Why does blood flow increase during exercise

Table of Contents Show

Shifts in Blood Flow
Vasodilation and Vasoconstriction
Blood Pressure Changes During Exercise

The McGill Physiology Virtual Lab

Exercise Physiology Laboratory

Cardio/CNS contribution

Many factors contribute to the changes observed during and immediately after exercise. The following will be covered:

Cardio-CNS contribution Respiratory contribution Changes at the muscular level Energy expenditure during exercise

Cardiac output may increase to 35L/min in well-trained athletes. In untrained individuals it can reach 20-25L/min. Most of the increase in cardiac output goes to the exercising muscles. There is an increase in blood flow to skin (dissipation of heat) and to the heart (increased work performed by the heart). Increased flows are the result of local arteriolar vasodilation. In both skeletal and cardiac muscles, vasodilation is mediated by local metabolic factors, and in the skin, it is achieved mainly by a decrease in the firing of sympathetic neurons supplying skin vessels. Simultaneously with vasodilation in these three regions, a vasoconstriction occurs in the kidneys and gastrointestinal organs, due to an increase in activity of sympathetic neurons supplying them.

Distribution of the systemic cardiac output at rest

and during strenuous exercise

Vasodilation of arterioles in the skeletal and heart muscles and skin causes a decrease in total peripheral resistance to blood flow. This decrease is partially offset by vasoconstriction of arterioles in other organs. But the vasodilation in muscle arterioles is not compensated, and the net result is a marked decrease in total peripheral resistance to blood flow.

The mean arterial pressure is the arithmetic product of the cardiac output and the total peripheral resistance (P=COxR). During exercise, the cardiac output increases more than the total resistance decreases, so the mean arterial pressure usually increases by a small amount. Pulse pressure, in contrast, markedly increases because of an increase in both stroke volume and the speed at which the stroke volume is ejected.

The cardiac output increase is due to a large increase in heart rate and a small increase in stroke volume.

The heart rate increases because of a decrease in parasympathetic activity of SA node combined with increased sympathetic activity.

The stroke volume increases because of increased ventricular contractility, manifested by an increased ejection fraction and mediated by sympathetic nerves to the ventricular myocardium.

End-diastolic volume increase slightly. Because of this increased filling, the Frank-Starling mechanism also contributes to the increased stroke volume (stroke volume increases when end-diastolic volume increases).

Cardiac output can be increased to high levels only if the peripheral processes favoring venous return to the heart are simultaneously activated to the same degree. Factor promoting venous return:

increased activity of the skeletal-muscle pump.
increased depth and frequency of respiration; respiratory pump.
sympathetically mediated increase in venous tone
greater ease of blood flow from arteries to veins.

Control of sympathetic outflow

One or more discrete control centers in the brain are activated by output from the cerebral cortex. Descending pathways from these centers transmit these centers’ activity to the appropriate autonomic preganglionic neurons eliciting the firing patterns typical for exercise. These centers become activated before the exercise started.

Once exercise is started, local chemical changes in the muscle can develop, particularly during high levels of exercise, because of imperfect matching between blood flow and metabolic demands. These changes activate chemoreceptors in the muscle. Afferent input from these receptors goes to the medullary cardiovascular centers. The result is a further increase in heart rate, myocardial contractility, and vasoconstriction in the nonactivated organs. Mechanoreceptors of the exercising muscle are also stimulated and provide an excitatory input to the medullary cardiovascular center.

The arterial baroreceptors

As mean and pulsatile pressure increase, baroreceptors should respond to increase parasympathetic and decrease sympathetic outflows, a pattern designed to counter the rise in arterial pressure. During exercise the exact opposite occurs: the arterial baroreceptors increase the arterial pressure during exercise. The reason is that one of neuronal component of the central command output goes to the arterial baroreceptors and ‘resets’ them upwards as exercise begins. The resetting causes a decrease firing frequency in the baroreceptors, signalling for decreased parasympathetic and increase in sympathetic outflow.

To continue with the next section: respiratory contribution, click here

When exercising your blood vessels expand and contract to pump blood toward your muscles.

Image Credit: nd3000/iStock/GettyImages

When you're working out, your heart rate increases and your blood vessels open up. The dilation of blood vessels during exercise helps your muscles get the energy they need to keep working. Over time, exercise makes your circulatory system healthier and more efficient.

During exercise, your blood vessels contract or expand to divert blood toward your muscles. Over time, the body creates new blood vessels to deliver more nutrients to your muscles.

Shifts in Blood Flow

According to an April 2015 review published in Physiological Reviews, almost 90 percent of the blood in your body is flowing through your organs when you are at rest. Your brain, liver, kidneys and heart need a constant supply of oxygen and other nutrients carried through the bloodstream.

When you exercise, the balance of blood flow in your body begins to shift. Slowly, more and more blood is redirected from your organs out to your muscles. The brain orchestrates this rearrangement as it sees fit.

Muscles that work harder demand more oxygen, so the brain begins to divert precious supplies of blood out to those muscles. The harder a given muscle works, the more blood is sent. However, there's a cut-off point beyond which your body can't send any more blood.

Eventually, the dilation of blood vessels during exercise can divert too much blood from your organs to your muscles. Your organs need fuel to survive, and your body needs to regulate blood pressure so that you don't pass out and collapse.

Vasodilation and Vasoconstriction

Your brain controls the dilation of blood vessels during exercise. There's a smooth layer of muscle inside your arteries that dilates or shrinks to help control blood flow, and it is composed of endothelial tissue.

Endothelial cells make up the bulk of this smooth layer of muscle. It produces nitrous oxide, which helps keep your blood flowing properly, according to Harvard Health Publishing. When nitrous oxide is released, it keeps the lining of the arteries smooth to prevent platelets from sticking together, an event that could lead to a heart attack or stroke.

In addition to causing vasodilation, exercise keeps your blood vessels blockage-free. It also causes your body to expand upon its existing number of blood vessels. According to a January 2016 research paper published in Comprehensive Physiology, your body makes new arteries, arterioles and capillaries.

This adaptation allows it to deliver more blood to the places that need it. Your capillaries are the smallest extension of the circulatory system. They look like little webs that extend from the bigger arteries and spread into the muscles to deliver more blood and oxygen.

These are important adaptations to exercise, but they can take a long time to achieve, according to the Gatorade Sports Science Institute. Even if you do increase your capillary count, it may decrease during periods of inactivity, so you need to keep up with your training.

Blood Pressure Changes During Exercise

During exercise, blood is moving through your body at an increased rate. Your heart is beating faster, which increases your blood pressure. How much your blood pressure rises during exercise is proportional to how hard you're working.

A September 2016 study published in BioMed Research International showed that the harder subjects worked, the more their systolic blood pressure rose. The systolic number is the number on top when blood pressure is measured.

Since exercise is supposed to be healthy, it might come as a surprise that it actually raises blood pressure. This elevation is only temporary, though. Over the long run, working out will lower your blood pressure at rest and throughout the day.

According to the Centers for Disease Control and Prevention, your blood pressure begins to drop two to three hours after exercise. This is called "post-exercise hypotension."

The same article explains that, over time, people with normal blood pressure levels will see a slight decrease if they're doing endurance training. However, those with high blood pressure should experience a sharp decrease and may even see a return to normal levels.