Page 80 - Cardiac Nursing

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56 PA R T I / Anatomy and Physiology
increases resistance 16-fold, because resistance is inversely propor- volumes of fluid with only small increases in transmural pressure;
tional to the fourth power of the radius. An increase in the hema- that is, they are compliant. Because of their ability to serve as a
tocrit (e.g., polycythemia caused by high altitude) can increase volume reservoir, the veins are referred to as capacitance vessels. At
blood viscosity, causing resistance to increase. 138 A limitation of increased pressures, the veins become distended and less compli-
Poiseuille’s law is that it is based on rigid tubes and predicts a lin- ant; thus, any given pressure change is associated with a smaller
ear relationship between pressure and flow. However, because of change in volume. Because of the compliant nature of the veins,
the elastic properties of blood vessels, the relationship between the venous system plays an important role in altering thoracic in-
pressure and flow is nonlinear. Depending on the starting pressure travascular volume. 24
and the vasoconstrictive state of the vessel, an initial increase in
pressure may distend the vessel but have limited effect on flow. See
Chapter 21 for a discussion of the calculation and interpretation MICROCIRCULATORY
of the systemic and pulmonary vascular resistance. EXCHANGE
Flow Through the Microvascular
THE VENOUS SYSTEM Circulation
˙
The venous system transports blood back to the heart from the Blood flow (Q) through the microcirculation (or any organ) is di-
microcirculation of each organ system and plays a crucial role in rectly related to the difference in pressure between the arterial end
P
P
the maintenance of thoracic intravascular volume. The veins also of the vascular segment (P A ) and venous pressure (P v P ) and is in-
13,14
R R
serve as a low-pressure reservoir with the capacity to contain a versely related to vascular resistance (R T ).
large and variable volume of blood (similar to a giant capacitor sit- P A P P V
P P
ting next to the right ventricle). The veins are innervated by - Flow R R
adrenergic fibers but not -adrenergic fibers. Only the splanchnic R T
P
and cutaneous veins receive extensive innervation. The veins con- In the absence of changes in arterial pressure (P A ), changes in lo-
strict in response to -adrenergic stimuli and dilate as the result of cal vascular resistance and intravascular pressure are caused by va-
withdrawal of the -adrenergic stimuli or in response to increased sodilation and vasoconstriction of the arterioles. Any alteration in
transmural pressure (i.e., passive vasodilation). There are no active the tone of the muscular venules contributes little to the change
vasodilatory mechanisms in the veins. 23 in resistance.
Microvascular Transport Mechanisms
Venous Pressure and Resistance
Solutes and water passively move across the endothelium as the
In the supine position, the pressure generated by the heart in the
large arteries is approximately 100 mm Hg. However, as demon- result of two processes, diffusion and ultrafiltration. Diffusion is
strated in Figure 2-14, the pressure decreases across the arterioles the result of the random kinetic motion of ions and molecules.
and capillaries, with a resultant pressure in the small veins of only Diffusion results in the net transport of substances along a con-
15 to 20 mm Hg. The right atrial pressure is approximately 0 to centration gradient from high to low concentration. Ultrafiltra-
5 mm Hg (depending on position, the state of hydration, and car- tion is the combined movement of fluid and solutes in a unilat-
diac output). Thus, the pressure driving blood flow from the left eral direction through a membrane, except that the movement
side of the heart to the capillaries is approximately 80 mm Hg, of the solutes is restricted by the membrane. The driving force
whereas the driving pressure from the postcapillary vessels to the for ultrafiltration is the difference between hydrostatic pressure
right atrium is only 15 to 20 mm Hg (difference between the and oncotic pressure across the membrane. Ultrafiltration is the
postcapillary vessels and the right atrium). Interestingly, in the up- primary mechanism for controlling plasma and interstitial fluid
13
right position this gradient is unchanged, despite the addition of volume.
hydrostatic pressure (determined by the height of a continuous Diffusion
column of blood between any given point and the heart). Concentration gradients, created by the production or consump-
Skeletal muscle contractions in the extremities (the muscle tion of specific substances, are the primary driving forces for dif-
pump) and respiration (respiratory pump) play an essential role in fusion (with the exception of the tight-junction capillaries, which
propelling venous blood from the veins to the right atrium (see are affected by electrical gradients). Because diffusion in or out of
Chapter 3). In addition, the venous valves prevent backward flow a blood vessel creates a concentration gradient along the vessel,
into the muscle. Valvular function is particularly important dur- diffusion exchange is strongly influenced by blood flow, particu-
ing standing and exercise and the pathogenesis of venous insuffi- larly for those substances that rapidly diffuse through the mem-
ciency. The valves also promote the one-way flow of blood brane wall. 14,139 The rate of diffusion of a solute across the capil-
through perforating veins that lie between the superficial and deep lary wall (J s J ) is proportional to the concentration gradient, that is,
J
veins.
the difference between the concentration in the plasma (C p ) and
C C
C
P
interstitial concentration (C i C ), the permeability (P s P ) of the en-
Venous Compliance dothelium to the solute, and the surface area (A) available for ex-
(
(
change.
When empty, the thin walls of the veins are flattened and the ves- J s J J P s P A(C p C i C )
P
C C
C
sels are elliptical. As the veins fill with blood, they passively change
to a circular shape. Because of this passive accommodation to an For substances that diffuse rapidly through the capillary endothe-
increase in volume, the veins are capable of receiving large lium (e.g., O 2 , CO 2 ), the transport of the solute depends on the