The Circulatory Responses to Exercise

1. The Circulatory Responses to Exercise Chapter 9 Powers & Howley 7th Edition

2. The Circulatory System • Cardiopulmonary system: works to maintain oxygen and carbon dioxide homeostasis in body tissues • The cardiovascular system includes the heart, vessels, and blood • The purposes of the CV system are: • Transport of O2 to tissues and removal of wastes • Transport of nutrients to tissues • Regulation of body temperature

4. Physical Characteristics of Blood

5. Organization of the circulatory system The heart is divided into four chambers (two pumps in one) • Right side: pumps “deoxygenated” blood to the lungs via the pulmonary circuit • Left side: pumps “oxygenated” blood to the various tissues via the systemic circulation • Heart wall composed of 3 tissues: • Epicardium: Outer layer • Myocardium: Middle layer (muscle responsible for contracting and forcing blood out of the heart) • Endocardium: Inner layer

7. Electrical Activity of the Heart Microanatomy Sinoatrial (SA) Node: Pacemaker of the heart Atrioventricular (AV) Node: Located in floor of right atrium Bundle of HIS, Bundle Branches, Purkinje Fibers Electrocardiogram (ECG): A recording of the electrical changes that occur in the myocardium during the cardiac cycle Contraction of chambers represented by P-wave, QRS complex, T-wave

9. Cardiac Cycle Cardiac Cycle - one complete sequence of contraction and relaxation of the heart

10. The Heart - Terminology • Stroke Volume - amount of blood ejected from the ventricles with each beat of the heart • Heart rate = beats per minute • Cardiac Output - the amount of blood pumped per unit of time, in liters per minute • Q = SV  HR • Arterio-venous O2 difference (a-VO2 diff) - The amount of O2 that is taken up from 100 ml of blood by the tissue during one trip around the systemic circuit • Fick Equation: VO2 = Q x (a-VO2 diff)

11. The Heart - Terminology • Blood Pressure: The force exerted by blood against the arterial walls, determined by the product of: • how much blood is pumped (Q) • resistance to blood flow (TPR) • SBP: the arterial pressure generated during ventricular systole (contraction phase) • DBP: the arterial pressure during ventricular diastole (relaxation phase)

12. Regulation of Cardiac Function Parasympathetic Nervous System • Causes HR to decrease • Parasympathetic tone: • Initial increase in HR during exercise is due to withdrawal of parasympathetic tone Sympathetic Nervous System • Causes HR to increase • Sympathetic Tone: • Increase tone => increased heart rate and increased force of contraction

14. Principles of Blood Flow • Pressure: Rate of blood flow is proportional to the pressure difference between the two ends of the vessel • Blood will flow from region of high pressure to a region of low pressure • Resistance: Blood flow is inversely proportional to resistance Length x viscosity Radius4 • Major Factor determining resistance is the vascular diameter/radius * • Resistance is inversely proportional to the 4th power of the radius Resistance =

16. The Vascular System

18. Cardiovascular Responses to Exercise Q, SV, HR, BP, a-VO2 diff • Relationship betweenVO2 and Q, HR, and SBP is essentially linear • Relationship between VO2 and SV (plateaus @ 40-50%), a-VO2 diff. (plateaus @ 40-50%), and DBP is nonlinear Arterio-venous O2 difference (a-VO2 diff) • Increases during exercise due to an increase in the amount of O2 taken up and used for oxidative production of ATP by skeletal muscle Redistribution of Blood Flow • Shifting of blood from inactive to active tissue

19. Steady State aerobic exercise

20. Distribution of cardiac output during light exercise

21. Long-Term, Moderate to Heavy Submaximal Aerobic Exercise (Cardiovascular Drift)

22. Distribution of Q during heavy exercise

23. Incremental Aerobic Exercise to Maximum

24. Distribution of Q during maximal exercise

25. Integrated Response in Exercise Activation of motor cortex and higher areas of brain (central command) • Reciprocal inhibition of parasympathetic activity => • Acceleration of heart rate (up to ~100 bpm) • Increase in sympathetic outflow => • General visceral vasoconstriction • Increased myocardial contractility, hence increase in BP, SV & HR (Q ) Regulation of local blood flow during muscle contraction & relaxation • Alterations in local / intrinsic metabolic conditions • due to increased production of local vasodilatory factors (i.e. hypoxia; decreased pH; increased PCO2, ADP, Mg++, Ca++, and temperature) • results in redistribution of blood flow

26. Integrated Response in Exercise Central command, cont. • Continued sympathetic outflow in conjunction with increases in epinephrine and norepinephrine • Continued constriction of vasculature in inactive tissues to maintain adequate perfusion pressure • Venous vessels stiffen to reduce their capacity • Facilitates venous return and maintains central blood volume Local regulation, cont. • Augmented local metabolic conditions • Further dilation of active muscle vasculature

28. Factors influencing Venous Return • Venoconstriction - Reducing the volume capacity of the veins to store blood • Muscle Pump - Mechanical action of rhythmic skeletal muscle contractions • One-way valves located in large veins

30. Arm versus Leg Exercise • HR and BP response to arm exercise is greater than during leg exercise • HR & BP during arm exercise exceeds expected values based on relative oxygen consumption • Greater sympathetic outflow to heart during arm work • Less arterioles dilated, therefore greater vascular resistance