The Microcirculation and Lymphatics

Functional Anatomy

Arterioles Are the Stopcocks of the Circulation

The arterioles, which range in diameter from about 5 to 100 μm, have a thick smooth muscle layer, a thin adventitial layer, and an endothelial lining (see Fig. 1.2 ). The arterioles give rise directly to the capillaries (5 to 10 μm in diameter) or in some tissues to metarterioles (10 to 20 μm in diameter), which then give rise to capillaries ( Fig. 8.1 ). The metarterioles can serve either as thoroughfare channels to the venules, which bypass the capillary bed, or as conduits to supply the capillary bed. There are often cross connections between the arterioles and venules as well as in the capillary network. Arterioles that give rise directly to capillaries regulate flow through their cognate capillaries by constriction or dilation. The capillaries form an interconnecting network of tubes of different lengths, with an average length of 0.5 to 1 mm.

Capillaries Permit the Exchange of Water, Solutes, and Gases

Capillary distribution varies from tissue to tissue. In metabolically active tissues, such as cardiac and skeletal muscle and glandular structures, capillaries are numerous. In less active tissues, such as subcutaneous tissue or cartilage, capillary density is low. Also, all capillaries do not have the same diameter. It is necessary for the cells to become temporarily deformed in their passage through these capillaries, because some capillaries have diameters less than those of the erythrocytes. Fortunately, normal red blood cells are quite flexible, and they readily change their shape to conform to that of the small capillaries.

Blood flow in the capillaries is not uniform; it depends chiefly on the contractile state of the arterioles. The average velocity of blood flow in the capillaries is approximately 1 mm per second; however, it can vary from zero to several millimeters per second in the same vessel within a brief period. Such changes in capillary blood flow may be random or they may show rhythmical oscillatory behavior of different frequencies. This behavior is caused by contraction and relaxation ( vasomotion ) of the precapillary vessels. Vasomotion is partially an intrinsic contractile behavior of the vascular smooth muscle, and it is independent of external input. Furthermore, changes in transmural pressure ( intravascular minus extravascular pressure ) influence the contractile state of the precapillary vessels. An increase in transmural pressure, whether produced by an increase in venous pressure or by dilation of arterioles, results in contraction of the terminal arterioles at the points of origin of the capillaries. Conversely, a decrease in transmural pressure elicits precapillary vessel relaxation (see myogenic response , Chapter 9 ).

Reduction of transmural pressure relaxes the terminal arterioles. However, blood flow through the capillaries cannot increase if the reduction in intravascular pressure is caused by severe constriction of the parent vasculature. Large arterioles and metarterioles also exhibit vasomotion. However, in the contraction phase, they usually do not completely occlude the lumen of the vessel and arrest blood flow, which may occur when the terminal arterioles contract ( Fig. 8.2 ). Thus flow rate may be altered by contraction and relaxation of small arteries, arterioles, and metarterioles.

Because blood flow through the capillaries provides for exchange of gases and solutes between the blood and tissues, the flow has been termed nutritional flow. Conversely, blood flow that bypasses the capillaries in traveling from the arterial to the venous side of the circulation has been termed nonnutritional, or shunt, flow (see Fig. 8.1 ). In some areas of the body (e.g., fingertips and ears), true arteriovenous shunts exist (see Fig. 12.1 ). However, in many tissues, such as muscle, evidence of anatomical shunts is lacking. Nevertheless, nonnutritional flow can occur, and the behavior has been termed physiological shunting of blood flow . This shunting is the result of a greater flow of blood through previously open capillaries, along with either no change or an increase in the number of closed capillaries. In tissues that have metarterioles, shunt flow may be continuous from the arterioles to the venules during low metabolic activity, at which time many precapillary vessels are closed. When metabolic activity rises in such tissues and more precapillary vessels open, blood passing through the metarterioles is readily available for capillary perfusion.

The true capillaries are devoid of smooth muscle and are therefore incapable of active constriction. Nevertheless, the endothelial cells that form the capillary wall contain actin and myosin, and they can alter their shape in response to certain chemical stimuli. There is no evidence, however, that changes in endothelial cell shape regulate blood flow through the capillaries. Hence changes in capillary diameter are passive and are caused by alterations in precapillary and postcapillary resistance.

The Law of Laplace Explains How Capillaries Can Withstand High Intravascular Pressures

The law of Laplace is illustrated in the following comparison of wall tension in a capillary with that in the aorta ( Table 8.1 ). The Laplace equation is:

$\text{T}=\Delta \text{Pr}$

where T is tension in the vessel wall, ΔP is transmural pressure difference (internal minus external), and r is radius of the vessel.

