Arterioles, Metarterioles, Capillaries, and Venules

Arterioles The smallest of the vessels that carry blood away from the heart, the arterioles, direct blood into the capillary networks. Both the tunica interna and tunica externa of arterioles are thin and the tunica media consists of only one or two layers of muscle cells that encircle the vessel. However, these arterioles are particularly important in regulating the amount of flow delivered to the tissues that they feed. These arterioles have a rich supply of sympathetic nerve fibers. When they receive a high number of signals from the sympathetic nervous system (i.e. a high degree of sympathetic tone), the arterioles constrict, increasing resistance to blood flow. If the signals from the sympathetic nervous system decrease (i.e. decreased sympathetic tone), the arterioles dilate, reducing the resistance to flow. Arterioles can also adjust their diameters in response to hormonal signals (such as angiotensin II) and local signaling molecules (such as prostaglandins). Fine adjustments to the diameter of the arteriole lumen have a direct effect on the flow of blood into the capillary network supplied by that particular arteriole. Arterioles are numerous and do not have individual names as the elastic and muscular arteries do.

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Figure 31  

Metarterioles

Metarterioles are short vessels that connect arterioles to the capillary networks. Metarterioles do not have a true tunica media. Instead, at the metarteriole-capillary junctions a single smooth muscle cell forms a ring around the metarteriole. Each encircling muscle cell acts as a precapillary sphincter, regulating the flow of blood into the capillaries that branch from the metarteriole. In response to stimuli, the muscle cell encircling the metarteriole contracts, reducing the size of the lumen. This prevents the flow of blood into the capillaries fed by that metarteriole. If most or all of the precapillary sphincters associated with a capillary network contract simultaneously, blood is moved directly from the arterial to the venous system through the metarteriole. In this situation, the metarteriole is acting as a thoroughfare channel, and the entire capillary network is bypassed. Because each metarteriole regulates blood flow into a specific number of capillaries, blood flow through any tissue is finely controlled. Blood delivery to a particular tissue can be quickly increased, decreased, or even temporarily halted in order to respond to the current metabolic activity of the tissues they supply.

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Capillaries

The smallest of all the vessels, the capillaries form extensive networks throughout the entire body. Capillary networks form the connections between the arterial and venous systems. The complexity of each capillary network varies in response to the metabolic needs of the tissues served. Tissues with high oxygen requirements, such as skeletal muscle, have 8 or 10 capillaries branching from each metarteriole. Tissues with lower needs, such as the intestinal tract have 2 or 3 capillaries from each metarteriole. There are even some tissues, such as nail beds, with metabolic needs so low that the ratio of metarteriole to capillary is one to one.

In addition to capillary network design, capillary wall construction further facilitates the rapid exchange of gases and solutes. Capillary walls consist of only a tunica interna. Decreasing the distance that substances must travel increases the diffusion rate. The artery and arteriole wall thickness prevents diffusion of gases and solutes from the blood until it arrives at the target tissue. In some tissues the demand for oxygen and nutrients is so high that even the thin barrier of the tunica interna prevents adequate diffusion rates. To accommodate the specific needs of each tissue and organ, there are three different types of

capillaries, based on wall structure. Following the exchange of gases and solute, capillary networks drain into postcapillary venules.

Types of Capillaries Capillaries are classified by the structure and arrangement of their endothelial cells and the underlying basement membrane. The three types, continuous capillary, fenestrated capillary, and sinusoid capillary, each have a characteristic structure that dictates the level of permeability. Continuous capillaries form smooth tubes with only narrow intercellular clefts between the adjacent endothelial cells. The basement membrane is whole and without pores. Small molecules such as glucose and oxygen can readily pass across and between the cells, but larger molecules such as plasma proteins and platelets, cannot pass across the membrane. Continuous capillaries have the lowest permeability rate of the three types and are common in muscle, nervous, and connective tissues. The endothelial cells of fenestrated capillaries have numerousfenestrae (“windows”). These are areas of the cells where the plasma membranes from opposite sides of the cell adhere and exclude the cytoplasm. Often the membrane of the fenestrations is constructed of a material even more porous than a typical phospholipid bilayer. The basement membrane of fenestrated capillaries has openings or pores that also increase the permeability of the vessels. Fenestrated capillaries allow for faster solute exchange but still exclude larger molecules from passing through. They are common in areas that require rapid absorption or filtration such as the small intestines, the kidneys and the choroid plexuses of the brain. Sinusoid capillaries are limited to areas where blood cells move in and out of the bloodstream. The endothelial cells are widely separated and contain large pores without membrane coverings. The basement membrane of sinusoid capillaries is sparse or missing. This structure allows large plasma proteins and even blood cells to enter and leave the capillaries. Sinusoid capillaries are found where blood cells are formed, such as red bone marrow, and where large proteins enter the bloodstream, such as the liver.

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Venules

At the venous end of a capillary network, the capillaries that branch from a single metarteriole reunite and empty into a venule. As blood moves into the venules, and thus the venous system, the return trip to the heart begins. The walls of venules and veins are much thinner than those of arteries and arterioles. Without blood within, arteries retain their shape but veins collapse. The postcapillary venules collect blood from the capillary networks. These smallest and most porous of the venules are involved in solute and gas exchange in the tissues along with the capillary networks. Postcapillary venules are also the site of white blood cell exit from the bloodstream in response to inflammation or infection. In a reversal of the branching of arteries into smaller and smaller vessels, venules coalesce to form larger and larger vessels, gaining vessel wall components and thickness as they enlarge.

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