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Chapter: Anatomy and Physiology for Health Professionals: Vascular System

The smallest-diameter blood vessels are capillaries, which connect the smallest arterioles to the smallest venules.



The smallest-diameter blood vessels are capillaries, which connect the smallest arterioles to the smallest venules. The walls of capillaries are also composed of endothelium and form the semipermeable layer through which substances in blood are exchanged with substances in tissue fluids surrounding cells of the body (FIGURE 19 -3). The capillaries are microscopic blood vessels with extremely thin walls. Their walls contain a thin tunica intima and nothing else. There-fore, this structure of the capillary wall differs from a vein or an artery. Sometimes, a single endothelial cell makes up the entire circumference of the wall of the capillary. There are three types of capillaries, which include: continuous, fenestrated, and sinusoidal.

Continuous Capillaries

In continuous capillaries, the endothelium forms a complete lining. Most of the body is supplied with continuous capillaries. Most capillaries average 1 mm in length with an average lumen diameter of 8 to 10 μm, causing red blood cells to move through them one at a time. Although most tissues have many capillaries tendons and ligaments are examples of tissues that only have a small amount. These tissues get their nutrients from the blood vessels of adjacent connec-tive tissues. In the eye, the cornea and lens lack vessels, receiving nutrients from the aqueous humor.

Fenestrated Capillaries

In fenestrated capillaries, window-like pores penetrate the endothelial lining, which allow rapid exchange of solutes and water between the intersti-tial fluid and plasma. Examples of these capillaries include the choroid plexus of the brain, and vessels in endocrine organs such as the hypothalamus, pancreas, and pineal, pituitary, and thyroid glands. Fenestrated capillaries are also located in absorptive areas of the intestinal tract, and in the kidneys’ filtration sites. The amount of pores and levels of permeability are differ-ent between various organs and between the actual capillaries themselves.

Sinusoidal Capillaries

Sinusoidal capillaries orsinusoidsare similar tofenestrated capillaries but are more irregular in shape, with a flattened appearance. Modified capillaries that are lined with phagocytes are called sinusoids. These capillaries have gaps between nearby endothelial cells. Their basement membrane is either absent or very thin. They allow free exchange of solutes and water, including plasma proteins, between interstitial fluid and blood. Through the sinusoidal capillaries, blood moves slowly, which increases the time available for exchange across the capillary walls. These capillaries are found in the bone marrow, liver, spleen, and endo-crine organs such as the adrenal and pituitary glands. In the liver, plasma proteins secreted by liver cells enter the bloodstream. In the sinusoidal capillaries of the liver, bone marrow, and spleen, phagocytic cells remove damaged red blood cells, cellular debris, and pathogens from the blood.

Capillary Beds

Capillaries form interwoven networks that are referred to as capillary beds or capillary plexuses. The term microcirculation is used to describe blood flowthrough the capillary beds, which lie between the arteri-oles and the venules. Just one arteriole often connects to dozens of capillaries, which empty into several venules. The two types of vessels in capillary beds, which exist in most areas of the body, include true capillaries, which are the actual exchange vessels, and vascular shunts, also called metarterioles or precapillary arterioles, which are short vessels directly connecting arterioles and venules at opposite ends of capillary beds.

A terminal arteriole feeds the capillary bed via the metarteriole, which is continuous with a thoroughfarechannel between a capillary and a venule. This channelthen joins the postcapillary venule, which drains the capillary bed. In each capillary bed, there are usually between 10 and 100 capillaries, varying per organ or type of body tissue. Most often, they branch from the metarteriole, returning to the thoroughfare channel. However, sometimes they emerge from the terminal arteriole, emptying directly into the venule.

A precapillary sphincter surrounds each true capillary’s root at the metarteriole. This sphincter is a cuff of smooth muscle fibers that function as a valve, regulating blood flow into the capillary. The blood that flows through a terminal arteriole moves either through the true capillaries or through the vascu-lar shunts. Open precapillary sphincters allow blood to flow through the true capillaries for tissue cell exchanges. Closed precapillary sphincters cause blood to flow through the vascular shunts, bypassing the ­tissue cells. More than one artery can supply blood to a capillary bed. These multiple arteries are known as collaterals, and fuse before becoming arterioles. Thisfusion is an example of arterial anastomosis, such as in the connections between the anterior and posterior interventricular arteries of the heart.

The amount of blood that enters a capillary bed is regulated by local chemical conditions as well as arte-riolar vasomotor nerve fibers. Based on body or body region conditions, a capillary bed may be nearly full with blood or bypassed almost totally. An example of differ-ing conditions involves eating. After a meal the digestive process causes the blood to circulate freely through the true capillaries of the gastrointestinal organs so break-down products may be received for absorption by the body. As you approach the time of your next meal and are getting hungry, the ­majority of these capillary path-ways are closed. A different example involves vigorous exercise. Blood is rechanneled from the digestive organs to the skeletal muscles,­ where it is needed more. Vigorous exercise after a meal “confuses” the body’s manage-ment of blood to the capillaries and may end up causing abdominal problems such as cramping or indigestion.

Capillary walls have thin slits where endothelial cells overlap. These slits have various sizes, affect-ing permeability. Capillaries of muscles have smaller openings than those of the glands, kidneys, and small intestine. Tissues with higher metabolic rates, such as muscles, have many more capillaries than those with slower metabolic rates, such as cartilage.

Some capillaries pass directly from arterioles to venules, whereas others have highly branched net-works (FIGURE 19-4). The precapillary sphincters con-trol blood distribution through capillaries. Based on the demands of cells, these sphincters constrict or relax so blood can follow specific pathways to meet tissue cellular requirements. Gases, metabolic byproducts, and nutrients are exchanged between capillaries and the tissue fluid surrounding body cells. Capillary walls allow diffusion of blood with high levels of ­oxygen and nutrients and also allow high levels of carbon­ dioxide and other wastes to move from the tissues­ into the capillaries. Diffusion occurs also between the blood and interstitial fluid in capillaries.

Plasma proteins usually cannot move through the capillary walls because of their large size, so they remain in the blood. Blood pressure generated when capillary walls contract provides force for filtration via hydrostatic pressure. Blood pressure is strongest when blood leaves the heart and weaker as the distance from the heart increases because of friction known as peripheral resistance between the blood and the vessel walls. Therefore, blood pressure is highest in the arteries, less so in the arterioles, and lowest in the cap-illaries. Filtration occurs mostly at the arteriolar ends of capillaries because the pressure is higher than at the venular ends. Plasma proteins trapped in capillaries create an osmotic pressure that pulls water into the capillaries, known as colloid osmotic pressure.

Capillary blood pressure favors filtration, whereas plasma colloid osmotic pressure favors reabsorption. At the venular ends of capillaries, blood pressure is decreased because of resistance so reabsorption can occur. The capillaries of the body provide direct access to most cells and are perfectly located and formed to easily exchange gases, hormones, nutrients, and other components between the interstitial fluid and blood.

Angiogenesis is the formation of new blood­vessels. It is directed by vascular endothelialgrowth factor (VEGF).Angiogenesis occurs in anembryo as the tissues and organs develop. It can also occur at other times in body tissues as a response to factors released by cells that are oxygen-starved or hypoxic. Angiogenesis is most clinically important inthe heart, where it occurs because of chronic constric-tion or occlusion of blood vessels.

1. Identify three types of capillaries.

2. What is the role of the precapillary sphincters?

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