TABLE 8.1

Vessel Wall Tension in the Aorta and a Capillary

	Aorta	Capillary
r (radius)	1.5 cm	5 × 10 −4 cm
h (height of Hg column)	10 cm Hg	2.5 cm Hg
ρ (density of Hg)	13.6 g/cm 3	13.6 g/cm 3
g (gravitational acceleration)	980 cm/s 2	980 cm/s 2
P (pressure)	10 × 13.6 × 980 = 1.33 × 10 5 dyne/cm 2	2.5 × 13.6 × 980 = 3.33 × 10 4 dyne/cm 2
w (wall thickness)	0.2 cm	1 × 10 −4 cm
T = Pr	(1.33 × 10 5 ) (1.5) = 2 × 10 5 dyne/cm	(3.33 ×10 4 ) (5 × 10 −4 ) = 16.7 dyne/cm
σ (wall stress) = Pr/w	2 × 10 5 /0.2 = 1 × 10 6 dyne/cm 2	16.7/1 × 10 −4 = 1.67 × 10 5 dyne/cm 2

Wall tension is the force per unit length tangential to the vessel wall. This tension opposes the distending force (ΔPr) that tends to pull apart a theoretical longitudinal slit in the vessel ( Fig. 8.3 ). Transmural pressure is essentially equal to intraluminal pressure, because extravascular pressure is usually negligible. The Laplace equation applies to very thin-walled vessels, such as capillaries. Wall thickness must be considered when the equation is applied to thick-walled vessels such as the aorta. This is done by dividing ΔPr (pressure × radius) by wall thickness (w). The equation now becomes:

$\sigma \left(\text{wall}s\text{tress}\right)=\Delta \text{Pr/w}$

Pressure in mm Hg (height of an Hg column) is converted to dynes per square centimeter, according to the following equation:

$\text{P}=\text{h}\text{ρ}g$

where h is the height of an Hg column in centimeters, ρ is the density of Hg in g/cm ³ , g is gravitational acceleration in cm/s ² ; and σ (wall stress) is force per unit area.

Thus at normal aortic and capillary pressures, the wall tension of the aorta is about 12,000 times greater than that of the capillary (see Table 8.1 ). In a person who is standing quietly, capillary pressure in the feet may reach 100 mm Hg. Under such conditions, capillary wall tension increases to 66.5 dynes/cm, a value that is still only one three-thousandths that of the wall tension in the aorta at the same internal pressure. However, σ (wall stress), which takes wall thickness into consideration, is only about tenfold greater in the aorta than in the capillary.

In addition to explaining the ability of capillaries to withstand large internal pressures, the preceding calculations also show that in dilated vessels, wall stress increases even when internal pressure remains constant.

The diameter of the resistance vessels is determined by the balance between the contractile force of the vascular smooth muscle and the distending force produced by the intraluminal pressure. The greater the contractile activity of the vascular smooth muscle of an arteriole, the smaller its diameter, until the small arterioles are completely occluded. This occlusion is caused by infolding of the endothelium and the consequent trapping of the cells in the vessel. With progressive reduction in the intravascular pressure, vessel diameter decreases, as does tension in the vessel wall. This constitutes the law of Laplace .

The Endothelium Plays an Active Role in Regulating the Microcirculation

For many years, the endothelium was considered to be an inert, single layer of cells that served solely as a passive filter that (1) permitted water and small molecules to pass across the blood vessel wall and (2) retained blood cells and large molecules (proteins) within the vascular compartment. However, the endothelium is now recognized as a source of substances that elicit contraction and relaxation of the vascular smooth muscle ( Fig. 8.4 ; also see Fig. 9.4 ).

As shown in Fig. 8.4 , prostacyclin (prostaglandin I ₂ , PGI ₂ ) can relax vascular smooth muscle via an increase in the cyclic adenosine monophosphate (cAMP) concentration. Prostacyclin is formed in the endothelium from arachidonic acid, and it may be released by the shear stress caused by the pulsatile blood flow. Prostacyclin formation is catalyzed by the enzyme prostacyclin synthase. The primary function of PGI ₂ is to inhibit platelet aggregation and adherence to the endothelium, thus preventing intravascular clot formation.

Of far greater importance in endothelially mediated vascular dilation is the formation and release of endothelium-derived relaxing factor (EDRF) (see Fig. 8.4 ), which has been identified as nitric oxide ( NO ). Stimulation of the endothelial cells in vivo, in isolated arteries, or in culture by acetylcholine or several other agents (such as adenosine triphosphate [ATP], adenosine diphosphate [ADP], bradykinin, serotonin, substance P, and histamine) produce and release NO. In blood vessels whose endothelium has been removed, these agents do not elicit vasodilation; some, including acetylcholine and ATP, can cause constriction. The NO (synthesized from l -arginine) activates guanylyl cyclase in the vascular smooth muscle. This process raises the cyclic guanosine monophosphate (cGMP) concentration and increases the activity of cGMP-dependent protein kinase (PKG), which in turn activates myosin light-chain phosphatase (MLCP). Myosin light-chain phosphatase induces relaxation by reducing the concentration of phosphorylated myosin regulatory light-chain subunits (MLC ₂₀ ) in vascular smooth muscle (see Fig. 9.2 ). NO release can be stimulated by the shear stress of blood flow on the endothelium, but the physiological role of NO in the local regulation of blood flow remains to be elucidated. The drug nitroprusside also increases cGMP, causing vasodilation. Nitroprusside acts directly on the vascular smooth muscle; its action is not endothelially mediated (see Fig. 8.4 ). Vasodilator agents, such as adenosine, H ⁺ , CO ₂ , and K ⁺ , may be released from parenchymal tissue and act locally on the resistance vessels (see Fig. 8.4 ).

