FORUM REVIEW ARTICLE
Red Blood Cell Function and Dysfunction:
Redox Regulation, Nitric Oxide Metabolism, Anemia
Viktoria Kuhn,
1,
*
Lukas Diederich,
1,
*
T.C. Stevenson Keller IV,
2
Christian M. Kramer,
1
Wiebke Lu¨ cksta¨ dt,
1
Christina Panknin,
1
Tatsiana Suvorava,
1
Brant E. Isakson,
2
Malte Kelm,
1
and Miriam M. Cortese-Krott
1
Abstract
Significance: Recent clinical evidence identified anemia to be correlated with severe complications of car-
diovascular disease (CVD) such as bleeding, thromboembolic events, stroke, hypertension, arrhythmias, and
inflammation, particularly in elderly patients. The underlying mechanisms of these complications are largely
unidentified.
Recent Advances: Previously, red blood cells (RBCs) were considered exclusively as transporters of oxygen
and nutrients to the tissues. More recent experimental evidence indicates that RBCs are important interorgan
communication systems with additional functions, including participation in control of systemic nitric oxide
metabolism, redox regulation, blood rheology, and viscosity. In this article, we aim to revise and discuss the
potential impact of these noncanonical functions of RBCs and their dysfunction in the cardiovascular system
and in anemia.
Critical Issues: The mechanistic links between changes of RBC functional properties and cardiovascular
complications related to anemia have not been untangled so far.
Future Directions: To allow a better understanding of the complications associated with anemia in CVD, basic
and translational science studies should be focused on identifying the role of noncanonical functions of RBCs in
the cardiovascular system and on defining intrinsic and/or systemic dysfunction of RBCs in anemia and its
relationship to CVD both in animal models and clinical settings. Antioxid. Redox Signal. 26, 718–742.
Keywords: red blood cells, nitric oxide, anemia, RBC deformability, hemolysis, cardiovascular disease, red
cell eNOS
Introduction
T
he main physiological role of red blood cells
(RBCs), or erythrocytes is to transport of gases (O
2
,CO
2
)
from the lung to the tissues and to maintain systemic acid/
base equilibria. In addition, RBCs are well equipped with
antioxidant systems, which essentially contribute to their
function and integrity. Damage of red cell integrity, defined
as hemolysis, has been shown to significantly contribute to
severe pathologies, including endothelial dysfunction. Re-
cent clinical and experimental evidence indicates that RBCs
may be directly involved in tissue protection and regulation of
cardiovascular homeostasis by exerting further noncanonical
functions, including nitric oxide (NO) metabolism and con-
trol of blood rheology, as well as erythrocrine function (i.e.,
by releasing bioactive molecules, including NO, NO metab-
olites, and ATP). Many hypotheses on the role of noncanonical
functions of RBCs in cardiovascular homeostasis have been
1
Cardiovascular Research Laboratory, Division of Cardiology, Pneumology, and Vascular Medicine, Medical Faculty, Heinrich Heine
University of Du
¨
sseldorf, Du
¨
sseldorf, Germany.
2
Department of Molecular Physiology and Biological Physics, Robert M. Berne Cardiovascular Research Center, University of Virginia
School of Medicine, Charlottesville, Virginia.
*These authors contributed equally to this work.
ª Viktoria Kuhn, et al., 2017; Published by Mary Ann Liebert, Inc. This Open Access article is distributed under the terms of the
Creative Commons License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly credited.
ANTIOXIDANTS & REDOX SIGNALING
Volume 26, Number 13, 2017
Mary Ann Liebert, Inc.
DOI: 10.1089/ars.2016.6954
718
put forward, and evidence of a central role played by RBCs in
cardiovascular protection is accumulating. However, many
aspects of RBC-mediated control of NO metabolism and
ATP release are still speculative or not universally accepted.
Anemia is a pathological condition characterized by a
decreased number of circulating RBCs and defined by he-
moglobin (Hb) concentrations in whole blood below 12 g/dL
in females and 13 g/dL in males (192). There is clinical ev-
idence that anemia is also associated with a series of severe
complications in cardiovascular disease (CVD) such as
thromboembolic events (e.g., venous thrombosis and stroke).
