Circulatory Responses to Exercise

Circulatory Responses to Exercise

• The cardiovascular (CV) system has several key functions including delivery, transport, maintenance, and removal of substances. Protection is also involved.
• The CV system consists of a fluid medium (blood), a network of channels (blood vessels), and a pump (the heart).
• Blood is composed of 55% plasma (90% water, 7% plasma proteins, 3% other), and 45% formed elements (predominantly red blood cells). The vascular system has varying vessel structures, including the aorta, arteries, arterioles, capillaries, venules, and veins. The vena cava is a large vein.
• The heart acts as two pumps in one, working in tandem for pulmonary and systemic circulation.
• The heart has specific structures like atria, ventricles, valves, and septa, for effective blood flow.
• Endurance training can cause changes in plasma volume and red blood cell percentage, impacting the overall composition of blood.
• Blood vessel structure varies based on their function and location in the body. For example, Capillaries have very thin walls allowing for the exchange of materials.
• Arterioles are resistance vessels, regulating blood flow to different parts of the body using intrinsic and extrinsic mechanisms. Intrinsic mechanisms adjust blood flow by factors like O2, CO2, H+, and K+. Extrinsic controls involve the sympathetic nervous system adjusting blood flow via hormones like adrenaline and noradrenaline.
• Blood flow is redistributed during exercise, shifting towards the exercising muscles and away from less active organs.
• Venous system (capacitance vessels) are important for accommodating blood volume changes without dramatic pressure changes.
• Venous return is aided by venous valves and skeletal muscle pumps.

Cardiac Output

• Cardiac output (Q) is the amount of blood pumped by the heart per minute (L/min). It is calculated as heart rate (HR) multiplied by stroke volume (SV).
• Stroke volume (SV) is the amount of blood ejected by the ventricles with each beat (ml).
• End-diastolic volume (EDV) and end-systolic volume (ESV) are factors contributing to SV.

Regulation of Heart Rate

• Heart rate is adjusted by the autonomic nervous system, with parasympathetic nerves slowing it and sympathetic nerves increasing it. Resting heart rate is relatively low due to parasympathetic tone, but increases significantly at the onset of exercise. This initial increase is due to the withdrawal of parasympathetic impulses. Further increase in rate comes from increasing sympathetic activity.

Factors Affecting Stroke Volume

• Stroke volume is influenced by the force of contraction and end-diastolic volume (EDV). Increased sympathetic nervous system activation increases force of contraction from circulating adrenaline and noradrenaline. The increase in EDV occurs via a length-tension relationship in the heart muscle. Training can impact left ventricular (LV) compliance, further affecting SV.

EDV - The Frank-Starling Mechanism

• The Frank-Starling mechanism: The force of contraction is directly proportional to the fibre length of heart muscle. Increased EDV stretches the heart muscle fibers, potentially increasing the force of contraction and subsequently SV.

Major Factors Affecting EDV

• Increased sympathetic activity to veins and blood volume, skeletal muscle pump, and increased inspiration affect venous return. These factors increase venous pressure, enhancing atrial pressure and ventricular EDV, leading to a subsequent increase in stroke volume.

Changes in Cardiac Output During Exercise

• Cardiac output increases due to increases in both heart rate (HR) and stroke volume (SV) initially. HR increases linearly until maximal heart rate is achieved. SV also increases then plateaus at approximately 40% VO2 max.
• Highly trained endurance athletes can substantially increase cardiac output beyond untrained individuals.

Transition from Rest to Submaximal Exercise to Recovery

• During exercise, cardiac output, heart rate, and stroke volume increase. During recovery, these parameters return gradually to baseline

Cardiovascular Changes During Prolonged Exercise

• During prolonged exercise, cardiac output remains relatively stable but cardiac drift occurs. Cardiac drift is an upward trend in heart rate over time. Stroke volume naturally decreases. Venous return also decreases.

Summary: Circulatory Adjustments With Exercise

• Oxygen delivery to exercising skeletal muscles occurs through increased cardiac output via redistribution from inactive to working tissues.
• Cardiac output rises with exercise as a linear function of oxygen uptake. SV plateaus at approximately 40% VO2 max. Sustained increased activity in cardiac output occurs due to increases in heart rate alone beyond this threshold.