Cardiovascular System and Exercise

Cardiovascular System Overview

• The circulatory and respiratory systems work together.
  • Respiratory system adds $O<em>2$ and removes $CO</em>2$.
  • Circulatory system ensures delivery.
• Cardiovascular system's main jobs:
  • Circulate oxygen.
  • Circulate nutrients.
  • Remove waste.
• Factors affecting blood volume to working tissues during exercise:
  • $Q$ or cardiac output $(SV * HR)$
  • Redistribution of blood flow
• Stroke volume regulation:
  • End diastolic volume
  • Average aortic blood pressure (afterload)
  • Contractility
• Arteries, Capillaries, & Veins
  • Veins have valves allowing one-way blood flow.
• Pulmonary & Systemic Circuits
  • Pulmonary circuit (right side):
    • Pumps deoxygenated blood from the right ventricle (RV) to the lungs via pulmonary arteries.
    • Returns oxygenated blood to the left atrium (LA) via pulmonary veins.
  • Systemic circuit (left side):
    • Pumps oxygenated blood from the left ventricle (LV) to the whole body via the aorta.
    • Returns deoxygenated blood to the right atrium (RA) via the vena cavae.

Heart's Function

• The heart functions like an engine, responsible for pumping blood throughout the body.
• Electrical Activity of the Heart
  • Contraction is caused by spontaneous electrical stimulation of the heart tissue (myocardium).
  • Electrical impulse starts in the right atrium and spreads throughout the entire heart.
  • Starting point: sinoatrial node (SA node), termed the "pacemaker" of the heart.
• The conduction system:
  • Sinoatrial node (SA node):
    • Pacemaker, initiates depolarization.
  • Atrioventricular node (AV node):
    • Passes depolarization to ventricles with a brief delay for ventricular filling.
  • Bundle Branches:
    • To left and right ventricle.
  • Purkinje fibers:
    • Throughout ventricles.
• ECG Waves
  • P wave: Atrial depolarization.
  • QRS complex: Ventricular depolarization (~0.15s later).
  • T wave: Ventricular repolarization.
• Regulation of Heart Rate
  • Factors influencing heart rate:
    • Autonomic control
    • Hormones/Chemicals (Epinephrine, beta-blocker)
    • Temperature
    • Emotions
    • Gender
    • Age
    • Nervous System Divisions

Parasympathetic & Sympathetic Control

• Parasympathetic Control
  • Via vagus nerve
  • Effect on HR: Slows HR
  • Rest: Low HR due to parasympathetic tone
  • Onset of Exercise: Increase in HR due to parasympathetic withdrawal
• Sympathetic Control
  • Via cardiac accelerator nerves
  • Effect on HR: Increases HR
  • After ~100 bpm, increase in HR is due to increased sympathetic stimulation
• The Cardiac Cycle
  • During exercise, there is an increased need to:
    • Deliver oxygen to working tissues.
    • Remove $CO_2$ and other metabolic byproducts.
  • The cardiac cycle reflects the contraction and relaxation phases of the heart:
    • Contraction/Pumping Phase: Systole
    • Relaxation/Filling Phase: Diastole
  • The cycle of contraction/relaxation has to happen.
  • Increased heart rate shrinks the time spent in both phases, especially diastole.
• Arterial Blood Pressure
  • Expressed as Systolic/Diastolic
    • Normal: Less than 120/80 mmHg
    • Hypertension: ≥130/80 mmHg
  • Systolic pressure: Pressure generated during ventricular contraction (systole).
  • Diastolic pressure: Pressure in the arteries during cardiac relaxation (diastole).
• Mean arterial pressure (MAP):
  • $MAP = DBP + 0.33 * (pulsepressure)$
• Pulse Pressure (PP):
  • $PP = SBP − DBP$
• Example:
  • $MAP = 80 + 0.33 * (120 − 80) = 93mmHg$