However, therapeutic interventions aimed to increase the
circulating number of RBCs (e.g., by transfusion of blood
or by administration of erythropoiesis-stimulating agents
[ESAs] to stimulate the production of RBCs by the bone
marrow), were not always effective in the tested cohorts (48,
91, 156). One possible explanation is that these treatments
have side effects and therefore may contribute themselves to
the negative outcome, for example, treatment with ESAs was
associated with increased thromboembolic events (45).
Interpretation of large cohort studies may be very complex
and requires recognition of many interacting features of
disease and normal physiology. This is particularly true for
studies evaluating the relationship between anemia and car-
diovascular complications, which may involve different as-
pects, including changes in number or function of RBCs, in
blood rheological properties, in systemic hemodynamics,
and overall cardiovascular physiology and pathology.
In this article, we aim to provide a chemical, biophysical,
and clinical perspective about the role of RBCs in the car-
diovascular system, with focus on noncanonical functions
of RBCs (Fig. 1). Specifically, we will describe (I) the role
of redox regulation in RBCs to maintain cell functionality
and integrity, including sources of reactive oxygen species
(ROS), enzymatic and nonenzymatic antioxidant systems,
and damage caused by dysregulation of the redox state; (II)
the complex role of RBCs in NO metabolism; (III) the in-
trinsic mechanical properties of RBCs and their effects on
blood rheology and hemodynamics; (IV) the pathophysi-
ology of specific anemic conditions, characterized by RBC
dysfunction and hemolysis, and present mice models applied
for basic and translational science studies; and (V) the clin-
ical aspects and therapeutic approaches for anemia in CVD,
outlining the open questions and proposing possible research
directions.
RBCs and Redox Regulation
The main function of RBCs is to transport oxygen from the
lungs to the tissues, where it is used as a source of electrons
and ATP synthesis in the mitochondria. Additionally, RBCs
transport carbon dioxide (CO
2
), which is produced as a result
of catabolic processes within the tissues, from the periphery
to the lungs to be exhaled. CO
2
may be transported in RBCs
by Hb through reaction of amino groups of the Hb chains and
formation of carbaminohemoglobin. However, most CO
2
in
the circulation is transported as bicarbonate ions (HCO
3
-
)
upon the carbonic anhydrase catalyzed reaction of CO
2
with
H
2
O, followed by H
2
CO
3
deprotonation in water. These
functions are intimately interconnected to each other: O
2
binding affinity to the ferrous heme (Fe
2+
) of Hb is regulated
by oxygen partial pressure (pO
2
), acid/base equilibria (pH),
and by the levels of 2,3-diphosphoglycerate; on the other
hand, CO
2
transport is dependent on the activity of carbonic
anhydrase and is directly involved in control of pH and
buffering capacity of RBCs. If the ferrous heme (Fe
2+
)iron
contained in the prosthetic group of Hb is oxidized to ferric
(Fe
3+
) heme to form methemoglobin (metHb), the affinity
of the protein toward oxygen is dramatically decreased.
To prese rve its funct io nali t y, H b ( wh ic h i s a lso the most
abundant cytoplasmic protein in RBCs) has to be main-
tained in the reduced state. The three main challenges herein
are the follow i ng: firs t, R BC s con tai n nume r ou s sou rce s of
oxidants (including high levels of molecular O
2
bound to
Hb) (89); second, RBCs carry high levels of iron within the
prosthetic group of Hb (89), which in its free soluble form is
a potent catalyst of ROS production via the Fenton r eaction;
and third, RBCs have limited capacity to restore damaged
elements due to loss of protein expression during erythro-
poietic maturation.
In the following section, we summarize (i) the sources of
oxidants in healthy RBCs, (ii) the antioxidant systems, in-
cluding (ii.a) antioxidant molecules and their redox couples,
such as reduced and oxidized glutathione (GSH/GSSG),
ascorbate/dehydroascorbate (vitamin C), and a-tocopherol
(vitamin E), (ii.b) the sources of reducing equivalents such
as nicotinamide adenine dinucleotide (NADH) and nicotin-
amide adenine dinucleotide phosphate (NADPH), and (ii.c)
the antioxidant enzymes such as superoxide dismutase
(SOD), catalase (Cat), glutathione peroxidase (GPx), and
peroxiredoxin 2 (Prx2); as well as (iii) the damage caused by
redox dysregulation in RBCs caused by oxidants (Fig. 2).
