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

Let's take a look at what vasoconstriction is and what this phenomenon in the human body is for.

The circulatory system is an essential key to understanding the long-term survival of the human being as a species. An adult person has, on average, between 4.5 and 6 liters of blood, or in other words, 7% of our body weight is this liquid. Blood transports nutrients, waste substances and oxygen to (and from) all our living cells. Therefore, it is impossible to conceive the life of a complex multicellular being without an irrigation system.

Beyond blood, if we think of the circulatory system, the first thing that comes to mind is the heart. This powerful organ is the key to life in vertebrates (and many invertebrates), as it tirelessly pumps blood to all 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 beating rhythm that exceeds 3,000 million contractions throughout our lives.

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 about them. However, what about the blood vessels, what is their functionality and what are their defining characteristics? Today we will tell you everything about vasoconstrictiona phenomenon that is essential to explain blood flow in living organisms.

What is vasoconstriction?

First of all, we must emphasize that a blood vessel is any vessel of the circulatory network that transports 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 of less than 100 micrometers in diameter that arise from the branching of arteries.
• Capillaries: the smallest vessels in living beings. They serve as a junction point between arterioles and venules where the exchange of essential substances, such as oxygen, takes place.
• Venules: collect blood from the capillaries. From here, the blood begins to return to the heart.
• Veins: these are the vessels that contain deoxygenated blood, usually rich in metabolic waste. They carry the fluid from the organs to the heart.

Now that we have briefly explored the types of blood distributing ducts in the human body, we are 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 of the vessels, particularly in the case of arteries and arterioles..

This process is the opposite of vasodilatation, that is, the increase in the diameter of the space through which blood flows in veins, arteries and arterioles. It should be noted that these processes are mediated by the vascular smooth musculature that lines the inner face of the aforementioned 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 muscular contraction, depends on calcium.. When a nerve impulse reaches the membranes of these smooth muscle fibers lining the walls of the blood vessels, it depolarizes and enables the entry of calcium ions from the extracellular plasma into the cytoplasm.

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

Epinephrine (and norepinephrine) activate the sympathetic nervous system (SNS), which directly activates the musculature.. By reacting with cellular adrenergic receptors, the cascade reaction is initiated that enables the influx of calcium ions and thus vasoconstriction.

Physiological functions of vasoconstriction

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

Here are some vital processes in which vasoconstriction is essential. Don't miss them.

1. Hemorrhage control

When an open wound occurs, living beings lose blood to a greater or lesser extent and give pathogens an easy source of entry into our body. As you can imagine, this situation is not at all favorable for individual survival, so local vasoconstriction mechanisms are triggered to prevent excessive blood loss and promote clotting.

When platelets reach the damaged area, they release serotonin (yes, the same one that is considered the neurotransmitter of joy), and this has a clear vasoconstrictor role in the vessels that are losing blood. in the vessels that are losing 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 close on their own.

2. Heat storage

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

In endotherms (living beings that generate metabolic heat), the warm blood from the core of the body passing through the superficial blood vessels of the skin exchanges heat with the environment, as it is always warmer than the environment. Therefore, when the situation presents a very cold climate, vasoconstriction phenomena occur in the body so that we can retain the heat inside our body.

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

3. Avoiding orthostatic hypotension

Orthostatic hypotension is a process based on a drop in blood pressure. is based on a drop in Blood Pressure as a result of prolonged standing or, alternatively, when someone stands up after lying down for a long time.. It occurs because blood accumulates in the legs and other areas of the lower extremities, preventing sufficient blood from reaching the brain momentarily. This causes syncope, dizziness and/or momentary fainting.

Selective vasoconstriction prevents orthostatic hypotension by preventing excess blood accumulation in one area of the body. This is part of a cyclical feedback that tries to maintain the homeostasis of the organism in the best possible way, or in other words, the balance with the environment.

Summary

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

As you will have seen, the circulation of the organism adapts 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 at random.

Bibliographical references:

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