🧭 Overview
🧠 One-sentence thesis
The cardiovascular system's primary function is to transport oxygen and nutrients to tissues while removing waste products, and it adapts significantly through exercise training to enhance cardiac output and oxygen delivery to working muscles.
📌 Key points (3–5)
- Primary functions: transport oxygen and nutrients to tissues, remove waste products, and regulate body temperature.
- Two-pump system: the right heart pumps blood through the lungs (pulmonary circuit); the left heart pumps blood through the rest of the body (systemic circuit).
- Closed circuit: blood circulates continuously through arteries → arterioles → capillaries → venules → veins, driven by pressure gradients.
- Common confusion: arteries vs. veins—arteries carry blood away from the heart (not necessarily oxygenated); veins carry blood toward the heart.
- Blood composition: red blood cells (RBCs) carry oxygen via hemoglobin; each RBC contains ~250 million hemoglobin molecules, each binding four oxygen molecules.
🫀 The heart as a dual pump
🫀 Two-pump structure
The excerpt describes the heart as "two distinct pumps":
- Right heart: circulates blood through the lungs (pulmonary circuit).
- Left heart: circulates blood through the rest of the body (systemic circuit).
Each side operates as a "pulsatile two-chamber pump":
- Atria (superior chambers): weaker pumps that deliver blood to the ventricles.
- Ventricles (inferior chambers): provide the primary pumping force.
The two sides are separated by the interventricular septum, preventing blood from mixing between right and left.
🔄 Closed circuit operation
The human circulatory system operates as a closed circuit, circulating blood to all body tissues.
- Blood flows continuously through connected vessels.
- A driving force generates pressure to move blood through the body.
- The system is "closed" because vessels are continuous.
Example: Blood leaves the heart → travels through arteries → returns via veins → re-enters the heart, completing a loop.
🩸 Blood vessels and capillary networks
🩸 Vessel types and functions
| Vessel type | Direction | Description |
|---|
| Arteries | Away from heart | Transport blood away from the heart; branch into smaller vessels |
| Arterioles | Away from heart | Microscopic vessels branching from arteries |
| Capillaries | N/A | Extensive networks where nutrient and gas exchange occurs |
| Venules | Toward heart | Small vessels carrying blood back toward the heart |
| Veins | Toward heart | Larger vessels that return blood to the heart |
💧 Capillary adaptation during exercise
- At rest: some muscle capillaries have minimal or no blood flow.
- During strenuous exercise: the number of open capillaries increases two- to three-fold.
- Why it matters: opening dormant capillaries reduces the distance oxygen and nutrients must diffuse to reach tissues.
Don't confuse: "more capillaries open" does not mean new capillaries are created; existing dormant capillaries simply receive blood flow.
🔴 Mixed venous blood
Blood returning to the right side of the heart is referred to as mixed venous blood.
- Venules converge into larger veins.
- Major veins from upper and lower body empty into the heart.
- Because venous blood comes from the entire body, it is "mixed."
🧬 Blood composition and oxygen transport
🧬 Blood components
Blood is primarily composed of plasma, the fluid portion, and cells.
- Plasma: the fluid portion.
- Red blood cells (RBCs): particularly important for gas transport.
🔴 Red blood cell characteristics
- Lifespan: four months.
- Typical proportion: 42% of blood in healthy college-aged males; 38% in females.
- Unique features: lack a nucleus and mitochondria.
- Energy source: derive energy mainly from glycolysis (minimal metabolic needs).
🧲 Hemoglobin and oxygen binding
Each RBC contains approximately 250 million hemoglobin (Hb) molecules, which are oxygen-carrying proteins.
- Each hemoglobin molecule has four sites that bind oxygen.
- When all sites are occupied, the RBC is saturated.
- Calculation: each saturated RBC can bind approximately one billion oxygen molecules (250 million Hb × 4 sites).
Example: If an RBC is only partially saturated (e.g., three of four sites occupied per Hb), it carries less oxygen than a fully saturated RBC.
