Chapter Twenty - The Circulatory System: Blood Vessels and Circulation
I. General Anatomy (p. 706)
A. Circulatory Routes (p. 706; Fig. 20.1)
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 to this is in the case of 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 Structure of Blood Vessels (p. 707; Figs. 20.2, 20.3; Transp. 374)
1. The walls of arteries and veins are made up of three layers, called tunics.
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 tunic, is the thickest layer of the vessel, made up mostly of smooth muscle. It is responsible for vasomotion.
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. 708)
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, or 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, spaces a short distance apart, rather than a continuous tunica media.
D. Capillaries (p. 709; 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 um away from the nearest capillary. Exceptions occur in tendons, ligaments, and the cornea.
2. Capillary Beds (p. 709; Fig. 20.5; Transp. 376)
a. 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.
b. At the origin of each capillary is a precapillary sphincter to regulate passage of blood into the capillary. About three-quarters of the body's capillaries are closed at any given time.
3. Types of Capillaries (p. 710; Fig. 20.6)
a. Two types of capillaries are distinguished by the sizes of gaps between or through endothelial cells.
b. 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.
c. 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.
d. 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. 711; Fig. 20.7)
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 located 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 back flow of blood when skeletal muscles relax. Valves are present mostly in medium-sized veins of the legs and arms.
II. Blood Flow, Pressure, and Resistance (p. 712)
A. Flow (p. 712)
1. 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.
2. Blood flow is an important measure of the amount of oxygen and nutrients delivered to a tissue and the rate of waste removal.
3. Inadequate blood flow can result in necrosis of the tissue or even death of the individual.
B. Blood Pressure (p. 712; Figs. 20.8, 20.9; Table 20.1)
1. Blood pressure 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 blood pressure.
4. Hypertension is a chronic resting systolic pressure of 130 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 also tends to rise with age.
7. Blood pressure is determined mainly by cardiac output, blood volume, and peripheral resistance.
C. Resistance (p. 714)
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 (p. 715)
a. 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. Vessel Length (p. 715)
a. Pressure and flow decline with increasing distance. If perfusion is good at a great distance from the heart, it is likely to be good elsewhere in the systemic circulation.
4. Vessel Radius (p. 715; Figs. 20.10, 20.11)
a. In a healthy person, the one real way to influence peripheral resistance is through altering the diameters of the blood vessels.
b. 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. Vessel radius exerts a very powerful influence over flow.
5. Poiseuille's Law (p. 716; Table 20.2)
a. Poiseuille's law is an equation that sums up the relationship between flow, pressure gradient, vessel radius, vessel length, and blood viscosity.
b. As blood leaves the heart, its velocity is great because the aorta is large and close to the heart; as blood progresses toward capillaries, its velocity slows because of friction, greater peripheral resistance (smaller lumens), and because of a greater distribution of blood over a larger area. From capillaries to vena cava, velocity increases again as many small vessels merge to spill blood into a single larger vessel.
D. Regulation of Peripheral Resistance (p. 717)
1. Local Control (p. 717)
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 then restored, the tissue exhibits reactive hyperemia.
d. Over time, hypoxic tissue can increase its own perfusion by angiogenesis.
2. Neural Control (p. 717; Figs. 20.12, 20.13; Transps. 377, 378)
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: baroflexes (autonomic responses to changes in blood pressure), chemoflexes (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).
3. Hormonal Control (p. 718)
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 Redirection of Blood Flow (p. 719; Figs. 20.14, 20.15; Transp. 379)
1. The body is able to adjust its blood routing according to the needs of the body. After a large meal, when a person rests in a chair, proportionately more blood is directed toward the organs of digestion and less toward skeletal muscle.
2. Physical exertion increases perfusion to the lungs. myocardium, and skeletal muscle while reducing perfusion to the digestive tract, skin, and kidneys.
III. Capillary Exchange (p. 720; Fig. 20.16; Transp. 380)
A. Diffusion (p. 720)
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 vise versa. Lipid-soluble substances diffuse through the plasma membrane. Other substances must pass through clefts, fenestrations, or membrane channels.
B. Transcytosis (p. 721)
1. Transcytosis accounts for only a small fraction of solute exchange. 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 it on the other side by exocytosis. Fatty acids, insulin, and albumin are transported this way.
C. Filtration and Reabsorption (p. 721; Fig. 20.17; Transp. 381)
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 which tends to draw water into the capillary by osmosis, thus opposing hydrostatic pressure.
