I. General Anatomy of the Blood Vessels (p. 750)

A. Circulatory Routes (p. 750; fig. 20.1; TR 687)

1. The usual route of blood flow around the body is heart to arteries to arterioles to capillaries to venules, then veins, and back to the heart.

2. An artery is any vessel that carries blood away from the heart. A vein is any vessel that carries blood toward the heart.

3. Usually, blood flows through only one capillary bed before returning to the heart. An exception is a portal system in which blood flows through two separate capillary beds on its return to the heart. Portal systems are found between the hypothalamus and anterior pituitary, in the kidneys, and between the intestines and liver.

4. Anastomoses are places where two veins or arteries merge. They can be arteriovenous shunts or arterial or venous anastomoses.

B. The Vessel Wall (p. 751)

1. The walls of arteries and veins are made up of three layers called tunics. (figs. 20.2, 20.3; TR 688)

a. The tunica externa (tunica adventitia) is the outermost layer, made up of loose connective tissue. It anchors the vessels in place and provides passage for the vasa vasorum that supply blood to the vessel wall.

b. The tunica media, or middle layer, is the thickest layer of the vessel, made up mostly of smooth muscle. It is responsible for vasoconstriction and vasodilation.

c. The tunica interna (tunica intima) consists of endothelium overlying a basement membrane and a sparse layer of fibroconnective tissue. It provides a smooth surrounding through which blood passes.

C. Arteries and Metarterioles (p. 752)

1. Arteries can withstand the surges of blood that occur with each beat of the heart. They are divided into categories by size.

a. Conducting (elastic) arteries are the largest.

b. Distributing (muscular) arteries distribute blood to specific organs.

c. Resistance (small) arteries are the primary means of controlling the routes of blood flow.

d. Metarterioles are short vessels that link arterioles and capillaries. They have individual muscle cells spaced a short distance apart, rather than a continuous tunica media.

D. Capillaries (p. 752; fig. 20.4)

1. Capillaries are the vessels through which materials are exchanged between blood and tissue fluid. They consist of endothelium only, and almost no body cell is more than 60–80 µm away from the nearest capillary. Exceptions occur in tendons, ligaments, and the cornea and lens.

2. Capillaries are organized into capillary beds of 10–100 capillaries, with a thoroughfare channel that can bypass the bed when needed and carry blood directly to a venule.

a. At the origin of each capillary is a precapillary sphincter to regulate passage of blood into the capillary. (fig. 20.5; TR 689)

b. About three-quarters of the body's capillaries are closed at any given time.

3. Two types of capillaries are distinguished by the sizes of gaps between or through endothelial cells.

a. Continuous capillaries occur in most tissues. Their endothelial cells are held tightly together by tight junctions, and have narrow intercellular clefts between them through which small solutes pass.

b. Fenestrated capillaries have endothelial cells riddled with holes that are covered by a thin mucoprotein diaphragm. These holes allow for more rapid passage of small molecules, which is necessary in the kidneys, small intestine, and endocrine glands, for example. (fig. 20.6; TR 690)

c. Sinusoids are blood-filled spaces in the liver, bone marrow, spleen, and other organs. Some are continuous capillaries, while others are fenestrated.

E. Veins (p. 754)

1. Venules collect blood from capillaries.

2. Venous sinuses are veins with very thin walls, large lumens, and no smooth muscle or vasomotion. Examples are found in the heart and brain.

3. Veins have much lower blood pressure than arteries because of their distance away from the heart.

4. Veins often have internal valves to prevent the backflow of blood when skeletal muscles relax. Valves are present mostly in medium-sized veins of the legs and arms.

5. About 54% of the blood is found in the systemic veins at rest. (fig. 20.7; TR 691)

II. Blood Pressure, Resistance, and Flow (p. 756)

A. Blood flow is the amount of blood flowing through an organ, tissue, or vessel in a given time. Perfusion is the rate of blood flow per given volume or mass of tissue. (p. 756)

1. Blood flow is an important measure of the amount of oxygen and nutrients delivered to a tissue and the rate of waste removal.

