How many layers tunicas surround a capillary




















Cardiovascular System: Blood Vessels. Search for:. Blood Vessel Structure and Function. Blood Vessel Structure Blood vessels are flexible tubes that carry blood, associated oxygen, nutrients, water, and hormones throughout the body. Learning Objectives Differentiate among the structure of arteries, veins, and capillaries.

Key Takeaways Key Points Blood vessels consist of arteries, arterioles, capillaries, venules, and veins. Vessel networks deliver blood to all tissues in a directed and regulated manner. Arteries and veins are composed of three tissue layers. The thick outermost layer of a vessel tunica adventitia or tunica externa is made of connective tissue. The middle layer tunica media is thicker and contains more contractile tissue in arteries than in veins.

It consists of circularly arranged elastic fibers, connective tissue, and smooth muscle cells. The inner layer tunica intima is the thinnest layer, comprised of a single layer of endothelium supported by a subendothelial layer. Capillaries consist of a single layer of endothelium and associated connective tissue. Key Terms tunica intima : The innermost layer of a blood vessel. Blood Vessel Function Blood vessels carry nutrients and oxygen throughout the body and aid in gas exchange. Learning Objectives List the functions of blood vessels.

The amount of elastic fibres in the tunica media decreases with these physiological changes. We now find a type of arteries which are termed. The tunica intima is thinner than in elastic arteries.

Subendothelial connective tissue other than the internal elastic lamina is often difficult to discern. The internal elastic lamina forms a well defined layer. The tunica media is dominated by numerous concentric layers of smooth muscle cells.

Fine elastic fibres and and a few collagen fibres are also present. The external elastic lamina can be clearly distinguished although it may be incomplete in places. The thickness and appearance of the tunica adventitia is variable. The basic structure of the walls of arteries does not change much as we come to the next type of arterial vessels. Size is used to differentiate them from muscular arteries.

The endothelium appears to rest on a still well-defined internal elastic lamina. The tunica media consists of concentric layers of smooth muscle cells.

It may be difficult to identify the external elastic lamina or to distinguish the tunica adventia from the connective tissue surrounding the vessel.

The smooth muscle of arterioles to some extent also that of muscular arteries regulate the blood flow to their target tissues and receive both sympathetic and parasympathetic innervation. Arterioles finally branch to give rise to the capillary network microcirculation. There are two easily recognizable features which distinguish these arteries from veins. If two vessels have a similarly sized lumen, the walls of arteries will be much thicker and more compact than the wall of veins.

At high magnification, the internal elastic lamina forms a pink streak immediately below the endothelial cell lining in arteries and even arterioles, while it is difficult to identify in veins.

The layer of subendothelial connective tissue is very thin, and the endothelium seems to rest on the internal elastic lamina. Smooth muscle cell nuclei are frequent in the tunica media. The external elastic lamina stains similar to the internal elastic lamina, but it is thicker and appears fibrous instead of forming a continuous band. Collagen fibres and a few connective tissue cell nuclei are visible in the tunica adventitia.

If you close the iris diaphragm of the microscope, the elastic layers will stand out very clearly, but remember to open the diaphragm once you have seen them.

In addition to the inner and outer elastic laminae, elastin stains will show fine elastic fibres in the tunica media and coarse elastic fibres between the collagen fibres of the tunica adventitia. The appearance of other structures will depend on the stain used together with the elastin stain. Draw either one large composite image containing the three tunics and the cellular and fibrous elements which form the tunics. Alternatively, you can draw a low power overview and supplement it with high magnification illustrations of the individual tunics.

The sum of the diameters of all capillaries is significantly larger than that of the aorta by about three orders of magnitude , which results in decreases in blood pressure and flow rate. Also, capillaries are very small vessels. The wall of a segment of capillary may be formed by a single endothelial cell. This results in a very large surface to volume ratio.

The low rate of blood flow and large surface area facilitate the functions of capillaries in. These functions are also facilitated by a very simple organisation of the wall of capillaries. Only the tunica intima is present , which typically only consists of the endothelium, its basal lamina and an incomplete layer of cells surrounding the capillary, the pericytes.

Based on the features of the endothelium three types of capillaries can be distinguished:. The endothelial cell and the basal lamina do not form openings, which would allow substances to pass the capillary wall without passing through both the endothelial cell and the basal lamina.

Endothelial cells and the basal lamina can act as selective filters in continuous capillaries. The endothelial cell body forms small openings called fenestrations, which allow components of the blood and interstitial fluid to bypass the endothelial cells on their way to or from the tissue surrounding the capillary.

The fenestrations may represent or arise from pinocytotic vesicles which open onto both the luminal and basal surfaces of the cell. The extent of the fenestration may depend on the physiological state of the surrounding tissue, i. The endothelial cells are surrounded by a continuous basal lamina, which can act as a selective filter. The basal lamina is also incomplete.

Discontinuous capillaries form large irregularly shaped vessels, sinusoids or sinusoid capillaries. They are found where a very free exchange of substances or even cells between bloodstream and organ is advantageous e.

Cardiac Muscle, sheep - Whipf's polychrome Large numbers of capillaries are present in almost all tissues. At least a few dozen cross sections are present in every sqr.

