Cardiac Output Q HR x SV Q cardiac

Cardiac Output • Q = HR x SV – Q = cardiac output – HR = heart rate – SV = stroke volume

Regulation of Stroke Volume • end diastolic volume (EDV) - volume of blood in ventricles at the end of diastole – Frank-Starling Law – increase in contractility increases volume pumped per beat – venous return • average aortic blood pressure • strength of ventricular contraction

Components of Blood • Plasma – Liquid portion of blood – Contains ions, proteins, hormones • Cells – Red blood cells • Contain hemoglobin to carry oxygen – White blood cells – Platelets • Important in blood clotting • Hematocrit – Percent of blood composed of cells

Hematocrit

• hematocrit is the percentage of whole blood which is composed of solid material – cells, platelets etc • the blood is composed primarily of water (~55 %) called plasma – the hematocrit would be 45 • can vary between 40 and 50

Cardiac Output during Exercise • Q increases in direct proportion to the metabolic rate required to perform task • linear relationship between Q and VO 2 • remember. . . Q = HR x SV

Stroke Volume and Heart Rate during Exercise • in untrained or moderately trained individuals stroke volume plateaus ~ 40% VO 2 max • at work rates > 40% VO 2 max, Q increases by HR alone • See fig 9. 17

Changes in Cardiovascular Variables During Exercise

The Fick Equation • VO 2 = Q x (a-v. O 2 diff) • VO 2 is equal to the product of cardiac output and arterial-mixed venous difference • an increase in either Q or a-v. O 2 difference will result in an increase in VO 2 max

Redistribution of Blood Flow • Increased blood flow to working skeletal muscle • Reduced blood flow to less active organs – Liver, kidneys, GI tract

Changes in Muscle and Splanchnic Blood Flow During Exercise

Redistribution of Blood Flow During Exercise

HR, SV, and CO During Prolonged Exercise

Prolonged Exercise • Cardiac output is maintained – Gradual decrease in stroke volume – Gradual increase in heart rate • Cardiovascular drift – Due to dehydration and increased skin blood flow (rising body temperature) .

Heat Exchange Mechanisms during Exercise

Increases in Temperature • Receptors on skin first sense changes – receptors also located in spinal cord and hypothalamus respond to core temp changes • Stimulates sweat glands - increases evaporation • Increases skin blood flow - vasodilation

Changes in Heat Production and Loss during Exercise

Take Home Message • During exercise, evaporation is the most important method of heat loss • Heat production must be matched with heat dissipation or hyperthermia will ensue

• Metabolic heat production increases in proportion to the exercise intensity • Convective and radiative heat loss do not increase with intensity as temp gradient between body and environment does not change significantly

Hyperthermia • Increased core temperature to the point that physiological functions are impaired • Contributing factors – dehydration – electrolyte loss – failure of cooling mechanisms to match heat production

Exercise in Hot/Humid vs. Cool Environment

Other factors related to hydration • Water as a solvent – Ionic concentration • Neuro-muscular coordination • Contractile function – Reactions • Macronutrient formation – Glycogen • Proper digestion and waste removal

Amount of fluids ingested • Small amounts of fluid ingestion do not entirely attenuate – Elevation in core temperature – Elevation in heart rate – Rating of perceived exertion

Moderate and Large fluid intake resulted in significantly different responses than No or Small fluid intake Small = 300 ml/hr Moderate=700 ml/hr Large=1200 ml/hr From Coyle SSE #50 GSSI

Effects of dehydration on cardiovascular parameters versus % body weight loss From Coyle SSE #50 GSSI

Recommendations • Drink as much as can be tolerated up to 1250 ml/h for 68 kg/150 lb individual • Drink should contain 4 -8 % CHO to optimize absorption • Adjust volume per body weight as ratio of 68 kg – E. g. 50 kg> 50/68 * 1250 = 925 ml/hr