Effects of acute exercise on the circulatory system

The primary goal of the circulatory system during exercise is to increase blood flow to the working muscles. This is accomplished by increasing MAP and decreasing local vascular resistance:

At the onset of exercise, signals from the cerebral cortex are transmitted to the vasomotor center in the medulla of the brainstem. This central command inhibits parasympathetic activity and also initiates the mass sympathetic discharge associated with exercise. Sympathetic activity (including release of catecholamines from the adrenal medulla) increases proportionally with the intensity of exercise.

Sympathetic stimulation of the heart results in:

• Increased myocardial contractility ^ increased SV

• Therefore, increased CO

Sympathetic stimulation of the veins and other blood reservoirs results in:

• Increased Pv ^ increased VR ^ increased EDV ^ increased SV

• Therefore, increased CO

In other words, the increase in cardiac output occurs by extrinsic (sympathetic stimulation) and intrinsic (increased VR and the Frank-Starling law of the heart) mechanisms. Venous return is also markedly increased by the compression of blood vessels in the working muscles. The increase in CO causes an increase in MAP, and the increase in MAP contributes to an increase in muscle blood flow.

Sympathetic stimulation of the arterioles results in:

• Increased TPR

Most arterioles of the peripheral circulation are strongly constricted by direct sympathetic stimulation. This widespread vasoconstriction serves two purposes. First, it contributes to the increase in MAP. Second, it is an important factor in the redirection of blood flow away from inactive tissues and toward the working muscles.

Resistance in the arterioles of the working muscles is regulated locally. As discussed previously, active hyperemia results in production of several factors that cause metabolic vasodilation. Exercising muscles generate CO2, H+ and K+ ions, heat, and adenosine. The vasodilator effect of these locally produced substances overrides the vasoconstrictor effect of the sympathetic system in the muscle. As a result, local vascular resistance is decreased. The combination of increased driving pressure and decreased local vascular resistance causes an increase in blood flow to the working muscles.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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