What happens to blood flow to skeletal muscles during exercise?

When you exercise the blood vessels that supply blood to your muscles and take blood away from your muscle dilate to allow for a massive increase in blood flow to your muscles. As you exercise your body needs large amounts of oxygen, glucose, amino acids, and a molecule called ATP to allow the muscles to contract and do work. As the muscles consume nutrients and perform work waste materials like lactic acid are produced and need to be carried away from the muscles, so they can be metabolized by the liver and eliminated or recycled.  It is because of this need to bring in large amounts of materials and remove waste products that blood flow increases dramatically during exercise. 

Blood flow is greater when you exercise because the blood vessels in your muscles dilate. Imagine water flowing through a fire hose compared to a garden hose. Adenosine triphosphate, or ATP, is the way your body uses biochemicals to store and use energy. When ATP gets used up in working muscles, the muscles themselves produce metabolic byproducts (for example, adenosine, hydrogen ions and carbon dioxide). As these byproducts leave the muscle cells, they cause small, thin-walled blood vessels (capillaries) within the muscle to expand or dilate, which is called vasodilation. The dilated capillaries allow increased blood flow, which delivers more oxygenated blood to the working muscle.

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Blood flow to an active muscle changes depending on exercise intensity and contraction frequency and rate.

Learning Objectives

• Summarize the factors involved in blood flow to skeletal muscles

Key Points

• The circulatory system is closely associated with skeletal muscle to provide efficient transfer of oxygen and nutrients required for contraction and the removal of inhibitory waste products.
• At rest, skeletal muscle uses approximately 20% of cardiac output, which can rise to 80% during exercise.
• Return of blood to the heart, especially from the legs, is facilitated by the skeletal muscle pump. Deep-lying veins are compressed by skeletal muscle, forcing blood through the circulatory system back to the heart.
• After repeated stimulation, vascular recruitment can lead to an increase in the number of capillaries present in a muscle tissue, facilitating better supply and more efficient removal of waste products.

• skeletal muscle pump: The mechanism whereby skeletal muscles aid the return of blood to the heart by compressing embedded veins.
• vascular recruitment: The increase in the number of capillaries in response to a stimulus; for example, repeated exercise results in an increase in the number of capillaries in a skeletal muscle.

Skeletal muscles are important in maintaining posture and controlling locomotion through contraction. For this reason, they receive approximately 20% of cardiac output at rest, which can increase up to a maximum of approximately 80% with exercise. Due to the requirements for large amounts of oxygen and nutrients, muscle vessels are under very tight autonomous regulation to ensure a constant blood flow, and so can have a large impact on the blood pressure of associated arteries.

Blood vessels are closely intertwined with skeletal muscle tissues lying between the fascicles, or bundles of muscle fibers. Each muscle is supplied by many capillaries. This close association reduces the diffusion distances, allowing for the efficient exchange of oxygen and nutrients required for contraction and the rapid removal of inhibitory waste products.

Skeletal Muscle: Skeletal muscle: 1] Bone, 2] Perimysium, 3] Blood vessel, 4] Muscle fiber, 5] Fascicle, 6] Endomysium, and 7] Epimysium Tendon.

Blood flow within muscles fluctuates as they contract and relax. During contraction, the vasculature within the muscle is compressed, resulting in a lower arterial inflow with inflow increased upon relaxation. The opposite effect would be seen if measuring venous outflow.

This rapid increase and decrease in flow is observed over multiple contractions. If the muscle is used for an extended period, mean arterial inflow will increase as the arterioles vasodilate to provide the oxygen and nutrients required for contraction. Following the end of contractions, this increased mean flow remains to resupply the muscle tissue with required nutrients and clear inhibitory waste products, due to the loss of the inhibitory contractile phase.

Skeletal muscles also play a key role in the movement of blood around the body. Veins embedded within a muscle are compressed during contraction of that muscle, causing an increase in blood pressure due to the presence of one-way valves within the veins. This increase in pressure drives the blood towards the heart. The skeletal muscles of the legs are particularly important skeletal muscle pumps as they prevent pooling of the blood in the feet and calves due to gravity.

Skeletal Muscle Pump: During contraction of the skeletal muscle the vein is compressed which increases blood pressure. Due to the presence of one way valves the blood can pass only in one direction, back towards the heart.

