Unit 4 Physiology
Keyto the Exercises
1.Preparation
Watcha video clip. After watching, please answer the following questions?
1 ) Because they are all made of cells. Asingle cell contains the information that
determines all the organisms' characteristics.
2 ) The structure and function of cellsdictate many of the basic behaviors of living
organisms.
3 ) Unicellular organisms.
Script
The diversity of life on earth isastounding. Organisms both large and small have
adapted to the many different habitats onthe planet. And along the way, they have
developed unique traits and methods ofsurvival. But at a microscopic level, all living
organisms are very similar. They are allmade of cells. A single cell contains the
information that determines all theorganisms' characteristics. A giant sequoia in
California begins as a single cell andeventually grows to be one of the largest trees in
the world. As the fundamental unit of life,the structure and function of cells dictate
many of the basic behaviors of livingorganisms. Animals eat, because their cells need
energy, and they breathe, because theircells need oxygen. Cells are able to function and thrive
on their own. Unicellular organisms are sosuccessful that they can be found in every habitat on
earth, no matter how extreme. Withoutcells, life, as we know it, would not exist. Life on earth
is diverse, but it all can be traced to asingle building block—the cell.
IIReading
TextA
ComprehensionCheck
Pairwork: Discuss and answer the following questions. Try not to back at the text.
1)Certain fundamental activities are commonto almost all cells and represent the
minimal requirements for maintaining cellintegrity and life. ( Para. 1 )
2 ) cell differentiation ( Para. 2 )
3 )"Hierarchy" refers to a systemof organized structures? in which differentiated cells
with similar properties aggregate to formtissues, which combine with other types of
tissues to form organs9which are linkedtogether to form organ systems. ( para. 3)
4) The criterion is differences instructure and function. ( para. 4 )
5 ) Yes, it means differently. In "connectivetissue", it means "an aggregate of a single
type of
type of specialized cell", while in "cardiactissue", it means "the general cellular fabric
of any organ or structure". ( Para. 6)
6 ) C. ( Para. 7 & 8)
7 ) Entitling the text as A Society ofCells", the author wants to emphasize that the
human body can be viewed as a complexsociety of differentiated cells, which are
structurally and functionally combined andinterrelated to carry out the functions
essential to the survival of the entireorganism. ( para. 11 )
LanguagePractice
1.Read the text carefully and then fill out the missing information in thefollowing short
passage.The beginning letter of each missing word has been given.
1 ) amebae/amebas 2 ) multicellular 3 )aggregates 4) solitary
5 ) tissues 6) epithelial 7 ) extracellular8) Organs
9 ) collection 10) interact
2.Paraphrase the following sentences. Pay special attention to the underlinedparts.
1 )... Each cell is made specific to servea particular function...
2 ) Of the cells that have become differentin the process of development, those with
similar properties group in a class to formtissues...
3)... The term "tissue"is alsocommonly used to mean the general organization of cells
of any organ or structure...
3.Identify the expression from each of the following sentences which correspondsin
meaningto one of the Chinese expressions given below. (These sentences are wellwritten.
Itis suggested that you read them aloud and learn them by heart. )
1)(be)common to共有的共同的
requirement( s) for需求必要条w必备条件
2)in turn继而依次
3)transform...Into...将······转化为·····。
4) differentiate into 分化 为
5)in various combinations以各种各样的组合
a hierarchy of多级的(结构)体系层次体系
6) (be) analogous to 与 · · · · · · 相似
7)(be viewed as被看作被视为
8)as opposed to相对于；而不是
4.Translate each of the following sentences into English with some expressionsyou have
identifiedin exercise 3.
1 ) Almost all of the cells that constitutethe basic units of human body exhibit the
fundamental functions common to all formsof life.
2) The remainder of the energy istransferred to another molecule that can in turn
transfer it to yet another molecule.
3) Once in the tissue, monocytes undergoanatomical and functional changes that
transform them into macrophages.
4) The atrium may be conveniently viewed asa continuation of the large veins.
5) B cells, upon activation, proliferateand differentiate into plasma cells, which secrete
antibodies.
6) The induction of one hormone's receptorsby another hormone may be essential to
the functioning of the first hormone.
7 ) When a ventricle with systolicdysfunction ( as opposed to a normal ventricle )
becomes completely filled with blood, itsforce of contraction actually decreases and
the situation worsens.
8) The relationship between a single musclefiber and a muscle is analogous to that
between a single neuron and a nerve.
Note: The underlining Points out the expressionsidentified in exercise 3.