2. Inadequate blood flow can result in necrosis of the tissue or even death of the individual.

B. Blood Pressure (p.756)

1. Blood pressure (BP) can be measured by various means, but most commonly a sphygmomanometer is employed.

2. Two pressures are recorded: systolic pressure, which indicates the peak arterial pressure during ventricular systole, and diastolic pressure, which is the minimum arterial pressure between heartbeats.

3. The difference between systolic and diastolic pressure is called pulse pressure, which is an important measure of the stress exerted on small arteries by the pressure surges of the heart. Another measure of stress on the blood vessels is mean arterial pressure (MAP).

4. Hypertension is a chronic resting systolic pressure of 140 mmHg or a diastolic pressure higher than 90 mmHg. Hypertension can weaken small arteries and cause aneurysms.

5. Hypotension is chronic low resting BP. It may be due to blood loss, dehydration, or anemia.

6. Blood flow in arteries is pulsatile, and when measured at points farther away from the heart, systolic and diastolic pressures are lower, with less difference between them. (fig. 20.8; TR 692)

7. Blood pressure is determined mainly by cardiac output, blood volume, and peripheral resistance, and it rises with age. (table 20.1)

C. Resistance (p. 758)

1. Peripheral resistance is the resistance that the blood encounters in the vessels as it travels away from the heart. It results from the friction of blood against the walls of the vessels and is proportional to three variables: blood viscosity, vessel length, and vessel radius.

2. Blood viscosity is mostly due to erythrocytes and albumin. A deficiency of either decreases peripheral resistance and increases blood flow; an excess increases peripheral resistance and reduces flow.

3. Pressure and flow decline with increasing vessel length. If perfusion is good at a great distance from the heart, it is likely to be good elsewhere in the systemic circulation.

4. In a healthy person, the one real way to influence peripheral resistance is through altering the diameters of the blood vessels.

a. The arterioles are the most significant point of control over peripheral resistance because of their number, their location on the proximal sides of capillaries, and because they are more muscular for their size compared to other blood vessels.

b. Vessel radius exerts a very powerful influence over blood velocity. (figs. 20.9, 20.10, TR 693; table 20.2, TR 746)

D. Regulation of Blood Pressure and Flow (p. 759)

1. Local Control

a. Autoregulation is the ability of tissues to regulate their own blood supply.

b. If a tissue is inadequately perfused, its metabolites accumulate and stimulate vasodilation. As the bloodstream delivers oxygen and carries away wastes, the vessels constrict.

c. If blood supply to a tissue is cut off and then restored, the tissue exhibits reactive hyperemia.

d. Over time, hypoxic tissue can increase its own perfusion by angiogenesis.

2. Neural Control

a. The vasomotor center of the medulla oblongata exerts sympathetic control over blood vessels throughout the body. Some sympathetic fibers induce vasoconstriction while others trigger vasodilation.

b. The vasomotor center is an integrating center for three autonomic reflexes: baroreflexes (autonomic responses to changes in blood pressure), chemoreflexes (autonomic reflexes to changes in blood chemistry, especially pH, oxygen, and carbon dioxide), and the medullary ischemic reflex (an autonomic response to insufficient perfusion of the brainstem). (fig. 20.11; TR 694)

c. A baroreflex is an autonomic negative feedback response to changes in blood pressure. (fig. 20.12; TR 695)

3. Hormonal Control

a. Angiotensin II is a potent vasoconstrictor, as are epinephrine and norepinephrine, except that the last two also dilate the coronary arteries and arteries leading to skeletal muscles. ADH is also a vasoconstrictor in high concentrations.

b. Atrial natriuretic factor has a vasodilator effect.

E. Vasomotion and Routing of Blood Flow (p.762)

1. The body is able to adjust its blood routing according to the needs of the body—e.g., when a person rests in a chair after a large meal, proportionately more blood is directed toward the organs of digestion and less toward skeletal muscle. (fig. 20.13; TR 696)

2. Physical exertion increases perfusion to the lungs, myocardium, and skeletal muscle while reducing perfusion to the digestive tract, skin, and kidneys. (fig. 20.14; TR 697)

III. Capillary Exchange (p. 763; fig. 20.15; TR 698)

A. Diffusion (p. 763)

1. Capillary exchange refers to the two-way movement of substances between capillaries and tissue fluid.

2. The most important mechanism of exchange is diffusion. Solutes more concentrated in tissue fluid diffuse into the blood, and vice versa.