Sources of oxidants in RBCs
Every 24 h, 3% of Hb undergoes autoxidation, producing
metHb and superoxide radical anion (Eq. 1) (50). Consider-
ing the abundance of Hb in RBCs (corresponding to 10 mM
heme), the reaction of autoxidation of Hb is therefore the
most abundant source of ROS in RBCs.
HbFe
2 þ
O
2
! HbFe
3 þ
þ O
2
[1]
However, only 1% of Hb is present in the metHb (Fe
3+
)
state. Hb-Fe
3+
can be reduced back to Hb-Fe
2+
in a reaction
catalyzed by the cytochrome b5 reductase using NADH as an
electron donor. Mutation of the gene for cytochrome b5 re-
ductase causes cyanosis and severe neurological problems
due to impairment of neuron myelination and increased
mortality during childhood.
Another source of oxidants is free iron (Fe
3+
), which may
dissociate from metHb (26). In the presence of free iron, the
prominent reaction ongoing is the Haber–Weiss reaction
(Eqs. 2 and 3), which includes the Fenton reaction (Eq. 3)
producing hydroxyl radicals.
Fe
3 þ
þ O
2
! Fe
2 þ
þ O
2
[2]
Fe
2 þ
þ H
2
O
2
! Fe
3 þ
þ
OH þ OH
[3]
The presence of ferritin in RBCs will contribute to scav-
enge free iron and therefore to limit the occurrence of the
Haber–Weiss reaction in RBCs.
RBC FUNCTION AND DYSFUNCTION 719
Another important oxidative pathway is the reaction be-
tween H
2
O
2
and both ferrous and ferric Hb, resulting in heme
degradation and release of free iron (3), as described in detail
in an older comprehensive review by Reeder (150). These
reactions lead to generation of the potent oxidizing ferrylHb
species as well as secondary radicals from the reaction be-
tween H
2
O
2
and either oxyHb or metHb. This pathway has
also been proposed to be important in mediating hemolytic
injury (24). Along with autoxidation (50), another important
oxidative pathway is the reaction between H
2
O
2
and both
ferrous and ferric Hb, resulting in heme degradation and re-
lease of free iron (3), as described in detail before (150).
Hb Fe
2 þ
þ H
2
O
2
! Hb Fe
4 þ
¼ O
2
þ H
2
O [4]
Hb Fe
3 þ
þ H
2
O
2
! Hb
þ
Fe
4 þ
¼ O
2
þ H
2
O [5]
Hb Fe
4 þ
¼ O
2
þ H
2
O
2
! Hb Fe
3 þ
þ O
2
þ H
2
O
[6]
Hb Fe
3 þ
þ O
2
! heme degradation products þ Fe
3 þ
[7]
As described in more detail in the RBCs and NO Meta-
bolism section, RBCs are thought to generate measurable
levels of NO. Therefore, taking into account the almost
diffusion-limited rate constant for the reaction between NO
and O
2
-
, it would seem at least reasonable that peroxynitrite
could be formed according to Eq. 8.
NO þ O
2
! ONOO
[8]
It is important to note that antioxidant systems abundant in
RBCs, including Prx and GPx, as well as metHb are also
potent peroxynitrite scavengers.
Antioxidant systems
The antioxidant systems of RBCs are based on both en-
zymatic and nonenzymatic mechanisms. RBCs carry (I) high
concentrations of antioxidant molecules, including glutathi-
one (GSH) and vitamins such as ascorbic acid (vitamin C)
and a-tocopherol (vitamin E), (II) sources of reduced
equivalents such as NADH and NADPH, and (III) antioxi-
dant enzymes, including SOD1, Cat, GPx, and the thior-
edoxin (Trx) system, as described in detail below. The
antioxidant systems in RBCs strongly contribute to keep
the levels of oxidants (including O
2
-
,H
2
O
2
and other re-
active species described in the section ‘‘Sources of oxidants
in RBCs’’) very low.
I. Antioxidant molecules and redox couples
GSH/GSSG redox couple. Antioxidant molecules abun-
dant in RBCs are the GSH/GSSG redox couple, ascorbate
and alpha-tocopherol. GSH is a tripeptide consisting of L-
glutamine, L-cysteine, and L-glycine synthesized by a c-
glutamine-cysteine ligase and a glutathione synthase. In
healthy human RBCs, 90%–95% of glutathione is present in
the reduced form GSH (200) that can be utilized for the re-
duction of ascorbate, oxidized proteins, and oxidized lipids
(184). Enzymes using GSH as reducing equivalent are glu-
taredoxin (GRx) and GPx. The enzyme responsible for GSH
recycling is glutathione reductase, which reduces glutathione
disulfide (GSSG) back to the reduced GSH via consumption
of NADPH (197).