5.0ral practice: describe the formation and function of the extracellular matrix.
The extracellular matrix consists of amixture of protein molecules (and, in some
cases, minerals) specific for any giventissue. The proteins of the extracellular matrix
consist of fibers-ropelike collagen fibersand rubber band-like elastin fibers-and a
mixture of other proteins that containchains of complex sugars ( carbohydrates). In
some ways, the extracellular matrix isanalogous to reinforced concrete. The fibers of
the matrix, particularly collagen, whichconstitutes one-third of all bodily proteins, are
like the reinforcing iron mesh or rods inthe concrete, and the carbohydrate-containing
protein molecules are the surrounding cement.With this formation, the matrix
provides a scaffold for cellularattachments, and transmits to the cells information, in
the form of chemical messengers, that helpsregulate their migration, growth, and
differentiation. (Para. 7 & 8)
TextB
ComprehensionCheck
Answerthe following questions. Try not to look back at the text.
1 ) Because erythrocytes contain largeamounts of the protein hemoglobin with which
oxygen and, to a lesser extent, carbondioxide reversibly combine. ( Para. 1 )
2) Erythrocytes look like a biconcavedisk-that is, a disk thicker at the edges than in the
middle, or like a doughnut with a centerdepression on each side instead of a hole.
This shape and their small size ( 7pm indiameter) impart to the erythrocytes a high
surface-to-volume ratio, so that oxygen andcarbon dioxide can diffuse rapidly to and
from the interior of the cell. ( Para. 2)
3) Normally, we can only find reticulocytesin the bone marrow¥ but they can also be
found in the blood in situations whenerythrocytes are unusually rapidly produced.
(Para. 3 )
4) As old erythrocytes are destroyed in thespleen (and liver), their iron is released into
the plasma and bound to an iron-transportplasma protein called transferrin. Almost
all of this iron is delivered bytransferrin to the bone marrow to be incorporated into
new erythrocytes. ( Para. 8)
5 ) When folic acid is not present inadequate amounts, impairment of cell division occurs
throughout the body but is most striking inrapidly proliferating cells? including
erythrocyte precursors. (Para. 9)
6) When there is a decreased oxygendelivery to the kidneys, the erythropoietin secretion
rate is increased markedly above basalvalues. As a result of the increase in
erythropoietin secretion, plasmaerythropoietin concentration, erythrocyte production
in bone marrow, blood hemoglobinconcentration, and the oxygen-carrying capacity of
the blood all increase; therefore, oxygendelivery to the tissues returns toward
normal. (Para. 13 )
LanguagePractice
Translatethe following sentences into Chinese.
1)红细胞中含有大量的血红蛋白，它们可以与氧和少部分的二氧化碳进行可逆性结合。
2)这样的形状和小尺寸(直径为7卜m)使红细胞具有较大的表面积与体积之比，这样一来
氧与二氧化碳就可以在红细胞内外之间迅速扩散。
3)然而，在红细胞生成异常迅速的情况下，许多网织红细胞会进人血液，这种现象对于临
床诊断很有帮助。
4)二个红细胞的平均寿命大约为12o天，这意味着每天体内大约有1%的红细胞被破坏
并由新生红细胞所取代，也就是每天总共2500亿个红细胞!
5)显著的铁平衡紊乱的结果是铁缺乏导致血红蛋白生成不足，或者机体铁过量造成严重
6)促红细胞生成素作用于骨髓，刺激红细胞的祖细胞增殖与分化成为成熟的红细胞。
7)最后，也存在与贫血恰好相反的(也就是红细胞异常增多)的情形，我们称之为红细胞增
多症。
III.video watching and speaking
ClipOne
pleasefollow the video clip to enjoy a physiology tour land do the exercises below.
1.Take notes while you are watching the video clip and then answer the followingquestions.
1 ) The cells are unified by a common setofinstructions that are copied for use and
passed from one cell to another.
2 ) The cells are diverse in size, shape andfunction.
3) Muscle, epithelial, and connectivetissues.
4) Different tissues combine to form anorgan. Yes，a vein is an organ.
5) Five levels. Cells, tissues, organs,organ systems, and the organism.
2.Listen and dictate. You are required to write down the exact words you hear.After
finishingit, you may follow the speaker to repeat the lines. Pay special attention tothe
pronunciationand intonation.
Answer See the Script below.
