Vasoconstriction: what it is, how it works, and what it is for

The circulatory system is an essential key piece to understand the survival of the human being as a species in the long term. An adult person has, on average, between 4.5 and 6 liters of blood, or what is the same, 7% of our body weight is this liquid. Blood carries nutrients, waste substances, and oxygen to (and from) all of our living cells. For this reason, conceiving the life of a complex multicellular being without an irrigation system is impossible.

Beyond blood, if we think about the circulatory system, the first thing that comes to mind is the heart. This powerful organ is the key to life for vertebrates (and many invertebrates), as it tirelessly pumps blood to all of our organs. It is estimated that this muscular organ is capable of pumping more than 7,000 liters of blood every 24 hours, with a continuous heartbeat rhythm that exceeds 3,000 million contractions throughout our life.

We could go on providing data on the circulatory system for hours, as the heart and blood have been extensively studied and this is reflected in the large amount of informative material on they. Anyway, what about the blood vessels? What is their functionality and what particularities define them? Today

we tell you everything about vasoconstriction, an essential phenomenon when it comes to explaining blood flow in living beings.

• Related article: "Vasodilation: what it is, how it works, and what it is for"

What is vasoconstriction?

First of all, we must emphasize that A blood vessel is any vessel in the circulatory network that carries blood., as indicated in the dictionary of the Clínica Universidad Navarra (CUN). Blood vessels are classified into 5 groups, which are as follows:

• Arteries: each of the vessels that carry oxygenated blood from the heart to the body capillaries.
• Arterioles: microcirculatory blood vessels with a diameter less than 100 micrometers that arise from the branching arteries.
• Capillaries: they are the smallest vessels in living beings. They serve as a point of union between arterioles and venules in which the exchange of essential substances, such as oxygen, occurs.
• Venules: collect blood from capillaries. From here, the blood begins to return back to the heart.
• Veins: they are the vessels that contain deoxygenated blood and, generally, rich in metabolic waste. They carry fluid from the organs to the heart.

Now that we've briefly explored the types of blood-distributing ducts in the human body, we're ready to dive into vasoconstriction. This phenomenon is defined as the reduction in the diameter of the internal space of the blood vessels resulting from the contraction of the muscular section thereof, particularly in the case of arteries and arterioles.

This process is the opposite of vasodilation, or what is the same, the increase in the diameter of the space through which blood passes in veins, arteries and arterioles. It should be noted that these processes are mediated by the vascular smooth muscle that lines the inner face of the previously mentioned vessels, since it contracts or relaxes depending on the physiological needs of the organism.

Mechanism of action

The mechanism of action of vasoconstriction, like that of all muscle contraction, depends on calcium. When a nerve impulse arrives at the membranes of these smooth muscle fibers that line the walls of the blood ducts, this depolarizes and allows the entry of calcium ions from the extracellular plasma to the cytoplasm.

One of the best known vasoconstrictor hormones/neurotransmitters is epinephrine (or adrenaline), which is involved in the fight-flight response in living things.

Epinephrine (and norepinephrine) activate the sympathetic nervous system (SNS), which directly activates the muscles. Through the reaction with cellular adrenergic receptors, the cascade reaction is initiated that allows the entry of calcium ions and, therefore, vasoconstriction.

Physiological functions of vasoconstriction

When blood vessels constrict, blood circulation slows or is completely blocked. Depending on the severity of the situation, it can be considered a normal physiological event or a pathological picture, since there are certain diseases that cause dangerous vasoconstriction (such as reversible cerebral vasoconstriction syndrome, among others).

Here are some vital processes in which vasoconstriction is essential. Do not miss it.

1. control of bleeding

When an open wound occurs, living beings lose blood to a greater or lesser extent and provide pathogens with an easy source of entry into our body. As you can imagine, this situation is not at all favorable for individual survival, so they put themselves in Local vasoconstriction mechanisms work to prevent excessive blood loss and promote coagulation.

When the platelets reach the damaged area, they release serotonin (yes, the same one that is considered the neurotransmitter of happiness), and this has a clear vasoconstrictor role. in vessels that are leaking blood. Thus, blood flow to the hemorrhagic core is reduced (or restricted), reducing acute blood loss. For this reason, patients with thrombocytopenia (low circulating platelet count) are very prone to bleeding wounds that do not heal on their own.

• You may be interested in: "Circulatory system: what it is, parts and characteristics"

2. heat storage

The temperature of the human being is around 37 degrees, and at less than 30 or above 42 death occurs in all cases. When we find ourselves in an exceptionally cold environment, we run the risk of suffering mild hypothermia (between 33 and 35 degrees) and, therefore, our body launches vasoconstriction mechanisms.

In endotherms (living beings that generate metabolic heat), the warm blood from the nucleus of the body that passes through the Superficial blood vessels of the skin exchange heat with the environment, since it is always hotter than the air. atmosphere. Therefore, when the situation presents a very cold climate, vasoconstriction phenomena occur in the body so that we can retain heat inside our body.

On the other side of the coin we have vasodilation at the superficial level, which is set in motion when endothermic animals are in environments that are too hot.. Many of the living beings that inhabit the savannah or desert (such as African elephants, Loxodonta africana) have ears with a large amount of very fine tissue. This is highly irrigated and its main function is the opposite of that of the previous case: to increase the blood contact surface with the environment to lose excess heat.

3. Avoid orthostatic hypotension

Orthostatic hypotension is a process that It is based on the drop in arterial blood pressure as a result of a prolonged standing position or, failing that, when someone stands up after being lying down for a long time.. It occurs because blood accumulates in the legs and other areas of the lower extremities, which prevents enough blood from reaching the brain momentarily. This causes syncope, dizziness and/or momentary fainting.

Selective vasoconstriction prevents orthostatic hypotension, since excess blood pooling in one area of ​​the body is prevented. This is part of a cyclical feedback that tries to maintain the homeostasis of the organism in the best possible way, or what is the same, the balance with the environment.

Summary

Thus, we can summarize that vasoconstriction is the process by which the blood vessel musculature reduces or blocks the flow of blood to a specific area. It should be noted that this capacity is found, above all, in those ducts with a thick muscular coat, such as medium-sized arteries and arterioles.

As you will have been able to verify, the circulation of the organism is adapted to the physiological needs of the species at all times, regardless of its simplicity or evolutionary origin. Vasoconstriction is yet another proof that, in the body of living beings, no process happens randomly.

Bibliographic references:

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