rationale example: “During isovolumic contraction pressure rises without volume change because all valves are closed.”

  • When given HR and a pressure or volume graph, derive:

    • Duration of cycle = \frac{60}{HR} seconds.

    • Stroke volume = difference between peak & trough of volume trace.

  • True vs. False prompts often hinge on the pulmonary circuit exceptions (artery = deoxygenated, vein = oxygenated) and on correct pressure direction.

Numerical & Formula Summary (All in One Place)

SV = EDV - ESV
EF = \frac{SV}{EDV} \times 100\%
Q\̇ = HR \times SV
Q\̇ = \frac{\Delta P}{TPR}
MAP = Q\̇ \times TPR
MAP = \frac{SBP - DBP}{3} + DBP
P{open\,aortic\,valve}:\; P{LV} > P_{aorta}

Ethical / Clinical Connections

  • Hypertension (“silent killer”) forces LV to operate against chronically high afterload, promoting hypertrophy & eventual failure.

  • Atrial arrhythmias tolerated (loss of atrial kick only 20–30 % of filling) vs. ventricular arrhythmias (immediately life-threatening).

  • Ejection fraction < 40 % often signals systolic heart failure; interventions aim to improve contractility (positive inotropes) or reduce afterload (vasodilators).

Real-World Scenarios & Examples

  • Standing still in formation → muscle pumps idle → venous pooling → syncope risk.

  • Exercise in heat: sweating ↓ plasma volume, ↑ HR (via sympathetic stimulation) to preserve Q\̇.

  • Ventricular pressure–volume loop shifts up/right during high-intensity exercise; shrinks in dilated cardiomyopathy.

Recap Checklist for Self-Testing

  1. Name every valve & state when it opens/closes.

  2. Draw Wiggers or PV loop & label phases.

  3. Work two MAP problems (one physiological, one clinical formula).

  4. Explain Frank–Starling in ≤2 sentences.

  5. Predict effect of ↑ afterload on SV, EF, PV loop shape.

  6. Describe autonomic steps from rest → sprint start.

Practice these, and you can replace the original lecture material confidently.