The endothelium can also synthesize endothelin , a very potent vasoconstrictor peptide (see Fig. 9.4A ). Endothelin can affect vascular tone and blood pressure in humans, and it may be involved in such pathological states as atherosclerosis, pulmonary hypertension, congestive heart failure, and renal failure.

The Endothelium is at the Center of Flow-Initiated Mechanotransduction

Blood vessels are continuously subjected to cyclic changes of blood pressure and flow. Endothelial cells that form the inner lining of blood vessels are linked with the glycocalyx on their luminal surfaces (see Fig. 8.7 ) and with the basement membrane on their abluminal surfaces (see Fig. 8.5 ). These interfaces are important signaling sites for transduction of the mechanical force (shear stress) imparted by the blood, into a signal for the regulation of endothelial cell function. The glycocalyx (composed of proteoglycans and glycoproteins) can extend up to 0.5 μm from the surfaces of endothelial cells. The fibrous network of the glycocalyx serves as a filter at the vessel wall in addition to that of endothelial cells. Also, the glycocalyx serves as a mechanotransducer of shear stress signals to the plasma membrane and cortical cytoskeletons of endothelial cells. For example, shear stress causes flow-mediated release of vasodilators, including NO and PGI ₂ (see Fig. 8.4 and Chapter 9 ). On the one hand, the ability of endothelial cells to release vasodilators is impeded when glycosaminoglycans in the glycocalyx are degraded enzymatically. On the other hand, when flow is laminar, the synthesis of glycocalyx components is increased, becoming a counterforce that sustains the ability of endothelial cells to sense and react to flow patterns.

The basement membrane also functions in mechanotransduction. Normally, the basement membrane underlying endothelial cells is rich in collagen and laminin. Integrins, a family of cell adhesion receptors, are found in the endothelial cell, where they anchor to the cytoskeleton and intracellular signaling molecules. In addition, integrins bind to collagen (α2β1, α1β1) and laminin (α6β1, α6β4) found in the extracellular matrix of the basement membrane. The result of this binding is an endothelial cell phenotype that is slow to proliferate. Injury, such as that produced by turbulent flow, promotes the deposition of fibronectin and fibrinogen in the extracellular matrix, where they bind integrins (α5β1, αvβ3). Thus by having fibronectin and fibrinogen present to bind other integrins, the endothelial cell expresses a phenotype that proliferates and migrates. The change of matrix structure and composition can trigger a reaction cascade that changes endothelial cell function to initiate inflammation and, eventually, atherosclerosis. Thus the balance between atheroprotective and atherogenic forces on the endothelial cell can be changed by a transition from laminar to turbulent flow.

The Endothelium Plays a Passive Role in Transcapillary Exchange

Solvent and solute move across the capillary endothelial wall by three processes: diffusion, filtration, and pinocytosis . The permeability of the capillary endothelial membrane is not the same in all body tissues. For example, liver capillaries are quite permeable, and albumin escapes from them at a rate several times greater than that from the less permeable muscle capillaries. Also, permeability is not uniform along the whole capillary; the venous ends are more permeable than the arterial ends, and permeability is greatest in the venules. The greater permeability at the venous ends of the capillaries and in the venules is attributed to the greater number of pores in these regions of the microvessels.

The sites at which filtration occurs have been a controversial subject for many years. Water flows through the capillary endothelial cell membranes through water-selective channels called aquaporins , a large family of intrinsic membrane proteins (28 to 30 kDa) that function as water channels. Each of these pore-forming proteins consists of six transmembrane segments that form a monomer; this structure is incorporated into membranes as homotetramers. The aquaporins are permeable to H ₂ O and other substances (glycerol, urea, Cl ^- ) having diameters of 3.4Å (0.34 nm) or less. Water also flows through apertures (pores) in the endothelial walls of the capillaries ( Figs. 8.5 and 8.6 ). Calculations based on the transcapillary movement of small molecules have led to the prediction of capillary pores with diameters of about 4 nm in skeletal and cardiac muscle. In agreement with this prediction, electron microscopy has revealed clefts between adjacent endothelial cells with gaps of about 4 nm (see Figs. 8.5 and 8.6 ). The clefts (pores) are sparse and represent only about 0.02% of the capillary surface area. In cerebral capillaries, there is a blood-brain barrier to many small molecules.

In addition to clefts, some of the more porous capillaries (e.g., in kidney and intestine) contain fenestrations (see Fig. 8.5 ) that are 20 to 100 nm wide, whereas in other sites (e.g., in the liver) the endothelium is discontinuous (see Fig. 8.5 ). Fenestrations and discontinuous endothelium permit passage of molecules that are too large to pass through the intercellular clefts of the endothelium.