🏗️ Heart structure and chambers
🏗️ Four-chamber system
The heart has four chambers:
- Two upper chambers: atria (right atrium and left atrium).
- Two lower chambers: ventricles (right ventricle and left ventricle).
Separation: the right and left sides are separated by the interventricular septum, a muscular wall that prevents blood mixing.
🚪 Heart valves
Blood moves from the atria to the ventricles through one-way valves called atrioventricular (AV) valves.
| Valve | Location | Also known as |
|---|
| Tricuspid valve | Right AV valve | Right AV valve |
| Mitral valve | Left AV valve | Bicuspid valve |
| Pulmonary semilunar valve | Right ventricle to lungs | N/A |
| Aortic semilunar valve | Left ventricle to aorta | N/A |
Function: Semilunar valves prevent backflow of blood from the pulmonary artery and aorta into the ventricles.
🔄 Blood flow pathway
The excerpt traces the pathway:
- Venules → larger veins → inferior and superior vena cava → right atrium.
- Right atrium → tricuspid valve → right ventricle.
- Right ventricle → pulmonary semilunar valve → pulmonary artery → lungs.
- Lungs (external respiration: O₂ loaded, CO₂ unloaded) → pulmonary veins → left atrium.
- Left atrium → mitral valve → left ventricle.
- Left ventricle → aortic semilunar valve → aorta → smaller arteries → arterioles → capillaries (tissues).
- Capillaries (internal respiration: O₂ unloaded, CO₂ loaded) → venules → veins → back to right atrium.
Don't confuse: Pulmonary veins carry oxygenated blood (from lungs to heart); pulmonary artery carries deoxygenated blood (from heart to lungs). The naming is based on direction relative to the heart, not oxygen content.
🧱 Myocardium and cardiac muscle
🧱 Heart wall layers
The heart walls have three layers:
- Epicardium (outermost): serous membrane; lubricative outer covering; contains blood capillaries, lymph capillaries, and nerve fibers.
- Myocardium (middle): responsible for muscular contractions that eject blood; separated by connective tissue; contains blood capillaries, lymph capillaries, and nerve fibers.
- Endocardium (innermost): protective inner lining of chambers and valves; composed of endothelial tissue; includes elastic and collagenous fibers for stretch.
💪 Cardiac muscle characteristics
Cardiac muscle, collectively known as the myocardium, is striated and contains the same contractile proteins as skeletal muscle: actin and myosin.
Differences from skeletal muscle:
- Cardiac fibers are shorter and typically branched.
- Fibers are interconnected end-to-end by intercalated disks (contain desmosomes that anchor neighboring cells).
- Gap junctions allow rapid transmission of action potentials → heart contracts as a single unit.
- Only one fiber type (similar to type I: highly aerobic, many mitochondria, high capillary density).
🔧 Myocardium thickness and hypertrophy
- Left ventricle: thickest myocardium because it must generate sufficient pressure to pump blood throughout the entire body.
- Hypertrophy: the left ventricle thickens in response to increased demand (vigorous aerobic activity) or disease (high blood pressure, valvular heart disease).
- Adaptation: the myocardium adapts to the condition, whether from exercise training or disease.
Don't confuse: Exercise-induced hypertrophy (normal adaptation) vs. disease-induced hypertrophy (pathological). The excerpt notes that experts once erroneously believed all cardiac hypertrophy was dangerous, but training-induced hypertrophy is now known to be a normal adaptation.
🩸 Coronary blood supply
- The myocardium has its own blood supply via the right and left coronary arteries.
- Critical importance: deficits in coronary blood flow result in myocardial damage.
- High oxygen demand: when coronary blood flow is disrupted for more than several minutes, permanent damage occurs.
- Limited regeneration: cardiac muscle fibers lack satellite cells, so they have limited regenerative capacity.
Myocardial infarction (heart attack): blockage of coronary vessels → oxygen deficit → death of cardiac muscle cells. Damage to a significant portion greatly diminishes pumping capacity. The excerpt notes that exercise training can provide cardiac protection during a heart attack.
