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26. 6. 2012.

Cardiovascular responses to acute exercise - part III

Blood flow

Acute changes in cardiac output and blood pressure during exercise allow for increased total blood flow to the body. These responses facilitate getting blood to areas where it is needed, primarily the exercising muscles. Additionally, sympathetic control of the cardiovascular system can redistribute blood so that areas with the greatest metabolic need receive more blood than areas with low demands.

Redistribution of blood during exercise

Blood flow patterns change markedly in the transition from rest to exercise. Through the action of the sympathetic nervous system, blood is redirected away from areas where elevated flow is not essential to those areas that are active during exercise. Only 15% to 20% of the resting cardiac output goes to muscle, but during high-intensity exercise, the muscles may receive 80% to 85% of the cardiac output. This shift in blood flow to the muscles is accomplished primarily by reducing blood flow to the kidneys and the primarily by reducing blood flow to the kidneys and the so-called splanchnic circulation that includes the liver, stomach, and intestines. Figure below illustrates a typical distribution of cardiac output throughout the body at rest and during heavy exercise. Because cardiac output increases greatly with increasing intensity of exercise, the values are shown both as relative percentages of the total blood available and as absolute flows.
Although several physiological mechanisms are responsible for the redistribution of blood flow through the body during exercise, they work together. To illustrate this, consider what happens to blood flow during exercise, focusing on the primary driver of the response, namely the blood flow requirements of the skeletal muscles.

As exercise begins, the active skeletal muscles rapidly sense the need for increased oxygen delivery. This need is met through sympathetic stimulation of vessels in those areas to which blood flow is to be reduced( e.g. the splanchnic and renal circulations). The resulting vasoconstriction in those areas allows for more of the increasing cardiac output to be redistributed to the exercising skeletal muscles. In the skeletal muscles, sympathetic stimulation to the constrictor fibers in the arteriolar walls also increases; however local vasodilating substances are released from the exercising muscle and overcome sympathetic vasoconstriction, producing an overall vasodilation in the muscle.
Many local vasodilating substances are released in exercising skeletal muscle. As the metabolic rate of the muscle tissue increases during exercise, metabolic waste products begin to accumulate. Increased metabolism causes an increase in acidity(increased hydrogen ions and lower pH), carbon dioxide, and temperature in the muscle tissue. These are some of the local changes that trigger vasodilation of, and increasing blood flow through, the arterioles feeding local capillaries. 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), the act of muscle contraction, and possibly other vasoactive substances(including adenosine) released as a result of skeletal muscle contraction.
When exercise is performed in a hot environment, there is an increase in blood flow to the skin to help dissipate the body heat. The sympathetic control of skin blood flow is unique in that there are both typical sympathetic vasoconstrictor fibers(similar to skeletal muscle) and sympathetic active vasodilator fibers interacting. During dynamic exercise, skin blood flow is initially and transiently decreased by an increase in sympathetic vasoconstrictor activity. As body core temperature rises, there is a reduction in this sympathetic vasoconstriction, causing a passive vasodilation. Finally, at a specific body core temperature threshold, skin blood flow begins to dramatically increase through the action of the sympathetic active vasodilator system. The increase in skin blood flow during exercise promotes heat loss, because metabolic heat from deep in the body can be released when blood moves close to the skin. This allows maintenance of the body temperature, although body temperature does increase with exercise.

Cardiovascular drift

With prolonged aerobic exercise or aerobic exercise in a hot environment, at a constant exercise intensity, SV gradually decreases and HR increases. Cardiac output is well maintained, but arterial blood pressure also declines. These alterations, illustrated in the figure below, have been referred to collectively as cardiovascular drift, and they are generally associated with increasing body temperature. Cardiovascular drift is associated with a progressive increase in the fraction of cardiac output directed to the vasodilated skiin to facilitate heat loss and attenuate the increase in body core temperature. With more blood in the skin for the purpose of cooling the body, less blood is available to return to the heart, thus decreasing preload. There is also a small decrease in blood volume resulting from sweating and from a generalized shift of plasma across the capillary membrane into the surrounding tissues. These factors combine to decrease ventricular filling pressure, which decreases venous return to the heart and reduces the end-diastolic volume. With the reduction in end-diastolic volume, SV is reduced(SV = EDV – ESV). In order to maintain cardiac output(Q = HR x SV). HR increases to compensate for the decrease in SV.

