Which of the following statement(s) is/are true concerning the physiology of the microvascular system?

a. Filtration of capillary fluid into the interstitial and the subsequent reabsorption is influenced by Starling’s law of ultrafiltration

c. Most of the resistance to systemic blood flow occurs at the arteriolar level

d. Adrenergic vasoconstriction can arrest blood flow to an entire capillary bed

Exchange of material between the vascular space and the cell of various tissues via the interstitial space is essential for organ viability and occurs at the capillary level. The filtration of capillary fluid into the interstitium and its subsequent reabsorption into the post capillary venule is governed by microvascular permeability in conjunction with the balance between hydrostatic and oncotic pressures. The relation of these forces to one another (and their net effects) are illustrated by what is termed Starling’s law of ultrafiltration. In normal circumstances, a net filtration from capillary to interstitium is effected by a relatively higher capillary hydrostatic pressure, whereas net reabsorption from the interstitium back into the post capillary venule occurs as hydrostatic pressure falls and oncotic forces predominate. Although the mechanisms controlling blood flow to the capillary bed are complicated and vary among the different tissues, certain concepts are useful. Poiseuille’s law describes the relation between flow of fluid through a tube and the tube length and radius, the fluid viscosity, and the pressure gradient between ends of the tube. The radius of the tube (or vessel) is the single most important variable, because flow is proportional to the radius to the fourth power. Vasoconstrictive and vasodilatory influences directly impact local blood flow, as well as flow to other tissues through secondary effects on the systemic pressure. This secondary effect of peripheral vasoconstriction maintains the pressure gradient for central perfusion of the heart and brain. Systemic blood flow meets most of its resistance at the arteriolar level. While the individual capillary radius is significantly smaller, the vast number of capillaries offers less total resistance. The vascular smooth muscle in arterioles has both a-and b- adrenergic receptors. Alpha stimulation affects vasoconstriction where beta stimulation affects vasodilatation. The efferent sympathetic fibers innervating the precapillary resistance vessels and the venous capacitance vessels release norepinephrine on stimulation, which induces smooth muscle contraction and narrowing of the caliber of the vessels. These contractions are potent enough that blood flow to entire capillary beds can be arrested by adrenergic vasoconstriction.