Blood return: blood from the myocardium drains into the coronary sinus via the veins of the heart and the great coronary vein, then empties into the right atrium as mixed venous blood.
🔄 The cardiac cycle and pressure changes
🔄 Cardiac cycle phases
The cardiac cycle encompasses all the events of a single heartbeat, characterized by a repeating pattern of contraction and relaxation.
Two primary phases:
- Diastole: relaxation; the heart fills with blood.
- Systole: contraction; blood is ejected from the ventricles.
Three-phase breakdown (starting in mid-to-late diastole):
- Phase one (ventricular filling): ventricles fill with blood as atrial pressure exceeds ventricular pressure → AV valves open. Atrial contraction pushes additional blood into ventricles.
- Phase two (systole):
- Isovolumetric contraction: blood volume in ventricles constant, but pressure builds; both AV and semilunar valves closed.
- Ventricular ejection: semilunar valves open (AV valves remain closed); blood ejected to body and lungs.
- Phase three (isovolumetric relaxation, early diastole): both AV and semilunar valves closed; atria begin filling with blood.
📊 Pressure changes and the Wiggers diagram
The excerpt describes a Wiggers diagram showing changes in atrial, ventricular, and aortic pressure throughout the cardiac cycle.
Key points:
- Pressure drives blood flow: blood moves from higher to lower pressure.
- Atrial filling: as atria fill during diastole, internal pressure gradually increases. ~70% of blood flows directly into ventricles before atria contract; atrial contraction forces the remaining ~30% into ventricles.
- Ventricular contraction: ventricular pressure rises sharply → AV valves close (prevents backflow into atria). When ventricular pressure exceeds pressure in pulmonary artery and aorta → semilunar valves open → blood forced into pulmonary and systemic circuits.
Heart sounds:
- "Lub" (first heart sound, S1): closing of AV valves (reverberation from sudden closure of mitral valve).
- "Dub" (second heart sound, S2): closing of semilunar valves (reverberation from sudden closure of aortic semilunar valve).
🩺 Blood pressure measurement
Blood pressure, the force exerted by the blood against the arterial walls, is generally measured as an indication of health.
- Influenced by: volume of blood pumped and resistance to flow.
- Normal values:
- Adult male: 120/80 mmHg.
- Adult female: 110/70 mmHg (tends to be lower).
- High blood pressure: diagnosed if >140/90 mmHg.
Systolic blood pressure (SBP): top number; pressure in arteries during ventricular contraction (systole).
Diastolic blood pressure (DBP): bottom number; pressure during cardiac relaxation (diastole).
At maximal exercise: SBP increases, DBP decreases.
Pulse pressure: difference between SBP and DBP.
- Formula: Pulse pressure = SBP – DBP
Mean arterial pressure (MAP): average pressure during a cardiac cycle; determines rate of blood flow through systemic circuit at rest.
- Formula: Mean arterial pressure = DBP + 0.33(pulse pressure)
- Note: formula assumes 33% of cardiac cycle is spent in systole. During exercise, systole may account for up to 66% of the cycle, so the formula must be adjusted.
⚡ Cardiac conduction system and ECG
⚡ Electrical conduction pathway
At rest, specialized mechanisms in the heart cause a succession of heart contractions called cardiac rhythmicity.
The intrinsic rhythm transmits action potentials throughout the heart muscle, causing regular contractions.
Conduction system structures:
- Sinoatrial (SA) node: pacemaker of the heart; discharges impulses at 60-100 bpm; highest automaticity.
- Interatrial tract (Bachmann's bundle): conducts impulse through left atrium.
- Internodal tracts: conduct impulse down right atrium.
- Atrioventricular (AV) node: located in lower right atrium near septum; secondary pacemaker (40-60 bpm).
- Bundle of His: located in septum region.
- Right and left bundle branches (RBB, LBB): divide from bundle of His.
- Purkinje fibers: terminal fibers forming an elaborate web; penetrate ventricular muscle mass.
Backup pacemakers: if SA node fails, AV node can function as secondary pacemaker (40-60 bpm); ventricular pacemaker cells can fire at 30-40 bpm or less.
