Skeletal muscle is invaluable for our day to day functioning by controlling voluntary action. However, due to the discrepancy in activity and size of skeletal muscle around the body, a highly regulated circulatory system is required.
In this article, we will look at the vasculature, metabolic adaptations and other applications of the skeletal muscle circulation.
Each muscle fibre is surrounded by a few capillaries. At rest, many of the capillaries are closed off from the rest of the circulation due to the contraction of pre-capillary sphincters (contractile smooth muscle cells that adjust blood flow to capillaries). This results in a higher vascular tone and the constriction of vessels.
These properties mean there is redundancy within the system, allowing it to cope with the increased workload during exercise. Relaxation of the pre-capillary sphincters allows increased perfusion to the tissue. This results in a further increase in blood flow due to vasodilation of the vessels.
Skeletal muscle accounts for 20% of cardiac output at rest. During demanding physical activity, this can increase to 80%. As metabolic activity increases, so does oxygen demand and in order to meet this demand, there is increased recruitment of capillaries. This is done by several mechanisms including the action of adrenaline (via beta-2 adrenoreceptors), the accumulation of local metabolites (eg. adenosine) and sympathetic activity which all promote vasodilation.
Skeletal Muscle Pump
Skeletal muscle is also important in helping venous blood to return to the heart, especially in the lower limbs. Veins within the muscle are compressed as the muscles contract, and this effectively acts as a secondary pump. This mechanism forces blood that would otherwise pool in the lower limbs back to the heart.
Clinical Relevance – Intermittent Claudication
Intermittent claudication is a symptom that describes muscle pain, typically in the lower leg, that usually occurs during exercise. The most common cause of intermittent claudication is peripheral vascular disease (PAD). PAD occurs due to atherosclerosis (a build-up of plaque in the arteries). This results in narrowing of the blood vessels and a lack of blood flow to the muscles. Like angina, pain is typically triggered by expertise, when oxygen demand to the muscles increases and is relieved by rest. If not treated, intermittent claudication can progress to critical limb ischaemia.
It is typically diagnosed when a patient presents with the typical history of leg pain on walking that is relieved by rest. An ultrasound can be performed to detect the presence of arterial disease.
Treatment involves modification of cardiovascular risk factors, such as ensuring lipid levels, diabetes and hypertension are well controlled using conservative or medical management strategies. Catheter-based interventions such as stenting and angioplasty can also be performed to try to remove the blockages. In severe cases, vascular surgery may be required, with an arterial bypass being the most common procedure within the legs.