Blood flow to skeletal muscles (ml/min)
Blood Flow Regulation in Skeletal Muscle at Rest and During Exercise Very strenuous exercise is one of the most stressful condi- tions that the normal circulatory system faces. This is true because there is such a large mass of skeletal muscle in the body, all of it requiring large amounts of blood flow. Also, the cardiac output often must increase in the nonathlete to four to five times normal, or in the well-trained ath- lete to six to seven times normal, to satisfy the metabolic needs of the exercising muscles. Rate of Blood Flow Through the Muscles During rest, blood flow through skeletal muscle averages 3 to 4ml/min/100g of muscle. During extreme exercise in the well-conditioned athlete, this can increase 25- to 50-fold, rising to 100 to 200ml/min/100g of muscle. Peak blood flows as high as 400ml/min/100g of muscle have been repoed in thigh muscles of endurance-trained athletes. Blood Flow During Muscle Contractions. Figure 21-1 shows a record of blood flow changes in a calf muscle of a human leg during strong rhythmical muscular exercise. Note that the flow increases and decreases with each muscle contraction. At the end of the contractions, the blood flow remains very high for a few seconds but then returns toward normal during the next few minutes. The cause of the lower flow during the muscle contrac- tion phase of exercise is compression of the blood vessels by the contracted muscle. During strong tetanic contrac- tion, which causes sustained compression of the blood vessels, the blood flow can be almost stopped, but this also causes rapid weakening of the contraction. Increased Blood Flow in Muscle Capillaries During Exercise. During rest, some muscle capillaries have little or no flowing blood. But during strenuous exercise, all the capillaries open. This opening of dormant capillaries diminishes the distance that oxygen and other nutrients must diffuse from the capillaries to the contracting muscle fibers and sometimes contributes a twofold to threefold increased capillary surface area through which oxygen and nutrients can diffuse from the blood to the tissues. Control of Blood Flow in Skeletal Muscles Local Regulation--Decreased Oxygen in Muscle Greatly Enhances Flow. The tremendous increase in muscle blood flow that occurs during skeletal muscle activity is caused mainly by chemicals acting directly on the muscle aerioles to cause dilation. One of the most impoant chemical effects is reduction of oxygen in the muscle tissues. When muscles are active they use oxygen rapidly, thereby decreasing the oxygen concentration in the tissue fluids. This in turn causes local aeriolar vaso- dilation because the aeriolar walls cannot maintain contraction in the absence of oxygen and because oxy- gen deficiency causes release of vasodilator substances. Adenosine may be an impoant vasodilator substance, but experiments have shown that even large amounts of adenosine infused directly into a muscle aery cannot increase blood flow to the same extent as during intense exercise and cannot sustain vasodilation in skeletal muscle for more than about 2 hours. Founately, even after the muscle blood vessels have become insensitive to the vasodilator effects of adeno- sine, still other vasodilator factors continue to maintain increased capillary blood flow as long as the exercise continues. These factors include (1) potassium ions, (2) adenosine triphosphate (ATP), (3) lactic acid, and (4) carbon dioxide. We still do not know quantitatively how great a role each of these plays in increasing muscle blood flow during muscle activity; this subject was dis- cussed in additional detail in Chapter 17. Nervous Control of Muscle Blood Flow. In addition to local tissue vasodilator mechanisms, skeletal muscles are provided with sympathetic vasoconstrictor nerves and (in some species of animals) sympathetic vasodilator nerves as well. Sympathetic Vasoconstrictor Nerves. The sympa- thetic vasoconstrictor nerve fibers secrete norepineph- rine at their nerve endings. When maximally activated, this can decrease blood flow through resting muscles to as little as one-half to one-third normal. This vasocon- striction is of physiologic impoance in circulatory shock and during other periods of stress when it is necessary to maintain a normal or even high aerial pressure. In addition to the norepinephrine secreted at the sym- pathetic vasoconstrictor nerve endings, the medullae of the two adrenal glands also secrete large amounts of nor- epinephrine plus even more epinephrine into the circu- lating blood during strenuous exercise. The circulating norepinephrine acts on the muscle vessels to cause a vaso- constrictor effect similar to that caused by direct sympa- thetic nerve stimulation. The epinephrine, however, often has a slight vasodilator effect because epinephrine excites more of the beta-adrenergic receptors of the vessels, which are vasodilator receptors, in contrast to the alpha vasocon- strictor receptors excited especially by norepinephrine 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 <a style="color: ; font-family: arial, helvetica, verdana, sans-serif;" href=" hyperemia</a> Totally800ml
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General physiology