Stroke Volume & Cardiac Output

• Stroke Volume (SV): Volume of blood pumped out of the left ventricle with each beat.
• Cardiac Output (Q): Amount of blood pumped by the heart per minute. To calculate SV:
  • Know how much blood filled ventricle at rest.
    • EDV = End Diastolic Volume
  • Know how much is left in ventricle after systole.
    • ESV = End Systolic Volume
  • The heart NEVER ejects 100% of the blood from the ventricle.
    • The proportion that is ejected is called the ejection fraction (EF).
    • Normal ~50-60% at rest.
• Regulation of Stroke Volume
  • End-diastolic volume (EDV):
    • Volume of blood in the ventricles at the end of diastole (“preload”).
  • Average aortic blood pressure:
    • Pressure the heart must pump against to eject blood (“afterload”).
  • Strength of the ventricular contraction (contractility):
    • Enhanced by circulating epinephrine and norepinephrine, and direct sympathetic stimulation of the heart.
• Frank-Starling Mechanism
  • The more the ventricles are "stretched" (i.e., greater EDV), the more forceful the contraction.
• Equations
  • $Q = SV * HR$
  • $SV = EDV − ESV$
  • $EF = SV /EDV$

Nervous System Effects on Heart Rate

• Parasympathetic nervous system
  • Effect: Slows HR by inhibiting SA and AV node
• Sympathetic nervous system
  • Effect: Increases HR by stimulating SA and AV node
• Resting HR
  • Low due to parasympathetic tone
• Onset of Exercise
  • Initial increase due to parasympathetic withdrawal up to approximately 100 beats/min, later increase due to increased SNS outflow
• Factors Influencing EDV
  • Rate of venous return to the heart
• Venous return can be increased by:
  • Venoconstriction—via SNS
  • Skeletal Muscle pump
  • Respiratory pump
    • During inspiration, the chest wall expands and the diaphragm descends.
    • Pressure in the pleural space decreases (becomes more negative).
    • This makes the lungs, right atria & ventricle, as well as the superior & inferior vena cava expand.
    • Pressure drops in the right atria, which increases venous return.
  • Muscle Pump
    • Muscle pump - actions of muscles push deoxygenated blood back towards the heart.
• Equation
  • $Q = HR ∗ SV (L/min) = (beats/min) ∗ (ml/beat)$

Cardiac Output with Exercise Training

• Values at Rest
  • Untrained Men:
    • $Q = 5.0$
    • $HR = 72$
    • $SV = 70$
  • Women:
    • $Q = 4.5$
    • $HR = 75$
    • $SV = 60$
  • Trained Men:
    • $Q = 5.0$
    • $HR = 50$
    • $SV = 100$
  • Women:
    • $Q = 4.5$
    • $HR = 55$
    • $SV = 80$
• Values at Maximal Exercise
  • Untrained Men:
    • $Q = 22$
    • $HR = 200$
    • $SV = 110$
  • Women:
    • $Q = 18$
    • $HR = 200$
    • $SV = 90$
  • Trained Men:
    • $Q = 34$
    • $HR = 190$
    • $SV = 180$
  • Women:
    • $Q = 24$
    • $HR = 190$
    • $SV = 125$
• Factors Affecting Blood Volume Delivery
  • $Q$
  • Redistribution of blood flow to active tissues.
    • Increase blood flow by dilating arteries, which decreases vascular resistance.
• Redistribution of Blood Flow
  • Increased blood flow to working skeletal muscle (At rest, 15–20% of cardiac output to muscle increases to 80–85% during maximal exercise).
  • Decreased blood flow to less active organs (liver, kidneys, GI tract).
  • Redistribution depends on metabolic rate (exercise intensity).
• Regulation of Local Blood Flow During Exercise
  • Skeletal muscle vasodilation:
    • Autoregulation where blood flow is increased to meet metabolic demands of tissue due to changes in $O<em>2$ tension, $CO</em>2$ tension, nitric oxide, potassium, adenosine, and pH.
  • Vasoconstriction to visceral organs and inactive tissues:
    • SNS vasoconstriction.
• Pressure & Resistance Impact on Blood Flow
  • All about hemodynamics—the relationship between flow (Q), pressure (Δ across system) & resistance!
• Resistance equation:
  • $Resistance = Length * Viscosity / Radius^4$

Resistance and Blood Flow

• Factors affecting resistance:
  • Length of the vessel
  • Viscosity of the blood
  • Radius of the vessel
• Flow is inversely proportional to resistance.
  • It’s hard to push a lot of blood through a thin tube! (Analogy).