Why do veins undergo vasoconstriction during physical activity?

How would this help increase blood circulation?

I believe that the answer is in the same cited source (BioSBCC).

Veins are considered a blood reservoir and undergo constriction to mobilize blood.

[… ] the veins act somewhat like a blood reservoir, containing 60% of the total blood volume at rest. [… ] When the body needs to mobilize more blood for physical activity, the sympathetic nervous system induces vasoconstriction of veins.

The result is an increase in circulating blood volume. This changes cardiac output and arterial blood pressure (CV Physiology: Blood Volume).

I also recommend reading on Biology.SE: Where does extra blood come from to fill your muscles during exercise?

There does not need to be a change in circulating blood volume during exercise; the role of vasoconstriction is more subtle than that.

Vasoconstriction increases the stiffness of venous vessels leading back to the heart. This makes them act more like stiff tubes than elastic reservoirs. As a result, any extra blood pumped by the heart is more likely to return to the heart rather than to be stored in the venous vasculature. The importance of regulating venous return in concert with cardiac function has been appreciated at least since the work of Guyton and colleagues in the 1950s.

In exercise the effective muscle pump arising from the combination of muscle contraction and valves in the veins preventing backflow is also important; both the muscle pump and venoconstriction promote venous return to the heart.

What Happens to Your Vessels When You Exercise?

Exercise causes the blood vessels to constrict and dilate at different moments during the activity. These reactions are due to the sympathetic and parasympathetic systems responding to the physical exertion brought about by exercise, notes. Poor cardiovascular health impairs blood vessel’s ability to dilate and constrict. So, regular exercise also helps keep your blood vessels flexible and working efficiently.


Endothelial cells form the lining of blood vessels. These cells have the critical ability to rearrange to remodel the vasculature network. This feature of endothelial cells enables blood vessel changes and adequate blood flow to allow tissue growth and repair throughout the body. The endothelial cells are closest to the lumen of both arteries and veins. Surrounding the thin endothelial cell layer is a basal lamina, followed by varying amounts of smooth muscle cells and connective tissue dependent on the vessel’s function. In contrast to arteries and veins, capillaries are only a single layer of endothelial cells and pericytes.[3]

Blood Pressure and Exercise

Exercises such as endurance and resistance training affect blood pressure differently. Endurance training requires sustained physical activity, so blood pressure tends to go up and down as the autonomic system regulates vasoconstriction and vasodilation throughout the workout, the "Journal of Human Hypertension" notes. Resistance training, on the other hand, causes spikes in blood pressure because it requires bursts of physical exertion rather than a gradual build up as in endurance training. However, both types of exercises tend to have long-term beneficial effects on resting blood pressure and vasoconstriction function.


Organs and systems


Vasoconstriction , affecting both cerebral and peripheral circulations, has been associated with LSD [ 5 ], but it is not usually significant at ordinary doses in people with a normal circulatory system.

Nervous system

Hallucinogen-induced mood disorder is associated with changes in affect, varying from euphoria to manic-like symptoms, panic/fear, and depression, often occurring within minutes and often varying in the same individual on different occasions. Changes in sensory perception, with a loss of ability to distinguish temporal or spatial reality and sensory hallucinations, particularly visual and tactile, are frequent, with a tendency to assume godlike attributes. These features often merge in a psychosis, particularly with repeated use. Whether chronic psychosis after LSD is the result of the drug or of a combination of the drug and predisposing factors is currently unanswerable [ 6–8 ].

The repeated use of LSD is associated not only with psychoses, but also with more specific neurological signs and symptoms, including ataxia, incoordination, dysphasia, paresthesia, and tremor. Convulsions have been reported. “Flashback,” or the return of hallucinogenic effects, occurs in almost a quarter of those who have used LSD, particularly if they have also used other CNS stimulants, such as alcohol or marijuana [ 2 , 9 ]. They can experience distortions of perception of objects, space, or time, which intrude without warning into reality, resulting in delusions, panic, and unusual images. A “trailing phenomenon” has also been reported, in which the visual perception of objects is reduced to a series of interrupted pictures rather than a constant view [ 10 ]. The frequency of these events may slowly abate over several years, but in a significant number their incidence later increases [ 1 , 3 ].