🔌 AV node functions
The AV node has three main functions:
- Slows conduction: allows time for atria to contract and empty blood into ventricles (atrial kick) before ventricles contract.
- Blocks rapid impulses: protects ventricles from dangerously fast rates when atrial rate is too rapid.
- Backup pacemaker: acts as secondary pacemaker if SA node fails.
📈 Electrocardiogram (ECG)
The electrocardiogram (ECG) is a recording of the heart's electrical activity.
- Captures: electrical processes of depolarization (spread of electrical stimulus through heart) and repolarization (return of stimulated muscle to resting state).
- Uses: identify arrhythmias, evaluate pacemaker function, assess response to medications, diagnose coronary artery disease (especially during exercise).
ECG setup: conductive gel pads (electrodes) placed on chest and body, connected to lead-cable system. Typical 12-lead ECG has six chest lead positions (V1-V6), two arm positions (RA, LA), two leg positions (LL, RL).
📊 ECG waveforms and intervals
Basic waveforms:
- P wave: atrial depolarization (spread of impulse from SA node throughout atria).
- QRS complex: ventricular depolarization (spread of impulse through ventricles). Note: atrial repolarization occurs simultaneously and is masked by QRS.
- T wave: latter phase of ventricular repolarization.
- U wave (not always present): thought to represent further ventricular repolarization.
Intervals and segments:
- PR interval: time from onset of atrial depolarization to onset of ventricular depolarization.
- ST segment: end of ventricular depolarization and beginning of ventricular repolarization.
- QT interval: (not explicitly defined in excerpt, but shown in figures).
- R-R interval: distance between successive R waves; used to measure heart rate.
Graph paper:
- Horizontal: each small square = 0.04 seconds.
- Vertical: each small square = 1 mm (voltage/amplitude).
Example: R-R interval spanning 20 small squares = 0.8 seconds (0.04 × 20). QRS complex spanning 14 small squares = 14 mm voltage (1 mm × 14).
🩺 Clinical significance
- ST segment depression: may signal myocardial ischemia (restricted blood flow).
- Elevated ST segment: can indicate myocardial injury.
- Heart rate (bpm): key metric.
- Normal sinus rhythm (NSR): 60-100 bpm at rest.
- Sinus bradycardia: resting HR <60 bpm.
- Sinus tachycardia: resting HR >100 bpm.
💓 Heart rate regulation and variability
💓 Autonomic regulation
Heart rate is regulated by the autonomic nervous system:
- Increase HR: increase sympathetic activity or decrease parasympathetic (vagal) activity.
📉 Heart rate variability (HRV)
Heart rate variability (HRV) refers to the variation in the time between heartbeats.
- Measurement: R-R interval on ECG tracing (in milliseconds).
- Counterintuitive finding: a wide variation in HRV is a good indicator of health.
- Why: reflects healthy balance between sympathetic and parasympathetic nervous systems.
Physiological significance:
- HRV reflects autonomic balance.
- Excellent noninvasive screening tool for cardiovascular diseases.
- Low HRV: predicts cardiovascular events (e.g., sudden cardiac death); indicates imbalance in autonomic regulation.
📐 Cardiac function terminology
📐 Key measures
- End-Diastolic Volume (EDV): volume of blood in ventricles at end of diastole; also known as "preload."
- End-Systolic Volume (ESV): volume of blood remaining in ventricles after ejection.
- Stroke Volume (SV): difference between EDV and ESV.
- Formula: SV (ml/beat) = EDV – ESV
- Ejection Fraction (EF): fraction (as percentage) of blood pumped out of ventricles relative to amount in ventricle before contraction.
- Formula: EF (%) = (SV/EDV) × 100
- Average in healthy adult: 60% at rest (60% of blood in ventricles is ejected per beat).
- Cardiac Output (Q): total amount of blood pumped by heart per minute.
- Formula: Q (L/min) = HR × SV
- "Q" stands for "quantity" of blood pumped per minute.