Script
A cell, the basic unit of life，forms allliving things. Unified by a common set of
instructions that are copied for use andpassed from one cell to another, the cells of a
complex organism show remarkable diversity.They are different in size and shape. The
folded cells of the kidney look verydifferent from the slender cells of nerves, and they
function differently. Yet for complexorganisms to survive\ similar cells must group
together as tissues that perform a commonfunction. In an arm, muscle tissue contracts.
In skin, epithelial tissue protects thesurface, while connective tissue provides flexibility.
Different tissues combine to form an organ,a functioning structure? such as a vein, or a
multilayered artery with tissues to lineit, make it elastic, contract it and hold it together.
Organs like the heart, capillaries andveins work together to form an organ system.
Finally, organ systems work together togive life to the organism.
ClipTwo
Watcha video clip and do the exercises below.
1.While you are watching the video clip, take notes for each of the followingaspects.
1)Major functions of blood: transportingoxygen and carbon dioxide, food molecules,
irons, waste, hormones and heat to everycell of our body defending the body
against the infections and other foreignmaterial
2)Function of the kidneys to maintain blooda vital organ filtering waste materials
from blood and producing urine monitoringthe blood to ensure sufficient oxygen
reaches every cell of the body
2.Fill in the blank in each of the following sentences to make them true.
1 ) a fluid
2) erythrocytes / red blood cells
3) Hemoglobin
4) nephrons
5) Erythropoietin / EPO
3.Tell your partner the process of erythropoiesis.
When the oxygen contained in the bloodpassing to the kidneys is low, or the
number of red blood cells decreases, thekidneys produce and release erythropoietin or
EPO-a hormone that stimulates unipotentstem cells to produce more red blood cells.
This process is called erythropoiesis.
Script
When we consider blood, we're faced with anorgan whose marvelous complexity is
matched only by its functional simplicity.Blood performs two major functions. The first
is transporting oxygen and carbondioxide,-food molecules, ions? waste? hormones and
heat to every cell in our body. The secondis defending the body against infections and
other foreign material. Blood is the onlyorgan in the body that is a fluid. Erythrocytes,
or red blood cells, comprise about 40% ofthe blood's volume. They contain
hemoglobin-a protein that gives blood itscharacteristic red color and enables it to carry
oxygen from the lungs and deliver it to allbody tissues.
Blood serves as a highly effective deliveryand disposal system for the body. To
remain a vital organ9 blood needs to bemaintained and replenished. One way is by
having wastes removed at regular intervals.The kidneys filter waste materials from
blood and produce urine. These functions
microscopic units within the kidneys. Eachbean-shaped kidney contains about one
million nephrons. The components of anephron include a thin-walled, bowl-shape
structure called Bowman's capsule andwithin that a glomerulus which is a tuft of
capillaries. In addition to theirfiltration responsibilities, the kidneys monitor the blood
to ensure sufficient oxygen reaches everycell in the body. When the oxygen content of
the blood passing through the kidneys islow, or the number of red blood cells decreases,
the kidneys produce and releaseerythropoietin or EPO-a hormone that stimulates
unipotent stem cells to produce more redblood cells. This process is called
erythropoiesis.
IV.Writing
Rewriteeach of the overloaded sentences below as two or more shorter sentences.
1 ) A man's internal body temperature is 37o c, and he is losing heat to the external
environment because it is at a lowertemperature. However, the chemical reactions
occurring within the cells of his body areproducing heat at a rate equal to the rate of
heat loss. under these conditions, the bodyundergoes no net gain or loss of heat, and
2) To identify mast cells, an adjacentsection was stained with alcian blue. The staining
shows that several mast cells are locatedin the media and adventitia region of the
intramural arteriole. However, the numberof alcian-blue-staining cells is lower than
that of cells that are positive for chymasemRNA shown in Fig. 5B.
COMMENT: The original sentence contains an embeddedidea ("but with the number
lower than that of cells that are positivefor thymase mRNA in shown Fig. 5B"). In
the revision, the embedded idea is put intoa separate sentence ( last sentence).
V.Autonomous Learning
VocabularyDevelopment
1.Fill in each of the following blanks with appropriate words.
1 ) leukocytes/white blood cells 2) bloodcells
3) blood clot 4) erythrocytes/red bloodcells
5 ) iron 6 ) oxygen
7 ) blood vessel 8) arteriole
9 ) vein 10) artery
2.Match each of the following terms with its meaning and write the appropriateletter to the
leftof each number.
1 ) e 2) d 3) a 4) b 5 ) c
3.Give the meaning for each of the following terms.
