General Anatomy of the Blood Vessels (p. 754)
1. Blood flows away from the heart in arteries and back to the heart in veins. Capillaries connect the smallest arteries to the smallest veins.
2. The wall of a blood vessel has three layers: tunica in tern a, tunica media, and tunica externa. The tunica interna is lined with a simple squamous endothelium.
3. Arteries are classified as conducting, distributing, and resistance arteries from largest to smallest. Conducting arteries are subject to the highest blood pressure and have the most elastic tissue; distributing and resistance arteries contain more smooth muscle relative to their size.
4. The smallest of the resistance arteries are arterioles. Metarterioles link arterioles with capillaries.
5. Certain large arteries above the heart contain baroreceptors called carotid sinuses for monitoring blood pressure, and chemoreceptors called carotid bodies and aortic bodies for monitoring blood CO2, O2, and pH.
6. Capillaries are the primary point of fluid exchange with the tissues. Their wall is composed of endothelium and basement membrane only.
7. The three types of capillaries are continuous capillaries, which form an uninterrupted tube; fenestrated capillaries perforated by patches of filtration pores; and sinusoids, which are irregular, highly porous
blood spaces in such tissues as liver, spleen, and bone marrow.
8. Capillaries are arranged in networks called capillary beds, supplied by a single metarteriole. Precapillary sphincters regulate blood flow through a capillary bed.
9. The smallest veins, or venules, also exchange fluid with the tissues. They converge to form medium veins, and medium veins converge to form large veins.
10. Venous sinuses are blood spaces with large lumens, thin walls, and no muscle. They occur in such places as the heart and brain.
11. Veins have relatively low blood pressure and therefore have thinner walls and less muscular and elastic tissue. Medium veins of the limbs have valves to prevent the backflow of blood.
12. Between the arteries and veins, blood normally flows through one capillary bed. Portal systems and anastomoses are exceptions to this rule.
Blood Pressure, Resistance, and Flow (p. 762)
1. Blood flow (mLimin) and perfusion (flow/g of tissue) vary with the metabolic needs of a tissue.
2. Flow (F) is directly proportional to the ~P!essure difference between two pOints (dP) and inversely proportional to resistance (B): F oc dP/ R.
3. Blood pressure (BP) is usually measured with a sphygmomanometer. Arterial pressures are expressed as systolic over diastolic pressure-for example, 120/80 mm Hg.
4. Pulse pressure is systolic minus diastolic pressure. Mean arterial pressure is the average pressure in a vessel over the course of a cardiac cycle, estimated as diastolic pressure + 1/3 of pulse pressure.
5. Chronic, abnormally high BP is hypertension and low BP is hypotension.
6. The expansion and contraction of
arteries during the cardiac cycle
reduces the pulse pressure and eases the strain on smaller arteries, but arterial blood flow is nevertheless pulsatile. In capillaries and veins, flow is steady (without pulsation).
7. Peripheral resistance is opposition to blood flow in the blood vessels. Resistance is directly proportional to blood viscosity and vessel length,
and inversely proportional to vessel radius to the fourth power (r4). Changes in vessel radius (vasomotion) thus have the greatest influence on flow from moment to moment.
8. Blood flow is fastest in the aorta, slowest in the capillaries, and speeds up somewhat in the veins.
9. Blood pressure is controlled mainly by local. neural. and hormonal control of vasomotion.
10. Autoregulation is the ability of a tissue to regulate its own blood supply. Over the short term, local vasomotion is stimulated by vasoactive chemicals (histamine, nitric oxide, and others). Over the long term, autoregulation can be achieved by angiogenesis, the growth of new vessels.
11. Neural control of blood vessels is based in the vasomotor center of the medulla oblongata. This center integrates baroreflexes, chemoreflexes, and the medullary ischemic reflex, and issues signals to the blood vessels by way of sympathetic nerve fibers. .
12. Blood pressure is regulated in various ways by the hormones angiotensin II, aldosterone, atrial natriuretic peptide, antidiuretic hormone, epinephrine, and norepinephrine.
13. Vasomotion often shifts blood flow from organs with less need of perfusion at a given time, to organs with greater need-for example, away from the intestines and to the skeletal muscles during exercise.
Capillary Exchange (p. 769)
1. Capillary exchange is a two-way movement of water and solutes between the blood and tissue fluids across the walls of the capillaries and venules.
2. Materials pass through the vessel wall by diffusion, transcytosis, filtration, and reabsorption, passing
through intercellular clefts, fenestrations, and the endothelial cell cytoplasm.
3. Fluid is forced out of the vessels by blood pressure and the negative hydrostatic pressure of the interstitial space. The force drawing fluid back into the capillaries is colloid osmotic pressure. The difference between the outward and inward forces is an outward net filtration pressure or an inward net reabsorption pressure.
4. Capillaries typically give off fluid at the arterial end, where the relatively high blood pressure overrides reabsorption; they reabsorb about 85% as much fluid at the venous end. where colloid osmotic pressure overrides the lower blood pressure.