Competition for blood supply

When the demands of exercise are added to blood flow demands for all other systems of the body, competition for a limited available cardiac output can occur. This competition for available blood flow can develop among several vascular beds, depending on the specific conditions. For example, there is a competition for blood flow between active skeletal muscle and the gastrointestinal system following a meal. McKirnan and coworkers studied the effects of feeding versus fasting on the distribution of blood flow during exercise in miniature pigs. The pigs were divided into two groups. One group fasted for 14 to 17h before exercise. The other group ate their morning ration in two feedings: half the ration was fed 90 to 120 minutes before exercise and the other half 30 to 45 minutes before exercise. Both groups of pigs then ran at approximately 65% of their VO2max.
Blood flow to the hindilimb muscles during exercise was 18% lower, and gastrointestinal blood flow was 23% higher, in the fed group than in the fasted group. Similar results in humans suggest that the redistribution of gastrointestinal blood flow to the working muscles is attenuated after a meal. As a practical application, these findings suggest that athletes should be cautious in timing their meals before competition to maximize blood flow to the active muscles during exercise.
Another example of the competition for blood flow is seen in exercise in a hot environment. In this scenario, the competition for available cardiac output is set up between the skin circulation for thermoregulatory purposes and the exercising muscles.

The remaining component is blood: the fluid that carries needed substances to the tissues and clears away waste products of metabolism. As metabolism increases during exercise, the functions of the blood become more critical for optimal performance.

Oxygen content

At rest, the blood’s oxygen content varies from 20ml of oxygen per 100ml of arterial blood to 14ml of oxygen per 100ml of venous blood returning to the right atrium. The difference between these two values(20ml – 14ml = 6ml) is referred to as the arterial-mixed venous oxygen difference, or (a-ṽ)O2 difference. This value represents the extent to which oxygen is extracted, or removed, from the blood as it passes through the body.
With increasing exercise intensity, the (a-ṽ)O2 difference increases progressively and can increase approximately threefold from rest to maximal exercise intensities(figure below). This increased difference really reflects a decreasing venous oxygen content, because arterial oxygen content changes little from rest up to maximal exertion. With exercise, more oxygen is required by the active muscles: therefore more oxygen is extracted from the blood. The venous oxygen content decreases, approaching zero in the active muscles. However, mixed venous blood in the right atrium of the heart rarely decreases below 4ml of oxygen per 100ml of blood because the blood returning from the active tissues is mixed with blood from inactive tissues as it returns to the heart. Oxygen use in the inactive tissues is far lower than in the active muscles.

Plasma volume

With the onset of exercise, there is an almost immediate loss of plasma from the blood to the interstitial fluid space. The movement of fluid out of the capillaries is dictated by the pressures inside the capillaries, which include the hydrostatic or blood pressure and the osmotic pressure exerted by the proteins in the blood, mostly albumin. The pressures that influence fluid movement outside the capillaries are the pressure provided by the surrounding tissue as well as the osmotic pressures from proteins in the interstitial fluid(figure below). As blood pressure increases with exercise, the hydrostatic pressure within the capillaries also increases. Thus, the increase in blood pressure forces water from the vascular compartment to the interstitial compartment. Also, as metabolic waste products build up in the active muscle, intramuscular osmotic pressure increases, which draws fluid out of the capillaries to the muscle. 

