Cardiovascular Response to Exercise

Cardiovascular Drift

• During prolonged, constant submaximal exercise, heart rate (HR) increases and stroke volume (SV) decreases while cardiac output stays constant.
• This happens due to a decrease in blood volume caused by sweating and fluid shifts, leading to increased skin blood flow for thermoregulation.
• The redistribution of blood flow reduces central venous filling pressure (preload), which decreases SV.
• HR increases to compensate and maintain cardiac output.

Partitioning of Cardiac Output during Exercise

• With increasing exercise intensity, there is sympathetic-mediated vasoconstriction in the kidneys, splanchnic regions, and inactive muscles.
• This redirects blood flow from these areas to active muscles, increasing their blood supply.
• The splanchnic and renal vascular beds each receive about 25% of cardiac output (CO) at rest, but utilize only 15-20% of available oxygen.
• Blood flow to these regions decreases during exercise, proportionate to the intensity of the work.
• Cutaneous blood flow initially decreases during exercise, but increases with rising body temperature as exercise duration and intensity increase.
• Skin blood flow eventually decreases when skin vessels constrict as overall oxygen consumption nears maximal values.
• Cerebral blood flow remains unchanged during exercise.

Blood Volume Changes

• Increased blood volume during exercise results mainly from elevated plasma volume (PV), but also involves a rise in red blood cell (RBC) volume.
• Increased PV with aerobic training is thought to be caused by increased plasma protein levels, particularly albumin, which raises osmotic pressure, retaining fluid in the blood.
• Exercise also triggers the release of antidiuretic hormone (ADH) and aldosterone, promoting water and sodium reabsorption in the kidneys, further increasing PV.
• Increased RBC volume with endurance training contributes to overall blood volume increases. While RBC count and hemoglobin concentration may rise, hematocrit (RBC volume to blood volume ratio) may decrease.

Oxygen Carrying Capacity and Performance Impact

• Highly trained athletes typically demonstrate higher absolute amounts of hemoglobin and RBCs.
• These changes enhance oxygen carrying capacity, improve oxygen delivery to muscles, and contribute to increased maximal oxygen consumption (VO2 max) and aerobic capacity.
• Oxygen extraction is the amount of oxygen removed from blood by tissues during blood flow.
• The arteriovenous oxygen difference (a-vO2 difference) represents the difference in oxygen concentration between arterial and venous blood.
• Submaximal exercise gains include more than 50% improvement in time to exhaustion with standardized exercise tests.

Ventilatory Responses to Exercise

• Minute ventilation (VE) is determined by tidal volume (VT) and breathing frequency (fb).
• Both VT and fb increase linearly with exercise until heavy exercise.
• At heavy exercise intensities, VT plateaus, and further VE increases are driven by increases in fb.

Metabolic Changes with Exercise Intensity

• Progressive exercise intensity leads to a range of metabolic changes.
• The "anaerobic threshold" or "lactate inflection point" occurs at approximately 65%-75% of VO2 max, where lactic acidosis develops as anaerobic glycolysis becomes more prominent in muscle energy supply.

This quiz covers key concepts related to cardiovascular drift and the partitioning of cardiac output during exercise. Understand how exercise affects heart rate, stroke volume, and blood flow distribution to active muscles. Test your knowledge of the physiological responses during prolonged physical activity.