Ascorbate/dehydroascorbate. Ascorbate is an essential
hydrophilic vitamin and an important reducing equivalent in
RBCs, which is (re)synthesized from dehydroascorbic acid at
the expense of GSH in a reaction catalyzed by a variety of
enzymes, especially GRxs (190), which are small cytoplas-
mic enzymes also catalyzing deglutathionylation, reduction
of protein disulfides, or Fe-S linkage formation. The major
transport mechanism of ascorbate into the RBC is attributed
to glucose transporter 1 (116, 130). It is involved in several
antioxidant mechanisms such as reduction of diffusible oxi-
dants and metHb, as well as maintenance of plasma mem-
brane redox system (PMRS) (51). The PMRS is a system
transferring electrons from the intracellular cytosolic RBC
compartment to the extracellular medium (plasma) by oxi-
dation of intracellular electron donors (e.g., ascorbate and
NADH), thus driving the exchange of electrons and keeping
plasma components in a reduced state. The physiological
significance of the PMRS is not completely understood, but it
was reported that the increased PMRS activity in erythrocytes
during aging may be a protective mechanism of the system
for efficient extracellular dehydroascorbate reduction and
ascorbate recycling under conditions of increased oxidative
stress (173). Thus, PMRS in RBCs may act as a compensa-
tory mechanism against increased ROS/oxidative stress and
represents a potential mechanism of how RBCs contribute to
redox regulation throughout the whole body (173).
a-Tocopherol (vitamin E). a-Tocopherol is an impor-
tant oxidant scavenger in RBCs. Due to its lipophilicity, a-
tocopherol accumulates in RBC membranes and plays a
central role in preventing lipid peroxidation, probably by lim-
iting the amplification of peroxidation chain reactions within
the plasma membrane (24). It was demonstrated that recycling
of a-tocopherol radicals, next to diffusion of new molecules out
of plasma, occurs by the oxidation of ascorbate (24, 121).
II. Sources of redox equivalents (NADH and NADPH). Like
other cell types, both NADH and NADPH are sources of
reducing equivalents in RBCs. In contrast to other cells,
the pentose phosphate cycle is the main source of reduc-
ing equivalents in the RBCs due to a lack of mitochondria.
Glucose-6-ph osphate dehydrogena se (G 6PDH) retrieves
glucose-6-phosphate from glycolytic ATP production and
starts NADP
+
reduction. Interestingly, the antioxidant de-
fense is tightly connected to the energy status of the RBC.
Excessive fasting can lead to decreased levels of reduced
NADPH in RBCs caused by decreased availability of glu-
cose to the glycolytic pa thway. I n addition , the lack of
riboflavins—components of the cosubstrate flavin adenine
dinucleotide (FAD) important for glutathione reductase
functionality—can impair antioxidant defense in fasting (10).
NADPH is of high importance for redox balance within the
RBC. Malfunction of enzymes within the pentose phosphate
pathway can have severe consequences on overall membrane
720 KUHN ET AL.
stability and permeability, as will be discussed in the section
‘‘Redox Dysregulation in RBCs and Hemolytic Anemia’’.
III. Enzymatic antioxidant systems (SOD, Cat, GPx,
Prx2). The enzymes known to participate in the processing
of oxidants in mature RBCs are SOD1, Cat, GPx, and Prx2
(129). The family of SOD enzymes comprises three isoforms
with different structural characteristics of the prosthetic
group (containing manganese or copper and zinc) as well as
compartmentalization and functional significance (118). Of
high importance for the mature RBC is the copper/zinc iso-
form 1 of SODs (SOD1), catalyzing the dismutation of the
superoxide anion (Eq. 9), which is formed in RBCs mostly by
Hb autoxidation (Eq. 1), into hydrogen peroxide (122).
2O
2
þ 2H
þ
! H
2
O
2
þ O
2
[9]
Hydrogen peroxide hereupon is degraded to oxygen and
water by Cat, GPx, or Prx2. In nonpathological conditions,
GPx has been shown to be the enzyme degrading most of the
H
2
O
2
by oxidizing reduced GSH into GSSG (Eq. 10). GSSG
is recycled by GSH reductase, which consumes NAPDH (38).