1 ) production of erythrocytes
2) deficiency of thrombocytes/platelets inthe blood
3) inflammation of veins
SupplementaryListening
Script
Blood is the essential fluid of life inhumans and other higher animals. Pumped by
the heart to the circulatory system, theblood carries oxygen, food, infection-fighting
antibodies and other vital materialsthroughout the body. About 55 % of the blood'
volume is made up of plasma, most of whichis water. plasma is the main vehicle. I
transporting blood cells, proteins andsalts. The rest of the blood's volume is made up of
blood cells called corpuscles. There arethree types of corpuscles. Red cells are the most
common. They contain a chemical substancecalled hemoglobin that draws oxygen from
the lungs and releases it to tissues. Redcells also return carbon dioxide to the lungs
where it is expelled. White cells are thebody's defense force, attacking any disease-
causing invaders. Shapeless,semitransparent bodies, white cells increase in number to
fight off infection. And platelets are cellfragments that help the blood to clot and
prevent the loss of this preciouslife-sustaining fluid.
Translationof the Texts
TextA
细胞的集群
细胞：生物体的基本单位
细胞是复杂多细胞生物体可分解成的、仍保留着作为生命特征的各项功能的最简结构单位。生物学中一个具有广泛意义的概念是：某些基本活动在几乎所有细胞都有共性，而且体现了维持细胞的完整性和生命的最低需求。因此，举例而言，人的一个肝细胞和一条阿米巴在以下几种方式上惊人地相似：与外在环境进行物质交换，从有机养分中获得能量，合成复杂分子，自我复制，探测所在环境的信号并产生应答。
人体起源于一个单细胞，即受精卵。受精卵分裂为两个细胞，其中每个细胞又依次分裂，产生四个细胞，以此类推。如果细胞增殖是发生的唯一事件，最终结果将产生一个由相同的细胞构成的球形团块。然而，在发育过程中，每个细胞为执行某一特定的功能，如产生力和运动（肌肉细胞）或产生电信号（神经细胞），而发生特化。非特化细胞转化成特化细胞的过程被称为细胞分化，它是当今生物学中最令人兴奋的研究领域之一。一个人的所有细胞都拥有相同的基因。那么，一个非特化细胞是如何接受指令并分化为一个神经细胞，而另一个分化为肌细胞，等等？构成这些“指令” 的外部化学信号是什么？它们如何以不同的方式影响各种细胞？在很大程度上，这些问题的答案都是未知的。
除了分化，细胞在发育过程中迁移到新的部位，并与其它细胞形成选择性连接，从而产生多细胞结构。人体细胞以这种方式被安排成不同的组合，形成一个多级的有组织的结构体系。具有相似特性的分化细胞聚集起来形成组织。这些组织与其他类型的组织结合在一起形成器官，器官再进一步结合形成器官系统（图1）。
图1 细胞组织的层次