There may be thousands in highly vascularized tissues. However, a "good" capillary is not that easy to find because of their small size and because the capillary walls are very thin, which often leads to the collapse of the capillary during tissue preparation. Cardiac muscle is highly vascularized. Each muscle cell is surrounded by one or more capillaries.

The capillaries roughly follow the course of the muscle cells. To find capillaries in transverse and longitudinal section is easiest to first find areas in which the muscle cells have been cut in these planes.

Only one or two red blood cells fit side by side in the capillary. Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed.

If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump the stroke volume and maintain adequate pressure and flow.

Artery walls would have to become even thicker in response to this increased pressure. The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system. An elastic artery is also known as a conducting artery, because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches. Figure 3. Comparison of the walls of an elastic artery, a muscular artery, and an arteriole is shown.

In terms of scale, the diameter of an arteriole is measured in micrometers compared to millimeters for elastic and muscular arteries. The artery at this point is described as a muscular artery. The diameter of muscular arteries typically ranges from 0. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction.

In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity has become less important. Rather, there is a gradual transition as the vascular tree repeatedly branches. In turn, muscular arteries branch to distribute blood to the vast network of arterioles. For this reason, a muscular artery is also known as a distributing artery. An arteriole is a very small artery that leads to a capillary.

Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin see Figure 3. With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure.

Because of this, you may see them referred to as resistance vessels. The muscle fibers in arterioles are normally slightly contracted, causing arterioles to maintain a consistent muscle tone—in this case referred to as vascular tone—in a similar manner to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure.

The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow. A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called perfusion.

Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid interstitial fluid. The diameter of a capillary lumen ranges from 5—10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as microcirculation. The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers.

There is some variation in wall structure: In a large capillary, several endothelial cells bordering each other may line the lumen; in a small capillary, there may be only a single cell layer that wraps around to contact itself. For capillaries to function, their walls must be leaky, allowing substances to pass through. The most common type of capillary, the continuous capillary , is found in almost all vascularized tissues. Continuous capillaries are characterized by a complete endothelial lining with tight junctions between endothelial cells.

Although a tight junction is usually impermeable and only allows for the passage of water and ions, they are often incomplete in capillaries, leaving intercellular clefts that allow for exchange of water and other very small molecules between the blood plasma and the interstitial fluid. Substances that can pass between cells include metabolic products, such as glucose, water, and small hydrophobic molecules like gases and hormones, as well as various leukocytes.

Continuous capillaries not associated with the brain are rich in transport vesicles, contributing to either endocytosis or exocytosis. Those in the brain are part of the blood-brain barrier. Here, there are tight junctions and no intercellular clefts, plus a thick basement membrane and astrocyte extensions called end feet; these structures combine to prevent the movement of nearly all substances.

Figure 4. The three major types of capillaries: continuous, fenestrated, and sinusoid. A fenestrated capillary is one that has pores or fenestrations in addition to tight junctions in the endothelial lining.

These make the capillary permeable to larger molecules. The number of fenestrations and their degree of permeability vary, however, according to their location. Fenestrated capillaries are common in the small intestine, which is the primary site of nutrient absorption, as well as in the kidneys, which filter the blood.

They are also found in the choroid plexus of the brain and many endocrine structures, including the hypothalamus, pituitary, pineal, and thyroid glands.

A sinusoid capillary or sinusoid is the least common type of capillary. Sinusoid capillaries are flattened, and they have extensive intercellular gaps and incomplete basement membranes, in addition to intercellular clefts and fenestrations. This gives them an appearance not unlike Swiss cheese. These very large openings allow for the passage of the largest molecules, including plasma proteins and even cells.

Blood flow through sinusoids is very slow, allowing more time for exchange of gases, nutrients, and wastes. Sinusoids are found in the liver and spleen, bone marrow, lymph nodes where they carry lymph, not blood , and many endocrine glands including the pituitary and adrenal glands. Without these specialized capillaries, these organs would not be able to provide their myriad of functions. For example, when bone marrow forms new blood cells, the cells must enter the blood supply and can only do so through the large openings of a sinusoid capillary; they cannot pass through the small openings of continuous or fenestrated capillaries.

The liver also requires extensive specialized sinusoid capillaries in order to process the materials brought to it by the hepatic portal vein from both the digestive tract and spleen, and to release plasma proteins into circulation.

A metarteriole is a type of vessel that has structural characteristics of both an arteriole and a capillary. Slightly larger than the typical capillary, the smooth muscle of the tunica media of the metarteriole is not continuous but forms rings of smooth muscle sphincters prior to the entrance to the capillaries. In arteries, the blood is pumped under pressure from the heart, so backflow cannot occur. However, passing through the capillary network results in a decrease in blood pressure, meaning that backflow of blood is possible in veins.

To counteract this, veins contain numerous one-direction valves that prevent backflow. Learning Objectives Differentiate among the structure of arteries, veins, and capillaries. Key Points Blood vessels consist of arteries, arterioles, capillaries, venules, and veins. Vessel networks deliver blood to all tissues in a directed and regulated manner. Arteries and veins are composed of three tissue layers. The thick outermost layer of a vessel tunica adventitia or tunica externa is made of connective tissue.



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