3. Net filtration pressure, then, 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. Variations in Capillary Filtration and Reabsorption (p. 722)
a. Capillary activity varies from one time to the next. During times when a tissue is metabolically active, its capillary flow increases. In active muscle, capillary pressure overrides reabsorption, and muscles accumulate fluid. Normally, however, capillaries reabsorb most of what they filter.
IV. Venous Return and Circulatory Shock (p. 722)
A. Mechanisms of Venous Return (p. 723; Figs. 20.18, 20.19; Transp. 382)
1. The flow of blood back to the heart, called venous return, is achieved by five mechanisms: pressure gradient (blood flows from a higher pressure to a lower one on its return to the heart); thoracic pump (breathing muscles help return blood to the heart); cardiac suction (within the ventricles due to the action of the chordae tendineae); the skeletal muscle pump (skeletal muscle squeezes against veins); and gravity (aiding return of blood from your head).
B. Venous Return and Physical Activity (p. 723)
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 polling is seen in the extremities. This can be troublesome to people who stand for long periods, and they may become dizzy and faint.
C. Circulatory Shock (p. 724)
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. Types and Causes of LVR Shock (p. 724)
a. Hypovolemic shock is due to a loss of blood from 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 extremities. This can be due to prolonged standing, or neurogenic shock (brainstem trauma or emotional shock which results in fainting), septic shock, or anaphylactic shock.
3. Responses to Circulatory Shock (p. 725)
a. When homeostatic mechanisms, like vasoconstriction or repositioning the extremities, 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 with its positive feedback loops.
c. In decompensated shock, 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. 725)
A. Brain (p. 725)
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 diameter 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 sensations, weakness, or aphasia, which results from spasms in diseased arteries. It is often an early warning of an impending stroke.
B. Skeletal Muscles (p. 726)
1. The skeletal muscles receive a variable blood flow depending on their state of exertion. At rest, most capillary beds are shut down. Blood flow increases up to 100-fold during exercise.
C. Lungs (p. 726)
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, which would interfere with gas exchange.
VI. Anatomy of the Pulmonary Circuit (p. 726; Fig. 20.20; Transp. 383)
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.
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.
C. After leaving the alveolar capillaries, pulmonary blood, now carrying oxygen, flows into venules and veins, then to the two pulmonary veins that leave the lungs and drain into the left atrium.
VII. Anatomy of the Systemic Arteries (p. 728; Tables 20.3 - 20.8; Transp. 385)
A. The systemic circuit supplies oxygen and nutrients to all the organs and removes their metabolic wastes. The body's systemic arteries are described in Tables 20.3 - 20.8, on p. 729-738 (Transp. 385), and depicted in Fig. 20.21, p. 728; Transp. 384.
1. The aorta and its major branches are shown in Table 20.3, p. 729; Transp. 385.
2. Arterial supply to the head and neck is depicted in Table 20.4, p. 730-731.
3. Arterial supply to the upper extremity is shown in Table 20.5, p. 732.
4. Arterial supply to the thorax is depicted in Table 20.6, p. 733.
5. Arterial supply to the abdomen is shown in Table 20.7, p. 734-736.
6. Arterial supply to the pelvic region and lower extremity is shown in Table 20.8, p. 737-738.
B. Pressure points at which a person's pulse can be detected are shown in Fig. 20.22, p. 739; Transp. 386.
VIII. Anatomy of the Systemic Veins (p. 740; Tables 20.9 - 20.14)
A. The body's veins are depicted in Fig. 20.23, p. 740 (Transp. 387), and described in Tables 20.9 - 20.14, p. 741-748.
1. Venous drainage of the head and neck is depicted in Table 20.9, p. 741-742.
2. Venous drainage of the upper extremity is shown in Table 20.10, p. 743-744.
3. The azygos system is described in Table 20.11, p. 744.
4. Major tributaries of the inferior vena cava are shown in Table 20.12, p. 745.
5. The hepatic portal system is depicted in Table 20.13, p. 746.
6. Venous drainage of the lower extremity and pelvic organs is shown in Table 20.14, p. 747-748.
CHAPTER ESSAY: Hypertension - The "Silent Killer" (p. 749)
i. Hypertension affects 30% of Americans over the age of 50, and 50% by the age of 74. It causes heart failure, stroke, and kidney failure.
ii. Primary hypertension (90% of cases) results from heredity, behavior, obesity, diet, race, and sex.
iii. Secondary hypertension is high blood pressure that results from identifiable disorders (i.e., Cushing syndrome, polycythemia).
IX. Connective Issues (p. 750)
A. Interactions between the circulatory system and other organ systems are listed on p. 750.
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