从结构和功能的角度看，在人体内约存在200种不同的细胞。然而，当根据其执行的广泛功能进行分类时，细胞分为以下四种类型：（1）肌肉细胞，（2）神经细胞，（3）上皮细胞，（4）结缔组织细胞。在每一个功能类别中，都有几种类型的细胞，执行有所区别的特定功能。例如，人体存在三种类型的肌肉细胞，即骨骼肌、心肌和平滑肌，它们彼此形状不同，收缩活动的控制机制不同，其在身体各个器官的分布也不同。
肌肉细胞专门负责生成机械力，以产生力和运动。神经细胞专司电信号的发起和传导（通常是长距离的）。上皮细胞专管离子和有机分子的选择性分泌和吸收。结缔组织细胞，顾名思义，主要功能是连接、锚定和支撑身体的结构。
组织
大多数特化细胞与其他类似的细胞相联系，形成组织。根据分化细胞的四大类别，有四大类别的组织：（1）肌肉组织，（2）神经组织，（3）上皮组织，（4）结缔组织。应当指出的是，“组织”一词有不同的用法。它的正式定义是一个单一类型特化细胞的集合。然而，它也常用来表示任何器官或结构的整体的细胞构成。例如，肾组织或肺组织，事实上它们中的任一个通常都包含四种类型的组织。
体内单个细胞的直接环境是细胞外液。事实上，这种流体散布在复杂的细胞外基质中。细胞外基质由具有组织特异性的蛋白质分子（或矿物质）的混合物构成。基质一般有两个功能：（1）它为细胞粘附提供支架，（2）它以化学信使的形式将信息传送到细胞，以便调节细胞的迁移、生长和分化。
细胞外基质的蛋白质由纤维（绳索样胶原纤维和橡皮筋样的弹性蛋白纤维）和包含复糖（碳水化合物）链的其他蛋白质的混合物构成。在某些方面，细胞外基质与钢筋混凝土类似。基质纤维，尤其是占身体总蛋白质三分之一胶原蛋白，如同混凝土中的钢筋铁网或铁杆。含碳水化合物的蛋白质分子则像周围的水泥。不过，后者的功能并不只是如在混凝土中充当惰性“填料”的水泥一般。其充当细胞间的粘附/识别分子，并在胞外信使分子和细胞的交流中发挥重要的纽带作用。
器官和器官系统
器官由4种组织按照不同比例组成，通常有以下组成模式：片状、管状、层状、束状、带状等等。许多器官由被称为功能单位的小的、类似的亚单位组成。功能单位执行器官的功能。
共同执行的整体功能的器官的集合构成器官系统。例如，肾、膀胱、肾脏与膀胱之间的管道、从膀胱到体外的管道共同构成泌尿系统。
综上所述，人体可以被看作一个由分化细胞构成的复杂的集群。这些细胞在结构和功能上相互结合、联系，以执行生物体存活所必需的各种功能。单个细胞构成这个集群的基本单位，并且，几乎所有细胞独自展示出一切生命形式中共同的基本活动。的确，许多细胞可以被取出并在试管里作为独立生存的生物体继续存活（这被称为in vitro，字面意思是“在玻璃器皿里”，与in vivo相对（字面意思是“在身体内”）。
TextB
红细胞
红细胞的主要功能是运输肺吸入的氧以及细胞产生的二氧化碳。红细胞中含有大量的血红蛋白，它们可以与氧和少部分的二氧化碳进行可逆性的结合。其中,氧与血红蛋白分子中的铁原子结合。女性的平均血红蛋白浓度为14g/100 ml,而男性为16 g/100 ml。
红细胞为双凹圆碟形，即边缘厚、中心薄的圆碟状，就象一个两面中央凹陷而没有洞的甜甜圈(图1)。这样的形状和小尺寸（直径为7μm）使红细胞具有较大的表面积与体积之比，这样一来氧与二氧化碳就可以在红细胞内、外之间迅速地扩散。红细胞的细胞膜含有个体特异的多糖与蛋白，这些物质赋予了血液不同的类型，即血型。
图1 电镜下的红细胞
红细胞生成的部位是骨骼内部被称作骨髓的柔软部分，确切地说，是红骨髓。在分化过程中，红细胞的前体细胞产生血红蛋白，但最终会丢失细胞核与各种细胞器—细胞合成蛋白质的机构。骨髓当中的新生红细胞仍含有少量的核糖体，它们会使红细胞在特殊染色下呈现出网状结构，这类细胞因此得名“网织红细胞”。正常情况下，只有丢失了核糖体的成熟红细胞才能离开骨髓进入全身循环。然而，在出现红细胞生成异常迅速的情况时，许多网织红细胞会进入血液，这种现象对于临床诊断很有帮助。
由于红细胞缺乏细胞核与细胞器，它们既无法自我复制也无法长时间维持自身的正常结构。一个红细胞的平均寿命大约为120天，这意味着每天体内大约有1%的红细胞被破坏并由新生红细胞所取代。也就是每天总共2500亿个红细胞！正常情况下，红细胞的破坏发生在脾脏和肝脏。这一过程中释放的铁绝大多数被保存起来，后面会做专门讲述。血红蛋白的主要降解产物为胆红素，它决定了血浆的颜色。
