Skeletal Muscle Circulation
Previous discussions have focused on the importance of regulating arterial blood pressure by either altering the cardiac output or changing arteriolar resistance. Both mechanisms involve alterations in the level of activity of the autonomic nervous system.
Since normal arterial blood pressure is maintained at a fairly constant level by mechanisms described, changes in resistance (arteriolar diameter) in various vascular beds permit redistribution of blood flow to meet varying tissue needs for oxygen supply and waste product removal. In addition to nervous control of peripheral resistance, some vascular beds also are influenced by special features, unique to that bed.
At rest, the cardiac output approximates 5600 ml/min in a normal man weighing 70 Kg. Skeletal muscle comprises about 50% of the body weight, but receives only 17% of the total cardiac output. Arterial blood entering the muscle capillaries has lost little oxygen prior to entering this vascular bed and still contains about 20 Vol% (20 ml O2/ 100 ml blood or 0.2 ml/ml). Venous blood draining this bed still has an oxygen content of about 13 Vol%. Thus, as blood transits the capillary bed of a resting individual it releases only 7 Vol% of the oxygen to the tissues. Because 17% X 5600 = 952 ml, and each ml of blood releases .07 ml O2, about 67 ml of oxygen (25 % of the total resting oxygen consumption of the body) is supplied to the muscles each minute. If the oxygen requirements of the muscle change, oxygen supply can be appropriately altered by either:
- changing the amount of oxygen
extracted from the blood or
- changing the amount of blood
supplied to the tissue.