Essentials of Anatomy and Physiology

The vascular system consists of the arteries,
capillaries, and veins through which blood
travels.

Arteries (and arterioles) (see Fig. 13–1)

1. Carry blood from the heart to capillaries; three lay-
   ers in their walls.


2. Inner layer (tunica intima): simple squamous
   epithelial tissue (endothelium), very smooth to pre-
   vent abnormal blood clotting; secretes nitric oxide
   (NO), a vasodilator.


3. Middle layer (tunica media): smooth muscle and
   elastic connective tissue; contributes to mainte-
   nance of diastolic blood pressure (BP).


4. Outer layer (tunica externa): fibrous connective tis-
   sue to prevent rupture.


5. Constriction or dilation is regulated by the auto-
   nomic nervous system.

Veins (and venules) (see Fig. 13–1)

1. Carry blood from capillaries to the heart; three lay-
   ers in walls.


2. Inner layer: endothelium folded into valves to pre-
   vent the backflow of blood.


3. Middle layer: thin smooth muscle, because veins
   are not as important in the maintenance of BP.


4. Outer layer: thin fibrous connective tissue because
   veins do not carry blood under high pressure.

Anastomoses—connections between vessels
of the same type

1. Provide alternate pathways for blood flow if one
   vessel is blocked.


2. Arterial anastomoses provide for blood flow to the
   capillaries of an organ (e.g., circle of Willis to the
   brain).


3. Venous anastomoses provide for return of blood to
   the heart and are most numerous in veins of the
   legs.

Capillaries (see Figs. 13–1 and 13–2)

1. Carry blood from arterioles to venules.


2. Walls are one cell thick (simple squamous epithelial
   tissue) to permit exchanges between blood and tis-
   sue fluid.


3. Oxygen and carbon dioxide are exchanged by
   diffusion.
   4. BP in capillaries brings nutrients to tissues and
   forms tissue fluid in the process of filtration.
   5. Albumin in the blood provides colloid osmotic
   pressure, which pulls waste products and tissue fluid
   into capillaries. The return of tissue fluid maintains
   blood volume and BP.
   6. Precapillary sphincters regulate blood flow into
   capillary networks based on tissue needs; in active
   tissues they dilate; in less active tissues they con-
   strict.
   7. Sinusoids are very permeable capillaries found in
   the liver, spleen, pituitary gland, and red bone mar-
   row to permit proteins and blood cells to enter or
   leave the blood.

Pathways of Circulation

1. Pulmonary: Right ventricle →pulmonary artery →
   pulmonary capillaries (exchange of gases) →pul-
   monary veins →left atrium.


2. Systemic: left ventricle →aorta →capillaries in
   body tissues →superior and inferior caval veins →
   right atrium (see Table 13–1 and Fig. 13–3 for sys-
   temic arteries and Table 13–2 and Fig. 13–4 for sys-
   temic veins).


3. Hepatic portal circulation: blood from the diges-
   tive organs and spleen flows through the portal
   vein to the liver before returning to the heart.
   Purpose: the liver stores some nutrients or regu-
   lates their blood levels and detoxifies potential poi-
   sons before blood enters the rest of peripheral
   circulation (see Fig. 13–7).

Fetal Circulation—the fetus depends on the

mother for oxygen and nutrients and for the

removal of waste products (see Fig. 13–8)

1. The placenta is the site of exchange between fetal
   blood and maternal blood.


2. Umbilical arteries (two) carry blood from the fetus
   to the placenta, where CO 2 and waste products
   enter maternal circulation.


3. The umbilical vein carries blood with O 2 and nutri-
   ents from the placenta to the fetus.


4. The umbilical vein branches; some blood
   flows through the fetal liver; most blood flows
   through the ductus venosus to the fetal inferior
   vena cava.


5. The foramen ovale permits blood to flow from the

The Vascular System 315

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