Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

U N I T 5

Right jugular trunk

Left jugular trunk

Right subclavian trunk

Thoracic (left lymphatic) duct Left subclavian trunk

Right lymphatic duct Right broncho- mediastinal trunk

Right lymphatic duct Right brachiocephalic vein

Internal jugular vein

Left lymphatic duct

Subclavian vein

Superior vena cava

Intercostal trunks

Thoracic duct

Inferior vena cava

Aorta

FIGURE 17-22. Lymphatic system showing the thoracic duct and position of the left and right lymphatic ducts (inset).

Therefore, it has been proposed that when the arterial pressure stretches the vessel, this in turn causes reactive vascular constriction that reduces the blood flow nearly back to normal. Conversely, at low pressures, the degree of stretch of the vessel is less, so that the smooth mus- cle relaxes, reducing vascular resistance and helping to return blood flow toward normal.

A phenomenon called reactive hyperemia is a mani- festation of local metabolic regulation of blood flow. When the blood supply to an area has been occluded and then restored, local blood flow through the tissues increases within seconds to restore the metabolic equi- librium of the tissues. This increased flow is called reac- tive hyperemia. The transient redness seen on an arm after leaning on a hard surface is an example of reactive hyperemia. Local control mechanisms rely on a continu- ous flow from the main arteries; therefore, hyperemia cannot occur when the arteries that supply the capillary beds are narrowed. For example, if a major coronary artery becomes occluded, the opening of channels sup- plied by that vessel cannot restore blood flow. Endothelial Control of Blood Flow One of the important functions of the endothelial cells lining the arterioles and small arteries is the synthe- sis and release of factors that can affect the degree of relaxation or contraction of the arterial wall. The most important of the endothelial relaxing factors is nitric oxide . The normal endothelium maintains a continu- ous release of nitric oxide, which is synthesized from the amino acid arginine and oxygen and reduction of inorganic nitrate. The production of nitric oxide can be stimulated by a variety of endothelial agonists, includ- ing acetylcholine, bradykinin, histamine, and throm- bin. Shear stress on the endothelium resulting from an increase in blood flow or blood pressure also stimulates

Arteriole

Smooth muscles

Precapillary sphincters

Arterial capillary

Thorough- fare channel

Venous capillary

Venule

FIGURE 17-23. Capillary bed. Precapillary sphincters control the flow of blood through the capillary network. Thoroughfare channels (i.e., arteriovenous shunts) allow blood to move directly from the arteriole into the venule without moving through nutrient channels of the capillary.