Page 68 - Cardiac Nursing

6/3
6/3
0
0
0
009
1
009
0/2
0/2
xd
2-0
68.
2-0
04
04
q
xd
q
68.
q
1
p
p
4 A
e 4
4 A
r
r
ta
p
ta
e 4
3
Pa
3
5:3
5:3
g
g
g
Pa
Pa
K34
K34
0-c
LWBK340-c02_ p pp042-068.qxd 06/30/2009 15:33 Page 44 Aptara a a
LWB
LWB
p
02_
02_
0-c
44 PA R T I / Anatomy and Physiology
Phenotype Organ (function)
Continuous bm CNS (blood brain barrier)
Lymph node (lymphocyte homing)
Muscle (metabolic exchange)
tj
Fenestrated Endocrine glands (secretion) ■ Figure 2-2 Different types of endothelial cells,
f GI tract (absorption) their distribution to different organs and specific
functional roles (bm, basal membrane, tj, tight junc-
Choroid plexus (secretion) tion, f, fenestrae, p, pores. (From Priese, K. R., &
Kuebler, W. M. [2006]. Normal endothelium. Hand-
Kidney glomeruli (filtration)
6
book of Experimental Pharmacology, 176[Pt. 1], 1–40.)
6
Discontinuous Liver (particle exchange)
p
Bone marrow (hematopoiesis)
Spleen (blood cell filter)
Tight-junction endothelium is located in the central nervous form a rich anastomosis with deeper veins via vessels that perfo-
system and retina. It is the least permeable. The endothelial cells rate the muscles. These perforating veins allow venous return from
are connected by tight junctions that effectively restrict passage of cold skin to be diverted to warm muscle, providing a thermal
all substances. Water- and lipid-soluble molecules pass directly short circuit, and they are particularly important for function of
through the endothelium, whereas ions and lipid-insoluble sub- the muscle pump, which is described in Chapter 3.
stances, such as glucose and amino acids, are transported by mem-
brane carriers. 19 Venous Valves
With the exception of the intrathoracic and intracerebral veins,
Venules the medium-sized veins contain valves that are oriented in the di-
rection of blood flow, thus preventing retrograde blood flow into
2
Venous capillaries extend to the postcapillary venules (nonmuscu- the muscle. The presence of competent valves, in conjunction
lar, 7 to 50 m) and collecting venules. Along with the capillar- with the muscle pump in the lower extremities, is crucial to the
ies, the nonmuscular venules act as exchange vessels. Smooth mus- ability to stand erect and in maintaining a reasonably low capil-
cle reappears in venules that are approximately 30 to 50 m in lary pressure, because the valves interrupt the hydrostatic column
diameter. These venules, which receive adrenergic innervation, are that extends from the right atrium to the feet after each leg mus-
referred to as the muscular venules, postcapillary resistance vessels, cle contraction. 20 After humans with normal valvular function
or capacitance vessels. 2,13,16 As discussed later in the section on stand up, the valves in dependent veins initially interrupt the hy-
microcirculation, postcapillary resistance tends to be far less than drostatic column. However, over a period of approximately 2 to 3
precapillary resistance and has almost no effect on overall systemic minutes, as the veins fill with blood, the valves can no longer in-
vascular resistance. The veins contain approximately 70% of total terrupt the hydrostatic column as volume continues to accumu-
blood volume, with approximately 25% of this volume in the late. At this time, there is a displacement of approximately
venules. 2 600 mL of blood from the central circulation into the legs and
21
pelvic organs. In conditions in which blood flow is high, the hy-
Veins drostatic effects associated with the loss of valvular function occur
within 2 to 3 seconds. If the hydrostatic effects are not overcome
In general, veins have a larger diameter and thinner, more com- by the muscle pump in the lower extremities, arterial hypoten-
pliant walls than arteries at equivalent branches of the vascular sion, and syncope result. This phenomenon is readily seen in the
2
tree. However, the thickness of the venous walls is variable. For soldier who faints while standing motionless at attention. There is
example, the veins in the legs and feet, which withstand the high also some evidence that there are microscopic venous valves lo-
hydrostatic pressure associated with standing, are thick-walled, cated in the small veins and also in the post-capillary venules. 22
whereas the veins near or above the level of the heart are thin- These microscopic valves may play a protective role against ve-
walled. The veins contain all three vascular layers found in the ar- nous hypertension when there is valvular insufficiency in larger
3
teries; however, these layers are often indistinct. Superficial veins veins.