5. About 15% of the tissue fluid is
reabsorbed by the lymphatic system.
6. Fluid exchange dynamics vary from place to place in the body (some capillaries engage solely in filtration and some solely in reabsorption) and from moment to moment (as vasomotion shifts the balance between filtration and reabsorption).
7. Accumulation of excess tissue fluid is edema. It results from increased capillary filtration, reduced reabsorption, or obstructed lymphatic drainage.
Venous Return and Circulatory Shock (p. 772)
1. Venous return, the flow of blood back to the heart. is driven by the venous blood pressure gradient, gravity, the skeletal muscle pump (aided by valves in the veins of the limbs), the thoracic pump. and cardiac suction.
2. Exercise increases venous return because the vessels dilate. the thoracic pump and skeletal muscle pump work more energetically. and cardiac output is elevated.
3. Inactivity allows blood to accumulate in low points in the body by gravity; this is called venous pooling. It can result in syncope (fainting) if too much blood drains away from the brain.
4. CirculatolJ' shock is any state of inadequate cardiac output. Its two basic categories are cardiogenic
shock and low venous return (LVR) shock.
5. The main forms of LVR shock are hypovolemic, obstructed venous return, and venous pooling shock.
6. Septic shock and anaphylactic shock combine elements of hypovole~ia and venous pooling.
7. Compensated shock is corrected by the body's homeostatic mechanisms. Decompensated shock is life threatening, incapable of self-correction. and requires clinical intervention.
Special Circulatory Routes (p. 775)
1. The brain receives a relatively stable total blood flow of about 700 mLimin. but flow shifts rapidly from one part of the brain to another during varying cerebral activities.
2. The brain regulates its own blood
J10w in response to changes in BP
- and pH.
. ~ -~
3. Transient ischemic attacks result
from brief periods of cerebral ischemia (poor blood flow). A cerebral vascular accident (stroke) results from a permanent loss of perfusion due to arterial blockage or rupture.
4. Skeletal muscles receive highly variable flow depending on their state of activity. Most muscle capillary beds are shut down at rest. During exercise, flow increases in response to muscle metabolites and sympathetic vasodilation.
5. The pulmonary circuit is the only route in which arteries carry less oxygen than veins do.
6. Pulmonary arteries have relatively low BP and slow flow, which allows ample time for gas exchange and promotes capillary reabsorption. The latter prevents fluid from accumulating in the lungs.
7. Pulmonary arteries, unlike systemic arteries, constrict in response to hypoxia, so less blood is sent to poorly ventilated areas of the lung.
Anatomy of the Pulmonary Circuit (p. 776)
1. The route of blood flow in the pulmonary circuit is right ventricle of the heart ~ pulmonary trunk ~ pulmonary arteries ~ lobar arteries ~ alveolar capillary beds ~ venules ~
pulmonary veins ~ left atrium of the heart.
2. The pulmonary circuit serves only to exchange CO2 for O2 in the blood. The metabolic needs of the lung tissue are met by a separate systemic blood supply to the lungs, via the bronchial arteries.
Anatomy of the Systemic Arteries (p. 777)
1. The systemic circulation begins with the ascending aorta. Table 20.3 describes the major branches of the aorta.
2. The head and neck receive blood from the common carotid and vnrtp.bral arteries. Table 20.4 describe~ the branches of these arteries.
3. The upper limbs receive blood from the subclavian arteries. Table 20.5 describes the branches of these arteries in the limb.
4. The thoracic organs receive blood from several small branches of thl) thoracic aorta and the subclavian and axillary arteries. Table 20.6 describes these branches.
5. After passing through the diaphragm, the descending aorta gives off a series of branches to the abdominal viscera. Table 20.7 describes these.
6. At its inferior end, the abdominal aorta forks into two common iliac arteries. whose distal branches supply the pelvic region and lower limb. Table 20.8 describes these.
7. Arteries tend to be deeper than veins, but there are several places where they come close enough to the surface to be palpated. These sites serve for taking a pulse and as emergency pressure points where compression can stop arterial bleeding.
Anatomy of the Systemic Veins (p. 790)
1. In venous circulation. blood flows through smaller veins that join to form progressively larger ones. Veins that merge to create a larger one are called tributaries.
2. The head and neck are drained by the jugular and vertebral veins. which ultimately converge to form the superior vena cava leading to the right atrium of the heart. Table
20.9 describes the tributaries that drain the head and neck.
. Table 20.10 describes tributaries in the upper limb that converge to drain the limb via the axillary and subclavian veins.
4. The thoracic viscera are drained by the azygos system, described in table 20.11.
5. The abdominal viscera are drained by tributaries of the inferior vena cava (IVC), described in table 20.12.
6. The digestive system is drained by the hepatic portal system of veins, described in table 20.13.
7. Table 20.14 describes tributaries of the lower limbs, which ultimately converge on the common iliac vein The two common iliac veins join tc form the IVC.