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The Microcirculation and the Lymphatic System 1616 H CHAPTER The Microcirculation and the Lymphatic System 1616 H. Glenn Bohlen, Ph.D. CHAPTER OUTLINE ■ THE ARTERIAL MICROVASCULATURE ■ TRANSCAPILLARY FLUID EXCHANGE ■ THE CAPILLARIES ■ THE REGULATION OF MICROVASCULAR ■ THE VENOUS MICROVASCULATURE PRESSURES ■ THE LYMPHATIC VASCULATURE ■ THE REGULATION OF MICROVASCULAR ■ VASCULAR AND TISSUE EXCHANGE OF SOLUTES RESISTANCE KEY CONCEPTS 1. Arterioles regulate vascular resistance and microvascular 9. Tissue hydrostatic and colloid osmotic pressures are minor pressures. forces for absorption and filtration of fluid across capillary 2. Capillaries are the primary sites for water and solute ex- walls. change. 10. The ratio of postcapillary to precapillary resistance 3. Venules collect blood from the capillaries and act as reser- is a major determinant of capillary hydrostatic voirs for blood volume. pressure. 4. Lymphatic vessels collect excess water and protein mole- 11. Myogenic arteriolar regulation is a response to increased cules from the interstitial space between cells. tension or stretch of the vessel wall muscle cells. 5. Water-soluble materials pass through tiny pores between 12. By-products of metabolism cause the dilation of adjacent endothelial cells. arterioles. 6. Lipid-soluble molecules pass through the endothelial cells. 13. The axons of the sympathetic nervous system release 7. The concentration difference of solutes across the capillary norepinephrine, which constricts the arterioles and wall is the energy source for capillary exchange. venules. 8. Plasma hydrostatic and colloid osmotic pressures are the 14. Autoregulation of blood flow allows some organs to main- primary forces for fluid filtration and absorption across tain nearly constant blood flow when arterial blood pres- capillary walls. sure is changed. he microcirculation is the part of the circulation where laries, are partially constricted by contraction of their vas- Tnutrients, water, gases, hormones, and waste products cular smooth muscle cells. If all microvessels were to dilate are exchanged between the blood and cells. The microcir- fully because of relaxation of their smooth muscle cells, the culation minimizes diffusion distances, facilitating ex- arterial blood pressure would plummet. Cerebral blood change, its most important function. Virtually every cell in flow in a standing individual would be inadequate, resulting the body is in close contact with a microvessel. In fact, most in fainting, or syncope. Regulation of vascular resistance in cells are in direct contact with at least one microvessel. As the microcirculation is an important aspect of total health. a consequence, there are tens of thousands of microvessels There is a constant conflict between the regulation of vas- per gram of tissue. The lens and cornea are exceptions be- cular resistance to preserve the arterial pressure and simulta- cause their nutrients are supplied by the fluids in the eye. neously to allow each tissue to receive sufficient blood flow A second major function of the microcirculation is to to sustain its metabolism. The compromise is to preserve the regulate vascular resistance and thereby interact with car- mean arterial pressure by increasing arterial resistance at the diac output to maintain the arterial blood pressure (see expense of reduced blood flow to most organs other than the Chapter 12). Normally, all microvessels, other than capil- heart and brain. The organs survive this conflict by increas- 262 CHAPTER 16 The Microcirculation and the Lymphatic System 263 ing their extraction of oxygen and nutrients from blood in the microvessels as the blood flow is decreased. The microvasculature is considered to begin where the smallest arteries enter the organs and to end where the smallest veins, the venules, exit the organs. In between are microscopic arteries, the arterioles, and the capillaries. De- pending on an animal’s size, the largest arterioles have an inner diameter of 100 to 400 ␮m, and the largest venules have a diameter of 200 to 800 ␮m. The arterioles divide into progressively smaller vessels to the extent that each section of the tissue has its own specific microvessels. The branching pattern typical of the microvasculature of differ- ent major organs and how it relates to organ function are discussed in Chapter 17. THE ARTERIAL MICROVASCULATURE Large arteries have a low resistance to blood flow and func- tion primarily as conduits (see Chapter 15). As arteries ap- proach the organ they supply, they divide into many small Scanning electron micrographs of smooth arteries both just outside and within the organ. In most or- FIGURE 16.1 ␮ muscle cells wrapping around arterioles of gans, these small arteries, which are 500 to 1,000 m in di- various sizes. Each cell only partially passes around large-diame- ameter, control about 30 to 40% of the total vascular re- ter (1A) and intermediate-diameter (2A) arterioles, but com- sistance. These smallest of arteries, combined with the pletely encircles the smaller arterioles (3A, 4A). 