What is vascular smooth muscle?

Different types of muscle

When we think of the word muscle, the image of bustling biceps or rippling abs may spring to mind. These are certainly striking examples of muscles, but they’re predominantly composed of just one type of muscle tissue: skeletal muscle. In fact, we all have two other less visible types of muscle tissue: cardiac and smooth muscle.

Let’s first go back to skeletal muscle. As its name suggests, this muscle type supports our skeleton and helps us to move. It’s the reason we can walk, pick up a cup of coffee, or keep our backs straight. Now, try to flex your bicep like Arnold Schwarzenegger or the late Bruce Forsyth. You’ll find that you’re able contract this skeletal muscle whenever you want. In this sense, we say that (most) skeletal muscles are under ‘voluntary’ control.

Next, try to deliberately contract your heart or churn your stomach. Are you able to do it? It ought to be impossible because the muscle tissues responsible for these movements are not, in contrast to skeletal muscle, under voluntary control.

There are two types of such ‘involuntary’ muscle tissue. Cardiac muscle makes up the walls of our heart. When cardiac muscle tissue contracts, blood is pumped around our body.

By contrast, smooth muscle* is typically found in the walls of hollow organs such as the stomach, intestines and bladder. Its function is to aid the passage of food, urine and other substances. For example, when intestinal smooth muscle contracts, food in the gut is moved onwards through the digestive system.

There’s also smooth muscle surrounding our blood vessels: namely our arteries and, to a lesser extent, our veins. We call this type of smooth muscle ‘vascular smooth muscle.’ When this type of muscle contracts and relaxes, the size of the lumen (the hole in the middle of a blood vessel) changes, allowing more or less blood to flow through the vessel.

KEY POINTS

• We have three types of muscle: skeletal, cardiac and smooth muscle.

• Smooth muscle is involuntary – we cannot consciously make it contract.

• Vascular smooth muscle is found in the walls of blood vessels.

* Smooth muscle is so-called because it appears smooth under a microscope. This is in contrast to skeletal and cardiac muscle, which have bands (called ‘striations’ across them).

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Why do we need vascular smooth muscle?

All of our organs and tissues need oxygen and nutrients to survive and function properly. We deliver these important molecules to our organs via our bloodstream. Blood is pumped from the heart through arteries and arterioles (smaller arteries) to supply tissues with oxygen, glucose, minerals, proteins and various other nutrients. Blood flow is also important for removing waste products (e.g. carbon dioxide) and toxins that may have been produced by an organ.

The problem our body faces is that the amount of blood an organ requires changes according to the demands of everyday life. For example, right now, I am sat down. My leg muscles still need an adequate blood supply to survive, but, as they are relaxed, their demands for energy are relatively low. Similarly, the muscles are not producing waste products at a high rate.

If I were to suddenly start running however, my leg muscles would quickly require more oxygen and glucose for energy. Waste products from muscle contraction (including carbon dioxide, ADP, lactic acid, chloride and magnesium ions) would also quickly build up. Consequently, my leg muscles now require a greater supply of blood. So, how should my body respond to this demand?

There are two obvious ways to increase the amount of blood flowing to any organ or tissue (in this case, my leg muscles).

• 1) One way is to simply get the heart to pump blood more quickly. Indeed, whenever you exercise, this is exactly what happens. Adrenaline, the ‘fight or flight’ hormone, is released from your adrenal glands, which binds to receptors in your heart, speeding up your heart rate.

• 2) The second way to is to widen or dilate the blood vessels supplying my muscles. This will allow more blood to flow to exercising muscles in order to meet their energy needs.

Vasodilatation and vasoconstriction

Vascular smooth muscle helps with this second strategy. When vascular smooth muscle relaxes, the lumen of blood vessels enlarges, allowing more blood to flow. We call this process ‘vasodilatation’ (also referred to as 'vasodilation').

Again, this process of vasodilatation is precisely what occurs during exercise. Adrenaline is released and binds to special receptors (called beta-2 receptors) on vascular smooth muscle. This causes vascular smooth muscle to relax, widening the blood vessel and allowing up to 20 times more blood to flow to exercising skeletal muscles, supplying them with oxygen and nutrients and clearing away waste products.

There’s one major drawback of increasing blood flow to exercising muscles in this way. As the volume of blood in our circulation is relatively fixed (around 5 litres), supplying ‘extra’ blood to muscles requires that we divert blood away from other organs. During exercise, we typically reduce the blood flow to organs that do not have a pressing demand for energy. Such organs include the gut, skin and any inactive skeletal muscles.

In order to reduce blood flow to these organs, our bodies simply do the opposite of ‘vasodilatation.’ That is: vascular smooth muscle contracts, blood vessels become narrower and less blood flows through them. This process is called ‘vasoconstriction.’

Our bodies are constantly fine-tuning this balance of vasodilation and vasoconstriction to meet the energy demands of different organs and tissues. We’ve already seen what happens during exercise. After a big meal, we do the opposite – we vasodilate blood vessels supplying the gut and digestive system, whereas we vasoconstrict other blood vessels and limit blood flow to resting skeletal muscles.

Regulation of blood pressure

Aside from altering blood flow to different organs, there’s another related reason as to why we need vascular smooth muscle.

Our circulatory system is a bit like a closed hydraulic system (think about the brakes on your car). We have a hydraulic pump (the heart), a system of connected tubes (our blood vessels) and hydraulic fluid (our blood). Hydraulic systems work because the fluid is kept under a certain pressure and the narrow width of the tubes provide a certain degree of resistance to the flow of fluid. If the pressure were to drop (say due to a leak of fluid), the hydraulic system would cease to work.