红细胞的生成所需的基本营养物质和其他任何细胞无异：氨基酸、脂类与碳水化合物。另外，铁和某些生长因子（包括叶酸、维生素B12等维生素）都是必需的。
铁前面已经提到过，铁是红细胞中的血红蛋白分子结合氧的元素。体内少量的铁通过尿液、粪便、汗液以及皮肤脱落细胞等途径流失的。另外，女性还会通过月经额外丢失一部分铁。为了维持铁的平衡，机体失去的这部分铁必须通过摄入含铁的食物来加以补偿。富含铁的食物有：肉类、动物肝脏、贝类、蛋黄，豆类、坚果以及谷物。显著的铁平衡紊乱的结果是：铁缺乏导致血红蛋白生成不足，或者机体铁过量造成严重的中毒症状（血色素沉着症）。
铁平衡的自稳调节的部位主要是小肠上皮。它会主动地从摄入的食物中吸收铁。正常情况下，只有一小部分摄入的铁被吸收。然而更重要的是，铁吸收的量的增加或减少受机体铁平衡状态的负调控--身体中铁越多，对摄入铁的吸收量就越少。
机体有相当大的铁储备。肝脏是主要的铁储备库，铁在此被铁蛋白缠绕。铁蛋白充当了机体铁缺乏时的缓冲器。身体铁总量的约50%存在于血红蛋白，25%的铁存在于身体细胞中的其他含血红素的蛋白（主要是细胞色素），还有25%以铁蛋白的形式存在于肝脏中。此外，机体铁的循环利用的机制是相当高效的(图2)。当衰老的红细胞在脾（与肝脏）被破坏时，细胞内的铁就被释放至血浆当中并与一种被称为“转铁蛋白”的血浆铁转运蛋白所结合。这种来源的铁几乎全都被转铁蛋白运送至骨髓被新生红细胞所吸收。红细胞铁的再循环具有极为重要的意义，因为其每天所涉及的铁量比机体吸收与排泄的铁量多20倍。与此相比，规模小得多的是：红细胞以外的其他细胞（其中部分持续地死亡并被取代）将铁由细胞色素中释放至血浆中，并以转铁蛋白为载体从血浆中摄取铁。
图2 铁平衡总图。以箭头的粗细示相关铁含量。铁稳态下，胃肠道吸收的铁量等于经尿液、皮肤以及月经所流失的铁总量。(改编自Crosby的作品)
叶酸与维生素B12 叶酸是多叶植物、酵母和动物肝脏中富含的一种维生素，为核苷酸碱基胸腺嘧啶的合成所必需。它因此为DNA的合成所必需，从而为正常的细胞分裂所必需。当这种维生素不足时，全身的细胞分裂受损，其中以快速增殖的细胞（包括红细胞的祖细胞）最为显著。因此，当叶酸缺乏时，机体产生的红细胞的数量会减少。
产生正常数量的红细胞还需要另一种极其微量（百万分之一克/天）的含钴分子——维生素B12 (也称为钴胺素)。原因是叶酸发挥作用离不开这种维生素。维生素B12只存在于动物性食物中，因此严格的素食者饮食会缺乏这种维生素。而且，维生素B12经胃肠道吸收的过程需要一种叫做内因子的蛋白，它由胃分泌。内因子缺乏也会造成维生素B12缺乏。
红细胞生成的调控 对于一个正常人，循环中的红细胞的总容积总是维持非常恒定。这是由于相关的反馈机制对骨髓产生这些细胞进行调控。前文叙述了铁、叶酸、维生素B12等物质对于红细胞的正常生成的必要性。然而，这些物质当中没有一种算是调控红细胞生成率的信号。
对红细胞生成的直接调控主要由被称作红细胞生成素的激素所施加的。这种激素主要由肾脏中的一群特殊的分泌激素的结缔组织细胞分泌入血。肝脏也会分泌这种激素，但数量上要少得多。促红细胞生成素作用于骨髓，刺激红细胞的祖细胞增殖与分化成为成熟的红细胞。
促红细胞生成素正常情况下分泌量相对较小，却足以刺激骨髓以适当的速率产生红细胞以取代日常损失的部分。当肾脏的供氧量下降时，促红细胞生成素的分泌速率就会在基础水平上显著上升。以下情况将发生上述现象：心脏泵血不足、肺部疾病、贫血（红细胞数量减少或血红蛋白浓度降低）以及暴露于高海拔的环境中。促红细胞生成素分泌增加会造成血浆促红素浓度、红细胞生成以及血液携氧能力的增加。因此，运输至组织的氧又恢复至正常(图3)。
图3 通过反射机制，肾脏供氧量下降引起促红细胞生成素分泌增加，从而使红细胞生成增多。
男性分泌的性激素，睾酮，也能够刺激促红细胞生成素的释放。这至少可以部分解释为什么男性的血红蛋白浓度高于女性。
贫血 贫血被定义为由于以下原因造成的血的携氧能力下降：（1）红细胞总数下降，但每个细胞的血红蛋白含量正常；（2）每个红细胞的血红蛋白含量下降；（3）以上两种原因同时存在。贫血的致病因素相当广泛，总结于表1。
最后，也存在与贫血恰好相反的（也就是红细胞异常增多）的情形，我们称之为红细胞增多症。一个典型例子是高原地区的居民。在这种情况下，红细胞数量的增多是一种适应性反应。然而，红细胞增多症会使血液流经血管变得更为困难。