In conditions of overproduction of O
2
-
and therefore higher
concentrations of H
2
O
2
, Cat takes over and reduces H
2
O
2
into
water (Eqs. 11 and 12) (89).
H
2
O
2
þ 2 GSH ! 2H
2
O þ GSSG [10]
catalase Fe
3 þ
þ H
2
O
2
! compound I þ H
2
O [11]
compound I þ H
2
O
2
! catalase þ O
2
þ H
2
O [12]
Although traditional descriptions of RBCs postulate the lack
of intracellular compartmentalization of enzymes due to a lack
of nuclei and organelles (including mitochondria), subcellular
localization of these antioxidant systems plays a fundamental
role. The association of Prx2 (which participates in reduction
of organic peroxides, Eq. 13) with the membrane (120) makes
it important for protection of RBC membrane constituents
against lipid peroxidation. A lack of this enzyme has greater
impact on the phenotype than lack of GPx or Cat (106).
Prx2 retrieves its electrons for hydrogen peroxide reduc-
tion from the reducing equivalent Trx (Eq. 14), which is
dependent on a sufficient supply of NADPH.
Prx(S
) þ H
2
O
2
! Prx(SO
) þ H
2
O [13]
Prx(SO
) þ Trx SHðÞ
2
! Prx(S
) þ Trx S
2
ðÞþH
2
O [14]
Damage caused by oxidation reactions
and redox dysregulation
The coordinated action of all antioxidant systems makes
RBCs very resistant against oxidation as well as an efficient
systemic redox buffering system. Malfunction of antioxidant
defense and/or conditions of increased oxidant production
have severe consequences for RBCs on a subcellular level.
These consequences include degradation of Hb (Eqs. 4–7)
and other proteins, disturbance of ionic homeostasis (Ca
2+
)
(49), formation of neoantigens (96), hindered RBC defor-
mation (37), interference with erythropoiesis (162), and en-
hanced exposure of phosphatidylserine (PS) (85). High
concentrations of polyunsaturated fatty acids in RBC mem-
brane make them susceptible to peroxidation leading to the
loss of RBC membrane integrity and decreased activity of
membrane enzymes [e.g., ATPase (198) and acetylcholin-
esterase (183)].
Biochemical consequences of hemolysis
Hemolysis in the circulation has been shown to exert
profound pathological effects, particularly on the cardiovas-
cular system. The consequence of hemolysis is particularly
evident in hemolytic anemia, where fragility of RBCs is
mainly due to redox dysbalance or hemoglobinopathies,
for example, sickle cell disease (SCD) (see the section ‘‘RBC
Dysfunction and Anemia’’ for more details), and observed in
transfusion of older blood (71). The main consequence of
RBC rupture is the release of intracellular content, in par-
ticular Hb and arginase 1. Loss of Hb compartmentalization
may lead to systemic NO scavenging and may affect endo-
thelial function (72, 153). According to Eqs. 4–7, Hb may
also react with H
2
O
2
(e.g., generated during inflamma-
tory conditions), promote ferrylHb formation, and induce
lipid peroxidation (3). Hb or heme has been proposed to ac-
tivate Toll-like receptor 4 and induce proinflammatory nu-
clear factor kappa-light-chain-enhancer of activated B
cell-dependent signaling (16, 67). Moreover, release of ar-
ginase 1 from the RBCs was proposed to contribute to argi-
nine depletion, leading to decreased endothelial nitric oxide
synthase (eNOS) activity (131).
Summary: redox buffering function of RBCs
To summarize, RBCs are particularly well equipped with
potent nonenzymatic and enzymatic antioxidant systems,
which are important to maintain Hb in a reduced oxygen
binding form, to limit oxidative modifications of membrane
lipids, structural proteins, channels, and metabolic enzymes,
and therefore to keep the cell alive and functional for its
(average) 120-day life. Failure or dysfunction of the antiox-
idant system may have severe consequences for the cell, in-
cluding loss of membrane integrity leading to hemolytic
anemia. Beyond this, RBC antioxidant pathways and their
ability to reduce extracellular antioxidants via the trans-
membrane electron transport system, along with RBC mo-
bility through the circulation, make them an ideal ROS
buffering component, which may contribute to overall sys-
temic homeostasis of redox balance.
