Smooth Muscle Examples and Function

The term smooth muscle refers to a muscle of the human body that is part of an involuntary muscle group. These muscles are extremely important to help the body perform a variety of crucial tasks, and can be found in many parts of the body. Keep reading to learn more about smooth muscle examples and how they function in the body.

diagram of artery with smooth muscle identification diagram of artery with smooth muscle identification
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Understanding Smooth Muscles

There are three types of muscles in the body: smooth muscles, cardiac muscles and skeletal muscles. Smooth muscles are unique in their largely involuntary response, and in their structure. Smooth muscles have a much stronger ability to contract than skeletal muscles, and are able to maintain contraction longer.

There are two types of smooth muscles:

  • visceral smooth muscle - all cells work together at the same time as one unit
  • multi-unit smooth muscle - cells can act and function independently of the others

The type of smooth muscle you’ll find depends on the organ they’re in or surrounding. You’ll find that most organs in your body have either visceral or multi-unit smooth muscle.

Where You Can Find Smooth Muscles

Unlike skeletal muscles, which are located near bones and control bodily movement, smooth muscles are found in and around organs. They’re primarily found in the walls of hollow organs where they can perform involuntary movements. You can find smooth muscle in the following places:

  • Walls of blood vessels
  • Walls of stomach
  • Ureters
  • Intestines
  • In the aorta (tunica media layer)
  • Iris of the eye
  • Prostate
  • Gastrointestinal Tract
  • Respiratory Tract
  • Small arteries
  • Arterioles
  • Reproductive tracts (both genders)
  • Veins
  • Glomeruli of the Kidneys (called mesangial cells)
  • Bladder
  • Uterus
  • Arrector pili of the skin
  • Ciliary muscle
  • Sphincter
  • Trachea
  • Bile duct

The specific roles of smooth muscles depend on where they are found in the body. The functions of smooth muscles are extremely crucial to the effectiveness of each organ with which the muscles are involved.

Functions of Smooth Muscles

Smooth muscles are hard at work in your body every moment of the day, even though you may not be aware of their activity. Take a moment to see what they are doing in different systems of your body.

Circulatory Smooth Muscle Functions

The main organ of the circulatory system is the heart, which uses cardiac muscle to contract. However, the blood vessels in the circulatory system use vascular smooth muscle cells to carry blood throughout the body. Smooth muscle functions in the circulatory system include:

  • Smooth muscle determines the flow of blood in the arteries.
  • In arteries, smooth muscle movements maintain the arteries' diameter.
  • Smooth muscles in arteries and veins are largely responsible for regulation of blood pressure.
  • Arteries have thick walls due to smooth muscle cells, which help them carry blood away from the heart to every part of the body.
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Digestive and Urinary Smooth Muscle Functions

The digestive system is almost entirely made of smooth muscle. From the lower section of the esophagus all the way to the rectum, smooth muscle pushes food through the digestive tract. The urinary system also includes a lot of smooth muscle. Functions of smooth muscle in the digestive and urinary systems include:

  • Smooth muscles move food through the digestive tract.
  • Folds in the stomach and small intestine, which are made of smooth muscle, help the body better absorb nutrients.
  • Smooth muscle composes all sphincters in the digestive system.
  • In the bladder, smooth muscle helps to push out urine.
  • When the bladder expands or contracts, folds of smooth muscle prevent refluxes of urine.

Integumentary Smooth Muscle Functions

Also known as the skin system, the integumentary system is the largest system in your body. Functions of smooth muscle in the integumentary system include:

  • Skin stands on end when piloerector muscles contract (also known as goosebumps).
  • Smooth muscles around blood vessels relax to allow more blood to flow into the skin.
  • Smooth muscle lining around secretory cells allows sweat to be excreted from skin ducts.

Nervous System Smooth Muscle Functions

All smooth muscle and smooth muscle cells are controlled by the nervous system. But the nervous system itself also includes several instances of smooth muscle, especially in the eye. Smooth muscles in the nervous system have the following functions:

  • The smooth muscles found in the eye’s pupillary sphincter are responsible for shrinking the size of the pupil.
  • The dilator muscle in the eye dilates the pupil.
  • The ciliary muscle is made of smooth muscle, and it alters the shape of the eye’s lens.
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Respiratory Smooth Muscle Functions

Several parts of the respiratory system have smooth muscle that aids in breathing. Lung diseases caused by smoking or pollution directly affect how well these muscles function, which can lead to compromised breathing. Here are some common functions of respiratory smooth muscles.

  • The bronchi and bronchioles use smooth muscle to bring air from the trachea into the lungs.
  • Smooth muscle in the lungs helps the airways to expand and contract as necessary.
  • During exercise or exertion, smooth muscles in the bronchi relax and dilate.

Reproductive Smooth Muscle Functions

Both male and female reproductive systems include smooth muscles. These muscles affect various bodily functions, depending on the specific organ. Their functions include:

  • During menstruation, smooth muscle in the uterus contracts to expel the uterine lining.
  • Smooth muscle helps sperm to move along the male reproductive tract.
  • To prevent urination during ejaculation by men, the sphincter muscle constricts.
  • The tunica dartos is a smooth muscle that contracts or expands to regulate temperature of the testicles.
  • The smooth muscle in the uterus helps a woman to push out her baby.
  • Smooth muscle cells in the ovaries contract when a woman ovulates.

Smooth Muscle Is Part of a Larger System

Now you see lots of smooth muscle examples, know where these muscles are in your own body, and understand the roles of these different muscles. For more information about the body, check out these examples of skeletal muscles and where they can be found. Or you can take a look at the whole body with a helpful overview of the major body systems.