Similarly, for blood to flow into organs and tissues, there must be a certain amount of pressure in arteries. We call this arterial blood pressure or simply ‘blood pressure.’ When we dilate blood vessels (vasodilatation), it’s easier for blood to flow through them. We say there is less resistance in the circulatory system and the blood pressure drops. If the pressure drops too low, blood will cease to flow into organs and they’ll gradually become starved of oxygen and nutrients. To counteract this, we can increase the resistance to blood flow in other parts of the circulatory system by narrowing arteries (vasoconstriction). This will help to stabilise blood pressure.

Conversely, if the pressure within arteries becomes too high, there is an increased strain on the heart and a risk that blood vessels may burst. By dilating blood vessels, the resistance to blood flow in the circulatory system decreases, helping to lower blood pressure.

By causing vasodilatation and vasoconstriction and adjusting the resistance to blood flow in this way, vascular smooth muscle helps us to regulate our blood pressure.

KEY POINTS

• Vascular smooth muscle causes blood vessels to widen (vasodilatation) or narrow (vasoconstriction).

• Vasodilatation results from relaxation of vascular smooth muscle - it increases the blood supply to tissues.

• Vasoconstriction results from contraction of vascular smooth muscles – it decreases the blood supply to tissues.

• During exercise, we vasodilate blood vessels supplying exercising skeletal muscles and vasoconstrict blood vessels to the gut, skin and inactive muscles.

How does smooth muscle contract and relax?

If you looked at vascular smooth muscle tissue under a microscope, you will notice it is composed of several individual spindle-shaped fibers. If you zoomed in further, you would see that these fibers contain small thread-like structures called ‘filaments.’

There are two types of filaments – thick filaments (which are predominantly made of a protein called myosin) and thin filaments (which are made from a protein called actin). The filaments are arranged so that they can slide over one another.

When the thick filaments slide over the thin filaments, tension is produced. This is essentially the basis of vascular smooth muscle contraction.

Calcium is required to trigger the process of filaments sliding over one another. For this reason, levels of calcium, both in the blood and in tissues, can influence vascular smooth muscle contraction and relaxation. In turn, this can affect how well you widen or narrow your blood vessels, which, as we’ve explored earlier, alters blood flow to muscles and other tissues.

Furthermore, factors that affect calcium levels, such as your Vitamin D intake or the levels of certain hormones (particularly parathyroid hormone [PTH]), will have an indirect effect on your vascular smooth muscle function.

Owing to the key role of calcium on vascular smooth muscle, your Vascular Smooth Muscle Contraction trait is heavily influenced by your Serum Calcium Level Trait.

KEY POINTS

• Vascular smooth muscle contraction relies on calcium.

• Changes to calcium levels in your body can affect the ability to contract and relax smooth muscle.

What things cause vascular smooth muscle to dilate or constrict?

Local factors

One of the elegant things about the human body is that it has evolved to automatically match blood flow to the energy demands of a tissue. For example, when we exercise a skeletal muscle, it produces carbon dioxide, lactate, potassium ions, adenosine and other waste products which accumulate in the bloodstream. These molecules act on vascular smooth muscle nearby, causing it to relax, thereby promoting vasodilatation and greater blood flow to the exercising muscle. This rather neat process of matching blood flow to energy demands is called autoregulation.

Molecules produced as a result of inflammation can also cause local changes in blood flow by either causing vascular smooth muscle relaxation or contraction.  

Nerves

Vascular smooth muscle also receives input from nerves. Specifically, it is controlled by the autonomic nervous system – the network of nerves that regulates involuntary processes like digestion, breathing and, you’ve got it, blood flow!

One branch of the autonomic nervous system is called the ‘sympathetic nervous system.’ The sympathetic nervous system is responsible for our ‘fight or flight response’ – an evolved bodily response to help us either encounter or run away from a threat (e.g. a predator/ your mother in law). Vascular smooth muscle in arteries and arterioles typically receives input from sympathetic nerves. When these nerves are stimulated, they produce a chemical called noradrenaline (or norepinephrine). This binds to special receptors (called alpha receptors) on vascular smooth muscle, causing it to contract and thereby giving rise to vasoconstriction. It is this sympathetic stimulation, for example, which limits blood flow to your digestive system during exercise.

Hormones

Another component of our ‘fight or flight response’ is the release of a hormone called adrenaline into the bloodstream. It is released from the adrenal glands, which are located just above each kidney. In addition to receptors for sympathetic nerves, vascular smooth muscle also contains receptors for adrenaline circulating in the blood. When adrenaline binds to one type of receptor (beta-2 receptors), it causes vascular smooth muscle to relax, thereby causing vasodilatation. This is the process by which blood flow to skeletal muscles is increased whenever we exercise.

There are also various hormones involved in the regulation of the blood pressure that cause vasoconstriction. In this regard, we have angiotensin II, produced by the kidneys and vasopressin (antidiuretic hormone/ADH), produced by the pituitary gland in the brain. Both of these hormones cause contraction of vascular smooth muscle, vasoconstriction and a resultant increase in blood pressure.

Key points

• Stimulation of sympathetic nerves causes contraction of vascular smooth muscle and reduced blood flow to the gut and skin during exercise.

• Adrenaline causes relaxation of vascular smooth muscle in blood vessels supplying exercising muscles.

• Inflammatory molecules, hormones produced by the kidneys and waste products of metabolism all influence blood flow.