You have been running hard for 1h already. You feel the blood flow increasing in your legs. You start feeling dizzy. You are bonking. So you try to eat while continue running so the training program can be successfully completed. However, as soon as you swallow the first bite of that energetic bar you like so much, stomach cramps start to appear, and you start throwing up. I bet you have been exposed to that story more than one time, but you will soon find out why is it due to.
When we hop on a bike, or get out to train for our eagerly awaited marathon, oxygen demand in the muscles is 15-25-fold greater than at rest. Therefore, O2 delivery to the working muscles should be increased so that we can keep on exercising. This is achieved by an increased cardiac output (check out this older post to review how is cardiac output enhanced during exercise) and a redistribution of blood flow from inactive organs to working muscles. Now that we all know how cardiac output is regulated, it’s time we got into blood flow redistribution!
We can only reach a 5-fold increase in cardiac output during exercise, but blood flow to muscles can increase 30-fold. You may wonder how this is possible, don’t you? Well, the answer remains in the distribution of blood flow. If we take some blood flow from inactive areas and destinate it to active tissues that demand more oxygen instead of sharing it evenly with all body tissues, blood flow to these them can be further increased. That is what our body smartly does when we perform physical activity: blood flow is redirected away from areas where elevated flow is not essential to areas that are active during exercise, thus destinating a larger percentage of that cardiac output to them.
Let’s get straight to the point!
What happens during exercise?
At rest and under normal conditions, almost half of the cardiac output is sent to the liver and kidneys, while resting skeletal muscles receive only about 15 to 20%. However, during exercise, blood is redirected to the areas where it is needed most.
During heavy endurance exercise, contracting muscles receive up to 80% or more of the blood flow instead of that 20% at rest. This shift in blood flow to the muscles is accomplished primarily by reducing blood flow to the kidneys and the so-called splanchnic circulation (which includes the liver, stomach, pancreas, and intestines). Besides, blood flow is also redirected to skin so that the excess of heat can be removed from the body by conduction – this has been reviewed in two newer blogs about the heat transfer mechanisms and the body’s thermostat. I recommend you have a look at these articles if you are interested in the incredible thermoregulatory skills of the human body!
Distribution of blood to various areas is controlled primarily at the level of the arterioles feeding local capillaries, through changes in the arteriolar diameter. These blood vessels have a strong muscular wall that can significantly alter vessel diameter, are highly innervated by sympathetic nerves, and have the capacity to respond to local control mechanisms.
During exercise, as the metabolic rate of the muscle tissue increases, metabolic waste products begin to accumulate, thus causing some local changes that trigger vasodilation of the arterioles. Such changes with increased metabolism include an increase in acidity, carbon dioxide, and temperature in the muscle. Also, local vasodilation is also triggered by the low partial pressure of oxygen in the tissue or a reduction in oxygen bound to hemoglobin (increased oxygen demand), both found during exercise due to increased metabolism. Besides, many vasodilator substances can be produced within the endothelium of arterioles, among which the nitric oxide (NO) plays a crucial role during exercise.
On the other side, as stated above, arterioles are highly innervated by sympathetic nerves of the autonomic root. As you probably already know, the sympathetic nervous system is stimulated during exercise. Among the consequences of such stimulation, there is a contraction of the smooth muscle wrapping the arterioles, thus vasoconstricting them. This results in a decreased blood flow to inactive areas (especially splanchnic and renal circulation), which allows for more of the (already increased) cardiac output to be distributed to the exercising skeletal muscles.
You may also be thinking how is blood flow to the active muscles increased if the sympathetic nervous system vasoconstricts all the arterioles, aren’t you? Well, everything in our body works in a complex and integrated manner. And yes. In the skeletal muscles, sympathetic stimulation to the constrictor fibers in the arteriolar walls also increases. However, in line with such high-integration, local vasodilating substances released by muscle fibers mentioned above overcome sympathetic vasoconstriction, producing an overall vasodilation in the muscle.
What happens in the skin?
When exercising, there is also an increase in blood flow towards the skin to help dissipate the body heat, especially when exercise is performed in a hot environment.
The sympathetic control of skin blood flow is unique in that arterioles that control the blood flow of skin capillaries are innervated by 2 populations of sympathetic nerves: the well-known sympathetic vasoconstrictor nerves and sympathetic vasodilator nerves, a less well-understood system that is activated during hyperthermia.
During dynamic exercise, as body core temperature rises, there is initially a reduction in sympathetic vasoconstriction, thus causing a passive vasodilation. Once a specific body core temperature threshold is reached, skin blood flow begins to dramatically increase by activation of the sympathetic active vasodilator system. This sends more blood to the skin to promote heat loss and limit the rate of rise in body temperature.
Why do we care nutrition-wise?
One of the main performance-enhancing strategies during endurance events is fuel intake during exercise. However, we already know that exercise actually decreases splanchnic circulation, and you probably already knew beforehand that digestion and intestinal absorption of ingested nutrients requires a proper splanchnic blood flow. Altogether, it arises a physiological limitation to food ingestion during exercise.
Here is when the so-called “train the gut” strategy that the renown Asker Jeukendrup started working with comes into action. It has a crucial role when pursuing the optimal performance during endurance events to minimize the physiological limits of nutrition during exercise (see this amazing article by Asker Jeukendrup for more information). However, this topic is beyond the scope of this blogpost and it will be faced in detail in future posts.
The cardiovascular system has a tremendous capacity to redistribute blood away from areas where the need is low to areas where there is an increased need. Under exercise conditions, blood flow is decreased in the splanchnic circulation and the kidneys to redirect it towards the active muscles and the skin. This is achieved through regulation of the arteriolar diameter in response to locally released substances by active muscles that trigger vasodilation of the arterioles and to activation of the sympathetic nervous system that results in vasoconstriction.
This has a negative impact on digestion and intestinal absorption, which becomes a concern to many athletes and performance nutritionists when ingesting food during endurance events to optimize performance. Fortunately, training the gut improves the body’s response to food intake during exercise.
Kenny, W. Larry, Jack H. Wilmore, and David L. Costill. 2012. “The Cardiovascular System and Its Control.” In Physiology of Sport and Exercise, 5th ed., 139–61. Champaign: Human Kinetics Publishers.
Kenny, W. Larry, Jack H. Wilmore, and David L. Costill. 2012. “Cardiorespiratory Responses to Acute Exercise.” In Physiology of Sport and Exercise, 5th ed., 181–204. Champaign: Human Kinetics Publishers.