Sensory systems

Apart from visual hallucinations (discussed under the section Nervous system in this monograph), diplopia, blurred vision, mydriasis, and other visual disturbances occur [ 11 ]. Pupillary dilatation, combined with altered sensory appreciation, has led to a number of instances of retinal damage after continued direct exposure to the sun [ 12 ].


The only hematological effect reported has been an increased rate of blood clotting associated with severe hyperthermia [ 13 ].


Retroperitoneal fibrosis has been reported, not unexpectedly, in view of the structural similarity between LSD and methysergide [ 3 ].

Body temperature

Hyperthermia can occur but does so very seldom with usual doses [ 14 ]. It has been produced experimentally with high doses.

Learning Outcomes

After completing this activity, the student will be able to:

Estimate venous blood pressures in different body regions.

Appreciate why venous pressure varies in different body regions.

Describe how venous blood pressure is affected by posture.

Briefly discuss a number of factors that influence venous return.

Assess the normal function of venous valves.

Determine the impact of the skeletal muscle pump on venous return.

Appreciate the significance of high and low CVPs.

Why does vasodilation and vasoconstriction occur when you exercise?

As you exercise, the blood vessels in your muscles dilate and the blood flow is greater, just as more water flows through a fire hose than through a garden hose. Your body has an interesting way of making those vessels expand. As ATP gets used up in working muscle, the muscle produces several metabolic byproducts (such as adenosine, hydrogen ions and carbon dioxide). These byproducts leave the muscle cells and cause the capillaries (small, thin-walled blood vessels) within the muscle to expand or dilate ( vasodilation ). The increased blood flow delivers more oxygenated blood to the working muscle.

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As you begin to exercise, blood from organs is diverted to the muscles.

Taking Blood from the Organs

When you begin to exercise, a remarkable diversion happens. Blood that would have gone to the stomach or the kidneys goes instead to the muscles, and the way that happens shows how the body's processes can sometimes override one another. As your muscles begin to work, the sympathetic nervous system, a part of the automatic or autonomic nervous system (that is, the brainstem and spinal cord) stimulates the nerves to the heart and blood vessels. This nervous stimulation causes those blood vessels (arteries and veins) to contract or constrict ( vasoconstriction ). This vasoconstriction reduces blood flow to tissues. Your muscles also get the command for vasoconstriction, but the metabolic byproducts produced within the muscle override this command and cause vasodilation, as we discussed above. Because the rest of the body gets the message to constrict the blood vessels and the muscles dilate their blood vessels, blood flow from nonessential organs (for example, stomach, intestines and kidney) is diverted to working muscle. This helps increase the delivery of oxygenated blood to working muscle further.


An arteriole is a very small artery that leads to a capillary. Larger arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin (see Figure 20.1.3). The smallest arterioles do not have a tunica external and the tunica media is limited to a single incomplete layer of smooth cells.

With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure. Because of this, you may see them referred to as resistance vessels. The muscle fibers in arterioles are normally slightly contracted, causing arterioles to maintain a consistent muscle tone—in this case referred to as vascular tone—in a similar manner to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure. The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow due to local metabolic demands.

Stretch and Breathe

The other main way that regular exercise affects your veins and arteries is that physical activity keeps your larger arteries flexible, meaning that they will be elastic enough to stretch and allow more blood to flow when it's needed, a little like a fire hose. As a result, your blood pressure is more likely to be sitting at normal levels during times of stress on the body instead of at dangerously elevated levels, easing the pressure on your heart and respiratory system.

Section Summary

Blood primarily moves through the body by the rhythmic movement of smooth muscle in the vessel wall and by the action of the skeletal muscle as the body moves. Blood is prevented from flowing backward in the veins by one-way valves. Blood flow through the capillary beds is controlled by precapillary sphincters to increase and decrease flow depending on the body’s needs and is directed by nerve and hormone signals. Lymph vessels take fluid that has leaked out of the blood to the lymph nodes where it is cleaned before returning to the heart. During systole, blood enters the arteries, and the artery walls stretch to accommodate the extra blood. During diastole, the artery walls return to normal. The blood pressure of the systole phase and the diastole phase gives the two pressure readings for blood pressure.