Approximately a 10% to 15% reduction in plasma volume can occur with prolonged exercise. Similarly, 15% to 20% decreases in plasma volume have been observed in 1 min bouts of exhaustive exercise. With resistance training, the plasma volume loss is proportional to the intensity of the effort, with losses of from 10% to 15%.
If exercise intensity or environmental conditions cause sweating, additional plasma volume losses may occur. Although the major source of fluid for sweating is the interstitial fluid, this fluid space will be diminished as sweating continues. This increases the osmotic pressure in the interstitial space(since proteins do not move with the fluid), causing even more plasma to move out of the vascular compartment into the interstitial space. Intracellular fluid volume is impossible to measure directly and accurately, but research suggests that fluid is also lost from the intracellular compartment and even from the red blood cells, which may shrink the size.
A reduction of plasma volume will impair performance under many circumstances. For long-duration activities in which dehydration occurs and heat loss is a problem, the total flow of blood to active tissues must be reduced to allow increasingly more blood to be diverted to the skin in an attempt to lose body heat. Note that a decrease in muscle blood flow occurs only in conditions of dehydration and only at high intensities. Severely reduced plasma volume also increases blood viscosity, which can impede blood flow and thus limit oxygen transport, especially if the hematocrit exceeds 60%.
In activities that last a few minutes or less, body fluid shifts are of little practical importance. As exercise duration increases, however, body fluid changes and temperature regulation become important for performance. For the football player, the Tour de France cyclist, or the marathon runner, these processes are crucial, not only for competition but also for survival. Deaths have occurred from dehydration and hyperthermia during or as a result of various sport activities.


When plasma volume is reduced, hemoconcentration occurs: the fluid portion of the blood is reduced, and the cellular and protein portions represent a larger fraction of the total blood volume. That is, they become more concentrated in the blood. This hemoconcentration increases red blood cell concentration substantially – by up to 20% or 25%. Hematocrit can increase from 40% to 50%. However, the total number and volume of red blood cells do not change substantially.
The net effect, even without an increase in the total number of red blood cells, is to increase the number of red blood cells per unit of blood; that is, the cells are more concentrated. As the red blood cell concentration increases, so does the blood’s per unit hemoglobin content. This substantially increases the blood’s oxygen-carrying capacity, which is advantageous during exercise and provides a distinct advantage at altitude at rest and during submaximal exercise.

Integration of the exercise response

As is evident from all of the changes in cardiovascular function that take place at the onset of exercise, the cardiovascular system is extremely complex but responds exquisitely to deliver oxygen to meet the demands of exercising muscle. Figure below is a simplified flow diagram that illustrates how the body integrates all these cardiovascular responses to provide for its needs during exercise. Key areas and responses are labeled and summarized to help illustrate how these complex control mechanisms are coordinated. It is important to note that although the body attempts to meet the blood flow needs of the muscle, it can do so only if blood pressure is not compromised. Maintenance of arterial blood pressure appears to be the highest priority of the cardiovascular system, irrespective of exercise, the environment, and other competing needs.

1)      Cardiovascular function is regulated by the medulla in the brain.
2)      Medulla regulates heart rate via the pacemaker in the right atrium.
3)      Right atrium is stimulated by sympathetic and parasympathetic nerves to raise and lower heart rate.
4)      The motor cortex of the brain stimulates the medulla in proportion to the amount of muscle recruited.
5)      Sensory nerves in the muscle send impulses to the medulla regarding the metabolic status of the muscle.
6)      High-pressure baroreceptors in the arteries send impulses to the medulla regarding arterial blood pressure.
7)      With increases in exercise intensity, sympathetic nerves cause reduction of blood flow to the stomach, liver, and intestines…
8)      … and the kidneys…
9)      … and the skin(unless there is a need for heat dissipation).
10)  Release of metabolites from exercising skeletal muscles causes vasodilation of the arterioles leading to the active fibers.
11)  However, muscle can accept too much blood flow; thus sympathetic nerves to exercising muscles must increase resistance somewhat to maintain blood pressure.
12)  Low-pressure baroreceptors in the right side of the heart and pulmonary circulation detect the extent to which the heart is filling with blood. With increased filling, the low-pressure baroreceptors send impulses to the brain to lower resistance to exercising skeletal muscles(11); thus, arterial pressure is maintained.

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