Page 101 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

4 Blood

(e.g., heme) can be protected from breakdown
Composition and Function of Blood
and renal excretion. The binding of small
The blood volume of an adult correlates with molecules to plasma proteins reduces their
his or her (fat-free) body mass and amounts to osmotic efficacy. Many plasma proteins are in-
ca. 4–4.5 L in women (!) and 4.5–5 L in men of volved in blood clotting and fibrinolysis.
70 kg BW ("; ! table). The functions of blood Serum forms when fibrinogen separates from
include the transport of various molecules (O 2, plasma in the process of blood clotting.
CO 2, nutrients, metabolites, vitamins, electro- Theformationofbloodcellsoccursinthered
lytes, etc.), heat (regulation of body tempera- bone marrow of flat bone in adults and in the
ture) and transmission of signals (hormones) as spleen and liver of the fetus. Hematopoietic tis-
well as buffering and immune defense. The suescontainpluripotentstemcells which,with
blood consists of a fluid (plasma) formed el- the aid of hematopoietic growth factors (see
ements: Red blood cells (RBCs) transport O 2 below), develop into myeloid, erythroid and
and play an important role in pH regulation. lymphoid precursor cells. Since pluripotent
White blood cells (WBCs) can be divided into stem cells are autoreproductive, their existence
neutrophilic, eosinophilic and basophilic is ensured throughout life. In lymphocyte
granulocytes, monocytes, and lymphocytes. development, lymphocytes arising from lym-
Neutrophils play a role in nonspecific immune phoid precursor cells first undergo special
defense, whereas monocytes and lymphocytes differentiation (in the thymus or bone marrow)
participate in specific immune responses. and are later formed in the spleen and lymph
Platelets (thrombocytes) are needed for he- nodes as well as in the bone marrow. All other
mostasis. Hematocrit (Hct) is the volume ratio precursor cells are produced by myelocytopoie-
of red cells to whole blood (! C and Table). sis, that is, the entire process of proliferation,
Plasma is the fluid portion of the blood in maturation, and release into the bloodstream
which electrolytes, nutrients, metabolites, vi- occurs in the bone marrow. Two hormones, er-
tamins, hormones, gases, and proteins are dis- ythropoietin and thrombopoietin, are involved
solved. in myelopoiesis. Thrombopoietin (formed
Plasma proteins (! Table) are involved in mainly in the liver) promotes the maturation
humoral immune defense and maintain on- and development of megakaryocytes from
cotic pressure, which helps to keep the blood which the platelets are split off. A number of
volume constant. By binding to plasma pro- othergrowthfactorsaffectbloodcellformation
teins, compounds insoluble in water can be in bone marrow via paracrine mechanisms.
transported in blood, and many substances Erythropoietin promotes the maturation and
proliferation of red blood cells. It is secreted by
Blood volume in liters relative to body weight (BW) the liver in the fetus, and chiefly by the kidney
" 0.041!BW (kg) + 1.53, ! 0.047!BW (kg) + 0.86 (ca. 90%) in postnatal life. In response to an oxy-
Hematocrit (cell volume/ blood volume): gen deficiency (due to high altitudes, hemoly-
" 0.40–0.54 Females: 0.37–0.47 sis, etc.; ! A), erythropoietin secretion in-
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12
Erythrocytes (10 /L of blood = 10 /µL of blood): creases, larger numbers of red blood cells are
" 4.6–5.9 ! 4.2–5.4 produced, and the fraction of reticulocytes
Hemoglobin (g/L of blood): (young erythrocytes) in the blood rises. The life
"140–180 ! 120–160 span of a red blood cell is around 120 days. Red
MCH, MCV, MCHC—mean corpuscular (MC), hemo- blood cells regularly exit from arterioles in the
globin (Hb), MC volume, MC Hb concentration ! C splenic pulp and travel through small pores to
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3
Leukocytes (10 /L of blood = 10 /µL of blood): enter the splenic sinus (! B), where old red
3–11 (64% granulocytes, 31% lymphocytes, blood cells are sorted out and destroyed
6% monocytes) (hemolysis). Macrophages in the spleen, liver,
3
Platelets (10 /L of blood = 10 /µL of blood): bone marrow, etc. engulf and break down the
9
" 170–360 !180–400
88 Plasma proteins (g/L of serum): cell fragments. Heme, the iron-containing
group of hemoglobin (Hb) released during
66–85 (including 55–64% albumin)
hemolysis, is broken down into bilirubin
(! p. 250), and the iron is recycled (! p. 90).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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