- Average: 4.0 L/min in women, 5.6 L/min in men (often stated as ~5 L/min).
🏃 Factors affecting cardiac output
🏃 Venous return and the muscle pump
During exercise, cardiac output increases proportionally to metabolic needs of muscles. Several factors influence cardiac output:
Muscle pump:
- Mechanism: rhythmic skeletal muscle contractions compress veins → push blood toward heart.
- One-way valves: veins contain valves that prevent blood from flowing away from heart.
- Process: muscles contract → veins compressed → blood pushed toward heart. Between contractions, blood fills veins, process repeats.
- Effect: accelerates venous blood return → increases EDV, stroke volume, and cardiac output.
- Limitation: if returning blood exceeds heart's pumping capacity, the heart becomes the limiting factor.
Venoconstriction:
- Reflex sympathetic constriction of smooth muscle in veins draining skeletal muscle.
- Reduces veins' capacity to store blood → moves blood toward heart.
- Endurance training enhances venous blood return → increases EDV.
🫀 Frank-Starling mechanism
Venous blood return is the primary controller of cardiac output and is more important than the heart itself in controlling cardiac output.
Frank-Starling law of the heart:
- The heart has a built-in mechanism to pump automatically whatever amount of blood flows into the right atrium from the veins.
- How it works: increased blood flow into heart → stretches walls of heart chambers → triggers Frank-Starling mechanism → cardiac muscle contracts with increased force.
- Ventricular contractility: like any muscle, myocardium can contract with greater force, directly affecting stroke volume.
- Result: more powerful ventricular contraction → empties extra blood that has returned from systemic circulation.
- Preload: the amount of blood return from the body.
🚧 Afterload and peripheral resistance
Afterload: aortic pressure or mean arterial pressure; represents a barrier to ejection of blood.
- To eject blood, pressure generated by left ventricle must exceed pressure in aorta.
- Total peripheral resistance from smaller openings or diseased vessels greatly affects cardiac output.
Ohm's law: Q = arterial pressure / total peripheral resistance
- Meaning: when total peripheral resistance changes, cardiac output changes quantitatively in the opposite direction.
- Healthy arteries: better vasodilation → blood travels more efficiently through systemic circuit.
- During exercise: afterload is minimized due to arteriole dilation in working muscles → decreases aortic pressure → easier for heart to pump large volumes of blood.
💉 Catecholamines
Circulating epinephrine and norepinephrine:
- In addition to increased sympathetic nervous system stimulation, catecholamines increase muscle contractility.
- Mechanism: increase amount of calcium available to myocardial cells. Calcium is necessary for muscle contraction activation → increases cross-bridge activation and force production.
Summary: Cardiac output is regulated by venous blood return (EDV), cardiac contractility, and cardiac afterload. During upright exercise, EDV, contractility, and cardiac output increase due to rhythmic skeletal muscle contractions and nervous system influence. Catecholamines also increase contractility and cardiac output.
🏋️ Cardiovascular responses to exercise
🏋️ Oxygen delivery during exercise
During maximal exercise, the metabolic need for oxygen in skeletal muscle can increase up to 25 times the resting values.
Two mechanisms for increased oxygen delivery:
- Increasing cardiac output.
- Redistributing blood flow from inactive organs to working muscles.
Blood flow redistribution:
- Blood directed away from gut by decreasing flow to splanchnic area (liver, kidneys, GI tract).
- At rest: ~15-20% of total cardiac output goes to skeletal muscle.
- During maximal exercise: 80-85% of total cardiac output goes to contracting skeletal muscle.
- Brain: percentage of blood reduced compared to rest, but absolute blood flow slightly increased above resting values → improved blood flow to brain.
- Coronary blood flow: increases during heavy exercise due to increased cardiac output → supplies myocardium with enough oxygen for increased contraction.
- Skin: blood flow increases during light and moderate exercise, but decreases during maximal exercise.
- Abdominal organs: blood flow decreases during maximal exercise compared to resting values.
Mechanism: increased arteriole vasodilation in vessels supplying muscles.
