The feet have a slightly higher Pv level during quiet standing compared to the level in a static column of blood from the right atrium to the feet. This indicates that the pressure drop from the foot veins to the right atrium is very small. Due to this low resistance, all the veins can be considered to have a common venous compliance in the circulatory system model. When a person transitions from quiet standing to walking, the Pv in the legs significantly decreases. The contraction of leg muscles and the operation of venous valves force blood from the veins towards the heart. This muscular contraction serves as an auxiliary pump, reducing the mean Pv in the legs and preventing venous pooling and lower capillary hydrostatic pressure. This ultimately reduces the tendency for edematous fluid accumulation in the feet during standing.

The circulatory system supplies the body's tissues with blood to meet the requirements for oxygen and nutrients. Capillaries, which consist of a single layer of endothelial cells, allow rapid exchange of gases, water, and solutes with interstitial fluid. Muscular arterioles, the major resistance vessels, regulate regional blood flow to capillary beds. Venules and veins primarily function as collecting channels and storage vessels. The lymphatic system, composed of lymphatic vessels, nodes, and lymphoid tissue, collects fluid and proteins that have escaped from the blood and transports them back into the veins for recirculation. This section provides a detailed examination of the smallest blood vessels in the body, as well as the lymphatic vessels.

The microcirculation refers to the circulation of blood through the smallest vessels in the body, including arterioles, capillaries, and venules. Arterioles, with a diameter of 5 to 100 µm, have a thick smooth muscle layer, a thin adventitial layer, and an endothelial lining. Arterioles directly give rise to capillaries (5 to 10 µm in diameter) or, in some tissues, to metarterioles (10 to 20 µm in diameter), which then give rise to capillaries. Metarterioles can bypass the capillary bed and connect to venules or connect directly to the capillary bed. Arterioles that directly give rise to capillaries regulate flow through these capillaries by constriction or dilation. The capillaries form an interconnected network of tubes with an average length of 0.5 to 1 mm.

Capillary density is high in metabolically active organs like the heart, skeletal muscle, and glands, while it is low in less active tissues such as subcutaneous tissue or cartilage. Capillary diameter also varies, with some capillaries having smaller diameters than erythrocytes, requiring temporary deformation of erythrocytes for passage. Fortunately, normal erythrocytes are highly flexible. Blood flow in capillaries primarily depends on the contractile state of arterioles. The average velocity of blood flow in capillaries is approximately 1 mm/second, but it can vary within a brief period. These variations in capillary blood flow may be random or rhythmic. Rhythmic oscillatory behavior of capillaries is caused by the contraction and relaxation (vasomotion) of the precapillary vessels, including arterioles and small arteries.
Precise measurement reveals that the final level of Pv in the feet during quiet standing is only slightly greater than that in a static column of blood extending from the right atrium to the feet. This finding indicates that the pressure drop caused by blood flow from the foot veins to the right atrium is very small. Because of this very low resistance, all the veins can be viewed as having a common venous compliance in the model of the circulatory system illustrated in . When an individual who has been standing quietly begins to walk, Pv in the legs decreases appreciably (see
Fig. 17.21 ). Because of the intermittent venous compression exerted by the contracting leg muscles, and because of the operation of the venous valves, blood is forced from the veins toward the heart. Hence, muscular contraction lowers the mean Pv in the legs and serves as an auxiliary pump. Furthermore, muscular contraction prevents venous pooling and lowers capillary hydrostatic pressure. In this way, muscular contraction reduces the tendency for edematous fluid to collect in the feet during standing.
The circulatory system supplies the tissues with blood in amounts that meet the body’s requirements for O2 and nutrients. The capillaries, whose walls consist of a single layer of endothelial cells, allow rapid exchange of gases, water, and solutes with interstitial fluid. The muscular arterioles, which are the major resistance vessels, regulate regional blood flow to the capillary beds. Venules and veins serve primarily as collecting channels and storage vessels. The lymphatic system is composed of lymphatic vessels, nodes, and lymphoid tissue. This system collects the fluid and proteins that have escaped from blood and transports them back into the veins for recirculation in blood. In this section, the network of the smallest blood vessels of the body, as well as the lymphatic vessels, are examined in detail.
The microcirculation is defined as the circulation of blood through the smallest vessels of the body: arterioles, capillaries, and venules. Arterioles (5 to 100 µm in diameter) have a thick smooth muscle layer, a thin adventitial layer, and an endothelial lining (see
Fig. 15.2 ). Arterioles give rise directly to capillaries (5 to 10 µm in diameter) or, in some tissues, to metarterioles (10 to 20 µm in diameter), which then give rise to capillaries (
Fig. 17.22 ). Metarterioles can bypass the capillary bed and connect to venules, or they can connect directly to the capillary bed. Arterioles that give rise directly to capillaries regulate flow through these capillaries by constriction or dilation. The capillaries form an interconnecting network of tubes with an average length of 0.5 to 1 mm.
Functional Properties of Capillaries
In metabolically active organs, such as the heart, skeletal muscle, and glands, capillary density is high. In less active tissues, such as subcutaneous tissue or cartilage, capillary density is low. Capillary diameter also varies. Some capillaries have diameters smaller than those of erythrocytes. Passage through these tiny vessels requires the erythrocytes to become temporarily deformed. Fortunately, normal erythrocytes are quite flexible.
Blood flow in capillaries depends chiefly on the contractile state of arterioles. The average velocity of blood flow •Fig. 17.22 Composite Schematic Illustration of the Microcirculation. Thecircular structures onthearterioleandvenulerepresentsmoothmusclefibers,andthebranching solid lines representsympatheticnervefibers.Thearrows indicatethedirectionofbloodflow.AV,arteriovenous.
in capillaries is approximately 1 mm/second; however, it can vary from zero to several millimeters per second in the same vessel within a brief period. These changes in capillary blood flow may be random or rhythmic. The rhythmic oscillatory behavior of capillaries is caused by contraction and relaxation (vasomotion) of the precapillary vessels (i.e., the arterioles and small arteries).