4. Arteries are capable of vasoconstriction as directed by the sympathetic impulses; when impulses are inhibited, vasodilation results.
C. Capillaries (p. 359; Figs. 13.20-13.21)
1. Capillaries are the smallest vessels, consisting only of a layer of endothelium through which substances are exchanged with tissue cells.
2. Capillary permeability varies from one tissue to the next, generally with more permeability in the liver, intestines, and certain glands, and less in muscle and considerably less in the brain (blood-brain barrier).
3. The pattern of capillary density varies from one body part to the next.
a. Areas with a great deal of metabolic activity (leg muscles, for example) have higher densities of capillaries.
4. Precapillary sphincters can regulate the amount of blood entering a capillary bed.
a. If blood is needed elsewhere in the body, the capillary beds in less important areas are shut down.
D. Exchanges in the Capillaries (p. 361; Fig. 13.22)
1. Blood entering capillaries contains high concentrations of oxygen and nutrients that diffuse out of the capillary wall and into the tissues.
a. Plasma proteins remain in the blood.
2. Hydrostatic pressure drives the passage of fluids and very small molecules out of the capillary at this point.
3. At the venule end, osmosis, due to the osmotic pressure of the blood, causes much of the tissue fluid to return to the bloodstream.
4. Lymphatic vessels collect excess tissue fluid and return it to circulation.
E. Venules and Veins (p. 362; Figs. 13.23-13.24)
1. Venules leading from capillaries merge to form veins that return blood to the heart.
2. Veins have the same three layers as arteries have and have a flap-like valve inside to prevent backflow of blood.
a. Veins are thinner and less muscular than arteries; they do not carry high-pressure blood.
A. The body's blood vessels can be divided into a pulmonary circuit, including vessels carrying blood to the lungs and back, and a systemic circuit made up of vessels carrying blood from the heart to the rest of the body and back.
B. Pulmonary Circuit (p. 367)
1. The pulmonary circuit is made up of vessels that convey blood from the right ventricle to the pulmonary arteries to the lungs, alveolar capillaries, and pulmonary veins leading from the lungs to the left atrium.
C. Systemic Circuit (p. 367)
1. The systemic circuit includes the aorta and its branches leading to all body tissues as well as the system of veins returning blood to the right atrium.
13.7 Arterial System (p. 367; Fig. 13.26)
A. The aorta is the body's largest artery.
B. Principal Branches of the Aorta (p. 367; Table 13.3)
1. The branches of the ascending aorta are the right and left coronary arteries that lead to heart muscle.
2. Principal branches of the aortic arch include the brachiocephalic, left common carotid, and left subclavian arteries.
3. The descending aorta (thoracic aorta) gives rise to many small arteries to the thoracic wall and thoracic viscera.
4. The abdominal aorta gives off the following branches: celiac, superior mesenteric, suprarenal, renal, gonadal, inferior mesenteric, and common iliac arteries.
C. Arteries to the Head, Neck, and Brain (p. 367; Figs. 13.27-13.29)
Arteries to the head, neck, and brain include branches of the subclavian and common carotid arteries.
The vertebral arteries supply the vertebrae and their associated ligaments and muscles.
In the cranial cavity, the vertebral arteries unite to form a basilar artery which ends as two posterior cerebral arteries.
The posterior cerebral arteries help form the circle of Willis which provides alternate pathways through which blood can reach the brain.
The right and left common carotid arteries diverge into the external carotid and internal carotid arteries.
Near the base of the internal carotid arteries are the carotid sinuses that contain baroreceptors to monitor blood pressure.
D. Arteries to the Shoulder and Upper Limb (p. 370)
1. The subclavian artery continues into the arm where it becomes the axillary artery.
2. In the shoulder region, the axial artery becomes the brachial artery that, in turn, gives rise to the ulnar and radial arteries.
E. Arteries to the Thoracic and Abdominal Walls (p. 370; Fig. 13.30)
1. Branches of the thoracic aorta and subclavian artery supply the thoracic wall with blood.
2. Branches of the abdominal aorta, as well as other arteries, supply the abdominal wall with blood.
F. Arteries to the Pelvis and Lower Limb (p. 372; Fig. 13.31)
1. At the pelvic brim, the abdominal aorta divides to form the common iliac arteries that supply the pelvic organs, gluteal area, and lower limbs.
2. The common iliac arteries divide into internal and external iliac arteries.
a. Internal iliac arteries supply blood to pelvic muscles and visceral structures.
b. External iliac arteries lead into the legs, where they become femoral, popliteal, anterior tibial, and posterior tibial arteries.
13.8 Venous System (p. 372)
A. Veins return blood to the heart after the exchange of substances has occurred in the tissues.
B. Characteristics of Venous Pathways (p. 372)
1. Larger veins parallel the courses of arteries and are named accordingly; smaller veins take irregular pathways and are unnamed.
2. Veins from the head and upper torso drain into the superior vena cava.
3. Veins from the lower body drain into the inferior vena cava.
4. The vena cavae merge to join the right atrium.
C. Veins from the Head, Neck, and Brain (p. 372; Fig. 13.32)
1. The jugular veins drain the head and unite with the subclavian veins to form the brachiocephalic veins.
D. Veins from the Upper Limb and Shoulder (p. 372; Fig. 13.33)
1. The upper limb is drained by superficial and deep veins.
2. The basilic and cephalic veins are major superficial veins.
3. The major deep veins include the radial, ulnar, brachial, and axillary veins.
E. Veins from the Abdominal and Thoracic Walls (p. 374; Fig. 13.34)
1. Tributaries of the brachiocephalic and azygos veins drain the abdominal and thoracic walls.
F. Veins from the Abdominal Viscera (p. 375; Figs. 13.35)
1. Blood draining from the intestines enters the hepatic portal system and flows to the liver first rather than into general circulation.
2. The liver can process the nutrients absorbed during digestion as well as remove bacteria.
3. Hepatic veins drain the liver, gastric veins drain the stomach, superior mesenteric veins lead from the small intestine and colon, the splenic vein leaves the spleen and pancreas, and the inferior mesenteric vein carries blood from the lower intestinal area.
G. Veins from the Lower Limb and Pelvis (p. 375; Fig. 13.36)
1. Deep and superficial veins drain the leg and pelvis.
The deep veins include the anterior and posterior tibial veins which unite into the popliteal vein and femoral vein; superficial veins include the small and great saphenous veins.
These veins all merge to empty into the common iliac veins.