What are Capillaries?

What Role Do Capillaries Play in the Circulatory System?

diagram of capillaries surrounding alveoli
What are capillaries, what types are there, and what is their function?. istockphoto.com

 

 

Definition: Capillaries

Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins. These vessels are often referred to as the "microcirculation."

Structure of Capillaries

Capillaries are very thin, approximately 5 micrometers in diameter, and are composed of only two layers of cells; an inner layer of endothelial cells and an outer layer of epithelial cells.

They are so small that red blood cells need to flow through them single file. If all the capillaries in the human body were lined up in single file, the line would stretch over 100,000 miles. It's been estimated that there are 40 billion capillaries in the average human body. Surrounding this layer of cells is something called the basement membrane, a layer of protein surrounding the capillary.

Capillaries in the Circulatory System

Capillaries may be thought of as the central portion of circulation. Blood leaves the heart through the aorta and the pulmonary arteries traveling to the rest of the body and to the lungs respectively. These large arteries become smaller arterioles and eventually narrow to form the capillary bed. From the capillaries, blood flows into the smaller venules and then into veins, flowing back to the heart.

Function of Capillaries

The capillaries are responsible for facilitating the transport and exchange of gasses, fluids, and nutrients in the body.

Gas Exchange

In the lungs, oxygen diffuses from the alveoli into capillaries to be attached to hemoglobin and be carried throughout the body. Carbon dioxide (from deoxygenated blood) in turn flows from the capillaries back into alveoli to be exhaled into the environment.

Fluid and Nutrient Exchange

Likewise, fluids and nutrients diffuse through selectively permeable capillaries into the tissues of the body, and waste products are picked up in the capillaries to be transported through veins to the kidneys and liver where they are thus processed and eliminated from the body.

Types of Capillaries

There are 3 primary types of capillaries:

  • Continuous - These capillaries have no perforations and allow only small molecular to pass through.  They are present in muscle, skin, fat, and nerve tissue.
  • Fenestrated - These capillaries have small pores which allow small molecules through and are located in the intestines, kidneys, and endocrine glands.
  • Sinusoidal or discontinuous - These capillaries have large open pores—large enough to allow a blood cell through. They are present in the bone marrow, lymph nodes, and the spleen, and are, in essence, the "leakiest" of the capillaries.
  • Blood-brain barrier - In the central nervous system the capillaries make up what is known as the blood-brain barrier. This barrier limits the ability of toxins (and, unfortunately, many chemotherapy agents) to pass through into the brain.  

Blood Flow Through Capillaries

Since the blood flow through capillaries plays such an important part in maintaining the body, you may wonder what happens when blood flow changes, for example, if your blood pressure would drop (hypotension.) Capillary beds are regulated through something called autoregulation, so that if blood pressure would drop, flow through the capillaries will continue to provide oxygen and nutrients to the tissues of the body.

With exercise, more capillary beds are recruited in the lungs to prepare for an increased need for oxygen in tissues of the body.

The flow of blood in the capillaries is controlled by precapillary sphincters. A precapillary sphincter is the muscular fibers that control the movement of blood between the arterioles and capillaries.

Physiology of Fluid Movement to and from the Capillaries - Capillary Microcirculation

Regulation of fluid movement between the capillaries and the surrounding interstitial tissues is determined by the balance of two forces: the hydrostatic pressure and osmotic pressure.

On the arterial side of the capillary, the hydrostatic pressure (the pressure that comes from the heart pumping blood and the elasticity of the arteries) is high. Since capillaries are "leaky" this pressure forces fluid and nutrients against the walls of the capillary and out into the interstitial space and tissues.

On the vein side of the capillary, the hydrostatic pressure has dropped significantly. At this point, it is the osmotic pressure of the fluid within the capillary (due to the presence of salts and proteins in the blood) that draws fluids back into the capillary. Osmotic pressure is also referred to as oncotic pressure and is what pulls fluids and waste products out of the tissues and into the capillary to be returned to the bloodstream (and then delivered to the kidneys among other sites.)

Number of Capillaries

The number of capillaries in a tissue can vary widely. Certainly, the lungs are packed with capillaries surrounding the alveoli to pick up oxygen and drop off carbon dioxide. Outside of the lungs, capillaries are more abundant in tissues that are more metabolically active.

The Capillaries "Visually" and in Medical Practice

Skin blanching - If you've ever wondered why your skin turns white when you put pressure on it the answer is the capillaries. Pressure on the skin presses blood out of the capillaries resulting in the blanching or pale appearance when the pressure is removed. 

Capillary refill - Doctors often check for "capillary refill." This is tested by observing how rapidly the skin becomes pink again after pressure is released and can give an idea of the health of the tissues. An example of this use would be in people with burns. A second-degree burn may reveal capillary refill to be somewhat delayed, but in a third-degree burn, there would be no capillary refill at all.

Emergency responders often check capillary refill by pushing on a fingernail or toenail, then releasing  pressure and waiting to see how long it takes for the nailbed to appear pink again. If color returns within two seconds (the amount of time it takes to say capillary refill), circulation to the arm or leg is probably okay. If capillary refill takes more than two seconds, the circulation of the limb is probably compromised and considered an emergency. There are other settings in which capillary refill is delayed as well, such as in dehydration.

Third spacing and capillary permeability - You may hear doctors talk about a phenomenon known as "third spacing." Capillary permeability refers to the ability of fluids to pass out of the capillaries into the surrounding tissues. Capillary permeability can be increased by cytokines (leukotrienes, histamines, and prostaglandins) released by cells of the immune system. The increased fluid (third spacing) locally can result in hives. When someone is very ill, this third spacing due to leaky capillaries may be widespread, giving their body a swollen appearance.

Capillary blood samples - Most of the time when you have your blood drawn, a technician will take blood from a vein in your arm. Capillary blood may also be used to do some blood tests, such as for those who monitor their blood sugar. A lancet is used to cut the finger (cut capillaries) and can be used for testing blood sugar and blood pH.

Conditions Involving the Capillaries

There are several both common and uncommon conditions which involve the capillaries. A few of these include:

Port wine stain - "birth mark"  - Around 1 in 300 children are born with "birth marks" consisting of an area of red or purple skin related to dilated capillaries. Most port wine stains are a cosmetic problem rather than a medical concern, but they may bleed easily when irritated.

Capillary malformation - arteriovenous malformation syndrome - Capillary malformation may occur as part of an inherited syndrome present in roughly 1 in 100,000 people of European ancestry. In this syndrome, there is more blood flow than normal through the capillaries near the skin, which results in pink and red dots on the skin. The may occur alone, or people may have other complications of this syndrome such as arteriovenous malformations (abnormal connections between arteries and veins) which, when in the brain, can cause headaches and seizures.

Systemic capillary leak syndrome - A rare disorder known as capillary leak syndrome involves leaky capillaries which result in constant nasal congestion and episodes of fainting due to rapid drops in blood pressure.

Macular degeneration - Macular degeneration, now the leading cause of blindness in the United States, occurs secondary to damage in the capillaries of the retina.

Sources:

Genetics Home Reference. Capillary malformation – arteriovenous malformation syndrome. Published 11/08/16. https://ghr.nlm.nih.gov/condition/capillary-malformation-arteriovenous-malformation-syndrome

U.S. National Library of Medicine. Capillaries. https://www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0022018/

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