3. Lipid-soluble substances diffuse through the plasma membrane. Other substances must pass through clefts, fenestrations, or membrane channels.

B. Transcytosis (p. 764)

1. In transcytosis, endothelial cells pick up droplets of fluid on one side of the plasma membrane by pinocytosis, transport the vesicles across the cell, and discharge the fluid on the other side by exocytosis.

2. Fatty acids, insulin, and albumin are transported this way, but transcytosis accounts for only a small fraction of solute exchange.

C. Filtration and Reabsorption (p. 764; fig. 20.16; TR 699)

1. Positive hydrostatic pressure inside the capillary and negative interstitial pressure work in the same direction to create a force that causes fluid to leave the capillary.

2. These two forces are opposed by colloid osmotic pressure (COP). The COP of the blood is mainly due the quantity of albumin. The difference between the COP of blood and that of tissue fluid is called the oncotic pressure, and it tends to draw water into the capillary by osmosis, thus opposing hydrostatic pressure.

3. Net filtration pressure is the difference between net hydrostatic pressure and oncotic pressure.

4. At the venous end, blood pressure is lower, but other pressures stay the same. The prevailing force is inward at the venous end because osmotic pressure overrides filtration pressure. This net reabsorption pressure causes the capillary to reabsorb fluid at the venous end.

5. Water carries dissolved solutes with it as it crosses the capillary wall, and the process is called solvent drag.

6. Capillary activity varies from one time to the next.

a. Normally, capillaries reabsorb most of what they filter, but there are exceptions.

b. During times when a tissue is metabolically active, its capillary flow increases. In resting tissue, the capillaries are collapsed.

D. Edema (p. 765)

1. Edema is the accumulation of excess fluid in a tissue.

2. Edema has three main causes.

a. Poor venous return causes pressure to back up into the capillaries, resulting in increased capillary filtration.

b. Reduced capillary reabsorption can be caused by an albumin deficiency.

c. Obstructed lymphatic drainage can lead to the accumulation of tissue fluid.

IV. Venous Return and Circulatory Shock (p. 766)

A. William Harvey proved that one-way valves exist in the veins. (p. 766; fig. 20.17)

B. Mechanisms of Venous Return (p. 766)

1. The flow of blood back to the heart, called venous return, is achieved by five mechanisms.

a. Pressure gradient. (Blood flows from a higher pressure to a lower one on its return to the heart.)

b. Thoracic pump. (Breathing muscles help return blood to the heart.)

c. Cardiac suction within the ventricles due to the action of the chordae tendineae.

d. The skeletal muscle pump. (Skeletal muscle squeezes against veins.) (fig. 20.18; TR 700)

e. Gravity aiding return of blood from your head.

C. Venous Return and Physical Activity (p. 767)

1. Exercise increases venous return, in part because the heart beats faster and respiratory rate is greater, but also because of the contraction of skeletal muscles.

2. If a person remains still, venous pooling is seen in the limbs. This can be troublesome to people who stand for long periods because they may become dizzy and faint.

D. Circulatory Shock (p. 768)

1. Circulatory shock is a state in which cardiac output is not sufficient to meet the body's needs. Cardiogenic shock occurs because the heart is not beating adequately, perhaps from a MI. All other forms are due to low venous return (LVR) shock.

2. There are three principal forms of LVR shock.

a. Hypovolemic shock is due to a loss of blood due to hemorrhage, dehydration, burns, or trauma.

b. Obstructed venous return shock occurs with a tumor or aneurysm.

c. Venous pooling (vascular) shock occurs when too much blood accumulates in the limbs. This can be due to prolonged standing, neurogenic shock (brainstem trauma or emotional shock that results in fainting), septic shock, or anaphylactic shock.

3. The body has several possible responses to circulatory shock.

a. When homeostatic mechanisms, such as vasoconstriction or repositioning the limbs, restore homeostasis, the person is said to be in compensated shock.

b. If homeostasis does not return, the person is in decompensated shock, which is life-threatening due to its positive feedback loops: Slow circulation leads to blood clots, poor cardiac output leads to MI, and ischemia and acidosis in the brainstem depress vasomotor and cardiac centers, causing further drops in circulation.