It is unclear whether the action of skeletal muscle pumps influences arterial flow or if this is maintained purely by the pumping of the heart.

Following repeated stimulus such as through exercise, the number of capillaries present in a muscle tissue can increase. This vascular recruitment increases the capillary surface area within a muscle, allowing for enhanced oxygen exchange with the muscle fibers, prolonging the period of aerobic respiration and thus muscle output, and facilitating a more rapid removal of inhibitory waster factors such as lactic acid, reducing fatigue.

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Volume 10, August 2019, Pages 146-155

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The regulation of skeletal muscle blood flow is important because skeletal muscle serves important locomotory functions in the body. Contracting muscle consumes large amounts of oxygen to replenish ATP that is hydrolyzed during contraction; therefore, contracting muscle needs to be able to increase its blood flow and oxygen delivery to support its metabolic and contractile activities. As in all tissues, the microcirculation, particularly small arteries and arterioles, is the most important site for the regulation of vascular resistance and blood flow within the muscle. Like cardiac muscle, each muscle fiber (cell) is surrounded by several capillaries. This reduces diffusion distances for the efficient exchange of gasses (O2 and CO2) and other molecules between the blood and the skeletal muscle cells.

Characteristics of Skeletal Muscle Blood Flow

1. Skeletal muscle accounts for about 20% of cardiac output at rest. During strenuous physical exertion, more than 80% of cardiac output can be directed to contracting muscles, which undergo extensive vasodilation to enable the increase in flow. When this occurs, systemic vascular resistance decreases, which attenuates the increase in arterial pressure that would otherwise occur with a large increase in cardiac output during strenuous exercise.
2. At rest, skeletal muscle blood flows may be 1-4 ml/min per 100g; maximal blood flows may reach 50-100 ml/min per 100g depending upon the muscle type. Therefore, blood flow can increase 20 to 50-fold with maximal vasodilation or active hyperemia.
3. Coordinated, rhythmical contractions (e.g., running) enhance blood flow by means of the skeletal muscle pump mechanism.
4. Sympathetic innervation produces vasoconstriction through alpha1 and alpha2 adrenoceptors located on the vascular smooth muscle. There is a significant amount of sympathetic tone at rest so that abrupt removal of sympathetic influences (e.g., by using an alpha-adrenoceptor blocker) can increase resting flow 2 to 3-fold.
5. Vascular beta2-adrenoceptors produce vasodilation when stimulated by agonists such as epinephrine.
6. There is evidence for sympathetic cholinergic innervation of skeletal muscle arteries in some species such as dogs and cats; however, there is no convincing evidence that this occurs in human skeletal muscle. In species having sympathetic cholinergic innervation, activation during exercise can cause vasodilation through the release of acetylcholine binding to muscarinic receptors.
7. There is tight coupling between oxygen consumption and blood flow.
8. Blood flow is determined by local regulatory (tissue and endothelial) factors such as tissue hypoxia, adenosine, K+, CO2, H+, and nitric oxide. During exercise, these local regulatory mechanisms override the sympathetic vasoconstrictor influences (termed functional sympatholysis).
9. Skeletal muscle blood flow shows a moderate degree of autoregulation.
10. Like the coronary circulation, muscle blood flow can be significantly compromised by extravascular compression that occurs during strong muscular contractions, especially during sustained tetanic contractions.

The figure below shows how blood flow changes during phasic contractions. An example of this would be measuring brachial artery inflow during rhythmical contraction of the forearm.

When the contractions first begin, blood flow briefly decreases because of compressive forces exerted by the contracting muscles on the vasculature within the muscle. Each time the muscles contract arterial inflow decreases due to extravascular compression, and then arterial inflow increases as the muscles relax. This is repeated each time the muscles contract and relax. If flow were measured in the outflow vein, the venous outflow would increase during contraction and decrease during relaxation - the opposite of what occurs on the arterial side of the circulation. After just a couple of seconds, mean and peak flows begin to increase (active hyperemia). After 15-20 seconds the increased flow will reach a steady state that is determined by the force and frequency of contraction, and the metabolic demands of the tissue. When contractions cease, blood flow may transiently increase because of the loss of compressive forces, and then over the next minute or so the flow will return to control.

Revised 12/10/20

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