Why does exercise increase venous return?
1. During exercise, cardiac output (Q or CO) increases exponentially according to the equation:
Q = Heart Rate (HR) x Stroke Volume (SV)
So if we start with a resting HR and SV and increase our HR and decrease our resistance (venous tone), we can see how Q is increased by increasing these two variables:
Figure 1: Venous return and cardiac output during graded-exercise testing in four patients with chronic venous insufficiency. The numbers below the curves indicate the time intervals between doses of dobutamine. Data from each patient were normalized to their maximal heart rate and stroke volume achieved during dobutamine infusion.
2. Active muscle contraction increases the venous return by increasing the pressure gradient that the blood flows backward.
3. During exercise, muscle contractions increase in intensity and duration, which result in more forceful compressions of the deeper veins against the surrounding bones and muscles, which also results in an increased venous return.
Notes: Several people have contributed to this article for this section. I will list them at the end of the post with their corresponding paragraphs. Please get in touch with me if you need clarification on their statements.
4. Moving from a recumbent position to an upright position also increases venous return because it decreases central blood volume (the brain releases an antidiuretic hormone protein). When we pull ourselves out of bed, we use many muscles, which causes more blood to return to the heart.
5. During exercise, the lower extremities produce two hormones (prostaglandin E2 and Nitric Oxide) that increase venous return by promoting vasodilation in skin vessels. This improves skin temperature even further, which causes an increase in cutaneous vasoconstriction (which will lead to an expansion of the limb volume).
Because of this, there is an increased pressure gradient for blood flow into deep veins because of the difference between arterial pressure and intravascular pressure (increased due to extravasation of fluid from the expanded tissues).
6. During exercise, perfusion rate decreases in skeletal muscle tissue but increases viscera (organs), increasing venous return.
7. Sympathetic stimulation of alpha receptors causes vasoconstriction of particular veins (such as the vena cava and some tributaries of the renal vein). As a result, blood diverted from these vessels increase flow to other areas or deep veins such as the femoral or iliac veins. This also adds more volume to them during exercise, which leads to increased venous return.
8. During isometric contraction (muscle contraction but no movement), it has been shown that there is an increase in radial force on muscles that results in the increased pull of blood into those muscles (it’s like squeezing something with your hands; it will cause more fluid to go out). This is called the ‘muscular pump,’ which occurs in skeletal muscles and results in venous and lymphatic return alterations.
9. During an isometric contraction, people also experience a decrease in limb volume, which leads to an increase in blood flow into other limbs that aren’t contracting (the other leg, for example). This is called the ‘Sprengel Effect.’
10. Venoconstriction will occur when you perform an intense contraction such as weightlifting or other training activity; we know we achieve more force during this type of activity. Therefore, we must recruit more muscle fibers, thus increasing venous return because there is more musculature working at one time, increasing the amount of vasoconstriction occurring (which will lead to high venous return).
When we train, our muscles are filled with more metabolites, which causes an increase in vasoconstriction effects at the local level when it comes to certain areas of training (metabolites are released when energy is expended during muscle contractions).
These metabolites cause constrictions at the capillary level, so less blood flow goes to that area. This will lead to higher venous return even though you might not perform or feel any muscular contractions. This also occurs when someone is warming up for a race.
The skeletal muscles that are inactive can experience low blood flow. Still, there is not necessarily decreased perfusion because of compensatory mechanisms such as the ‘Boyle’s Law’ phenomenon, which states that increased pressure on a fluid (blood) will cause it to flow more readily (the system wants equilibrium if there is an increase of pressure we compensate by increasing blood flow).
Wearing tight clothes and g-suits will cause vasoconstriction, which can lead to high venous return and mean that you’ll feel like you won’t be able to deal with the G’s and increases your risk for developing decompression sickness.
Swimming in chlorinated water leads to osmotic stress, so some individuals experience redness or discomfort and dizziness due to hypotension occurring after swimming in chlorinated pools. This also leads to possible fainting.
Dehydration has also been shown as a significant factor in neurological problems like dizziness or blurred vision because you lose essential electrolytes (which also lead to vasoconstriction) when you pee out the water.
If there is a decrease in blood flow in specific areas, an individual can experience tingling sensations, pain, numbness, and muscle paralysis, leading to injury.