By Dillon Lim - Medicine Student @ Brasenose College, Oxford
If you were to, say, do a few squats, the blood flow to your quadriceps muscles would increase – but how does the body know to do this? The contraction of a muscle like your quadriceps not only requires a greater supply of glucose and O2 but requires a greater removal of CO2 from the tissue. If you continued exercising, it would also be important for the blood to remove lactate and cations such as H+ and K+ from the muscle. To trigger the necessary vasodilation (dilation of the blood vessels), tissues respond to different biochemical or physiological signals. Regulation of local blood flow is an important part of our body’s constant adjustment of its internal conditions to support tissue demand.
The first mechanism we will discuss is metabolic hyperaemia – i.e., increases in blood flow generated by the products of metabolism. In skeletal muscle like the quadriceps, the most important signal is K+. In excitable cells, the vast majority of K+ is held within the cell, and most of the Na+ outside. With each action potential in the muscle, some of the Na+ and K+ switch places. Given enough time (and oxygen) between contractions, cation concentrations will rebalance on the “correct” side of the cell membrane; if the muscle is being continually worked, however, more K+ will find itself on the outside of the cell. This K+ has the effect of depolarising the smooth muscle located at the entrance to capillary beds – the pre-capillary sphincters (it never builds up to a high enough concentration to be able to depolarise the skeletal muscle). This results in the walls of the blood vessel relaxing and blood flow to the muscle increasing.
In cardiac muscle, the preferred signal is adenosine – its concentration increases when ATP (the cellular energy currency) is expended faster than it is generated. In the cerebral circulation, the signal used is H+, which tends to increase in tissues with less oxygen. All these by some mechanism or another dilate the precapillary sphincter and thus increase the blood flow in a very local manner.
What we also have at a higher level are “neurogenic” responses – control mediated by the nervous system. If before you started those squats you spent some time gearing yourself up, your body’s sympathetic nervous system would probably respond – this is colloquially known as the fight-or-flight response. While the K+-related mechanism is specific to the capillary bed, the sympathetic response is more powerful, acting on arterioles further up. As well as vasodilating arterioles to skeletal muscle, sympathetic activity also vasoconstricts in places like the gut, helping to redirect blood to where it is needed the most. Sympathetic activity will also make your heart pump faster and harder to get blood to the periphery quicker. All this can occur even before you start the exercise! Even if you haven’t consciously prepared, your body will later recognise the feeling of exercise, and kick the sympathetic system into action – just slightly later.
Further reading:
Autoregulation of organ blood flow. https://www.cvphysiology.com/Blood%20Flow/BF004.
Adrenergic and cholinergic receptors in blood vessels. https://www.cvphysiology.com/Blood%20Pressure/BP010b.
Local control of blood flow. https://journals.physiology.org/doi/full/10.1152/advan.00074.2010.
Neural control of circulation. https://journals.physiology.org/doi/full/10.1152/advan.00114.2010.