V. Special Circulatory Routes (p. 769)

A. Brain (p. 769)

1. Cerebral blood flow, although relatively stable, is governed by autoregulation in response to changes in BP and chemistry. Whenever BP fluctuates, the cerebral arteries adjust their diameters to compensate.

2. A cerebrovascular accident (CVA, or stroke) is death of brain tissue caused by ischemia resulting from cerebral atherosclerosis, thrombosis, or ruptured aneurysm. The consequences of CVA vary widely from minimal damage to death.

3. A transient ischemic attack (TIA) is a temporary feeling of dizziness, loss of vision or other senses, weakness, or aphasia, which results from spasms in diseased arteries. It is often an early warning of an impending stroke.

B. Skeletal Muscles (p. 769)

1. The skeletal muscles receive a variable blood flow depending on their state of exertion.

2. At rest, most capillary beds are shut down. Blood flow increases up to 100-fold during exercise.

C. Lungs (p. 769)

1. Pulmonary arteries have a low BP and thus also a low hydrostatic pressure. This lower pressure allows for more time for gas exchange and, since oncotic pressure overrides hydrostatic pressure, these capillaries are engaged almost entirely in absorption.

2. Since capillaries in the lungs primarily absorb, this prevents accumulation of fluid in the lung that would interfere with gas exchange.

VI. Anatomy of the Pulmonary Circuit (p. 770)

A. The pulmonary circuit begins with the pulmonary trunk that branches into the right and left pulmonary arteries, each of which leads to a lung. (p. 770; fig. 20.19; TR 701)

B. In the lung, pulmonary arteries branch into lobar arteries that carry blood to each lobe of a lung. These arteries further subdivide and lead to capillary beds that surround each alveolus. (p. 770)

C. After leaving the alveolar capillaries, pulmonary blood, now carrying oxygen, flows into venules and veins, ultimately leading to the two pulmonary veins that leave the lungs and drain into the left atrium. (p. 770)

VII. Anatomy of the Systemic Arteries (p. 770; fig. 20.20; TR 702, 703; tables 20.3–20.8)

A. The systemic circuit supplies oxygen and nutrients to all the organs and removes their metabolic wastes.

B. The names of the blood vessels often describe their location by indicating the body region traversed.

C. There is a great deal of variation in the circulatory system from one person to another. This discussion focuses on the most common pathways:

1. The aorta and its major branches. (fig. 20.21; TR 704, 705; table 20.3, p. 773)

2. Arterial supply to the head and neck. (figs. 20.22, 20.23; TR 706–709; table 20.4, p. 774)

3. Arterial supply to the upper limb. (fig. 20.24; TR 710, 711; table 20.5, pp. 776–77)

4. Arterial supply to the thorax. (fig. 20.25; TR 712, 713; table 20.6, pp. 777–78)

5. Arterial supply to the abdomen. (figs. 20.26, 20.27, 20.28; TR 714–719; table 20.7, pp. 779–81)

6. Arterial supply to the pelvic region and lower limb. (figs. 20.29, 20.30; TR 720–722; table 20.8, pp. 782–83)

D. Pressure points at which a person's pulse can be detected are shown in figure 20.31, p. 784. (TR 723, 724)

VIII. Anatomy of the Systemic Veins (p. 786; fig. 20.32; TR 725, 726; tables 20.9–20.14)

A. Veins occur in both deep and superficial groups

B. Deep veins run parallel to the arteries and often have similar names. It can usually be assumed that they drain the same structures as the corresponding arteries supply.

1. Venous drainage of the head and neck. (fig. 20.33; TR 727–732; table 20.9, pp. 786–87)

2. Venous drainage of the upper extremity. (fig. 20.34; TR 733–736; table 20.10, pp. 788–89)

3. The azygos system. (fig. 20.35; TR 737, 738; table 20.11, pp. 789–90)

4. Major tributaries of the inferior vena cava. (fig. 20.36; TR 739, 740; table 20.12, pp. 790–91)

5. The hepatic portal system. (fig. 20.37; TR 741, 742; table 20.13, pp. 791–92)

6. Venous drainage of the lower limb and pelvic organs. (fig. 20.38; TR 743–745; table 20.14, p. 794)

IX. Circulatory Disorders (pp. 795–96; table 20.15)