RBCs and NO Metabolism
RBCs were considered for a long time as a powerful
scavenger of endothelial cell-derived NO, participating in
systemic NO metabolism mainly by limiting NO bioavail-
ability. Recent evidence indicates that RBCs participate in
systemic NO metabolism and regulation of vascular tone and
integrity, but in a different way than initially thought.
NO is constitutively produced within the endothelium by
eNOS and it is thought to freely diffuse into smooth muscle
cells, where NO binds to the Fe
2+
-heme center of a soluble
RBC FUNCTION AND DYSFUNCTION 721
guanylyl cyclase leading to vasodilation. Hb is thought to
rapidly inactivate the NO signal by binding NO to the oxy-
genated Fe
2+
-heme metal center, which is followed by sub-
sequent formation of metHb and nitrate with a high reaction
constant of &6–8 · 10
7
M
-1
s
-1
(Eq. 15) (81). Moreover,
under hypoxic conditions, NO can also rapidly react with
deoxygenated Hb, leading to formation of ni-
trosylhemoglobin (Eq. 16):
HbFe
2 þ
O
2
þ NO ! HbFe
3 þ
þ NO
3
[15]
HbFe
2 þ
þ NO ! HbFe NO [16]
Since RBCs contain 10 mM Hb (corresponding to 97% of
erythrocytes’ dry weight) (81), they were thought to represent
an irreversible sink of endothelium-derived NO under any
conditions. Interestingly, the nature of NO as an endothelial-
derived relaxing factor was initially not universally accepted
because of these reactions: the reasoning of many researchers
in this field was that the presence of a highly efficient scav-
enger in direct vicinity of the main source of NO would
produce a concentration gradient leading to diffusion of NO
into the blood stream (instead of into the smooth muscle) and
thereby prevent vasodilatory function. However, in vitro
experiments have shown that there is a 1000-fold discrepancy
between the reaction with free Hb or intact RBCs (111).
Thanks to its small size and lack of charge, NO can easily
diffuse through plasma membranes, but its diffusion into the
RBCs is limited by transmembrane and intracellular resis-
tances (185), as well as by the presence of an unstirred layer
of solvent around the RBC (extracellular resistance) (46).
This may explain lower consumption of NO by RBCs com-
pared with free Hb.
This view changed radically after the discovery of RBC-
mediated hypoxic vasodilation: under conditions of decreased
oxygen saturation, RBCs were shown to induce vasodilation
of vessel strips by exporting NO bioac tivity (146). Formation
of nitrosothiols, and in particular S-nitrosohemoglobin, was
proposed to be the signal mediating hypoxic vasodilation. In
FIG. 1. RBC function and dysfunction: redox regulation, NO metabolism, and anemia. (A) Intrinsic RBC properties
and function. Beside their canonical role in transport of gases and nutrients, RBCs are well equipped with redox buffer
systems and are important modulators of NO metabolism. Their intrinsic mechanical properties allow them to deform/
change their shape in response to changes in flow and to changes in vessel diameter, thus participating in control of blood
rheology. (B) Effects of RBCs in blood. A second way for RBCs to control blood rheology is via their concentration
(hematocrit), which critically defines blood viscosity and blood rheology. In addition, RBCs interact with PLTs resulting in
a complex cell–cell communication involving membrane adhesion molecules, NO metabolism, and redox regulation. (C)
Effects on systemic hemodynamics. In addition to control of vascular tone and cardiac function, intrinsic RBC properties
and overall blood rheology are contributors to systemic vascular hemodynamics. (D) Anemia. RBC dysfunction mainly
results in a number of anemic conditions, which are characterized by a decrease in blood Hb concentration and circulating
number of RBCs. Redox dysregulation results mainly in hemolytic anemia and release of Hb, affecting redox metabolism
and NO scavenging. Anemia affects systemic hemodynamics and myocardial performance. Furthermore, patients with CVD
show disturbances in hemostasis and thromboembolism and increased mortality, which cannot be effectively treated by
blood transfusion or substitution of ESAs. CVD, cardiovascular disease; ESA, erythropoiesis-stimulating agent; Hb, he-
moglobin; NO, nitric oxide; PLT, platelet; RBC, red blood cell. To see this illustration in color, the reader is referred to the
web version of this article at www.liebertpub.com/ars
722 KUHN ET AL.