Page 283 - Color Atlas of Biochemistry

274 Tissues and organs
Blood: composition and functions
Human blood constitutes about 8% of the
body’s weight. It consists of cells and cell frag- p. 294). The neutrophil granulocytes¸ mono-
ments inanaqueous medium,the blood
plasma. The proportion of cellular elements,
known as hematocrit, in the total volume is
approximately 45%.
A. Functions of the blood
The blood is the most important transport
medium in the body. It serves to keep the
“internal milieu” constant (homeostasis) and
it plays a decisive role in defending the body
against pathogens.
Transport. The gases oxygen and carbon
dioxide are transported in the blood. The
blood mediates theexchangeofsubstances
between organs and takes up metabolic end
products from tissues in order to transport
them to the lungs, liver, and kidney for excre-
tion. The blood also distributes hormones
throughout the organism (see p. 370).
Homeostasis. The blood ensures that a bal-
anced distribution of water is maintained be-
tween the vascular system, the cells (intra-
cellular space), and the extracellular space.
The acid–base balance is regulated by the
blood in combination with the lungs, liver,
and kidneys (see p. 288). The regulation of
body temperature also depends on the con-
trolled transport of heat by the blood.
Defense. The body uses both non-specific
and specific mechanisms to defend itself
against pathogens. The defense system in-
cludes the cells of the immune system and
certain plasma proteins (see p. 294).
Self-protection. To prevent blood loss
when a vessel is injured, the blood has sys-
tems for stanching blood flow and coagulat-
ing the blood (hemostasis; see p. 290). The
dissolution of blood clots (fibrinolysis) is
also managed by the blood itself (see p. 292).
B. Cellular elements
Thesolid elements in theblood arethe eryth-
rocytes (red blood cells), leukocytes (white
bloodcells), andthrombocytes(platelets).
The erythrocytes provide for gas transport
in the blood. They are discussed in greater
detail on pp. 280–285.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.
The leukocytes include various types of
granulocyte, monocyte, and lymphocyte. All
of these have immune defense functions (see
cytes, and the macrophages derived from
monocytes are phagocytes. They can ingest
and degrade invading pathogens. The lympho-
cytes are divided into two groups, B lympho-
cytes and T lymphocytes. B lymphocytes
produce antibodies, while T lymphocytes reg-
ulate the immune response and destroy virus-
infected cells and tumor cells. Eosinophilic and
basophilic granulocytes have special tasks for
defense against animal parasites.
Thrombocytes are cell fragments that arise
in thebonemarrow from large precursor
cells, the megakaryocytes. Their task is to
promote hemostasis (see p. 290).
C. Blood plasma: composition
The blood plasma is an aqueous solution of
electrolytes, nutrients, metabolites, proteins,
vitamins,trace elements,and signalingsub-
stances. The fluid phase of coagulated blood is
known as blood serum.Itdiffers from the
plasma in that it lacks fibrin and other coag-
ulation proteins (see p. 290).
Laboratory assessment of the composition
of the blood plasma is often carried out in
clinical chemistry. Among the electrolytes,
there is a relatively high concentration of
+ 2+ –
Na ,Ca ,and Cl ions in the blood in compar-
ison with the cytoplasm. By contrast, the con-
+ 2+
centrations of K ,Mg ,and phosphate ions
are higher in the cells. Proteins also have a
higher intracellular concentration. The elec-
trolyte composition of blood plasma is similar
to that of seawater, due to the evolution of
early forms of life in the sea. The solution
known as “physiological saline” (NaCl at a con-
–1
centration of 0.15 mol L )is almostisotonic
with blood plasma.
A list of particularly important metabolites
in the blood plasma is given on the right.