🔬 Oxygen uptake by muscles
Arterial-(mixed blood) venous O₂ difference (a-vO₂ difference):
- Represents amount of O₂ taken up from 100 ml of blood by tissues during one cycle of systemic circuit.
- Calculation: change in O₂ blood content between arteries and veins.
- During intense exercise: increase in a-vO₂ difference indicates enhanced oxygen extraction at muscle capillaries.
- Cause: rise in amount of O₂ taken up and used for oxidative phosphorylation (utilization of O₂ in skeletal muscle).
- Endurance exercise training increases muscles' O₂ extraction abilities.
🏆 Chronic cardiovascular adaptations to aerobic training
🏆 The Fick equation
The Fick equation (1870), developed by Adolf Eugene Fick, describes the relationship between oxygen delivery and utilization by the tissues with whole-body oxygen consumption.
Formula: VO₂ = Q × (a – v)O₂ difference
- VO₂: rate at which oxygen is being consumed.
- Q: cardiac output.
- (a – v)O₂ difference: arterial-venous oxygen difference.
Much of endurance performance is related to the cardiovascular and respiratory systems' ability to deliver sufficient oxygen to meet the needs of metabolically active muscles.
💪 Cardiac hypertrophy
- Cardiac muscle adaptation: like skeletal muscle, cardiac muscle can undergo morphological changes when stimulated by exercise training.
- Left ventricular hypertrophy: induced by exercise → increase in chamber size → allows increased filling → increases stroke volume and cardiac output.
- Increased ventricular wall thickness: endurance training increases left ventricular wall thickness → increased ventricular mass → increased contractile force → lower end-systolic volume (ESV).
- Historical misconception: experts once believed cardiac hypertrophy was always dangerous (pathological state from severe hypertension, myopathies). Now known that training-induced hypertrophy is a normal adaptation.
- Correlation: left ventricular mass is highly correlated with VO₂max and improved performance.
📈 Stroke volume adaptations
Aerobic training affects stroke volume through:
- Adaptations to left ventricular dimensions.
- Increases in contractility.
- Greater blood volume.
Chronic adaptation: stroke volume at rest is substantially higher after endurance training than before.
Blood volume changes:
- Plasma volume: expands with training → increases EDV.
- Red blood cell volume: may also increase (finding is inconsistent).
- Hematocrit: ratio of RBC volume to total blood volume. May decrease in trained athletes due to greater increase in plasma volume (even if actual RBC number increases).
- Benefit of decreased hematocrit: reduced blood viscosity → decreases peripheral resistance to blood flow.
Frank-Starling mechanism: increased blood volumes stretch ventricular walls → increased force of contraction. More blood enters left ventricle → greater percentage ejected with each contraction with greater force → increased stroke volume.
💓 Resting and submaximal heart rate
Resting heart rate (RHR):
- Sedentary individual with initial RHR of 80 bpm can decrease RHR by ~1 bpm per week of aerobic training (at least for first few weeks).
- After 10 weeks: RHR can decrease from 80 to 70 bpm or lower.
Submaximal heart rate:
- After endurance training, submaximal HR is 10-20 bpm lower during exercise at the same absolute workload.
- Reflects increased cardiac output, higher stroke volumes, and increased blood volume.
- Implication: trained heart performs less work than untrained heart at the same workload.
Maximal heart rate:
- Generally does not change or may decrease slightly with endurance training.
Mechanisms: training appears to influence parasympathetic activity in the heart; decrease in sympathetic activity may play a small role. Mechanisms not fully understood.
📊 Summary of adaptations
The excerpt provides a flow chart summarizing factors that enhance cardiovascular endurance performance:
- Increases in blood volume (plasma + RBC content).
- Increases in stroke volume.
- Increases in ventricular volume and ventricular muscle mass.
- Increases in venous return and end-diastolic volume.
- Ultimately result in increased cardiac output and decreased resting heart rate.
Overall: Endurance training programs improve the consumption, distribution, and utilization of oxygen within skeletal muscles, with the cardiorespiratory system adapting to the training stimulus to facilitate these developments.