1A, 2A, and the arterioles of the microcirculation, constitute the resistance small insets of 3A and 4A are at the same magnification. The en- blood vessels; together they regulate about 70 to 80% of larged views of 3A and 4A are at 4-times-greater magnification. the total vascular resistance, with the remainder of the re- (Modified from Miller BR, Overhage JM, Bohlen HG, Evan AP. sistance about equally divided between the capillary beds Hypertrophy of arteriolar smooth muscle cells in the rat small in- and venules. Constriction of these vessels maintains the rel- testine during maturation. Microvasc Res 1985;29:56–69.) atively high vascular resistance in organs. Constriction re- sults from the release of norepinephrine by the sympathetic larger vessel, but may encircle a smaller vessel almost 2 nervous system, from the myogenic mechanism (to be dis- times (see Fig. 16.1). cussed later), and from other chemical and physical factors. Vessel Wall Tension and Intravascular Pressure Arterioles Regulate Resistance by the Contraction Interact to Determine Vessel Diameter of Vascular Smooth Muscle The smallest arteries and all arterioles are primarily respon- The vast majority of arterioles, whether large or small, are sible for regulating vascular resistance and blood flow. Ves- tubes of endothelial cells surrounded by a connective tissue sel radius is determined by the transmural pressure gradient basement membrane, a single or double layer of vascular and wall tension, as expressed by Laplace’s law (see Chap- smooth muscle cells, and a thin outer layer of connective ter 14). Changes in wall tension developed by arteriolar tissue cells, nerve axons, and mast cells (Fig. 16.1). The vas- smooth muscle cells directly alter vessel radius. Most arte- cular smooth muscle cells around the arterioles are 70 to 90 ␮ rioles can dilate 60 to 100% from their resting diameter and m long when fully relaxed. The muscle cells are anchored can maintain a 40 to 50% constriction for long periods. to the basement membrane and to each other in a way that Therefore, large decreases and increases in vascular resist- any change in their length changes the diameter of the ves- ance and blood flow are well within the capability of the sel. Vascular smooth muscle cells wrap around the arteri- Њ microscopic blood vessels. For example, a 20-fold increase oles at approximately a 90 angle to the long axis of the ves- in blood flow can occur in contracting skeletal muscle dur- sel. This arrangement is efficient because the tension ing exercise, and blood flow in the same vasculature can be developed by the vascular smooth muscle cell can be al- reduced to 20 to 30% of normal during reflex increases in most totally directed to maintaining or changing vessel di- sympathetic nerve activity. ameter against the blood pressure within the vessel. In the majority of organs, arteriolar muscle cells operate at about half their maximal length. If the muscle cells fully relax, the diameter of the vessel can nearly double to in- THE CAPILLARIES crease blood flow dramatically (flow increases as the fourth Exchanges Between Blood and power of the vessel radius; see Chapter 12). When the mus- Tissue Occur in Capillaries cle cells contract, the arterioles constrict, and with intense stimulation, the arterioles can literally shut for brief periods Capillaries provide for most of the exchange between of time. A single muscle cell will not completely encircle a blood and tissue cells. The capillaries are supplied by the 264 PART IV BLOOD AND CARDIOVASCULAR PHYSIOLOGY brane. Lipid-soluble molecules, such as oxygen and carbon dioxide, readily pass through the lipid components of en- dothelial cell membranes. Water-soluble molecules, how- ever, must diffuse through water-filled pathways formed in the capillary wall between adjacent endothelial cells. These pathways, known as pores, are not cylindrical holes but complex passageways formed by irregular tight junctions (see Fig. 16.2). The capillaries of the brain and spinal cord have virtually continuous tight junctions between adjacent endothelial cells; consequently, only the smallest water-soluble mole- cules pass through their capillary walls. In all capillaries, there are sufficient open areas in adjacent tight junctions to provide pores filled with water for diffusion of small mole- cules. The pores are partially filled with a matrix of small fibers of submicron dimensions. The potential importance of this fiber matrix is that it acts partially to sieve the mol- The various layers of a mammalian capil- FIGURE 16.2 ecules approaching a water-filled pore. The combination of lary. Adjacent endothelial cells are held to- the fiber matrix and the small spaces in the basement mem- gether by tight junctions, which have occasional gaps. Water-sol- uble molecules pass through pores formed where tight junctions brane and between endothelial cells explains why the ves- are imperfect. Vesicle formation and the diffusion of lipid-soluble sel wall behaves as if only about 1% of the total surface area molecules through endothelial cells provide other pathways for were available for exchange of water-soluble molecules.
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